Human kidney

Human kidneys form an important organ of the excretory system. It is also a vital part of the urinary system. The soluble, non irritant and solid substances that are not utilized by the body are excreted by the kidney. Kidney also secretes out excess water. Apart from kidney, the human skin also excretes these substances, though to a smaller extent. The volatile substances are excreted out by the human lungs and the heavy metals are secreted out mainly by gastro intestinal tract. To a lesser extent liver and saliva also excrete out these substances. Fats are excreted out through skin (sebum) and liver (bile).

When the body is at rest, approximately 25% of the total output of blood from the heart is distributed in kidneys (through renal arteries). The kidney filters the blood and secretes out the unnecessary substances.
Human kidney is the main organ for excretion in the human body. It also performs homeostatic and endocrine functions. The urinary system, as a whole is comprised of those organs which produce urine and then carry out its elimination from the body.

Role of kidney as an organ of the urinary system- To absorb water etc. for formation of urine from the excretable material of the blood plasma that has been supplied to it by the renal artery.

There are also other organs in the urinary system. Their names and functions are described below.
Ureters- They are the ducts inside the human body for carrying the urine (formed in the kidneys) to the urinary bladder.
 Bladder- The organ acts as a urine reservoir. When you feel a pressure for urinating, it means that the urinary bladder is filled at a higher level.
Urethra- The passage through which the urine is passed from the urinary bladder to the external environment. In men, penis helps in urine excretion.
Formation of urine and its elimination are key functions through which the body maintains its homeostasis (regulation of the body’s internal environment).

LS (lateral section) of the Human Kidney

Kidney Anatomy – External

The Renal Vein, Renal Artery and the Ureter enter the kidney through the opening known as Hilus. All the vessels and the kidney itself are embedded in peri-renal fat (a mass of fat tissues). This mass extends into the central cavity or the renal sinus.
Renal Vein- They connect the kidney to the inferior vena cava and carry the blood purified by the kidneys.
Renal artery- They arise out of the abdominal aorta and supply blood to the kidneys. The renal artery has a radius of approx. 0.25 cm. One third of the cardiac output of blood is carried to the kidney by these renal arteries for purification. The right renal artery is longer than the left renal artery.
Ureter- They are made of smooth muscle fibers. Each Ureter from each of the kidney carries urine from it to the urinary bladder. The diameter of the ureters is 3 to 4 mm. They are 25 to 30 cm long. They arise from the pelvis of the kidney and cross at the brim of the pelvis (in front the iliac arteries). They enter the urinary bladder from the backside after passing through the sides of the pelvis.

Kidney Anatomy- Internal

The main three internal parts of the human kidney are renal cortex, renal medulla and renal pelvis.
The Renal Medulla- It is composed of conical masses that are called renal pyramids. The apex regions of the entire renal pyramids combine together to form the renal papilla. The papilla projects themselves inside the lumen of minor calyces.

Renal Cortex- The renal cortex is located in between the medullary pyramids. These regions are also called as Renal Columns of Bertini.
Renal Pelvis- The short or minor calyces that surround the renal papilla have a cup shape. They combine and converge to form major calyces. All the major calyces then combine to form the renal pelvis.

The Renal Blood Vessels

 Both the cortex and medulla have a complex network of vessels that carry blood. Through the renal pyramids all the blood arteries pass. These arteries are called Interlobar Arteries. These Interlobar arteries arch around the renal pyramid’s base and form the Arcuate Artery.The Arcuate arteries form a series of branches and enter the renal cortex. They are then called the Interlobular Arterioles. These arterioles give rise to afferent arterioles. The afferent arterioles divide themselves to form the glomerulus (a capillary network). All the glomerulus combine with each other to form the efferent arteriole (afferent means towards an organ and efferent means away from an organ).
The efferent arteriole goes back in the medulla and is called the Vasa Recta.

The shape, size and location of human kidneys

The human kidneys have a bean shape. They are located just behind the vertebral column in the abdomen. More specifically, they are located in the 12th thoracic to the 3rd lumbar segment. The average size dimensions of human kidney are:

Length-10 to 12 cm.
Width- 5 to 6cm.
Thickness- 3 to 4 cm.
Weight- A healthy male kidney weighs 150 gm in an adult male and 135 gm in adult female.

In most of the cases, the right kidney is smaller than the left kidney by a fraction.

The renal artery, renal vein, nerves and the ureters pass through a concavity that is present at the medial border. The concavity is called Hilus.If we section the human kidney vertically, then we will find opposite the Hilus a dark reddish region that is called as Cortex. Rest of the lighter area is known as Medulla. The medulla is divided in 10-15 conical areas that are known as Renal Pyramids. The boundaries of these lateral pyramids are made of projection of the dark reddish cortex and are called Columns of Bertin. The brownish lines of the pyramid converge towards the apex which is also called Area Cribrosa.
At the renal medulla certain very important substances are formed which help in regulation of body’s blood pressure. These are PGA-2, PGF-2 and PGF-2 alpha.

Nephron- the smallest unit of Kidney

The functional microscopic unit of kidney is Nephron. Each human kidney contains about 1 million nephrons. The main function of homeostasis and excretion is performed by these nephrons only. The nephrons drain what they absorb into the Pelvis of Ureter. The drained fluid or urine passes down the ureters (ducts) and is stored in the urinary bladder.
Uriniferous Tubule is the name given to the combination of Nephron (secretory part) and duct (non secretory part).
According to their positions, Nephrons are divided in two categories:

The Superficial Nephron or Superficial Glomeruli- They form the major chunk of Nephrons (85%) and occupy the outer two third area of the Renal Cortex. They are smaller in size in comparison to the Juxtameduallary Nephrons and are fully functional under normal conditions.
Juxtamedullary Nephrons or the Juxtamedullary Glomeruli- They occupy the inner one third area of the renal cortex. They constitute the rest of the 15 % of the total Nephrons. They are larger in size than the superficial Nephrons and work only under stress.

Parts of Nephron

The human nephron is formed of different parts. They are described below.

1. Renal or Malphigian Corpuscle- The malphigian corpuscle is 200 µm in diameter. The corpuscle is only found in the renal corpuscle. It is comprised of 2 parts.

A. Glomerulus and, B.  Bowman’s capsule

A. Glomerulus- The capillary which invaginates the Bowman’s capsule is called Glomerulus. The Glomerulus lies within the Bowman’s capsule and has around 20 to 50 capillary loops. The afferent arteriole brings blood to the glomerulus and efferent arteriole takes out the filtered blood away from the glomerulus. The afferent arteriole is wide and short and the efferent arteriole is long and narrow. The arrangement is in such a way that the glomerular blood pressure is much higher (70 mm of Hg) than the capillary/artery blood pressure. This facilitates filtration of blood. The combined surface area of the glomerular capillaries or the total filtration bed of the human body is 1.73 sq meter.
B. Bowman’s Capsule
 The end of the Nephron is called the Bowman’s capsule. It has 2 layers viz. parietal and visceral.
The filtering layer/membrane of the Malphigian corpuscle consists of -

1. The endothelial cell layer.
2. A basement membrane.
3. The epithelial cells of the visceral layer of the bowman capsule. After developing into the visceral layer the epithelial cells are also known as Podocyte or Glomerular Epithelial Cell. The podocytes give rise to tentacle like cytoplasmic processes called End Feet or Pedicles. These have large number of minute branches.
4. The endothelium of the glomerulus also contains deep cells/stalk cells /mesangial cells. They are found between the glomerulus and the basal lamina. In certain pathological conditions, these cells turn into phagocytes and proliferative.

2. Renal Tubule- The glomerulus has 2 poles. The first pole is the vascular pole. At the vascular poles the blood vessels are attached. The renal tubule begins at the opposite pole or the tubular pole. The length of human renal tubule is about 3cm. it is 20-60 µm in width. If we measure the length of all the renal tubules put together in the 2 million nephrons that are present in the 2 human kidneys, then we will have a 65 km long renal tubule! The renal tubule has a neck just below the glomerulus. After the neck, the renal tubule is divided into following parts.

1. Proximal Convoluted tubule/Pars convolute- The length of PCT is approx. 14mm. The external diameter of PCT is 60 µm and interior diameter is 15-25 µm. The pct has high amounts of twists and runs. The cells of PCT are very active and have large number of enzymes. Thus they play greater role in metabolism. Two third of water, all glucose and parts of sodium, phosphate and chloride are reabsorbed form the glomerular filtrate in the PCT.
2. Henle’s loop-

It is a loop that appears after the PCT. It has a U shape and it dips to variable length inside the medulla. The parts of Loop of Henle are:

a. Descending limb
b. Ascending limb (thin walled)
c. Ascending limb (thick walled).

In the superficial nephrons, the length of Henle’s loop is very less. It also dips only slightly in the medulla in the juxta medullary nephrons; the loop is of considerable length and passes deep inside the renal medulla. The average length of Henle’s loop is 20 mm.
The membrane of descending loop is more permeable to water than it is to urea and NaCl. Thus more of the water is removed in this area. With the loss of NaCl, the fluid becomes less dense in the ascending limb. The thick portion of the ascending limb is also known as the functional proximal tubule. Active chloride re-absorption occurs in both the parts of ascending limb.

3. Distal or second convoluted tubule- The DCT is located in the cortex region of the nephron. The average length of DCT ranges from 4.6 mm to 5.2mm.its diameter ranges from 20 µm to 50 µm. It is lined by cubical epithelial cells which do not have brush borders. The DCT’s lumen diameter is larger than that of the PCT.

The central part of DCT comes in contact with the juxtaglomerular cells of the afferent vessel. This region of cells has closely packed nuclei and is termed as Macula Densa (as they have a dense structural network). The juxtaglomerular cells, macula densa and Lacis (extra glomerular mesangial cells) cells together form the juxtaglomerular apparatus. Vasopressin or ADH is a hormone that plays key role in determining the re-absorptive capacity of the DCT. Hence it also plays a vital role in deciding the urinary output.

Straight tubule or collecting tubule- The 20 mm long tubule is lined by cuboidal cells.

Duct of Bellini -Several straight tubules from the different nephrons join with each other to form the Duct of Bellini. The duct opens at the apex of the renal pyramid.

Juxtaglomerular Apparatus

The term juxtaglomerular apparatus was coined by Goormaghtigh. The apparatus includes

1. The afferent arteriole’s granular juxtaglomerular cells.
2. The DCT’s macula densa, and
3. Lacis Cells- found at the entry and exit points of afferent and efferent arterioles of each glomerulus.

The entire three parts combine together to form the juxtaglomerular complex. The apparatus plays a key role in control of blood pressure, in erythropoesis, in maintaining salt balance and renal flow.

Working process of human kidneys

The nephron is the smallest unit of kidney which does the filtering work. Each nephron filters a very small volume of blood. Much  of the material that is filtered by the glomerulus is reabsorbed by the tubules (including Proximal Convoluted Tubule and Distal Convoluted Tubule).

Selective Re-absorption
1. Protein- The glomerular filtrate has a small amount of protein. This protein is reabsorbed in the PCT.
2. Glucose- Glucose is completely reabsorbed in the PCT.
3. Water-The Glomeruli of all the nephrons in adult kidneys filter around 170 liters of water a day (both the   kidneys). Out of this approx. 1.5 liters is excreted out per day.
4. Blood cells- The nephrons do not allow passage to blood cells and large protein molecules. They are not absorbed by the permeable membranes of the nephron and are not excreted in urine.
5. Sodium and chloride- Both sodium and chloride are absorbed in the ascending limb of Henle’s Loop. The Glomeruli filter around 560 gm of Na+ per day. Out of this 5 gm is excreted in the urine. The rest is reabsorbed in the Proximal Tubule (490 gm) and distal tubule(65gm).
6. Potassium- Only 5 to 10% of the filtrate of potassium is excreted through the urine.  The rest is reabsorbed in the proximal tubule.
7. Bicarbonate- All the bicarbonate filtered by the glomerulus is reabsorbed in the renal tubule.
8. Phosphate- much of the phosphate is reabsorbed in the proximal tubule.
9. Re-absorption of other substances-  Other substances that are reabsorbed in the tubule are  ketone bodies, beta-hydroxybutyric acid, uric acid, sulphate, ascorbic acid, ketone bodies, creatin and some amino acids.
10. Urea- Is filtered in PCT and other parts.
The GFR or glomerular filtration rate is 125 ml per minute.

Functions of kidney

1.It helps to maintain the water balance in the body and regulates plasma volume.
The kidney performs various functions. The functions of human kidney are listed below.
2. It is the main excretory organ of the body. It excretes all the waste products. Of vital importance to excretion is the excretion of the nitrogenous and sulphur containing waste products that are formed in the body during protein metabolism. Such waste if gathered in the body can make the blood toxic.
3. It eliminates the drug residues and other toxic material from the body.
4. It maintains the hydrogen ion concentration and also maintains the electrolyte balance of the body.
5. The kidney also manufactures some new substances in the body during its waste processing process. These are inorganic phosphates, ammonia and hippuric acid. Ammonia preserves the acid base equilibrium of the body. Thus kidneys can be said to be the junk processor of the human body. They excrete the waste and also process some wastes into useful compounds.
6. During the condition of hypoxia, the kidneys regulate the blood pressure of the body. They do this by liberating rennin from the juxtaglomerular apparatus.
7. Kidneys help in metabolizing the Vitamin D inside the body.
8. Kidneys help in the regulation of erythropoiesis (process of RBC production).
9. The kidneys help in maintaining the osmotic pressure at desired levels in the tissues and blood of humans.
10. The kidney also selectively reabsorbs certain material from the blood. The process is vital for keeping the concentration optimum of certain constituents of blood. 

Diseases associated with the human kidney

Congenital kidney disorders
When a person is born with an impaired kidney or set of kidneys, then it is a case of congenital disease. The abnormalities include deformed kidneys, for example a single horseshoe kidney in which both the kidneys join at the base to form a single kindly that has a horseshoe shape. Other deformations include a single kidney, two ureters for a single kidney and both the kidneys on one side. A person can live with a single kidney though.
When the waste products accumulate in the body then the kidney fails or undergoes a functional abnormality. Heavy ingestion of toxic materials can lead to such an event. Also, over usage of certain drugs, malnutrition etc can also be the causes for kindly failure. The patient develops shortness of breath, weakness, confusion and lethargy as signs of kidney failure. The kidney fails to remove excess of potassium from the body because of which abnormality in heart functioning occurs. This may result in cardiac arrest and death. Sometimes the kidney failure is irreversible and cannot be cured.
Acute kidney diseases

1. Pyelonephritis - Pyelonephritis is an infection of kidneys by any harmful microorganism. It is a common infectious disease and can be treated successfully with antibiotics. In this condition the symptoms include fever, pain in abdomen, and painful and frequent urination.
2. Kidney stones- When stone occurs in the kidney they need to be broken to smaller particles. Then they flush out of the kidney through urine. They are made of hard crystals that separate in the urine formation process and accumulate in the kidneys.
3. Acute kidney failure- It occurs when someone has an injury that significantly reduces his blood flow, in conditions of severe dehydration, stones, infections and exposure to toxic chemicals and drugs.
4. Nephritic syndrome- the syndrome occurs when there is high level of protein the urine and small level of protein in bloodprotenuria. Te symptoms  include swelling across the eye, hands and feet. Diabetes mellitus and other diseases can give rise to this condition.

The diagnosis for kidney failure is done by blood tests for creatinine, BUN, and glomerular filtration rate. If you want to prevent failing of kidneys then you must take lifelong measures for controlling blood pressure and diabetes. Some measures include exercising regularly and eating healthy balanced diet full of fruits and vegetables.
When the kidneys fail permanently then the patient has undergo a kidney transplant or dialysis. In dialysis or hemodialysis, an artificial machine known by the name dialysis machine acts as an artificial kidney.
Renal hypertension
If a person has hypertension then he may have arteriolar sclerosis. In arteriolar sclerosis, the glomerular capillaries and arterioles thicken and harden. If the hypertension gets severe, then the sclerosis worsens and causes ischemia (inadequate blood supply), fibrosis (thickening and scarring) of the cortex and destruction of small vessels. In such condition, renal failure occurs.
Reno vascular hypertension
A renovascular disease may also cause malignant hypertension. This occurs when the renal artery’s diameter is decreased because of the accumulation of atherosclerotic plaque. This results in elevation of blood pressure. Because of increased blood pressure, the walls of the glomerular capillaries and small arterioles get damaged. The vessels break down (rupture) and cause excessive bleeding (hemorrhage) and scarring (infarction). Thus fibrosis of the renal cortex and atrophy (decrease in effectiveness) of cells occur in the renal cortex.
Atherosclerosis of renal artery
 There are three layers of the renal artery. They include the internal layer or intima, the middle layer or media and the external layer or adventitia. When arteriosclerotic plaque or lipid plaque gets deposited in the intima, it causes hardening of renal artery and fibrosis. Severe deposition results in hypertension.

Chronic disorders of the kidney
Chronic (long lasting) disorders are serious conditions as they result in kidney deterioration with time.
1. Glomerulonephritis- The Glomeruli of the kidneys become inflamed in a condition known as glomerulonephritis. High blood pressure and diabetes are a common cause of this disease.
2. Tumor or cancer in the kidney can result in kidney impairment and malfunctioning. When untreated, such conditions may result in and ESRD or end stage renal disease. When ESRD occurs, the kidneys fail to function and shut down completely.
 As we age, the functioning of Kidneys is reduced.

Human embryo development

The union of sperm with egg cell  takes place in fallopian tube of female reproductive system. After this union  diploid Zygote is formed with two sets of chromosome one set  of  chromosomes from sperm and another set   from ova.The two sets of genetic material from two sources recombine to recreate and a unique genetic combination, which unfolds gradually during embryonic development forming a baby. The baby inherits some traits from father and some from mother. The period in which the fertilized zygote develops in to fully matured baby in side uterus is called gestation period. The gestation period for  human is 9 months.

The fertilized zygote undergoes rapid cell divisions called Clevage which starts 30 hrs after fertilization. The zygote undergoes first mitotic division and forms two identical cells called Blastomeres. The clevage continues to form mass of 16 cells called Morula.The size of morula is same size as that of zygote. By 72 hrs Morula reaches uterus. Morula continues dividing and forms ball of 100 cells called Blastula. It is hollow at centre as is called Blastocoel. It  gets implanted in endometrial tissue. In rare conditions one out of 300 pregnancies, Blastula some where other than uterus in fallopian tube or abdominal cavity (Ectopic pregnancy) causes severe complications. 

The next stage in embryonic development is the formation of the body plan.The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation.  Each of these layers is called a germ layer, which differentiate into different organ systems.The three germs layers are the  outer ectoderm, , and middle mesoderm  and inner most endoderm

Organogenesis is the process by which the three germ tissue layers of the embryo, which are the ectoderm, endoderm, and mesoderm, develop into the internal organs of the organism by cell differentiation, the process by which a less-specialized cell becomes a more-specialized cell type. This must occur many times as a zygote becomes a fully-developed organism. In vertebrates, one of the primary steps during organogenesis is the formation of the neural system.

The ectoderm

It forms epithelial cells and tissues, as well as neuronal tissues.It gives rise to the nervous system and the epidermis. During the formation of the neural system, special signaling molecules called growth factors signal some cells at the edge of the ectoderm to become epidermis cells. The remaining cells in the center form the neural plate.

 The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube. In further development, the neural tube will give rise to the brain and the spinal cord .

The endoderm

It consists of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the digestive tube. It also forms the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas; the epithelium of the auditory tube and tympanic cavity; the trachea, bronchi, and air cells of the lungs; the urinary bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus. Additionally, the endoderm forms internal organs including the stomach, the colon, the liver, the pancreas, the urinary bladder, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestines.

The mesoderm gives rise to the muscle cells, bones and connective tissue in the body.

Week 3:

The embryo grows to a length of about 2 millimeters long  with rounded head and a tapering tail. There are four extra-embryonic membranes which protect the developing embryo they are the Yolk sac, the Allantois , the Amnion, and the Chorion.

 Amnion surrounds the body of the embryo like a fluid-filled balloon.It allows fetus to be suspended in amniotic fluid, protecting from mechanical injuries, allows normal fetal movement and accommodates growth.The normal amount of amniotic fluid  is typically 500 to 1000 ml.Amniocentesis is procedure of removing a small amount of amniotic fluid by inserting a needle through the mother’s abdomen and into the amniotic cavity.Fetal cells present in the amniotic fluid can be cultured and examined for various chromosomal and metabolic defects.Traditionally after birth of baby, amnion and and placental tissues were discarded, but these tissues were found quite promising for stem cell therapy as they contain pluripotent stem cells that  can form variety of tissues.

Yolk sac usually contains reserve food  'Yolk'  for developing embryo. It is well observed in embryos of fishes and amphibians. In human embryo Yolk sac is devoid of yolk and is vestigial as there is ample supply of nutrients from mother's blood.

 Allantois is large sac which is vestigial again in human embryos but in most aves and reptiles it stores urinary wastes and serves respiration.Allantois is continuous with forming urinary bladder of embryo.

Blood vessels begun to form and by day 20, the embryo has developed arteries and veins which circulates embryos blood. Cells aggregate along the embryo's dorsal surface to form neural tube. Neural tube eventually develop into the brain and spinal cord.

Week 4:

The embryo takes curved C shape as  eye stalks and ear pits appear. Upper and lower limb buds become noticeable. The umbilical cord and  facial areas develop. It`s size is approx 3-4 mm in this stage. Body tapers to spiraled tail which is prominent in 4 week embryos. Lung, liver, pancreatic, and gall bladder buds emerge in the form of mesodermal somites which are group of differentiating cells.Fourth week is marked by  rapid neural and cardiac development. On either sides of neural tube feature spinal cord primitive bones develop.  Nerves, muscle, and connective tissues emerge around the primitive bone formations.

Week 5:

the embryo is about 7 to 9 millimeters long and has all of its internal organs. The mouth, stomach, and urinary bladder are present. Nose pits and eye lenses are visible. A few days after upper limb bud extension, the lower limb buds evolve further.The external ears takes their form, and upper limb buds extend to form paddle like hands.  Extensive brain development occurs at this time, and the head grows big, causing it to  appear large compared to the body. The umbilical cord becomes more clearly defined.

Week 6:

The trunk of embryo straightens and upper limb continues developing. Bone, cartilage, and muscles become defined around the spinal cord and in the embryonic chest. Early in this week, tooth buds appear. These buds will become the baby teeth that are lost in childhood. Rib cells line up horizontally along the trunk sides, and skin.The regions of the brain that will become the cerebral hemispheres are very prominent at this time. The embryo appears more human by this point. It is about 11 to 14 millimeters long, and its rate of heart beat is 140 to 150 beats per minute.

Week 8:

Nerve cells in the brain form at a rate of about 100,000 a minute. The top of the head becomes more rounded and erect. Between day 52 and day 56, the fan shaped toes go from being webbed to separated. The fingers are entirely distinct. The eyelids close over the eyes and become fused shut until about the twenty-sixth week. External genital sex organ differences begin to develop. All appearances of the tail are gone. By day 56, the embryo is roughly 27 to 31 millimeters long.

The first three months of embryonic development are known as the first trimester. At the end of the first trimester, the embryo looks like an adult, with all major organs having been formed. It is about 7.5 cms long.  Additional time in the mother's womb is needed to permit further development of the organs.
At the beginning of the second trimester, the growing organism is no longer called an embryo, but a fetus. Fetal development continues through the second and third trimesters until it is ready for birth at the end of the ninth month.When fetus reaches the end of 9th month, uterus contractions begin due to Oxytosin hormone associated with labor pains. Child birth takes place by rupture of embryonic membranes by dilation of  cervical muscles and vagina. parturition. The process of normal childbirth is occurs in three stages the  dilation of the cervix,  birth of the baby fallowed by detachment  and exit of placenta  through vagina. The umbilical cord is clamped in two places and is cut in between.Respiration initiates normally by usual first baby cry. The newly born baby given warm water bath and swallowed amniotic fluid is removed out using a sterilized tube. The baby is allowed to feed on mother`s first milk called colostrum which has high concentrations of nutrients and antibodies. Antibodies in colostrum protect the mucous membranes in the throat, lungs, and intestines of the baby.

The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer, which differentiate into different organ systems

Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/

The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer, which differentiate into different organ systems

Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/

The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer, which differentiate into different organ systems

Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/

The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer, which differentiate into different organ systems

Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/

Menstrual cycle

The menstrual cycle is the series of changes that occur in the women`s body in anticipation of fertilization and  pregnancy. Menarche is first menstrual cycle of a girl. It can happen as early as about age 9 or up to age 15.About once a month, the uterus grows a new lining called endometrium to get ready for reception of fertilized egg.When there is no fertilized egg to start pregnancy the uterus sheds its endometrium. This is monthly menstrual bleeding. There are three phases in menstrual cycle which are controlled by hormones secreted by pitutary gland and ovaries. follicle stimulating hormone (FSH) and luteinizing hormone (LH) and prolactin are hormones secreted by Pitutary gland. Ovaries produce Oestrogen and progesterone  hormones

The entire duration of a Menstrual cycle can be divided into four main phases:

1. Menstrual phase (From day 1 to 5)
2. Follicular phase (From day 1 to 13)
3. Ovulation phase (Day 14)
4. Luteal phase (From day 15 to 28)

Menstrual phase

Menstrual phase begins on the first day of menstruation and lasts till the 5th day of the menstrual cycle. The following events occur during this phase.The uterus looses its inner lining of soft  endometrial tissue,unfertilized ova, blood vessels and blood in the form of menstrual fluid through vagina.This phenomena is called weeping of uterus.Blood loss of ten ml to eighty  ml is considered normal.  This fluid may be bright red, light pink or even brown.Women usually feel abdominal discomfort and use of sanitary napkins during this period is common practice to maintain hygienic conditions.

Follicular phase

This phase starts from first day and lasts for thirteen days. The pituitary gland secretes FSH hormone that stimulates the egg cells in the ovaries to grow and mature.One of these egg cells begins to mature in a sac like structure called follicle. It takes thirteen days for the egg cell to reach complete maturity. When the egg cell matures, its follicle secretes FSH that stimulates the uterus to develop a lining of blood vessels and soft cushiony tissue called endometrium. The lining of  uterus begins to grow thick in response to this increase in estrogen hormone.

Ovulation phase

The pitutary gland on 14th day secretes LH  hormone that causes the ovary to release the matured egg cell. The egg cell is directed into the fallopian tube by the cilia of the fimbriae. Fimbriae are finger like projections present at the end of the fallopian tube close to the ovaries .The increase in LH triggers ovulation. The egg then travels into the uterus. If you have regular 28-day menstrual cycles, ovulation usually occurs on day 14. However, most women have different menstrual cycle lengths. In general, ovulation occurs 11 to 16 days before your upcoming period. As the egg moves down the fallopian tube over several days, the lining of the uterus continues to grow thicker and thicker. It takes about three to four days for the egg to travel toward the uterus. If fertilization is to occur, it must happen within 24 hours of ovulation or the egg degenerates.

Luteal phase

This phase begins on the 15th day and lasts till the end of the cycle. The following events occur during this phase.The egg cell released during the ovulation phase stays in the fallopian tube for 24 hours.If a sperm cell does not impregnate the egg cell within that time, the egg cell disintegrates. After ovulation, the graffian follicle becomes a hormone-producing structure called the corpus luteum. The cells of the corpus luteum produce estrogen and large amounts of progesterone, with the latter hormone stimulating the uterine lining development in preparation for implantation of a fertilized egg. If fertilization does not occur, the corpus luteum degenerates about two weeks after ovulation. Because of this, progesterone levels drop and the stimulation for the lining is lost. This causes the lining to shed as a new menstrual cycle starts.

The loss of the corpus luteum can be prevented by fertilization of the egg. If  fertilization  occur within 24 hours of ovulation. About five days after fertilization, the fertilized egg enters  uterus and becomes embedded in the lining. With implantation, cells that will eventually become the placenta begin to produce the “pregnancy hormone” or human chorionic gonadotropin (HCG). It interrupts your menstrual cycle by providing continual stimulation of the corpus luteum to produce progesterone. This prevents the loss of endometrium.

Female reproductive system

Female reproductive system is located in lower abdomen of women. It`s main function is to incubate the developing fetus and deliver baby into world. It also produces ova (female gamete) every 27-30 days associate with Menstrual cycle and produce couple of female sex  hormones which are required for secondary sexual characters, secretion of milk and pregnancy.The female reproductive organs can be subdivided into the internal and external genitalia .The internal genitalia are those organs that are within the true pelvis. These include the vagina, uterus, cervix, uterine tubes (oviducts or fallopian tubes), and ovaries. The external genitalia lie outside the true pelvis. These include the perineum, mons pubis, clitoris, urethral (urinary) meatus, labia majora and minora, vestibule, greater vestibular (Bartholin) glands, Skene glands, and periurethral area.

External genitalia

The vulva, also known as the pudendum, is a term used to describe those external organs that may be visible in the perineal area (see the images below). The vulva consists of the following organs: mons pubis, labia minora and majora, hymen, clitoris, vestibule, urethra, Skene glands, greater vestibular (Bartholin) glands, and vestibular bulbs^.The boundaries include the mons pubis anteriorly, the rectum posteriorly, and the genitocrural folds (thigh folds) laterally.

Mons pubis

The mons pubis is the rounded portion of the vulva where sexual hair development occurs at the time of puberty. This area may be described as directly anterosuperior to the pubic symphysis.

Labia

The labia majora are 2 large, longitudinal folds of adipose and fibrous tissue. They vary in size and distribution from female to female, and the size is dependent upon adipose content. They extend from the mons anteriorly to the perineal body posteriorly. The labia majora have hair follicles.

The labia minora, also known as nymphae, are 2 small cutaneous folds that are found between the labia majora and the introitus or vaginal vestibule. Anteriorly, the labia minora join to form the frenulum of the clitoris.

Hymen

The hymen is a thin membrane found at the entrance to the vaginal orifice. Often, this membrane is perforated before the onset of menstruation, allowing flow of menses. The hymen varies greatly in shape.

Clitoris

The clitoris is an erectile structure found beneath the anterior joining of the labia minora. Its width in an adult female is approximately 1 cm, with an average length of 1.5–2.0 cm. The clitoris is made up of 2 crura, which attach to the periosteum of the ischiopubic rami. It is a very sensitive structure, analogous to the male penis. It is innervated by the dorsal nerve of the clitoris, a terminal branch of the pudendal nerve.

Vestibule and urethra

Between the clitoris and the vaginal introitus (opening) is a triangular area known as the vestibule, which extends to the posterior fourchette. The vestibule is where the urethral (urinary) meatus is found, approximately 1 cm anterior to the vaginal orifice, and it also gives rise to the opening of the Skene glands bilaterally. The urethra is composed of membranous connective tissue and links the urinary bladder to the vestibule externally. A female urethra ranges in length from 3.5 to 5.0 cm.

Skene and Bartholin glands

The Skene glands secrete lubrication at the opening of the urethra. The greater vestibular (Bartholin) glands are also responsible for secreting lubrication to the vagina, with openings just outside the hymen, bilaterally, at the posterior aspect of the vagina. Each gland is small, similar in shape to a kidney bean.

Vestibular bulbs

Finally, the vestibular bulbs are 2 masses of erectile tissue that lie deep to the bulbocavernosus muscles bilaterally.

Internal genitalia

Vagina

The vagina is a canal that joins the cervix (the lower part of uterus) to the outside of the body. It also is known as the birth canal.The vagina extends from the vulva externally to the uterine cervix internally. It is located within the pelvis, anterior to the rectum and posterior to the urinary bladder. The vagina lies at a 90º angle in relation to the uterus. The vagina is held in place by endopelvic fascia and ligaments.

                                                                                                   

Uterus
The uterus is the inverted pear-shaped female reproductive organ that lies in the midline of the body, within the pelvis between the bladder and the rectum. The uterus does not lie in line with the vagina but is usually turned forward (anteverted) to form approximately a right angle with it.It is thick-walled and muscular, with a lining that, during reproductive years, changes in response to hormone stimulation throughout a woman’s monthly cycle.

The uterus can be divided into 2 parts: the most inferior aspect is the cervix, and the bulk of the organ is called the body of the uterus (corpus uteri). Between these 2 is the isthmus, a short area of constriction.

The body of the uterus is globe-shaped and is typically situated in an anteverted position, at a 90º angle to the vagina. The upper aspect of the body is dome-shaped and is called the fundus; it is typically the most muscular part of the uterus. The body of the uterus is responsible for holding a pregnancy, and strong uterine wall contractions help to expel the fetus during labor and delivery. The uterus is composed of three layers of tissue namely outer peritoneum, middle myometrium and innermost endometrium.Endometrium begins to reach full development at puberty and thereafter exhibits dramatic changes during each menstrual cycle

Cervix

The cervix is the inferior portion of the uterus, separating the body of the uterus from the vagina. The cervix is cylindrical in shape, with an endocervical canal located in the midline, allowing passage of semen into the uterus. The external opening into the vagina is termed the external os , and the internal opening into the endometrial cavity is termed the internal os. The internal os is the portion of a female cervix that dilates to allow delivery of the fetus during labor. The average length of the cervix is 3-5 cm.

Fallopian tubes

 Their primary function is to transport sperm toward the egg, which is released by the ovary, and then to allow passage of the fertilized egg back to the uterus for implantation.

The uterine tubes exit the uterus through an area known as the cornua and form a connection between the endometrial and peritoneal cavities. Each tube is approximately 10 cm in length and 1 cm in diameter and is situated within a portion of the broad ligament called the mesosalpinx. The distal portion of the uterine tube ends in an orientation encircling the ovary.

The uterine tube has 3 parts. The first segment, closest to the uterus, is called the isthmus. The second segment is the ampulla, which becomes more dilated in diameter and is the typical place of fertilization. The final segment, furthest from the uterus, is the infundibulum. The infundibulum gives rise to the fimbriae, fingerlike projections that are responsible for catching the egg that is released by the ovary.

Ovaries

The ovaries are small, oval-shaped glands that are located on either side of the uterus. The ovaries produce eggs and hormones. They are about 4 cm (1.6 inches) long, 2 cm  wide, and 1.5 cm (0.6 inch) thick; the two ovaries weigh 4–8 grams . The ovaries are held in place by several ligaments (bands of fibrous connective tissue), including the broad ligament, the suspensory ligament, and the ovarian ligament.The ovaries are responsible for housing and releasing the ova (ovulation), or eggs, necessary for reproduction. At birth, a female has approximately 1-2 million eggs, but only 300 of these eggs ever mature and are released for the purpose of fertilization.  A cross-section of the ovary reveals many cystic structures that vary in size. These structures represent ovarian follicles at different stages of development and degeneration. An ovarian follicle consists of a developing oocyte surrounded by one or more layers of cells called follicular cells.The ovarian follicle with mature oocyte is called Graffian follicle. The Graffian follicle ruptures releasing oocyte and after wards it become a yellow body called Corupus Luteum which secretes Progesterone hormone.

Prepared by Kiran.B

Male reproductive system

The male reproductive system is located in lower abdomen of males. It consists of  scrotum, testes, spermatic ducts,  glands, and penis.The function of male reproductive system is to produce millions of sperms within viscous fluid called semen and depositing it in to vagina, with the use of erectile organ called Penis.Males are driven by natural instincts to carry out this biological act as it is very essential for perpetuation of race.Testes also produces male sex hormone Testosterone which is responsible for development of secondary sexual characters in boys.

Scrotum

The scrotum is a sac-like organ made of skin and muscles that accommodates the testes. It is located below to the penis in the pubic region. The scrotum is made up two pouches with a testis located in each pouch. . When the testes become too warm to support spermatogenesis, the scrotum relaxes to move the testes away from the body’s heat.This mechanism maintains -2 degrees less than usual body temperature of 37 degrees centigrade which is necessary for normal production of sperm. In contrary the scrotum can shrink closer to body during cool climate keeping it warm.

Testes

There are two testes, also known as testicles which are responsible for production of sperms and testosterone. The testes are egg shaped glandular organs around 1.5 to 2 inches long and an inch in diameter. Each testis is found inside its own pouch on one side of the scrotum and is connected to the abdomen by a spermatic cord and cremaster muscle. The cremaster muscles contract and relax along with the scrotum to regulate the temperature of the testes. The inside of the testes is divided into small compartments known as lobules. Each lobule contains a section of seminiferous tubule lined with germinal epithelial cells. These epithelial cells contain many stem cells that divide by mitotic division followed by meotic division and form sperm cells through the process of spermatogenesis.Seminiferous tubules open into reticulate network called Rete testis. Rete testis opens into Head of epididymis through serveral efferent ductules (Vas efference) as you see in figure below.

Epididymis

The epididymis is folded tube like structure on testes. The epididymis is made up of  long, thin tubules that are tightly coiled into a small mass. Sperm produced in the testes moves into the epididymis to mature before being passed on through the male reproductive organs. The length of the epididymis delays the release of the sperm and allows them  to mature.

 Vas Deferens

Within the scrotum, a pair of spermatic cords connects the testes to the abdominal cavity. The spermatic cords contain the vas deferens along with nerves, veins, arteries, and lymphatic vessels that support the function of the testes. The vas deferens , also known as the ductus deferens, is a muscular tube that carries sperm superiorly from the epididymis into the abdominal cavity to the ejaculatory duct. The ductus deferens is wider in diameter than the epididymis and uses its internal space to store mature sperm. The smooth muscles of the walls of the ductus deferens are used to move sperm towards the ejaculatory duct through peristalsis a wave like movement.

Seminal Vesicles

The seminal vesicles are a pair of lumpy exocrine glands that store and produce some of the liquid portion of semen. The seminal vesicles are about 2 inches in length and located posterior to the urinary bladder and anterior to the rectum. The liquid produced by the seminal vesicles contains proteins and mucus and has an alkaline pH to help sperm survive in the acidic environment of the vagina. The liquid also contains fructose to feed sperm cells so that they survive long enough to fertilize the oocyte.

Ejaculatory Duct

The ductus deferens passes through the prostate and joins with the urethra at a structure known as the ejaculatory duct. The ejaculatory duct contains the ducts from the seminal vesicles as well. During ejaculation, the ejaculatory duct opens and expels sperm and the secretions from the seminal vesicles into the urethra.

Urethra

Semen passes from the ejaculatory duct to the exterior of the body via the urethra, an 8 to 10 inch long muscular tube. The urethra passes through the prostate and ends at the external urethral orifice located at the tip of the penis. Urine exiting the body from the urinary bladder also passes through the urethra.

Prostate gland

The prostate is a walnut-sized exocrine gland that borders the inferior end of the urinary bladder and surrounds the urethra. The prostate produces a large portion of the fluid that makes up semen. The prostate gland contributes about 30 percent of the seminal fluid; the constituents of its secretions are mainly citric acid, acid phosphatase, calcium, sodium, zinc, potassium, protein-splitting enzymes, and fibrolysin (an enzyme that reduces blood and tissue fibres) The prostate also contains smooth muscle tissue that can constrict to prevent the flow of urine or semen.

Cowper’s Glands

The Cowper’s glands, also known as the bulbourethral glands, are a pair of pea-sized exocrine glands located inferior to the prostate and anterior to the anus. The Cowper’s glands secrete a thin alkaline fluid into the urethra that lubricates the urethra and neutralizes acid from urine remaining in the urethra after urination. This fluid enters the urethra during sexual arousal prior to ejaculation to prepare the urethra for the flow of semen.

Penis

The penis is   copulatory organ located superior to the scrotum. The penis is roughly cylindrical in shape and contains the urethra and the external opening of the urethra.It has expanded sensitive end called glans penis which is purposeful during penetration into vagina. Large pockets of spaces corpora cavernosa (erectile tissue) in the penis allow it to fill with blood and become erect. The erection of the penis causes it to increase in size and become turgid and retains the state for a while due to  blocking of veins carrying blood away from penis. The function of the penis is to deliver semen into the vagina during sexual intercourse.At the beginning of the glans penis, a circular fold of skin, commonly called the foreskin (or prepuce), extends forward to cover the glans. At birth or during early childhood, the foreskin may be removed by an operation called circumcision.Sensitive spots around rim of glans of penis give a feel of excitement and pleasure during the intercourse and finally semen gets ejaculated in a state called Climax. In addition to its reproductive function, the penis also allows  the excretion of urine through the urethra to the exterior of the body.

Semen

Semen, also called seminal fluid.Semen is the fluid produced by males for sexual reproduction and is ejaculated out of the body during sexual intercourse. Semen also contains other liquids, known as seminal plasma, which help to keep the sperm cells viable.Semen contains sperm, the male reproductive cells, along with a number of nourishing components and enzyme called Hyaluronidase suspended in a liquid medium. The chemical composition of semen gives it sticky consistency and a slightly alkaline pH. The total volume of semen for each ejaculation of a human male averages between 2 and 5 ml.In human beings each ejaculation contains normally 200 to 300 million sperm.These traits help semen to support reproduction by helping sperm to remain within the vagina after intercourse and to neutralize the acidic environment of the vagina. In healthy adult males, semen contains around 100 million sperm cells per milliliter. These sperm cells fertilize oocytes inside the female fallopian tubes.This semen can be artificially ejected by stimulating the penis.