Human Circulatory System

The human circulatory system comprises both cardiovascular system and  lymphatic system. The chief constituent of human cardio vascular circulatory system are heart, blood vessels and blood which carries oxygen, metabolic wastes, nutrients,hormones to all parts of the body. 

Cardio vascular system

The cardiovascular system is a part of human circulatory system. Its main function is to provide the necessary materials to all the cells of the body that are required for their growth, maintenance and repair. The cardiovascular system also removes the metabolic wastes away from the cells and tissues. The fluids and materials of cardio vascular system flow through pressure gradients. The heart acts as the central pump for the flow of fluids. Apart from heart, the cardio vascular system also includes other parts such as arteries, capillaries and veins. Circulation of blood is the main function of all the parts of cardio vascular system.
We can divide the cardiovascular circulatory system in 2 parts. These are:

The systemic circulation-it passes through all the tissues of the body.
The pulmonary circulation- it passes through the lungs.
The circulatory system also helps the body to maintain blood pressure levels.

Arteries

The 2 outer layers of arteries are very thick. The modification is because the arteries have to withstand high amounts of blood pressure. The three layers of human arteries are tunica adventitia or the outer layer, the tunica media or the middle layers and the tunica intima or the inner most layer.
The tunica adventitia is formed of white fibrous connective tissues. It runs parallel to the blood vessels and has an elastic membrane that is located very close to the tunica media. The outer covering of tunica adventitia is non elastic. The non elastic membrane prevents the layer to stretch and thereby prevents its rupture or distension. The layer has vasa vasorum or special kind of vessels that supply blood to these layers.
The tunica media is composed of smooth muscle cells that are arranged in circulation.
The tunica intima has a single layer of endothelium. This endothelium layer rests upon internal elastic membrane which is made up of elastic tissues. The internal elastic membrane has folds which prevent injury to the endothelial cells that may occur because of pulsation. The inner layer of the tunica intima has endothelial lining consisting of very delicate fibro elastic areolar connective tissue.

Arteriole

the smallest branch of artery is called arteriole. The walls of arterioles are thicker than the arteries and have narrow lamina. These vessels have adequate vasomotor innervations. They distribute blood by vasoconstriction and vasodilation. The lumen of arterioles is modified so that the pressure and flow of the vascular systems can be maintained to optimum level.
Arterioles also have 3 layers. The tunica intima is composed of an inner elastic membrane and endothelial cell lining. The tunica media has elastic fibrils and muscle cells. The loose connective tissues form tunica adventitia.

Capillaries

Capillaries connect the arteries to veins. Flat endothelial cells link the capillaries and form their walls. The basal lamina separates the capillaries from the supportive bed of connective tissues. The endothelial cells of the capillaries have an elongated or ovoid nucleus. These endothelial cells are stretched and taper at their ends. Reticular and collage nous fibers surround the capillaries. Capillaries are also surrounded by fibroblasts, macrophages, mesenchymal cells and other cells of neighboring nerve fibers. There are also rouget cells that have long branching processes which surround the capillaries.

       The diameter of human capillaries is around 7 to 9 µ. The capillaries can be grouped into 2 types. These are the muscular types and the fenestrated type. The muscular type of capillaries are found in lung, smooth muscles, cardiac muscles, central nervous system and other tissues. They consist of endothelial cell layers that lie uninterrupted and are of equal thickness. Slight budges are present that are formed because of cell nuclei.

    The fenestrated type capillaries are found in endocrine glands, renal glomerulus, intestinal mucosa and in other organs. Here the endothelial lining is not continuous and ahs numerous pores that have length of 300 to 500 angstrom.Electron microscopic studies have revealed that all capillaries are composed of small vesicles that lie on the basal and luminal surfaces of endothelial cells. These vesicles are called as pinocytotic vesicles. The pinocytotic vesicles help in transporting the fluid across cell membranes.

Veins

Veins have all three layers of arteries but they differ in size. The media and intima layers of the veins are thinner because they have less elastic and muscular components. Still the veins are very strong as they have connective tissue components. The endothelial cells of tunica intima are less elongated in veins. The tunica media of veins have more amounts of collagen fibers and lesser mounts of muscle and elastic tissue. The tunica adventitia of veins if highly developed and has elastic, collagen and muscle fibers, the cerebral veins, retinal veins, meningital veins and some other veins do not have smooth muscles.

Valve of veins- Almost all veins have valves especially the lower limb veins. These valves prevent backflow of blood. The intima folds to form semi lunar pockets which act as flaps or valves.

Blood

Blood is a specialized connective tissue. When fresh, blood is red, opaque, thick and slightly alkaline. The cells that are found in blood include red blood cells, white blood cells, other forms elements and the platelets. These cells are suspended in a liquid intercellular substance that is called as plasma. Approximate specific gravity of whole blood is 1.060.   Blood assumes around 7% of the adult human weight which is around 5 liters. 

Compositon of blood

Functions of blood

Higher forms of living organisms have more specialized cells. Thus, in addition to supply and excretion, blood also carries out many other functions. Some of the important functions of blood are listed below.

1. Transport of nutrition- Blood carries the digested nutritive substances for the intestines to other parts of the body. According to need, it also transfers these substances inside the body from one place to another. For example, the nutritive substances are transferred from the food storage depots inside the body to specific tissues where they are required by blood.
2. Transport of gases (oxygen and carbon-di-oxide) - blood carries oxygen from the lungs to all the tissues of the body and takes back carbon-di-oxide to the lungs for its removal from the body.
3. Transport of other essential elements- Blood carries vitamins, minerals, hormones, enzymes and other essential substances inside the body to the places they are required within the body.
4. Regulation of the temperature of the body- blood has abundant quantity of water which due to its specific heat helps in regulation of body temperature. Blood distributes the body heat to all the body parts through conduction. Excessive heat is also dissipated in the external atmosphere through the blood water evaporation from skin and from lungs.
5. Drainage of body wastes- The waste products of the body (waste generated during metabolic activities) are carried by the blood to their respective organs of secretion (for example kidney and lungs).
6. Coagulation- Blood has a property to coagulate. This property helps to prevent excessive loss of blood (haemorrhage)
7. Regulation of blood pressure- Blood regulates blood pressure by changing its viscosity and volume. It is termed as the haematocrit value of blood. 
8. A key pillar of the immune/defensive system of the body- In response to a foreign toxic agent the blood develops antibodies. Blood also contains WBC’s or white blood cells which engulf the pathogens and destroy them.
9. Maintains ion balance- Blood maintains ion balance of the body which is necessary for proper functioning of all the cells of the body.
10. Maintains water balance in the body.

Blood cells

Blood cells are classified into three types- erythrocytes, leucocytes and platelets.

Red blood corpuscles or erythrocytes

The adult human erythrocytes are circular, non nucleated and biconcave discs.These are most numerous cells in the blood.Immature RBC have all cell organelles. After maturation, organelles like nucleus, mitochondria, lysosomes, Golgi bodies,endoplasmic reticulum, ribosomes etc disappear.In some mammals like camel, Lama RBC have nucleus.In adulsts RBC are formed only in the marrow of long bones while in embryonic stages, they are formed in the liver and spleen. RBC have life span of 120 days. Old Red blood cells are destroyed mostly in spleen and to some extent in liver. Therefore spleen is called "Grave yard of Red blood cells".

 They are composed of proteins and lipids which form a meshy framework. Haemoglobin fills these meshes. They have a red color but under a microscope they show yellow or light brown color. A delicate outer membrane envelops the RBC’s. The constituents of this membrane are phosphatides, proteins and cholesterol. The middle RBC layer is composed of lipids while the inner and outer layers are made of proteins. The membrane of RBC is highly selective and does not give passage to cations like sodium and potassium and to bigger molecules. Anions like chlorine and crystalloids like urea can freely pass through.

Composition of RBC

Each cell is composed of an envelope that has no color.  Inside the envelope there is 65% water, 35% solids and a semi-liquid material. The solid content is composed of 33% haemoglobin which is bound to a stromal network. The stromal network consists of cholesterol, phospholipid, protein, neutral fact and cholesterol ester. There are organic constituents inside the RBC like urea, creatinine, amino acids, adenyl pyrophosphate etc. These are present in traceable quantities. The lipid content of RBC is composed of 60% phospholipid, 30% cholesterol and 10 % cholesterol esters and fats. It also contains salts like potassium phosphate etc. The normal average RBC count in an adult male is 5.4 million and in adult female is 4.5 million per cubic millimeter of blood.

Fate of RBC

The average life period of a mature RBC is 120 days after which it is engulfed by the phagocytic cells of the red bone marrow, liver and spleen. Its constituents are excreted or absorbed for further processing. The old and senile RBC’s become brittle. They become flask shaped and throw out poikilocytes. The RBC disintegrates by gradual breakage of these poikilocytes away from the cell. The reticulo endothelial cells absorb the broken fragments of RBC’s. The reticulo endothelial cells of spleen, liver etc engulf and digest the old RBC’s. Haemoglobin is released in the intracellular breakdown of RBC’s. The haemoglobin degrades to choleglobin. It is next decomposed to haem and protein. The protein content forms amino acid while the haem content gets stored in the body in the form of haemosiderin and ferritin. These constituents allow for the formation of new haemoglobin. The remaining part of haem molecule gets converted into bilirubin and biliverdin. Biliverdin and bilirubin circulate in the blood stream by combining with plasma alpha 1 globulin. Once they enter the liver, they decompose into monobilirubin and dibilirubin glucuronide. After further processing, some part of materials is absorbed while the rest is excreted.

Functions of RBC’s

1. To carry oxygen and carbon-di-oxide inside the body.
2. To maintain ion balance of the body
3. To maintain viscosity of blood
4. To provide new material like bilirubin, biliverdin etc after degradation.

Haemoglobin

The red pigment found in blood is called haemoglobin. Haemoglobin is a kind of chromoprotein and has two parts. The first part which forms 96% of haemoglobin is called globin or histone. The other part that forms 4% of the haemoglobin is a prosthetic group which contains iron. It is called haem. Haem consists of 4 pyrrole groups joined together to form a protoporphyrin compound. Hame can also be said to be metalloporphyrin compound where the metal is iron. Iron forms about 0.34% of hemoglobin. Around 3 gm of iron is present is adult human blood. Iron is in ferrous form. Globin aids haem so that it can contain the iron in ferrous state. It also helps the iron ion to combine with oxygen for its transportation inside the body. The molecular weight of haemoglobin is 68,000. Each haemoglobin molecule contains 8 atoms of sulphur and 4 atoms of iron.

White blood cells/corpuscles or leucocytes

Leucocytes are a type of blood cells. They are different from erythrocytes. They have abundant nucleoprotein. They also contain ascorbic acid, cholesterol, glycogen, lipids and many enzymes. There is lot of variation in WBC count in a normal human body and the cell count may vary from hour to hour as well. The normal WBC range is between 4000 to 11000 cells per cubic mm of blood. The WBC count is thus less than the RBC count. The ratio of WBC to RBC is 1:700.

Classification of WBC’s or leucocytes.

WBC are two types namely Granulocytes and Agranulocytes.

Granulocytes

These cells have granular cytoplasm. They are formed in the red bone marrow after birth. They are of 3 types viz Neutrophils, Eosionophils  and Basophils. 

a. Neutrophil-These cells can be stained with neutral dyes hence they are called so. Their count varies from 3000 to 6000 per cubic mm of blood. They are around 10 to 12 µm in diameter. The number of nucleus lobes can be more than 7 in a neutrophil. They are also called polymorphs as they have multi-lobed nucleus. Generally 3 to 4 lobed nucleus cells are found. With maturity the number of lobes increases. The cytoplasms of these cells have neutrophilic granules. These cells are amoeboid and phagocytic in nature. The enzymes found in these cells are lipase, protease, nucleotidase, phosphates etc.  Also lutathione, glycogen, ascorbic acid etc are found. The enzyme and other content of the neutrophils help them in their phagocytic activity.  

b. Eosinophil-These cells can be stained with acid dye called eosin hence they are called Eosinophils or  acidophils. They have 2 or 3 lobed nucleus. Their strength is 1 to 4% and count varies from 150 to 400 per cubic mm of blood. They have a diameter ranging from 10 to 12 µm. The cytoplasm has course granules and the nucleus has 2 lobes. The eosinophils do not show any phagocytic activity. They are amoeboid though. They contain histamine.  

c. Basophil- These cells can be stained with basic dyes- therefore they are called basophils.The nucleus is lobed and the diameter of the cells varies from 8 to 10µm. These cells are phagocytic in nature and play vital role in healing process.

Agranulocytes

These cells do not have any granular material in their cytoplasm.The nucleus in these cells is large.There are two types namely Lymphoctes and Monocytes.  

a. Lymphocytes- they can be small or large. The lymphocyte strength is 1500 to 2700 per cubic mm of blood and the count varies from 1500 to 2700 per cubic mm.  

b. Monocytes- they vary from 5-10% in strength. Their count varies from 350 to 800 per cu. mm. They have larger diameter which varies from 16 to 18 µm. The young nucleus is round or oval. The older cells have convoluted nucleus that can be kidney or horse shaped. The cells have non granular cytoplasm and an eccentric nucleus. 

Functions of white blood cells or leucocytes

1. Phagocytosis- the monocytes and the neutrophils engulf foreign bacteria and particles and destroy them.
2. Antibody formation- the lymphocytes play a vital role in the manufacture of beta and gamma fractions of the protein globulin which act as antibodies.
3. Fibroblast formation – the lymphocytes are converted to fibroblast in areas where inflammation occurs. They thus help in cell and tissue repair.
4. Heparin secretion- basophils secrete heparin that prevents clotting of blood inside the blood vessels.
5. Trephone manufacture- trephons are plasma proteins manufactured by leucocytes. Trephones aid growth, nutrition and repair of tissues and cells.
6. Antihistamine function-histamines are found in eosinophils.  They relive the body in allergic reactions.

Blood platelets

The platelets have no nucleus. They are round or oval in shape and are covered by a membrane. The average size of a platelet cell is 2.5µm. When seen through microscope, they appear in clusters. The cell has 2 parts viz. the hyalomere and the chromatomere.

Hyalomere- The hyalomere consists of homogenous fine granular material. It contains microfilaments and microtubules. Thrombosthenin is contained in the microfilament. The pigment is capable of contraction like the actin and myosin found in muscles.

Chromatomere- The chromatomere consists of alpha granules (oval granules with dia.0.2µm and length 0.3µm. The alpha granules have lysosomal function that is important during platelet aggregation, in clot resolution and in phagocytic activity. They also contain mitochondria, sydersomes (ferritin containing vesicles) and very dense granules (contain 5 hydroxytryptamine). Apart from these, glycogen granules, ribosome, tubules and vesicles are also present.

Platelets contain protein and phospholipids (mostly cephalin). The strength of platelets varies from 250000 to 450000 per cubic mm of blood.

Functions of blood platelets

1. To initiate blood clotting.
2. To repair capillary endothelium
3. To prevent haemorrhage.
4. Fasten clot retraction

Formation of blood cells

Blood cells are formed in the bone marrow. The other name of bone marrow is myeloid tissue. In later post natal life, bone marrow only consists of the marrows or inner cavities of bone. The cancellous spaces and the medulla cavities of the bone contain the cellulovascular bone marrow tissue. Red bone marrow is the active bone marrow and it only manufactures new blood cells. The yellow bone marrow or the inactive bone marrow does not produce blood cells. As age progresses more and more red marrow is converted in yellow marrow. The yellow marrow does changes into red marrow when there is urgent need of new blood cells. The red marrow constitutes around 3 to 6 percent of total body weight in an adult.

Haematopoietic function of bone marrow

The red bone marrow can form the red blood cells and also other blood cells. It produces myeloid elements for the formation of WBC’s. The red bone marrow thus forms all of the blood cells including RBC’s or erythrocytes, WBC’s or leucocytes (includes monocytes, lymphocytes, granulocytes) and platelets. The bone marrow also destroys the aged, damaged and defective RBC’s. The macrophages of bone marrow digest these imperfect RBC’s.