Components of Circulatory System

The circulatory system consists of 3 components:

1. Blood vessels

2. Heart

3. Blood

Blood Vessels

These are hollow, tubular vessels which conduct the blood from the heart to the tissues and from the tissues to the heart. There are 3 type of blood vessels, arteries, capillaries and veins.

Arteries

Arteries are vessels which carry blood away from the heart. They are thick walled vessels. They are elastic in nature, have a narrow lumen, are deep seated in the body parts and have no valves in them. The blood flowing through the arteries carry oxygenated blood in them, except the pulmonary artery which carries deoxygenated blood to the lungs. The average diameter of a an artery is 500 mm.

The arterial wall has three coats namely tunica interna, tunica media and tunica adventitia. The tunica interna is composed of endothelial cells. The tunica media is the middle layer and is compound of elastic and simple muscle fibers. The tunica adventitia is composed of white connective tissue and elastic fibers. The smallest branches of an artery are called arterioles. The walls of the arteries are supplied with a set of blood vessels called vaso vasonum.

Capillaries

The arterioles further divide into smaller vessels called meta arterioles which have a diameter of 70 mm which in turn divide into capillaries. They are the thinnest blood vessels and their walls are formed of a single layer of endothelial cells. These form a connective link between the arterioles and the veins. They were discovered by Marcello Malpighi in 1661. The endothelium allows the exchange of materials like the nutrients, CO₂, O₂, hormones and waste products between the blood and the surrounding tissue cells through the tissue fluid.

T. S. of artery and vein

                       

Capillary bed showing arterioles and capillaries

Veins

The arteriole capillaries join to form venous capillaries which then join to form the venules and veins. The veins are thin walled vessels. The walls of the veins have all the three layers as in an artery, but they are comparatively thinner, less elastic and less muscular. The pressure of the blood in a vein is low and the speed is slow. It has a wide lumen and is superficial.

Action of Semi-lunar Valve in a Vein

Valves are present in most of the veins to prevent the backward flow of blood. Veins are also supplied with vasa vasorum, the nutrient blood vessels.

Blood

The blood is a fluid connective tissue. It is opaque and somewhat sticky. It is more viscous than distilled water with a viscosity of 4.7 and slightly alkaline in nature, Oxygenated blood is bright red while the deoxygenated blood is purple coloured.

Blood flows in the blood vessels due to the pumping action of the heart. It forms about 6-10% of the body weight and about 30-35% of the ECF. Adult humans contain about 5 litres of blood.

The blood is formed of 2 parts,

a) Plasma

b) Blood corpuscles

Plasma

It forms about 55-60% of the blood volume. It is the liquid portion of blood. It contains 90% water and the remaining 10% is formed of organic and inorganic substances. These materials include proteins, glucose, nitrogenous wastes, enzymes, hormones and minerals.

About 200-300 gm of plasma proteins are present in the total volume of blood. They are serum albumin, serum globulin, fibrinogen and prothrombin.

Blood Corpuscles

Red and white cells from the human blood

The blood has 3 types of blood corpuscles.

a) Erythrocytes or Red blood corpuscles or R.B.Cs.

b) Leucocytes or White blood corpuscles or W.B.Cs.

c) Blood platelets or thrombocytes.

Erythrocytes

(erythros = red; cyton = cell) the erythrocytes are oval, biconvex and nucleated in all vertebrates except mammals. In mammals the erythrocytes are circular, biconcave and non-nucleated. The total number of RBCs per microlitre is called RBC count. Normal RBC count in an adult human male is 5 - 5.5 million per cubic millimeter of blood while in an adult female, it is 4.5 - 5 million per cubic millimetre. The instrument used to determine RBC count is called haemocytometer.

Each RBC is composed of an envelope and a spongy elastic substance called stroma. Inside the meshes of the stroma is present the iron pigment haemoglobin. Haemoglobin is a conjugated protein composed of a protein part globin and a non protein pigment haem. Haem is an iron containing porphyrin.

4 molecules of haem + globin

Haemoglobin.

 

On combining with oxygen, haemoglobin is converted to oxyhaemoglobin (H₆O₂).

The erythrocytes develop from the bone marrow. The life span of RBC is around 120 days. As the erythrocyte becomes older and older, the cell membrane becomes fragile and the old RBCs are disintegrated in the spleen which is said to be the graveyard for red cells.

The haemoglobin is broken down into 2 important components, denatured globulin and iron. The iron is stored in the liver and is used to make new haemoglobin. The globin part is converted into bilirubin a yellow pigment and biliverdin a green pigment used in colouring the bile.

 

A scanning electron micrograph of red blood cells of a mammal