You have learnt that all living cells have to be provided with nutrients, O2 and other essential substances. Also, the waste or harmful substances produced, have to be removed continuously for healthy functioning of tissues. It is therefore, essential to have efficient mechanisms for the movement of these substances to the cells and from the cells. Different groups of animals have evolved different methods for this transport. Simple organisms like sponges and coelenterates circulate water from their surroundings through their body cavities to facilitate the cells to exchange these substances. More complex organisms use special fluids within their bodies to transport such materials. Blood is the most commonly used body fluid by most of the higher organisms including humans for this purpose. Another body fluid, lymph, also helps in the transport of certain substances.

Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements. Plasma is a straw coloured, viscous fluid constituting nearly 55 percent of the blood. 90-92 percent of plasma is water and proteins contribute 6-8 percent of it. Fibrinogen, globulins and albumins are the major proteins. Fibrinogens are needed for clotting or coagulation of blood. Globulins primarily are involved in defense mechanisms of the body and the albumins help in osmotic balance.

Erythrocytes, leucocytes and platelets are collectively called formed elements and they constitute nearly 45 per cent of the blood. Erythrocytes or red blood cells (RBC) are the most abundant of all the cells in blood. A healthy adult man has, on an average, 5 millions to 5.5 millions of RBCs mm–3 of blood. RBCs are formed in the red bone marrow in the adults. RBCs are devoid of nucleus in most of the mammals and are biconcave in shape.

Two such groupings – the ABO and Rh – are widely used all over the world. ABO grouping is based on the presence or absence of two surface antigens on the RBCs namely A and B. Similarly, the plasma of different individuals contain two natural antibodies. The distribution of antigens and antibodies in the four groups of blood, A, B, AB and O are given in the text. Hence ‘O’ group individuals are called ‘universal donors’.

Blood exhibits coagulation or clotting in response to an injury or trauma. This is a mechanism to prevent excessive loss of blood from the body. Fibrins are formed by the conversion of inactive fibrinogens by the enzyme thrombin. An injury stimulates the platelets in the blood to release certain factors which activate the mechanism of coagulation.

The fluid released out is called the interstitial fluid or tissue fluid. It has the same mineral distribution as that in plasma. Exchange of nutrients, gases, etc., between the blood and the cells always occur through this fluid. An elaborate network of vessels called the lymphatic system collects this fluid and drains it back to the major veins. The fluid present in the lymphatic system is called the lymph.

The circulatory patterns are of two types – open or closed. Open circulatory system is present in arthropods and molluscs in which blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses. Annelids and chordates have a closed circulatory system.

Human circulatory system consists of a muscular chambered heart, a network of closed branching blood vessels and blood, the fluid which is circulated. The heart has four chambers, two relatively small upper chambers called atria and two larger lower chambers called ventricles.

To begin with, all the four chambers of heart are in a relaxed state, i.e., they are in joint diastole. As the SAN generates an action potential which stimulates the atria to contract, the ventricles then contract following. The sequential events in the heart which is cyclically repeated is called the cardiac cycle.

ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart, such as atrial contraction and ventricular depolarization.

The blood flows strictly by a fixed route through Blood Vessels—the arteries and veins. This pathway constitutes the pulmonary circulation and systemic circulation. The pulmonary circulation starts by the pumping of deoxygenated blood by the right ventricle to the lungs while the systemic circulation delivers oxygenated blood throughout the body.

Normal activities of the heart are regulated intrinsically, i.e., auto regulated by specialised muscles (nodal tissue). A special neural centre in the medulla oblongata can moderate the cardiac function through autonomic nervous system (ANS).

Hypertension, Coronary Artery Disease (CAD), and Angina are some common disorders affecting the circulatory system. Hypertension occurs when blood pressure remains high, CAD is caused by arterial blockages, and Angina is recognized by chest pain due to insufficient oxygen to the heart.