Body Fluids and Circulation — Class 11 Biology

This chapter describes the body fluids blood and lymph and the human circulatory system. Blood is a fluid connective tissue with plasma (a straw-coloured fluid that is mostly water with proteins such as fibrinogen, globulins and albumins) and formed elements, namely erythrocytes (red blood cells with haemoglobin), leucocytes (granulocytes and agranulocytes) and platelets. Human blood is grouped by the ABO system, based on A and B antigens, and the Rh system; matching is essential before transfusion, and Rh incompatibility can cause erythroblastosis foetalis. Coagulation forms a fibrin clot to prevent blood loss. Lymph is the colourless tissue fluid that drains back to the veins and carries lymphocytes and absorbed fats. Circulation may be open or closed; vertebrate hearts range from two-chambered in fishes to four-chambered in birds and mammals, which have complete double circulation. The human heart, with two atria and two ventricles guarded by valves, is myogenic, with the sino-atrial node acting as the pacemaker. The cardiac cycle of systole and diastole produces heart sounds and an electrocardiogram (ECG), and the cardiac output is the stroke volume times the heart rate. Cardiac activity is regulated by the autonomic nervous system and adrenal hormones. Disorders include hypertension, coronary artery disease, angina and heart failure.