Summary — Body Fluids and Circulation

This chapter has traced body fluids and circulation from the level of individual formed elements to the working of the whole heart. Blood, a fluid connective tissue of about five to five-and-a-half litres in an adult, consists of plasma (roughly fifty-five percent, mostly water carrying plasma proteins, nutrients, hormones, gases and excretory products) and formed elements (roughly forty-five percent): red blood cells specialised by their biconcave, nucleus-free structure for carrying haemoglobin and transporting gases; white blood cells, both granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes), specialised for defence; and platelets, cell fragments specialised for triggering clotting.

The ABO and Rh blood grouping systems classify blood by the antigens present on the red cell surface and the corresponding antibodies present in the plasma, a pattern that directly determines which transfusions are safe (group O as universal donor, group AB as universal recipient) and that explains the clinical importance of Rh incompatibility in a second and later Rh-positive pregnancy of an Rh-negative mother. Blood coagulation is a cascade in which injury triggers the release of thromboplastin, which converts prothrombin to thrombin, which in turn converts soluble fibrinogen into the insoluble fibrin mesh that traps blood cells to form a clot. Lymph, filtered from plasma across capillary walls into the tissue spaces, drains excess tissue fluid back into the blood, carries absorbed fat from the intestine, and, via the lymphocytes and lymph nodes along its path, contributes to immune defence.

The human heart has four chambers (two thin-walled atria, two thick-walled ventricles, the left thicker than the right), four one-way valves (tricuspid, bicuspid/mitral, and a pulmonary and an aortic semilunar valve), and connects to three categories of vessel — thick-walled arteries carrying blood away from the heart, thinner-walled veins (with internal valves in the limbs) carrying blood back to it, and single-cell-thick capillaries where actual exchange with the tissues occurs. Its rhythmic beating follows the cardiac cycle: joint diastole, then a brief atrial systole, then a longer ventricular systole, producing the two heart sounds ('lub' and 'dub') at valve closure, all within about zero-point-eight seconds at a resting heart rate. Cardiac output (stroke volume × heart rate, ≈5 L/min at rest) measures the heart's pumping performance, while the electrocardiogram records its underlying electrical activity as the P wave (atrial depolarisation), QRS complex (ventricular depolarisation) and T wave (ventricular repolarisation). …