Zoology · Ch 3 — Body Fluids and Circulation
Summary
Summary
Vertebrates circulate blood, a fluid connective tissue, to carry essential substances to the cells and to remove wastes from them. A second fluid, lymph (tissue fluid), also helps transport certain substances.
Blood and its grouping
Blood is made of a fluid matrix, the plasma, and the formed elements — red blood cells (erythrocytes), white blood cells (leucocytes) and platelets (thrombocytes). Human blood is classified into the A, B, AB and O groups on the basis of the presence or absence of two surface antigens, A and B, on the red blood cells. A separate grouping is based on the presence or absence of another antigen, the Rhesus factor (Rh), on the red cell surface.
Lymph
The fluid that fills the spaces between cells in the tissues is derived from blood and is called tissue fluid or lymph. It is almost the same as blood except that it differs in its protein content and its formed elements.
The circulatory system and the heart
All vertebrates and a few invertebrates have a closed circulatory system. The human system consists of a muscular pumping organ, the heart, a network of vessels, and the fluid blood. The heart has two atria and two ventricles, and its muscle is auto-excitable. The sino-atrial node (SAN) generates the greatest number of action potentials per minute (about 70 to 75 per minute) and so sets the pace of the heart's activity — hence it is called the pacemaker.
The cardiac cycle
The action potential makes the atria, and then the ventricles, undergo contraction (systole) followed by relaxation (diastole). Systole drives blood from the atria into the ventricles and onward into the pulmonary artery and the aorta. This repeated, cyclic sequence of events is the cardiac cycle, and a healthy person completes about 72 such cycles per minute. Each ventricle pumps out roughly 70 mL of blood during a cycle, called the stroke (beat) volume. The volume pumped by each ventricle per minute is the cardiac output, equal to the stroke volume multiplied by the heart rate (about 5 litres). The heart's electrical activity can be recorded from the body surface using an electrocardiograph, and this recording, the electrocardiogram (ECG), is of clinical importance.