Circulation and blood
The heart is a four-chambered double pump whose rhythm is set and conducted by specialised cardiac tissue. It drives blood through a closed system of arteries, veins and capillaries that differ in their walls and pressure. Blood carries, regulates and defends. It is a connective tissue of cells suspended in plasma, its cells are made in the red bone marrow and the lymphoid tissue, and its group is identified by the antigens on its red cells.
The heart
The heart is the pump that keeps blood circulating through the body, carrying oxygen and nutrients to tissues and removing waste products. It is a triangular muscular organ the size and shape of a closed fist, with four chambers: two superior atria and two inferior ventricles. The atria are thin-walled and receive blood, whereas the ventricles are thick-walled and muscular because they must pump blood to all parts of the body.
The bulk of the heart consists of contractile myocardium, the thick middle layer of the heart wall, composed of cardiac muscle. The endocardium is the membrane that lines the internal chambers and the valves. The epicardium covers the external surface and is the innermost of the two layers of the pericardium. The pericardium is a two-layered fibrous sac that contains the heart, with lubrication between its layers. A fibrous skeleton provides strength, elasticity and support for the contractile myocardial cells and the valves.
Four valves keep blood flowing in the correct direction through the heart and prevent backflow. The tricuspid valve, with three cusps, lies between the right atrium and the right ventricle, and the mitral or bicuspid valve, with two cusps, lies between the left atrium and the left ventricle. The pulmonary semilunar valves sit at the beginning of the pulmonary artery. The aortic semilunar valves, with three half-moon-shaped cusps, sit at the beginning of the aorta, between it and the left ventricle. The closing of the valves produces the characteristic heartbeat sound. Each beat has two distinct sounds: the first ("lup") is caused by vibration and closing of the tricuspid and mitral valves during contraction, and the second ("dup") by closing of the semilunar valves as the heart muscle relaxes.
The heart works as two separate pumps, right and left, separated by a thick fibrous septum. In an adult, systemic blood enters the right atrium through the superior and inferior venae cavae and passes through the tricuspid valve to the right ventricle. From there it passes through the pulmonary semilunar valve into the pulmonary artery and on to the lungs, where it is oxygenated. It returns from the lungs to the left atrium, passes through the bicuspid valve to the left ventricle, and leaves through the aortic semilunar valve into the aorta and out to the body.
The conduction system
The heart keeps its rhythm through specialised masses of cardiac tissue. The sinoatrial (SA) node, the pacemaker, sits in the wall of the right atrium near the opening of the superior vena cava. The atrioventricular (AV) node is specialised heart muscle in the septal wall of the right atrium, and it receives impulses from the SA node and transmits them to the bundle of His. The AV bundle, or bundle of His, lies in the septum and is a small group of atypical cardiac muscle fibres that propagates the atrial contraction rhythm. The Purkinje fibres lie in the walls of the ventricles and are modified cardiac muscle fibres that transmit impulses rapidly. The SA node depolarises first, and the impulse then travels to the AV node, through the bundle of His and the bundle branches, and along the Purkinje fibres before it stimulates the myocardial cells of the ventricles and the heart contracts.
Arteries, veins and capillaries
Blood is transported through a series of closed vessels, an estimated 62,000 miles of them. Arteries are thick-walled and elastic and carry blood away from the heart under high pressure. Veins are thin-walled and carry blood toward the heart under low pressure, so they have valves to ensure that blood keeps moving only forward. The blood in a vein may be oxygenated or not. Capillaries connect arterioles and venules, and gas and nutrient exchange happens in them. They are so small that red blood cells must pass through in single file, and this single-file flow facilitates exchange by diffusion and osmosis.
Functions and composition of blood
Blood transports dissolved gases, nutrients, waste products, enzymes and hormones; regulates the pH and electrolyte concentration of body fluids; restricts fluid losses; defends against pathogens and toxins; and regulates body temperature.
Blood is a connective tissue whose cells are widely spaced from one another in an intercellular matrix that includes several important proteins. Its cells are erythrocytes, leukocytes and thrombocytes. Erythrocytes are mature red blood cells and contain the pigment haemoglobin. Leukocytes are white blood cells, involved in the body's defence, and their five types are neutrophils, basophils, eosinophils, lymphocytes and monocytes. Thrombocytes are blood platelets, small disc-shaped cellular elements responsible for clotting.
Plasma is the fluid portion of the blood, in which the cells are suspended, and it contains water, electrolytes and glucose. It accounts for 55% of the volume of whole blood and is approximately 92% water; the remaining 8% is electrolytes and dissolved organic compounds. Plasma resembles interstitial fluid, the fluid in the spaces of a tissue, except for its relatively high concentration of dissolved proteins, among them albumins, globulins and fibrinogen. Albumin, the most abundant, contributes to the osmotic pressure of the blood and provides a transport mechanism for insoluble materials. Globular proteins bind and transport hormones, lipoproteins and metalloproteins, and the immunoglobulins are the antibodies, proteins that attack antigens. Under the proper stimulation, fibrinogen molecules aggregate into large insoluble strands of fibrin, which form the basis of a blood clot. Removing fibrinogen from plasma leaves a fluid called serum.
Blood cell formation
Two kinds of connective tissue make blood cells. Myeloid tissue, the red bone marrow, lies in adults chiefly in the sternum, ribs and hip bones, with small amounts in some other bones such as the vertebrae, clavicles and cranial bones, and it produces all types of blood cell except some lymphocytes and monocytes. Lymphoid tissue is found chiefly in the lymph nodes and produces lymphocytes and monocytes.
Hematopoiesis is the formation and development of blood cells. In it, early stem cells with the potential for renewal, proliferation and differentiation give rise to large numbers of mature cells through a series of intermediate cells, influenced by specific hematopoietic growth factors. By 2007 about 20 such growth factors had been identified. They are active in extremely low concentrations and usually act at several points throughout hematopoiesis, facilitating a stem cell's change into a mature functional cell.
Blood groups
Blood types are identified by the presence of certain antigens on the red blood cells. Every person's blood belongs to one of four types, A, B, O or AB, where "A" stands for the A antigen on the red cell membrane, and so on. About 41% of white Americans are classified as type A.
A person with type A blood has plasma with anti-B antibodies. If that person is given type B blood in a transfusion, the recipient's antibodies agglutinate the donor's red cells, and this can lead to harmful effects or even death. Type O blood, which has neither A nor B antigens, is often called universal donor blood because it can be given without the danger of anti-A or anti-B antibodies clumping its red cells. Type AB is universal recipient blood because its plasma contains neither anti-A nor anti-B antibodies, so it cannot clump any donor's red cells that carry A or B antigens.