Cardiovascular system

The Cardiovascular System

All body systems are linked by the cardiovascular system, a transport network that affects every part of the body. To maintain homeostasis, the cardiovascular system must provide for the rapid transport of water, nutrients, electrolytes, hormones, enzymes, antibodies, cells, and gases to all cells. In addition, it contributes to body defences and the coagulation process and controls body temperature. The term cardiovascular refers to the cardiac (heart) muscle, the vascular system (a network of blood vessels that includes veins, arteries, and capillaries), and the circulating blood. Thus, the three primary components of the cardiovascular system are:

• Heart
• Circulating blood
• Blood vessels (the circulatory system)

The Heart

The human heart is a muscular organ about the size of a man’s closed fist. The heart contains four chambers and is located slightly left of the midline in the thoracic cavity. The two atria are separated by the interatrial septum (wall), and the interventricular septum divides the two ventricles. Heart valves are positioned between each atrium and ventricle so that blood can flow in one direction only, thereby preventing backflow. The right atrium of the heart receives o2-poor blood from two large veins: the superior vena cava and the inferior vena cava. The superior vena cava brings blood from the head, neck, arms, and chest; the inferior vena cava carries blood from the rest of the trunk and the legs. Once the blood enters the right atrium, it passes through the heart valve (right atrioventricular, or tricuspid, valve) into the right ventricle. When blood exits the right ventricle, it begins the pulmonary circuit—it enters the right and left pulmonary arteries. Arteries of the pulmonary circuit differ from those of the systemic circuit because they carry deoxygenated blood.

Like veins, they are usually shown in blue on colour-coded charts. These vessels branch into smaller arterioles and capillaries within the lungs, where gaseous exchange occurs (o2 is picked up, and Co2 is released). From the respiratory capillaries, blood flows into the left and right pulmonary veins and then into the left atrium. The left atrium also has a valve (left atrioventricular, bicuspid, or mitral valve). Blood flows through the mitral valve into the left ventricle. When blood exits the left ventricle, it passes through the aortic semilunar valve and into the systemic circuit by means of the ascending aorta. The systemic circuit carries blood to the tissues of the body. If a valve malfunctions, blood flows backwards and a heart murmur results. The right side of the heart pumps o2 poor blood to the lungs to pick up more o2; the left side pumps o2-rich blood toward the legs, head, and organs. The heart’s function is to pump sufficient amounts of blood to all cells of the body by contraction (systole) and relaxation (diastole). Because the lungs are close to the heart, and the pulmonary arteries and veins are short and wide, the right ventricle does not need to pump very hard to propel blood through the pulmonary circuit. Thus, the heart wall of the right ventricle is relatively thin. On the other hand, the left ventricle must push blood around the systemic circuit, which covers the entire body. As a result, the left ventricle has a thick, muscular wall and a powerful contraction.

Blood pressure increases during ventricular systole and decreases during ventricular diastole. Blood pressure not only forces blood through vessels but also pushes it against the walls of the vessels like air in a balloon. Therefore, it can be measured by how forcefully it presses against vascular walls.

The average heart beats 60 to 80 times per minute. Children have faster heart rates than adults, and athletes have slower rates because more blood can be pumped with each beat. During exercise, the heart beats faster to supply muscles with more blood. During and after meals, it also beats faster to pump blood to the digestive system. During a fever, the heart pumps more blood to the skin surface to release heat. Remember that all

responses are designed to maintain homeostasis. The heart rate (pulse rate) is measured by feeling for a pulse and counting the pulses per minute.

The Vessels and Circulation

Three kinds of blood vessels exist in the human body:

• Arteries
• Veins
• Capillaries

This intricate system travels to every inch of the human body through repeatedly branching vessels that get smaller and smaller as they move away from the heart (arteries) and then get larger again as they return toward the heart (veins). The largest artery (aorta) and veins (venae cavae) are approximately 1 inch wide.

Arteries

Arteries are highly oxygenated vessels that carry blood away from the heart (efferent vessels). They branch into smaller vessels, called arterioles, and into capillaries. Arteries appear brighter red in colour, have thicker elastic walls than veins do, and have a pulse.

Veins

Blood is carried toward the heart by the veins (afferent vessels). It is remarkable that the blood in veins flows against gravity in many areas of the body; these vessels have one-way valves and rely on weak muscular action to move blood cells. The one-way valves prevent the backflow of blood. All veins (except the pulmonary veins) contain deoxygenated blood. Veins appear bluish in colour under the skin and have thinner walls than arteries. You should become familiar with the principal veins of the arms and legs. The antecubital area of the forearm is most commonly and generally the largest and best-anchored vein. Others in the antecubital area that are acceptable are the basilic vein and the cephalic vein.

Capillaries

Capillaries are tiny microscopic vessels that connect or link arteries (arterioles) and veins (venules) and may be so small in diameter as to allow only one blood cell to pass through at any given time. They are the only vessels that permit the exchange of gases (o2 and Co2) and other molecules between blood and surrounding tissues.

Capillaries do not work independently but are a part of an interconnected network. Each arteriole ends in dozens of capillaries (capillary bed) that eventually feed-back into a venule (when gas/ the nutrient exchange has been completed). Blood in the capillary bed is a mixture of arterial and venous blood.

Capillary bleeding occurs slowly and evenly because of the smaller size of the vessels and the low pressure within the vessels. Capillary bleeding is usually considered minor and is easily controlled with slight pressure, or sometimes bleeding stops without intervention. Capillary blood is a colour between the bright red of arterial blood and the dark red of venous blood.

The Blood

Circulating blood provides nutrients, oxygen, chemical substances, and waste removal for each of the billions of individual cells in the body and is essential to homeostasis and to sustaining life. Any region of the body that is deprived of blood and 02 soon becomes oxygen-deficient, and the tissues may die within minutes. This condition is called hypoxia.

Human bodies contain approximately 4.73 litres of whole blood, which is composed of water, solutes (dissolved substances), and cells. The volume of blood in an individual varies according to body weight; for instance, adult men usually have 5 to 6 litres of whole blood, whereas adult women usually have 4 to 5 litres.

Abnormally low or high blood volumes can seriously affect other parts of the cardiovascular system. Whole blood is normally composed of approximately 2.84 litres, or about 55 to 60 percent, of plasma and 1.89 litres, or about 40 to 45 percent, of cells. Thus, if a blood specimen is withdrawn into a test tube from a vein and centrifuged, about 55 percent will be plasma, and 45 percent will be formed elements (cells). The plasma portion contains approximately 92 percent water and 8 percent solutes. Solutes include proteins, such as albumin (maintains water balance in the blood); fibrinogen (for blood clotting); metabolites, such as lipids; glucose; nitrogen wastes; amino acids; and ions, such as sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), and chloride (Cl).

Haemostasis and Coagulation

Haemostasis (not to be confused with homeostasis) is a complex series of processes in which platelets, plasma, and coagulation factors interact to control bleeding while at the same time maintaining circulating blood in the liquid state. It enables the human body to retain blood in the vascular system by preventing blood loss. When a small blood vessel is injured, the haemostatic process (clotting response) repairs the break and stops the haemorrhage by forming a plug or blood clot.

This intricate process involves the following phases:

• Vascular phase—Once a blood vessel is injured, a rapid constriction of the vessel (vasoconstriction) decreases the blood flow to the surrounding vascular bed.
• Platelet phase—Platelets degranulate, clump together and adhere to the injured vessel in order to form a plug and inhibit bleeding.
• Coagulation phase—Many specific coagulation factors (including fibrinogen, clotting factors, and calcium) are released and interact to form a fibrin meshwork or blood clot. This clot seals off the damaged portion of the vessel.
• Clot retraction—This occurs when the bleeding has stopped. The entire clot retracts to heal tom edges by bringing them closer together.
• Fibrinolysis—When the final repair and regeneration of the injured vessel occurs, the clot slowly begins to break up (lysis) and dissolve as other cells carry out further repair. The entire process is fast, intricate, self-sustaining, and remarkable.

It is important to focus briefly on the coagulation process (the third phase), which is a result of numerous coagulation factors. For simplicity, it is divided into two systems: intrinsic and extrinsic. All coagulation factors required for the intrinsic system are contained in the blood, whereas the extrinsic factors are stimulated when tissue damage occurs. For example, blood vessels are lined with a single layer of flat endothelial cells and are supported by collagen fibres. Normally, endothelial cells do not react with or attract platelets; however, they do produce and store some clotting factors. When the clotting sequence begins due to a vessel injury, endothelial cells react with degranulated platelets in forming the fibrin plug.1 Bleeding from small arteries and veins can be controlled by the hemostatic process; however, large- or medium-sized veins and arteries require rapid surgical intervention to prevent excessive bleeding.