The anatomy and physiology of the cardiovascular system

Anatomy and Physiology of the Cardiovascular System

The heart is a pump responsible for maintaining adequate circulation of oxygenated blood around the vascular network of the body. It is a four-chamber pump, with the right side receiving deoxygenated blood from the body and pumping it to the pulmonary circulation and the left side receiving oxygenated blood from the lungs and pumping it to the systemic circulation. The myocardium is a specialized form of muscle, consisting of individual cells joined by electrical connections. The contraction of each cell is produced by a rise in intracellular calcium concentration leading to spontaneous depolarization, and as each cell is electrically connected to its neighbor, contraction of one cell leads to a wave of depolarization and contraction across the myocardium. This depolarization and contraction of the heart is controlled by a specialized group of cells localized in the sinoatrial node in the right atrium. These cells generate a rhythmical depolarization, which then spreads out over the atria to the atrioventricular node. The atria then contract, pushing blood into the ventricles. The electrical conduction passes via the atrioventricular node to the bundle of His, which divides into right and left branches and then spreads out from the base of the ventricles across the myocardium. This leads to a contraction of the ventricles, forcing blood up and out into the pulmonary artery and aorta. The atria then refill as the myocardium relaxes. The contraction is referred to as systole and normally lasts for about 250 ms. The relaxation period, when the atria and ventricles refill, is called diastole. The time given for diastole is directly dependent on the heart rate.

There are two main coronary arteries, the left and right coronary arteries, and these branch further to form several major branches. The coronary arteries lie in sulci running over the surface of the myocardium, covered over by the epicardium, and have many branches that terminate in arterioles supplying the vast capillary network of the myocardium. Even though these vessels have multiple anastomoses, significant obstruction to one or other of the main branches will lead to ischemia in the area supplied by that branch.

Cardiovascular disease includes dysfunctional conditions of the heart, arteries, and veins that supply oxygen to vital life-sustaining areas of the body like the brain, the heart itself, and other vital organs. If oxygen doesn't arrive the tissue or organ will die. Ischemic heart disease is the term for obstruction of blood flow to the heart. This disorder results because excess fat or plaque deposits narrow the veins that supply oxygenated blood to the heart. Excess buildup of fat or plaque is respectively termed arteriosclerosis and atherosclerosis. Equally significant would be inadequate oxygen flow to the brain, which causes a stroke.

High blood pressure often results from this excess fat or plaque buildup because of the extra effort it takes to circulate blood. Even though the heart works harder, blockages still shortchange the needed blood supply to all areas of the body. Kidney disorders, which leave extra fluids, sodium, and toxins in the body, obesity, diabetes, birth control pills, pregnancy, smoking, excess alcohol, stress, and thyroid and adrenal gland problems can also cause and exacerbate a high blood pressure condition.

Proper ranges of cholesterol are also important in the prevention of heart attack or stroke. Total blood cholesterol above 200 mg/dl, LDL cholesterol above 130 mg/dl, HDL cholesterol below 35 mg/dl; and lipoprotein (a) level greater than 30 mg/dl are indicators of problematic cholesterol. Cholesterol is not actually a damage mechanism but is more an indicator of compromised liver function, and increased risk of heart attack.

Damage to the heart tissues from cardiovascular disease or from heart surgery will disrupt the natural electrical impulses of the heart and result in cardiac arrhythmia. Sudden fluctuations in heart rate can cause noticeable palpitations, with an associated faintness, or dizziness, and if severely abnormal could interfere with blood flow and even initiate a heart attack.

Infection of the heart, carditis and endocarditis, is an additional complication that can occur as a result of a weak immune system, liver problems, heart surgery, or from an autoimmune disorder like rheumatic fever. Endocarditis is quite common in persons with compromised immune systems from HIV or AIDS. If not appropriately handled, permanent heart muscle damage can occur from the infection.

Strategies to prevent cardiovascular disease include exercise, smoking cessation, and limited use of alcohol and diet modification. Regular exercise increases the strength of the heart with a concomitant increase in stroke volume. The risk of developing heart disease is about half in regular exercisers compared to sedentary individuals. Exercise can also help reduce high cholesterol and high blood pressure.