The Circulatory System

The human circulatory system or cardiovascular system consists of blood vessels, the heart and blood.

The piping system in humans is extensive. There are tens of thousands of kilometers of blood vessels in a human body to ensure that every cell of the body is within diffusion distance of a capillary. We divide blood vessels into 5 basic types:

arteries

arterioles

capillaries

venules

veins

As the heart pumps blood out into arteries, the artery walls are pushed outward, increasing the fluid capacity of the vessel. As the walls stretch, the elastic fibers act much as rubber bands and resist the stretching. When we measure blood pressure, we are measuring the push back force of this elastic material.

The instrument used to measure blood pressure is called a sphygomanometer. It consists of an inflatable cuff and a pressure gauge. The cuff is wrapped around one's upper arm and inflated sufficiently to momentarily stop arterial blood flow. A stethoscope is placed over the large arteries opposite the elbow and as air is released from the cuff, one listens for sounds while watching the pressure gauge. The pressure value, measured in units of mm of Hg when one first hears heart sounds is the systolic pressure and a measure of highest pressure attained in the artery as blood is forced through it.

This pressure drops rapidly as blood is forced through the artery lessening its diameter. Arteries have insufficient time to return to an unstretched diameter before the next heart beat and so the arteries are always under some amount of pressure. When one can no longer hear heart sounds, the pressure in the artery has reached its lowest value, the diastolic pressure.

Blood pressures greater than 140/90 are considered to be elevated. The causes of high blood pressure are not well understood. You should have your blood pressure checked periodically. If it is elevated, your physician may prescribe blood pressure lowering drugs. You should take the drugs as prescribed even though having high blood pressure does not appear to be painful. High blood pressure has detrimental effects on several organ systems (cardiovascular accidents, atherosclerosis, kidney failure, coronary artery blockage, kidney damage and failure) and can lead to organ failure and an early death if not treated.

The major arteries branch into smaller vessels called arterioles. The amount of blood available to a particular tissue depends in large part on the state of contraction of of the muscle cells in the walls of these vessels. When a person needs more oxygen, these muscle cells relax in the necessary arterioles, allowing their diameter to increase. Tissues requiring less oxygen see the arterioles serving them constrict a bit diminishing their supply of blood.

From the arterioles, blood passes into the capillaries, the narrowest and most "porous" channels of the cardiovascular system. The diameter of these vessels is just sufficient to allow a red blood cell to pass. It is at the capillary level that exchanges occur between cells and blood. A human body contains about 40,000 kilometers of capillaries-- a very large surface area. The porous nature of capillary walls allows gases and water to be readily exchanged but prevents cells and most proteins from leaving the capillary.

Blood flows from capillaries into venules which eventually empty into larger thin-walled veins. Venous blood pressure is very low, its having been dissipated by passage through the extensive capillary system. Skeletal muscle pressure against veins forces blood to move. One-way flaps or valves prevent backflow.

The human heart is a fist sized, muscular organ possessing four chambers. Every minute, the heart re-circulates the body's entire blood volume (about 5 liters). The two thin-walled atria (singular=atrium) act as receiving chambers and fill with blood as the heart relaxes. Contraction of the thicker-walled ventricles pushes blood out of the heart.

Deoxygenated blood enters the right atrium of the heart from the superior and inferior vena cava (veins). The atria contract, forcing blood from the right atrium into the right ventricle. The right ventricle forces deoxygenated blood out of the heart into the pulmonary arteries (they carry deoxygenated blood) to the lungs where carbon dioxide is exchanged for oxygen.

The oxygenated blood returns to the left atrium of the heart via the pulmonary veins (carrying oxygenated blood). As the atria contract, blood from the left atrium is forced into the left ventricle. Ventricular contraction forces oxygenated blood from the left ventricle into the aorta from which it is subsequently delivered to all parts of the body.

The right side of the heart drives the pulmonary circulation, pumping deoxygenated blood into the capillary beds of the lungs. The oxygenated blood returns to the left side of the heart and propelled by the left side of the heart through the systemic circulation system.

The average resting heart beat is 72 beats per minute.

The heart muscle itself is not oxygenated by blood passing through it. Large coronary arteries branch from the aorta as it leaves the heart and supply the heart muscle with blood. The coronary arteries get their name by their supposedly looking like a crown upon the heart.

Obstructions within the coronary arteries can deprive the heart muscle of necessary oxygen. Such deprivation can lead to spasms, arhythmias and death of portions of heart musculature. These are called myocardial infarcts. If sufficient cardiac muscle is involved, organismal death may occur. Debilitation often results.

Sometimes one suffers severe pains in the chest due to insufficient amounts of oxygen being delivered to the heart musculature. The phenomenon is called angina. Angina pains are scary and the individual usually seeks medical aid. After appropriate studies, what is generally found is that one or more of the coronary arteries are occluded. Sometimes these occlusions are treated by balloon angioplasty, a procedure where a long thin tube is threaded into an arm artery and then up into the blocked or partially blocked coronary artery. A balloon at the end of the tube is then inflated to squeeze the blocking plaque material, making the overall diameter of the artery larger. The tube is then removed.

In other instances, the occlusion is so severe that much more radical procedures must be resorted to: coronary bypass surgery. In this procedure, a piece of vein is taken from the individual's leg and grafted into place between the aorta and and heart muscle, bypassing the blocked coronary artery. Often several blocked arteries are bypassed during a single operation.

Coronary bypass surgery is common in the US. Careful attention to diet and exercise are necessary following surgery to prevent new blockage from occurring. In spite of these efforts, many individuals present for a second surgical event a few years after their first surgery.

The one-way movement of blood through the heart is ensured by the proper operation of valves. When you listen to a heart beat with a stethoscope, you hear a "lub-dub" sound. This is caused by the closure of the heart valves. Improper valve closure changes this sound and results in a heart "murmur." Sometimes these murmurs are of no consequence; other times they are the result of a disease process and may warrant surgical repair.

The contraction of heart muscle is a well-controlled event. It initiates in a small piece of cardiac tissue called the sino-atrial node. This pacemaker generates a wave of electrical activity that spreads across the walls of the atria and causes their contraction. It then spreads to the ventricles causing their contraction just as the atria relax.

The rate of heart muscle contraction is controlled by a number of factors. In general, impulses that reach the pacemaker from the parasympathetic nerves decrease the heart contraction rate. Impulses that arrive from the sympathetic nerves increase its contraction rate.

Sometimes the pacemaker region of the heart is damaged (infarct) or fails to properly initiate contractions. In these instances, an artificial pacemaker, actually an electrical shock device, can be surgically implanted in the heart muscle to initiate regular contractions.

Blood is complex tissue consisting of cells suspended in a straw-colored liquid called plasma. The average adult has about 10 pints of blood, about 55 percent of which is plasma. Plasma is mostly water, but also contains many proteins and dissolved salts.

The erythrocytes or red blood cells of blood are red due to their containing an abundant quantity of a red pigment called hemoglobin. Red blood cells are flattened, disk-shaped cells and are the most abundant cell type in the human body (about 5 billion in each ml of blood). Mature, circulating red blood cells lack a nucleus, ribosomes and mitochondria and are largely bags of hemoglobin.

Leukocytes or white blood cells represent a much smaller population of blood cells. There are five classes of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils and basophils. Their numbers change as a function of disease. Some infections see a large rise in the number of neutrophils. Others see an increase in the numbers of lymphocytes.

One of the more common laboratory procedures your physician may request is a blood count. This will include a measurement of amount of hemoglobin, a red blood cell count, a white blood cell count and an assessment of the percentage of each white blood cell type.

With most of us, when we are cut, we bleed and fairly rapidly, a clot forms, stopping further blood loss. The cardiovascular system is set up to minimize blood loss due to injury. Local trauma releases substances that causes blood vessels to constrict. Circulating blood platelets become trapped at the site of injury and in conjunction with various clotting factors, act to plug the hole. One of the several clotting factors is fibrinogen which is converted to fibrin.

The conversion of fibrinogen to fibrin is complex reaction involving several factors. There are human genetic diseases (hemophilia) that are a consequence of lack of synthesis of one or more of these factors.

At one time or another, you will have cut yourself with an extremely sharp edged object such as a razor blade. You will then notice that this cut does NOT readily stop bleeding. The reason is that there is insufficient tissue trauma to release enough initiating chemicals to start the clotting reactions.

This discussion has only treated the closed circulatory system found in humans and other higher animals. Some animals (insects, spiders, most mollusks) have an open circulatory system where blood is pumped through vessels that dump into a large hemocoel (an open space) in which internal organs are immersed. In this way, these internal organs are oxygenated, and exchange carbon dioxide.

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