What is the Cardiovascular System?

The cardiovascular system is made up of the heart and all of the blood and lymphatic vessels in the body. The heart acts as a pump to drive the blood; a complex fluid made up of digested food materials, respiratory gasses (oxygen being carried in, carbon dioxide being carried out), and protective and regulatory chemicals through the approximately 60,000 miles of vessels in the body. In order to perform all of it’s necessary functions, the heart constantly works interactively with the:

• Lungs and respiratory system-monitoring oxygen/carbon dioxide balance.
• Urinary tract- monitoring fluid levels.
• Digestive system- delivering digested nutrients.
• Endocrine system- delivering hormones.
• Skin- blood flow to the skin determines heat loss from the body, allowing the control of body temperature.
• All tissues- removal of excess hydrogen ions, maintenance of proper levels of calcium, magnesium, and potassium ion levels in the tissues.

To truly appreciate health a person must consider the health and nutritional status of each organ, the interactive relationship between all of the organs and attempt to see the parts of the body working together as a complete system. When our defense systems weaken, blood can also act as transport for disease-causing entities such as viruses, bacteria, parasites, and their toxins. The circulatory system has immune defense cells (white blood cells) and the lymphatic system that act as a waste removal system and defense mechanism against these and other foreign invaders.

Anatomy of the Circulatory System

The Importance of Oxygen

The cells in your body need oxygen, and it is the job of your blood to bring oxygen from your lungs to various tissues and organs. When you breathe, oxygen passes through the walls of air sacs in your lungs and is picked up by your blood. Newly oxygenated blood travels along the pulmonary circuit to your heart, where it is pumped to other parts of your body via the systemic circuit. Once the blood reaches these other tissues, the oxygen it contains is released and exchanged for carbon dioxide. Deoxygenated blood is returned to your heart where it is pumped back to your lungs to drop off carbon dioxide and pick up oxygen, completing the cycle.

If your browser supports JavaScript, you can point to the magnifying glass icons at right to see the oxygen and carbon dioxide molecules in motion.

The Systemic and Pulmonary Circulation

The circulatory system consists of the Systemic Circulation (which comprises blood supply to the entire body except the lungs) and the Pulmonary Circulation (which is circulation to the lungs for removal of carbon dioxide, other gases, and volatile compounds.)

The systemic circulation begins at the left side of the heart where the left atrium receives blood which has just returned from the pulmonary circulation, and is rich in oxygen. The blood is received from the left atrium into the left ventricle, which then pumps it out through the body's main artery, the aorta. Other arteries then branch off the aorta and carry blood all over the body, finally branching into small arterioles which feed the capillaries which feed the tissues, by allowing oxygen and nutrients to pass through their thin walls. Likewise, deoxygenated blood and waste products enter the capillaries which combine to form venules, which join to form veins, which dump into the inferior and superior vena cavas. The vena cavas empty into the right atrium. The right ventricle receives the blood from the right atrium and then pumps it through the pulmonary artery to the lungs, finally reaching the alveoli where carbon dioxide is released and oxygen enters, to be finally returned via the pulmonary veins to the left atrium.

Hepatic Circulation

Within the systemic circulation, there is a bypass to the liver called the portal circulation. Capillaries carrying nutrient-rich blood from the stomach, spleen, and intestines, join to form venules (small veins) which meet to form veins. These then merge into the portal vein which conveys the blood to the liver. Nutrients pass from the capillaries into the liver cells for processing and storage, or reentry into the general circulation. The liver produces plasma proteins, and cholesterol. It also takes up glucose, stores it as glycogen, and converts it as need back to glucose for energy. The liver also regulates the level of amino acids in the blood by converting any excess into glucose, proteins, other amino acids, or urea (which is passed to the kidneys). Along with the kidneys, the liver acts to clear the blood of drugs and toxins, by altering their chemical structure, and excreting them into the bile, which leaves via the hepatic duct to the gallbladder. Merging with the portal blood, is oxygen rich blood conveyed from the left ventricle (about one quarter of the hearts output) via the hepatic artery, which forms many branches in the liver to feed its cells.

Autonomic Controls

The role of the central control mechanisms is to regulate the body's global functions. Maintaining blood flow against gravity when we get up suddenly, for example, involves such regulation. Heat control is another example. Blood flow is a most effective way to move heat. Channeling extra blood into muscles, the heart, and other tissues involved in emergency activity, is another example of global functioning. The main central control reflecting the level of activity and mobilization in the body as a whole is the autonomic nervous system, comprised of the parasympathetic and sympathetic pathways. In general, the sympathetic nervous system heightens activity in the body - quickening the heart beat and breathing rate as if it were preparing the body for a fight or flight response. The parasympathetic system has the opposite effect. (see the Autonomic Nervous System article) While the body is at rest, the parasympathetic system essentially lets the tissues' metabolic demands set the pace of circulation. When the body is mobilized or active, the sympathetic nervous system can shut down metabolism and shift the resulting surplus blood supply to zones necessary for immediate action and survival, the muscles, heart and brain. This sympathetic response of shifting blood supply, along with the accompanying mechanisms of dilated bronchi, accelerated heart rate, inhibition of the metabolic process, etc., is known as the "stress response." The "stress" response is of particular concern with cardiovascular diseases. The problem is that, unlike our forebears, our stress responses are not accompanied by physical exertion and movement which dissipates the energy and accompanying neurotransmitters and hormones. It is actually counter productive and harmful to distort metabolic functions so that blood levels of sugars and cholesterol are increased and to stimulate the heart beat and increase blood pressure if they cannot be diffused through physical exercise. However, in today's society with much of our stress taken sitting down, a wind-up stagnation of energy occurs without much hope of healthful dissipation. Add to this, the over-consumption of food (especially empty calories and low quality fats), alcohol and tobacco, and much of the circulatory problems effecting the sedentary over-stressed human can be explained.

The Results of Inadequate Maintenance

High cholesterol levels (through undissipated stress and over-consumption) lead to fatty infiltration of the artery walls and, in time, to arteriosclerosis. This, with persistent constriction of blood vessels in the sedentary stress response and associated increase in the quantity of body fluids, is most likely to cause elevated blood pressure. This in turn creates an extra workload on the heart, and coupled with sympathetic stimulation, can produce arrhythmias. Arteriosclerosis of the heart's own blood supply, the coronary arteries, can produce angina and coronary heart attacks. Changes in blood chemistry and the blood-vessel lining resulting from the elevated cholesterol and homocysteine levels, free radicals and oxidants, coupled with undissipated stress responses and arteriosclerosis can produce spontaneous blood clotting, thromboses and embolisms. Persistent heart strain, high blood pressure, and fluid retention can lead to heart failure. Additionally, smoking constricts the blood vessels, raises blood pressure, reduces available oxygen, and exacerbates all of the above. A detailed of many cardiovascular conditions can be found here.

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