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The Cardiac/Circulatory System
The main function of the cardiovascular system is to deliver oxygen to all other organs and systems of the body and to remove waste gases, such as carbon dioxide, which is produces when the oxygen is used up.
The cardiac/circulatory system consists of:
Blood and blood elements
Blood is one of the connective tissues of the body. It is a sticky, opaque fluid with both solid and liquid components. Living blood cells, the formed elements, are suspended in a nonliving fluid called plasma. Plasma is more than 90 percent water.
There are three formed elements in the blood: red blood cells, platelets, and white blood cells.
The oxygen-carrying elements of blood are the red blood cells (RBC's), which are formed in the bone marrow. They pick up oxygen in the lungs and release it in the capillaries (see below) to tissues and cells. RBCs are also called erythrocytes.
RBCs can't reproduce themselves. They have a fairly short useful life.
RBCs circulate for about 120 days until defective and are then removed from the blood by the spleen.
Platelets, another formed element in the blood, are cell fragments that aid in blood clotting. They are also called thrombocytes.
White blood cells are the third formed element in the blood. They are important in the immune system. Another name for them is leukocytes.
The circulatory system
The structures of the circulatory system are organized much like a system of roads, with highways, secondary roads, and alleys.
Arteries, the largest blood vessels in the blood transport system, are the "highways." They are lined with a slick coating that keeps blood flowing smoothly. Blood enters these first on being pumped out of the heart.
The blood moves next into the arterioles, which are smaller arteries. These are the "secondary roads."
Smaller yet are capillaries, tiny hairlike structures in the tissues. These are integrally related to the surrounding tissues (much as alleys are with their surrounding neighborhoods).
Blood enters the capillaries from the arterioles. It is here that the actual exchange of blood and nutrients with body tissues occurs.
Capillaries drain into the veins.
The veins empty into the large veins entering the heart.
Coronary arteries supply oxygen and nutrients to the heart.
This "road system" brings oxygen-poor blood from the right side of the heart to the lungs and freshly oxygen-rich blood back to the left side of the heart. It also brings oxygen-rich blood from the left heart to the body tissues and returns oxygen-poor blood to the right heart.
The heart
The heart is an hollow organ about the size of a fist and weighs less than a pound. It works as a pump, propelling blood into the blood vessels and to all tissues of the body.
The heart wall (myocardium) is made of thick bundles of cardiac muscle arranged in spirals. This is the part that actually contracts.
Cardiac muscles are involuntary (meaning we can't control their movements). Their contractions provide the force that pumps blood.
The heart has four hollow chambers or cavities: two upper atria and two lower ventricles. Like the arteries, the chambers are lined with a slick coating that keeps the blood flowing smoothly.
The upper atria are the receiving chambers. Blood flows here under low pressure from the veins.
The lower ventricles are the actual pumps. When they contract, blood is propelled out of the heart and into the circulatory system.
Why blood pressure is important
Blood pressure is the force exerted by the blood against the artery walls.
If blood pressure stays within certain limits, no undue strain is put on the arteries.
Blood pressure is measured in two phases. Systolic pressure is the peak pressure exerted whien the left ventricle contracts while pumping out blood. Diastolic pressure is the reduced pressure felt just before the next beat, when the heart is relaxed and blood is flowing into it.
The two readings are presented as a fraction, with systolic over diastolic.
Although there's no real "normal" blood pressure, the average range for healthy young adults is 115/75 to 128/80.
Severe high blood pressure can cause strokes or heart attacks. Even slightly high blood pressure can shorten your life if it's chronic.
High blood pressure can also create weak spots in the artery walls. When an artery wall sags and balloons out, it's called an aneurysm. This can be life-threatening.
LDL Cholesterol vs. HDL Cholesterol
The critical issue in understanding cholesterol is knowing the difference between cholesterol that helps the body and cholesterol that harms one.

Low-density lipoprotein (LDL) cholesterol, which is derived from animal fats, is associated with increased risk for coronary heart diseases. This is "bad" cholesterol. High LDL levels cause a buildup of arterial plaque-a fatty sludge made up of oxidized cholesterol-thickened muscle layers in the arteries, and blood platelets stuck together in mounds of debris. This clogging of the arteries forces the heart to work harder and harder to push blood through the system.

Conversely, high-density lipoprotein (HDL) cholesterol, which comes from polyunsaturated fatty acids, actually helps reduce the amount of LDL in the blood. This is "good" cholesterol. It can also help lower blood pressure, relax coronary arteries, and inhibit platelet stickiness. All of these effects contribute to a healthy heart.

Therefore, in the interest of good health, levels of HDL cholesterol should be high and levels of LDL cholesterol low.
How the cardiac/circulatory system works when the body is in balance
Brain/Central Nervous System
The autonomic nervous system (which controls automatic functions) helps regulate heart rate and blood pressure.
Muscle/Skeletal System
Activity of the skeletal muscles (the ones that allow you to move) increases the efficiency of cardiovascular functioning. If you build up these muscles through exercise, you are building up your heart muscle as well!
Energy/Metabolic System
Several hormones influence blood volume, blood pressure, and the power of the heart's contractions.
The hormone erythropoietin stimulates the production of RBCs.
In return, hormones "travel" through the body via the blood.
Digestive/Detox System
The digestive system provides nutrients to heart and blood vessels.
It absorbs iron needed to create hemoglobin, the oxygen-carrying component of blood.
This system also absorbs the water necessary for normal blood volume.
The cardiac system transports nutrients absorbed by the digestive tract to all tissues of the body. It also distributes hormones of the digestive tract.
Immune System
The spleen destroys aged RBCs, stores iron, and removes debris from the blood.
Immune cells protect cardiovascular organs from specific disease-causing agents, called pathogens.
Blood is the source of lymph, a fluid found between cells.
Blood provides the route for immune elements (such as white blood cells) to travel throughout the body.
The respiratory system provides oxygen that is pumped throughout the body by the heart.
It also disposes of carbon dioxide, the gas that is left over when oxygen is used up. It does this every time you exhale.
Blood transports the respiratory gases, oxygen, and carbon dioxide to and from the organs.
Aneurysm: a weakened artery wall which has sagged and ballooned out.
Arerial plaque: a fatty sludge made up of oxidized cholesterol.
Arterioles: the "secondary roads" of the body. Blood enters these vessels after living the arteries.
Atrium: the upper chambers of the heart. These are the chambers which receive blood from the veins.
Blood pressure: the force exerted by the blood against artery walls.
Carbon dioxide: the gas created after the body has used up the oxygen that we take in during breathing. It is made of one carbon and two oxygen atoms.
Contractions: the powerful tightening of muscle which causes any sort of movement. For example, heart beats are contractions of heart muscle.
Diastolic: the reduced pressure felt just before the next heart beat, when the heart is relaxed and blood is flowing into it.
Erythrocytes: also known as red blood cells. These cells carry oxygen throughout the body.
Erythropoientin: the hormone which stimulates the production of red blood cells.
Leukocytes: white blood cells.
Lymph: a fluid found between cells.
Myocardium: the wall of the heart.
Oxidized cholesterol: blood cholesterol which has been attacked by free radicals.
Pathogens: any disease-causing agent, for example, bacteria and virus cells.
Plasma: the clear, yellowish fluid portion of blood, lymph, or intramuscular fluid in which cells are suspended.
Platelets: the cells which allow the blood to clot and prevent us from bleeding to death.
Systolic: the peak pressure exerted when the left ventricle contracts while pumping out blood.
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