Blood Pressure

The heart forces blood into the aorta under pressure, which can be measured and is called your blood pressure. Blood pressure depends on the strength of the heart's contraction and the number of beats per minute. It also depends on the volume of blood in the heart and blood vessels and the elasticity of the arteries.

There are two phases of blood pressure. When the heart is contracting, the highest pressure generated in the heart and arteries is known as systolic pressure. As the heart relaxes, blood pressure declines, and the lowest pressure level is known as diastolic pressure. For example, if blood pressure is recorded as "120 over

The Blood

Blood is a very complex fluid that both feeds and cleanses the body. It is the means by which oxygen, tiny food particles, and other nutrients are delivered to tissue and, conversely, waste products are removed and eventually discarded by the lungs, liver, and kidneys.

The blood consists of plasma, which is a straw-colored solution, and three formed elements suspended in the plasma: red blood cells, white blood cells, and platelets. Blood travels through an im mense network of arteries and veins. Arteries typically carry bright red, oxygenated blood from the heart to the tissues, whereas veins carry dark purple, unoxygenat-ed blood back to the heart. The tiniest blood vessels linking the two kinds of vessels are called capillaries. Capillaries are so small that blood cells travel through some capillaries in single file. The diameter of a capillary can be as small as three or four microns — and there are approximately twenty-five thousand microns in an inch!

The amount of blood in your body depends on your size and some other factors. A person of 160 pounds has about five quarts of blood.

Plasma is mostly water but contains hundreds of other substances, including proteins, digested food, waste products, and electrolytes, which are mainly minerals in solution. There are substances

in blood that cause clotting in response to injury. There are also dissolved gases and chemical transmitters called hormones. Hormones, which originate in various glands, activate or deactivate certain bodily functions.

Serum is a term often confused with plasma. Serum is plasma that has had the clotting elements removed.

Each of the three formed elements in blood has a specific function. Red blood cells are produced in the bone marrow and are also called erythrocytes. One ounce of blood contains billions of red blood cells. Their main job is to cany oxygen from the lungs to the body and to carry carbon dioxide from the tissues to the lungs. White blood cells are also called leukocytes and help protect the body against disease and infection. They are somewhat larger than the red blood cells and are also produced in the bone marrow. There are several types of white blood cells, each with a different function. There are millions of white blood cells per ounce of blood.

Platelets are disk-shaped structures produced in the bone marrow. They are much smaller than red or white blood cells. They are responsible for helping to stop bleeding if a blood vessel is damaged. They clump or stick together around the edges of a damaged blood vessel. As they pile up, they form a seal that helps to start the blood-clotting process so a permanent plug can form.

Platelet

Superior Vena Cava

Inferior Vena Cava

Sinoatrial Node:

Also referred to as the sinus node, or S-A node. This is the true pacemaker of the heart. Its cells rhythmically discharge electrical impulses that cause the heart to contract. These impulses also travel to the A-V node.

Atrioventricular Node:

A specialized nervetype tissue located in the wall of the right ventricle, also called the A-V node. It receives electrical impulses from the sinoatrial node that cause it to relay electrical impulses that cause the heart to contract.

Bundle of His:

A special nerve-type tissue extending from the atrioventricular node (A-V node) along the ventricular septum.

It helps conduct electrical impulses from the A-V node through the ventricles.

Superior Vena Cava

Inferior Vena Cava

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