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Biology - The Heart

The heart has four chambers, two on the right and two on the left. On each side of the heart, right and left, the top half is called the atrium and the bottom half is called the ventricle. So, the human heart has a right atrium, a left atrium, a right ventricle, and a left ventricle. Blood has to move throughout the body all of the time. The heart is a pump whose job is to keep the blood moving throughout the body all of the time. Blood goes around the body and through the heart in a continuous cycle, and that cycle is called the circulation. 

We will start at the point in the circulation at which the blood leaves the heart to begin its tour around the body. The blood leaves the heart from the left side specifically from the left ventricle. After leaving the left ventricle, the blood tours the entire body. It enters a huge blood vessel which is called the aorta. The aorta is the largest artery in the human body. Remember that arteries take blood away from the heart. Shortly after it takes off from the left ventricle, the aorta divides into smaller arteries and then into even smaller arteries, and these small arteries go off in all different directions throughout the body. When the arteries get very small, they are called arterials and when they get very, very small, they are called capillaries. Some of these capillaries go to the skin. They have the ability to constrict in cold weather, thus retaining body heat and to dilate in warm weather, so these capillaries are involved in thermal regulation which is an important function of the skin. 

Capillaries coalesce to form bigger and bigger blood vessels eventually forming veins. Veins always take blood towards the heart. Finally, all of the blood ends up in two large veins and these two veins return the blood to the heart. One vein is called the anterior vena cava and the other is called the posterior vena cava. The anterior and the posterior vena cava take the blood into the right side of the heart, specifically into the right atrium. On each side of the heart, there are two trap doors between the atria and the ventricles. There is a trap door between the right atrium and the right ventricle. There is also a trap door between the left atrium and the left ventricle. The trap door between the right atrium and the right ventricle is called the tricuspid valve. The trap door between the left atrium and the left ventricle is called the bicuspid valve or mitral valve. Because of these trap doors, blood that’s on the right side of the heart can move from the right atrium to the right ventricle. Blood that’s on the left side of the heart can move from the left atrium to the left ventricle. Blood cannot move however directly from the right side of the heart to the left side of the heart. The blood doesn’t stay in the right ventricle. Blood is always on the move. The next thing it does is to leave the right ventricle and pass into the pulmonary circulation. 

The pulmonary circulation is another system of arteries, arterials, capillaries, venules and veins separate from the one we talked about before. The pulmonary circulation involves only the lungs. When blood leaves the right ventricle, it immediately enters a large artery called the pulmonary artery. This pulmonary artery then divides into a right pulmonary artery which goes to the right lung and a left pulmonary artery which goes to the left lung. The pulmonary arteries carry deoxygenated blood. Within the lung, each pulmonary artery branches repeatedly to form capillaries, then the capillaries coalesce to form venules, small veins, larger veins, and finally all of the blood from the right lung is collected in two large veins called the right pulmonary veins and all of the blood from the left lung is collected in two large veins called the left pulmonary veins. The pulmonary veins carry oxygenated blood. All four of the pulmonary veins, two from the right lung and two from the left lung, then conduct blood into the heart’s left atrium. From the left atrium, it passes through a valve into the left ventricle. From the left ventricle, it passes out into the aorta and the circulatory cycle begins again. 

Contraction of the atrium is called diastole. Contraction of the ventricles is called systole. Because during systole blood is pushed from the left ventricle out into the entire body, all of the arteries in the body feel a push of blood called the arterial pulse. The heart contracts and relaxes automatically about 72 times per minute throughout life. The signal for every contraction originates in the heart itself. The place of origin is located in the right atrium and it is called the sinoatrial node. It is also called the SA node. When you think of the signal to contract spreading throughout the heart, just think of the letters SABP. The signal begins at S, the sinoatrial node. It next reaches A, the atrioventricular node. From the atrioventricular node, the signal passes to B, the bundle of His and from there it travels through both ventricles via P, the Purkinje fibers. 

Blood contains three types of cells: red blood cells, white blood cells and platelets. It also contains proteins. Red blood cells carry the blood gases, oxygen, and carbon dioxide. The white blood cells are called leukocytes, which form pus and eat foreign particles like bacteria and lymphocytes which we will discuss later. Platelets are necessary for blood clotting. All the blood cells originate from precursor cells in the bone marrow. A hematocrit is the percentage of whole blood volume that is occupied by red blood cells. Plasma is whole blood without the cells. Serum is the fluid that is left over after the plasma has clotted. As blood flows through the capillary, two types of pressure are at work, hydrostatic and osmotic. Hydrostatic pressure causes fluids to be pushed out of the capillary and osmotic pressure causes fluids to be pulled into the capillary. At the arterial end, the hydrostatic pressure is greater than osmotic pressure, so fluids are pushed out of the capillary. At the venous end, the osmotic pressure is greater than the hydrostatic pressure, so fluids are pulled into the capillary. This is the fate of 99% of the fluid. The rest of the fluid gets picked up by the lymph system.