The heart & external structure
The mammalian heart is a muscular double pump divided into two sides. The right side pumps Deoxygenated blood to the lungs to be Oxygenated. The left side pumps Oxygenated blood to the rest of the body. Pressure forces the blood along the arteries.
Firm dark red muscle surrounds the two main pumping chambers - the ventricles. Above the ventricles are two thin-walled chambers - the atria. These are much smaller than the ventricles and easy to overlook.
The coronary arteries lie over the surface of the heart. They carry oxygenated blood to the heart muscle itself. As the heart is a hard-working organ, these arteries are very important. If they become constricted, it can have severe consequences for the health of the heart and of the animal. Restricted blood flow to the heart muscle reduces the delivery of oxygen and nutrients such as fatty acids. This may cause angina or a heart attack.
At the top of the heart are a number of tubes. These are the veins that carry blood into the heart and the arteries that carry blood out of the heart.
The heart & internal structure
Divided into four chambers. Two upper chambers are atria which receive blood from the major veins. Deoxygenated blood from the body, flows from the vena cava into right atrium. Oxygenated blood from lungs flows from the pulmonary vein into left atrium.
From atria, blood flows down through atrioventricular valves into the ventricles. These valves are thin flaps of tissue arranged in a cup shape. When the ventricles contract, the valves fill with blood and remain closed. This ensures that the blood flows upwards into major arteries and not back into the atria. String like tendinous cords are attached to the flimsy valves and stop them from tuning inside out.
The septum, a wall of muscle, separates the ventricles from each other. This ensures that oxygenated blood in left side & deoxygenated blood in right side are kept separate.
Deoxygenated blood leaving right ventricle flows into the pulmonary artery leading to the lungs. Oxygenated blood leaving the left ventricle flows into the aorta. This carries blood to a number of arteries that supply all parts of the body. At the base of the major arteries, where they exit the heart, are valves called semilunar valves, which prevent blood returning to the heart as the ventricles relax.
The muscle of each chamber contract to create increased pressure in the blood. the higher the pressure created in the heart, the further it will push the blood.
Right atria: Very thin muscle walls, because chambers do not need to create much pressure. Their function is to push blood into the ventricles.
Right ventricle: Thicker walls than atria. Enables it to pump blood out of the heart. However, the walls are much thinner than those of the left ventricle. Right ventricle pumps deoxygenated blood to the lungs. The lungs in the chest cavity are beside heart, so blood doesn't need to travel far. Also, lungs contain a lot of very fine capillaries that are in close contact with the walls of the alveoli. The alveoli walls are very thin and there is very little or no tissue fluid. So capillaries are not supported and could easily burst. The pressure of the blood must be kept down to prevent the capillaries in the lungs busting.
Left ventricle: Very thick walls, can be two or three times thicker than right ventricle. The blood from the left ventricle is pumped out through the aorta and needs sufficient pressure to overcome the resistance of the systematic circulation.
The cardiac cycle
The cardiac cycle cont.
1. Atrioventricular valve closes.
2. Semilunar valve opens.
3. Semilunar valve closes.
4. Atrioventricular valve opens.