The heart is an amazing specimen. Without the heart, our body tissues would not get the oxygen exchange and other nutrients required to survive. The heart is responsible for carrying out the job of transporting our blood through our entire body, including itself. When looking at how the heart works, it is interesting to look at both the anatomy and the physiology of the heart. I had this opportunity recently during a dissection in my biology lab.
Once inside the thoracic cavity, you see that the heart sits between the two plural cavities, in a space called the mediastinum. The apex (the bottom tip) of the heart tips to the left and fits into the cardiac notch of the left lung. The heart is protected by the pericardium. This is a two layer sac filled with fluid that protects the heart from friction during contractions. Once through the pericardium, the outer layer of the heart is the epicardium. The middle layer of the heart is the myocardium, and the inner layer is the endocardium. It is the myocardium that contracts the heart. You can see this layer of muscle tissue once the heart has been opened.
The myocardium of the heart is striated or striped, cardiac muscle tissue. This is the thickest layer of the heart because of the force needed to push blood through the ventricles. There are two ventricles, a left and a right. The ventricles sit inferiorly (below) the right and left atrium. The left ventricle needs more force to pump the blood through the body and therefore has the greatest, thickest myocardium. Together these four chambers work to pump 5-6 liters of blood through our body every minute. But our blood cannot go through the body, come back to the heart, and go around again. The blood returning to the right atrium of the heart via the Superior and Inferior Vena Cava is low in oxygen, and needs a refill. This is why we have a Pulmonary and Systemic Circulation.
During the Pulmonary Circulation, the low oxygenated blood gets pumped into the right ventricle and travels through the Pulmonary Arteries into the lungs to retrieve oxygen and exchange the waste gases. It then travels back to the heart into the left atrium via the Pulmonary Veins and is now oxygen rich. The oxygenated blood is now pumped into the left ventricle and ready to enter Systemic Circulation.
Systemic Circulation is responsible for carrying the oxygen rich blood from the Ascending and Descending Aorta to the smaller arteries, arterioles, to the capillary beds where the nutrient and gas exchange occur with every tissue in our body. The blood leaves by way of the left ventricle and returns into the right atrium, thus starting the process all over again.
In a sense you have two circulatory systems operating almost simultaneously. How is it then that the low oxygen blood does not mix with the rich oxygen blood? How is it that the heart knows when to pump and how fast to pump?
First of all, the heart has four doorways that maintain a one-way route. These doorways are called semilunar and atrioventricular valves. The Aortic and Pulmonary Semilunar Valve keep the blood that has left the heart from leaking back into the heart. The Bicuspid and Tricuspid Valve close off both atria after the blood has been pushed into the two ventricles. As the atrium relax and fill with blood, we call it diastole. When they are contracting and ejecting blood into the ventricles, it is called systole. The atrium and ventricles function in opposite patterns, when one set is in diastole the other is in systole. The semilunar and AV valves open and close systematically during this cardiac cycle, ensuring the one- way direction of blood flow through the heart. As these valves close we hear the lubb-dupp of our heart beat.
Did you know that the heart can beat once out of the body? Even if the heart is dissected, it can still beat just for a little while. The heart is made of autorhythmic fibers. These muscle cells are excited and beat all by themselves. But if each cell can beat on its own, it would not function as a whole unit. To fix this we have a natural pacemaker called the SA Node. The SA Node is in charge of all the other heart cells. When it gives the signal, all the cells beat in rhythm. The signal travels over to the AV Node (secondary pacemaker). The AV Node then sends the signal down the ventricles through the Bundle of Hiss, and finally the electric signal reaches the Perkenje Fibers and then the ventricles contract. Our heart beat driven by the SA Node is approximately 100 beats per minute.
Maybe that sounds high, but there is one more little part of our body that helps regulate our beating heart. The Medulla Oblongata is part of the brain stem that houses the cardiac center. The Medulla receives sensory signals from other parts of the brain and the Parasympathetic Nervous System. This slows down our heart beat to approximately 75 beats per minute.
The heart is truly amazing....