by Kim Stairs

Created on: May 09, 2009

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

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