The heart is the pump in the transport system that circulates blood throughout the body via the blood vessels. Dissolved in the blood are nutrients, oxygen, and waste products that are transported to and from body cells. The heart is about the size of a person’s fist and is located centrally in the thorax in a cavity called the mediastinum. The heart tips slightly to the left.

The heart is enclosed in a double-walled sac called the pericardium. The outer fibrous pericardium protects the heart and anchors it to surrounding structures such as the diaphragm, sternum, and great vessels. Deep to the fibrous pericardium is the double-layered serous pericardium. The outer serous pericardium is called the parietal pericardium and is fused to the fibrous pericardium. The inner serous pericardium is the visceral pericardium and covers the external heart surface. Between the two serous layers is the slippery serous fluid which fills the pericardial cavity. This prevents friction damage to the heart as it contracts. Inflammation of the pericardium (pericarditis) may result from bacterial pneumonia. It hinders the production of serous fluid and roughens the membrane surface, creating a rustling sound.

Composed of 3 layers (outer epicardium, middle myocardium, inner endocardium). The heart itself has its own blood supply via the coronary arteries. The myocardium is composed mainly of cardiac muscle and forms the bulk of the heart. It is the layer that contracts. Within this layer the branching cardiac muscle cells are tethered to each other by connective tissue fibers called the fibrous skeleton of the heart. The epicardium is the same thing as the visceral pericardium. The endocardium is a sheet of endothelium on the inner heart surfaces. It is a slick surface that reduces friction due to blood flow and is continuous with endothelium of the blood vessels.

4 chambers: 2 superior atria (atrium, singular) and 2 inferior ventricles. They are separated by an inner partition called the interatrial septum or interventricular septum, depending upon which chambers it separates. Functionally the atria are receiving chambers for blood returning to the heart. They contract minimally and thus are relatively thin-walled. External to the atria are the auricles which increase the atrial volume somewhat. The interatrial septum has a shallow depression called the fossa ovalis which marks the spot where an opening (foramen ovale) existed in the fetal heart.

Blood enters the right atrium via three veins:

a. superior vena cava – returns blood from body regions superior to diaphragm

b. inferior vena cava – returns blood from body regions inferior to diaphragm

c. coronary sinus – collects blood draining from the myocardium itself.

Four pulmonary veins enter the left atrium. The pulmonary veins transport blood from the lungs back to the heart.

The ventricles are the pumps that pump blood out of the heart. The right ventricle pumps blood to the pumonary arteries which carries blood to the lungs. The left ventricle ejects blood into the aorta which carries blood to the body. Marking the internal ventricle walls are irregular muscle ridges called trabeculae carneae. Other muscle bundles (papillary muscles) are projecting and stalk-like and play a role in valve function.

The heart is actually 2 side-by-side pumps, each serving a separate blood circuit. The pulmonary circuit receives blood returning to the heart from the body and sends it through the lungs. In the lungs the blood gives up waste carbon dioxide and takes on oxygen. The systemic circuit supplies the entire body with oxygenated blood.

The right side of the heart is the pulmonary circuit pump. It pumps deoxygenated blood to the lungs. The left side is the systemic circulation pump. It receives oxygenated blood from the lungs and pumps it to the body tissues via the arteries. The workload differs between the two circuits. The pulmonary circuit is short and low-pressure, whereas the systemic circuit is longer and encounters 5 times as much friction. Therefore the left ventricle walls are much thicker than the right ventricle walls.

Blood flows through the heart in one direction. This one-way flow is enforced by 4 heart valves: one pair of atrioventricular valves and one pair of semilunar valves, which open and close in response to differing blood pressures on their two sides.

The atrioventricular valves (AV) are located at the junctions of the atria and their respective ventricles and prevent backflow into the atria when the ventricles are contracting.

The right AV valve, the tricuspid, has 3 valve flaps or cusps. The left AV valve, the bicuspid, had two. The left AV valve is also called the mitral valve. Attached to each of the AV valve flaps are tiny white collagen cords called chordae tendineae ("heart strings") that anchor the cusps to the papillary muscles protruding from the ventricular walls.

When the heart is completely relaxed, the AV valve flaps hang open. Blood flows through the atria into the ventricles. When the ventricles begin to contract, the pressure rises in the ventricle and forces the blood superiorly against the valve flaps. The flaps meet, closing the valve. The chordae tendineae and papillary muscles serve as guy wires to anchor the valve flaps in the closed position. If the cusps where not anchored in this manner, they would be blown upward into the atria.

The aortic and pulmonary semilunar valves guard the bases of the large arteries issuing from the ventricles and prevent backflow into the associated ventricles. Each of the semilunar valves has three pocket-like cusps. When the ventricles contract, the pressure in the ventricles are greater than the pressure in the aorta and pulmonary arteries and the valves are forced open. The blood exits the ventricle. When the ventricles relax and the blood begins to flow backward toward the heart, it fills the cusps and effectively closes the valves.

In valvular stenosis (narrowing) the valves become stiff and the heart must contract more forcibly than normal.

6. Summary of blood flow through the heart. Use this space to list in order every blood vessel, heart chamber, and heart valve that a red blood cell would pass through on its way from the superior vena cava to the aorta. Also designate where the lungs would be in that sequence of structures.

The cells of the heart receive no nutrients or oxygen from the blood flowing through the heart. Their blood supply is provided by the right and left coronary arteries. The coronary arteries branch greatly and supply alternative routes for blood delivery. Thus there is a backup system when these arteries become occluded. Full occlusion of a coronary artery leads to tissue death and heart attack.

After passing through the capillary beds of the myocardium, the cardiac veins join together to form an enlarged vessel, the coronary sinus, which empties into the right atrium.

The sinoatrial (SA) node is a small mass of cells in the right atrium that spontaneously depolarize (initiate a contraction) about 60 - 70 times a minute. This is the pacemaker of the heart. The rate of contraction can be altered by the sympathetic nervous system, hormones, and electrolytes. From the SA node the depolarization wave spreads through gap junctions throughout the atria and reaches the atrioventricular (AV) node located in the inferior portion of the interatrial septum. The atria contract. Then the nerve fibers from the AV node spreads the depolarization wave through the ventricles, and the ventricles contract from inferior to superior forcing the blood into the arteries (pulmonary and aorta).

The most important extrinsic influences on heart rate are exerted by the autonomic nervous system. When the sympathetic nervous system is activated, norepinephrine is released which causes the heart to beat faster and to contract more forcibly. Activation of the parasympathetic division opposes the sympathetic effects and reduces heart rate via the release of acetylcholine.

Hormones such as epinephrine produces the same effects as norepinephrine. Thyroxine causes a slower but more sustained increase in heart rate when it is released in large quantities.

Reduced blood levels of ionic calcium depress the heart. Hypercalcemia increases heart irritability, which can lead to spastic heart contraction.

Too much sodium inhibits transport of calcium, thus blocking heart contraction. Excess potassium interferes with depolarization and may lead to heart block and cardiac arrest.

Other factors including age, gender, exercise, and body temperature also influence heart rate.