Anatomy and Physiology Lecture Notes

Unit 7 – Circulatory System - The Heart

Approximately the size of a person's fist, the hollow, cone-shaped heart weighs less than a pound. The pointed apex is pointed toward the left hip and rests on the diaphragm, approximately at the level of the fifth intercostal space. Its broader posterosuperior aspect, the base, from which the great vessels of the body emerge, points toward the right shoulder and lies beneath the second rib.

The human heart essentially is two separate hearts enclosed in a membrane called the pericardium. The pericardium surrounds the heart and secretes a fluid that reduces friction as the heart beats. Fibrous tissues in the pericardium protect the heart and anchor it to surrounding structures, such as the diaphragm and sternum.

The heart walls are composed of three layers:

• The epicardium, or visceral pericardium, is actually part of the pericardium.
• The myocardium consists of thick bundles of cardiac muscle, the layer that actually contracts.
• The endocardium is a thin, glistening sheet of endothelium that lines the heart chambers and helps blood flow smoothly through the heart. It is continuous with the linings of the blood vessels leaving and entering the heart.

The upper chambers of the heart, right and left atria (atrium), receive blood returning to the heart. As a rule, they are not important in the pumping activity of the heart. Blood flows into the atria under low presure from the veins and then continues on to fill the ventricles.

The lower chambers, right and left ventricles, pump blood out of the heart. The left ventricle is the thickest chamber of the heart because it has to do most of the work to pump blood to all parts of the body.

Vertically dividing the right and left sides of the heart is a common wall called the septum. The septum prevents the mixing of oxygen-poor and oxygen-rich blood.

Circulation Through the Heart

Pulmonary Circulation: The right side of the heart pumps blood from the body into the lungs, where oxygen-poor blood (deoxygenated), gives up CO2 and picks up O2.

Systemic Circulation: The left side of the heart pumps oxygen-rich blood (oxygenated), from the lungs to the rest of the body except the lungs.

Oxygen-poor blood from the body enters the right side of the heart through two large blood vessels. The superior (upper) vena cava brings blood from the upper part of the body to the heart. The inferior (lower) vena cava brings blood from the lower part of the body to the heart. Both vena cava empty into the right atrium. When the heart relaxes (between beats), pressure in the circulatory system causes the atrium to fill with blood. When the heart contracts, blood is squeezed from the right atrium into the right ventricle through flaps of tissue called the atrioventricular (AV) valve, that prevents blood from flowing back into the right atrium. The valve that separates the right atrium and ventricle is called the tricuspid valve. The general purpose of all valves in the circulatory system is to prevent the backflow of blood. They also ensure that blood flows in only one direction. The specific purpose of the tricuspid valve is to prevent backflow of blood from the right ventricle to the right atrium when the right ventricle contracts. When the heart contracts a second time, blood in the right ventricle is sent through a semilunar (SL) valve known as the pulmonary valve into the pulmonary arteries to the lungs. These are the only arteries to carry oxygen-poor blood. At the base of the pulmonary arteries, the pulmonary valve prevents blood from traveling back into the right ventricle.

Oxygen-rich blood leaves the lungs and returns to the heart by way of blood vessels called pulmonary veins. These are the only veins to carry oxygen-rich blood. Returning blood enters the left atrium, it passes through flaps of tissue called the atrioventricular (AV) valve to the left ventricle. The valve that separates the left atrium and ventricle is called the mitral valve or biscuspid valve. From the left ventricle, blood is pumped through a semilunar valve called the aortic valve into the aorta artery that carries blood to every part of the body except the lungs. At the base of the aorta is an aortic valve that prevents blood from flowing back into the left ventricle.

The Heart Valves The two artioventricular valves prevent backflow into the atria when the ventricles contract: The left AV-valve, the biscupid or mitral valve consists of two cusps, or flaps, of endocardium. The right AV-valve, the tricuspid valve has three cusps. The two semilunar valves close the two large arteries as the ventricles relax: The right SL-valve, the aortic valve has three cusps that fit tightly together. The left SL-valve, the pulmonary valve also has three cusps.

The cardiac cycle is the sequence of events in one heartbeat.

In its simplest form, the cardiac cycle is the simultaneous contraction of the two atria, followed a fraction of a second latter by the simultaneous contraction of the two ventricles.

A heartbeat has two phases:

• Phase 1 - systole - contraction.

Occurs when the ventricles contract, closing the AV valves and opening the SL valves to pump blood into two major vessels leaving the heart.

• Phase 2 - diastole - relaxation.

Occurs when the ventricles relax, allowing the back pressure of the blood to closed SL valves and opening AV valves. The cardiac cycle also creates the heart sounds:

Each heartbeat produces two sounds, often called lubb-dub, that can be heard with a stethoscope.

• The first sound, the loudest and longest, is caused by the ventricular systole (contraction) closing the AV valves.
• The second sound is caused by the closure of the aortic and pulmonary valves (SL).

If any of the valves do not close properly, an extra sound called a heart murmur, may be heard.

Heart Muscle Contraction:

The heart consists of muscle cells that contract in waves. When the first group is stimulated, they in turn stimulate neighboring cells. Those cells stimulate more cells. This chain reaction continues until all cells contract. The wave of activity spreads in such a way that the atria and the ventricles contract in a steady rhythm.

The wave begins in a small bundle of specialized heart muscle cells embedded in the right atrium called the sinoatrial node (SA). The SA-node is the natural pacemaker of the heart. It initiates each heartbeat, without stimulation from the nervous system, and sets the pace for the heart rate.

The impulse spreads from the pacemaker through the cardiac muscle cells in the right and left atrium, causing both atria to contract almost simultaneously. When the impulse initiated by the SA-node reaches another special area of the heart known as the atrioventricular (AV) node. The AV-node is located in the septum between the right and left ventricles. The AV-node relays the electrical impulse to the muscle cells that make up the ventricles. The ventricles contract almost simultaneously a fraction of a second after the atria, completing one full heartbeat. These contractions causes the chambers to squeeze the blood, pushing it in the proper direction along its path.

Cardiac Output (CO) is the amount of blood pumped out of each side of the heart (each ventricle) in 1 minute. It is the product of the heart rate (HR) and the stroke volume (SV). Stroke volume is the volume of blood pumped out by a ventricle with each heartbeat. In general, stroke volume increases as the force of ventricular contraction increases.

Using the normal resting values for heart rate (75 beats per minute) and stroke volume (70 ml - about 2 ounces - per beat), the average adult cardiac output can be easily figured.

CO = HR X SV

CO = (75 beats / min) ( 70 ml / beat)

CO = 5250 ml / min

Regulating Stroke Volume:

A healthy heart pumps out about 60% of the blood that enters it. The critical factor controlling stroke volume is how much cardiac muscle cells are stretched just before they contract. Venous return, the amount of blood entering the heart and distending its ventricles, is the determining factor. Anything that increases the volume or speed of venous return also increases stroke volume and force of contraction.

• A slow heartbeat allows more time for the ventricles to fill.
• Exercise speeds venous return because it results in increased heart rate and force.
• The enhanced squeezing action of active skeletal muscles on veins returning blood to the heart, the so-called muscular pump, also plays a major role in increasing benous return.

On the other hand, low venous return, such as might result from sever blood loss or an extremely rapid heart rate, decreases stroke volume, causing the heart to beat less forcefully.

Regulating Heart Rate:

For most of us, at rest our heart beats between 60 and 80 beats per minute. Under certain conditions, that number can increase to as many as 200 beats per minute.

• The sympathetic nervous system increases heart rate.

During times of physical or emptional stress, the SA-node and AV-node - and even the cardiac muscle itself - can be stimulated to increase heart rate.

• The parasympathetic nervous system decreases it.

When demand declines, the vagus nerves slow and steady the heart.

• Various hormones and ions can have a dramatic effect on heart rate.

Epinephrine, which mimics the effect of the sympathetic nerves, and thyroxine both increase heart rate.

Reduced Ca⁺² in the blood depresses the heart, while a low level of K⁺ causes the heart to beat feebly and without rhythm.

• A number of physical factors, including age, gender, exercise, and body temperature, influence heart rate.

Cardiac Circulation:

Although the heart chambers are continuously bathed with blood, the blood contained in the heart does not nourish the myocardium. The blood supply that oxygenates and nourishes the heart is provided by the right and left coronary arteries. The coronary arteries branch from the base of the aorta and encircle the heart in the atrioventricular groove at the junction of the atria and ventricles. The coronary arteries and their major branches are compressed when the ventricles are contracting and fill when the heart is relaxed. The myocardium is drained by several cardiac veins, which empty into an enlarged vessel on the backside of the heart called the coronary sinus. The coronary sinus, in turn, empties into the right atrium.

Heart-related Problems:

Pericarditis is an inflammation of the pericardium. This can lead to a decrease in the amount of serous fluid surrounding the heart, which in turn causes the pericardial layers to bind and stick together, forming painful adhesions that interfere with heart movements.

Heart valves are basically simple devices, and the heart - like any mechanical pump - can function with "leaky" valves as long as the damage is not too great. However, severe valve deformities can seriously hamper cardiac function.

• An incompetent valve forces the heart to pump and repump the same blood because the valve does not close properly and blood backflows.
• In valvular stenosis, the valve flaps become stiff, often because of repeated bacterial infection of the endocardium (endocarditis). This forces the heart to contract more vigorously than normal.
  
  In each case above, the heart's workload increases, and ultimately the heart weakens and may fail. Under such conditions, the faulty valve can be replaced with a synthetic valve, or a valve taken from a pig heart.

Angian pectoris is a crushing chest pain caused by low levels of oxygen reaching the myocardium. While the cause of this decreased flow to the heart tissue may vary, the pain is a warning that should never be ignored. Prolonged angina may cause the death of ischemic (blood-starved) heart cells, forming an infarct. The resulting myocardial infarction is commonly called a "heart attack" or "coronary".

Fibrillation is a rapid uncoordinated shuddering of the heart muscle. This contraction of the heart muscle is described as looking like a "bag of worms". The cause is related to either or both of the heart "nodes" and makes the heart totally useless as a pump. This is th major cause of death from heart attacks in adults.

Tachycardia is a rapid heart rate (over 100 beats per minute). Bradycardia is a heart rate that is substantially slower than normal (less than 60 beats per minute). Neither condition is pathological, but prolonged tachycardia may progress to fibrillation.

Congestive heart failure (CHF) is a progressive decrease in the efficiency of the heart. This condition reflects a weakening of the heart by coronary artherosclerosis (clogging of the coronary vessels with fatty buildup), persistent high blood pressure, or multiple myocardial infarcts - leading to repair with non-contracting scar tissue.

Pulmonary congestion occurs when only the left side of the heart fails. The right side of the heart continues to pump blood to the lungs, but the left side is unable to send the returning blood into the systemic circulation. As blood vessels within the lungs become swollen with blood, the pressure causes leaking into the lung tissue, causing pulmonary edema (lung swelling).

Peripheral congestion occurs when only the right side of the heart fails, causing blood to back up in the systemic system. Edema is most noticeable in the distal parts of the body - the feet, ankles, and fingers become swollen and puffy. Failure of one side of the heart puts a greater strain on the opposite side, and eventually the whole heart fails.