4.3: Heart

Chambers and the Cardiac Cycle

The heart consists of four chambers (Figure 4.3). Blood from the veins enters the two upper chambers, called atria. Blood from the lungs returns to the heart through several pulmonary veins, which deliver it to the left atrium. This blood has a high concentration of oxygen, which was added as the blood passed trough the lungs. The oxygen is needed by all the cells in the body.

Figure 4.3 The internal structure of the heart and adjoining blood vessels. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

While blood from the lungs is entering the left atrium, blood from the body is flowing into the right atrium via two large veins. This blood has had most of its oxygen removed by body cells and contains a high concentration of a waste product called carbon dioxide, which was produced by body cells. It also carries many useful substances (e.g., nutrients and hormones) added by various organs.

The blood flows easily from each atrium into the ventricle just below it because the ventricles relax and tend to widen at this time ((Figure 4.4). The flow is aided by a relatively weak contraction of the atria. Once the ventricles have been filled, they contract powerfully, squeezing the blood and pumping it into the arteries. The ventricles contract for a fraction of a second and then relax again.

(b) Blood Pressure **

Figure 4.4 (a) Cardiac cycle and (b) blood pressure (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission)

**The video of the heart beating at https://upload.wikimedia.org/Wikipedia/commons/5/56/Cardiac-Cycle-Animated.gif. It shows an animation of an opened human heart performing cardiac cycles. The red line in the graph below the animation shows the rising and falling blood pressure of the "Aortic pressure" in a main artery, the aorta. Notice that the blood pressure rises as the ventricles contract during systole and falls as the ventricles relax during diastole. In this example, the systolic pressure rises to 120mmHG, and the the diastolic pressure falls to 80mmHG, hence the blood pressure is “120/80”. The small pressure bump at the beginning of diastole is from the pulse wave bouncing back from lower arteries.

The blood from the left ventricle is pushed very forcefully into a large artery, the aorta. Branches from the aorta deliver this oxygen‑rich and nutrient‑rich blood to all parts of the body except the lungs. Special branches from the aorta – coronary arteries – transport some blood to the walls of the heart.

The right ventricle pumps blood through the pulmonary arteries to the lungs. Most of the carbon dioxide in this blood is removed while the blood is in the lungs. At the same time, oxygen is added to the blood for delivery to the rest of the body.

When the ventricles contract and force blood into the arteries, the blood pressure rises quickly to a peak value called systolic pressure (Figure 4.4). When the ventricles relax and blood in the arteries flows into the capillaries, the arterial blood pressure drops to a low value called diastolic pressure. Diastolic pressure does not reach zero because the ventricles remain relaxed for only a fraction of a second before contracting again. Also, as will be described later, the elasticity of large arteries helps prevent it from falling too low.

While the ventricles are contracting, the atria relax and then begin to fill with the next volume of blood that will enter the ventricles and be pumped to the body.

This completes one heartbeat or cardiac cycle. By repeating this process over and over, the heart keeps the blood circulating. The blood must pass through the heart twice to make one complete circuit around the body (Figure 4.1). The rate of flow depends on the amount pumped per minute: the cardiac output (CO). Cardiac output equals the amount pumped by each beat of either the left or the right ventricle [stroke volume (SV)] times the number of beats per minute [heart rate (HR)]. Therefore, CO = SV x HR.

The highly coordinated and well‑timed operation of the heart chambers is controlled by special muscle cells. A patch of these cells in the right atrium signals when each beat is to begin. For this reason, the patch of cells is called the pacemaker, a name shared by the artificial electronic devices sometimes used to restore the proper heart rate to a diseased heart. Other cells send the signal through the atria and then the ventricles. As the signal spreads, causing other muscle cells to contract, the contracting cells produce electrical impulses that can be detected and recorded. The recording is called an electrocardiogram (ECG or EKG).

Valves

Valves are located within the openings leading from the atria to the ventricles and from the ventricles to the arteries. The movement of blood from the atria into the ventricles and from the ventricles into the arteries pushes the valves open. When the ventricles begin to contract, some of the blood within them begins to move backward toward the atria. Similarly, when the ventricles relax, blood in the arteries starts to flow back into them. This causes the valves to swing shut, stopping the backward flow of blood. Thus, the valves ensure that the blood moves only in the correct direction (Figure 4.4).

Layers

The heart wall is composed of three layers: the endocardium, myocardium, and epicardium.

Endocardium

The inner lining of the heart is called the endocardium (Figure 4.5). This layer must be very smooth and must have no gaps that allow blood to contact the underlying collagen. Blood that contacts rough spots or collagen will clot, and clots formed in the heart can move into arteries and block blood flow.

Figure 4.5 Layers of the heart with coronary vessels (a) & (b). (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Myocardium

The middle layer of the heart - the myocardium - is a thick layer that constitutes most of the wall of the heart. The myocardium consists mostly of heart muscle (cardiac muscle), though it also contains fat tissue and collagen fibers. Contraction of the cardiac muscle provides the force that pumps the blood.

The myocardium in the atria is thin because the atria pump blood only into the neighboring ventricles. The myocardium of the right ventricle is of moderate thickness because it must pump blood somewhat farther through the lungs. The myocardium of the left ventricle is much thicker because it pumps blood farther and through many vessels in all other regions of the body.

Epicardium

The outer layer of the heart – the epicardium – contains some connective tissue coated with a smooth, slippery layer of epithelial cells. This coating allows the beating heart to move easily within the pericardial cavity. At the top of the heart, the epicardium tethers the heart to other structures in the chest so that it does not shift out of position.

Coronary Blood Flow

The heart muscle must have a steady supply of energy to pump blood continuously. It gets this energy through a complicated series of chemical reactions that combine oxygen with nutrients such as blood sugar. These materials must be delivered to the myocardial cells by the blood flowing through the coronary arteries (Figure 4.6). The heart muscle cannot get materials directly from the blood inside the heart chambers because molecules do not pass easily through the thick wall of the heart.

Figure 4.6 Coronary arteries; (a) anterior view (b) posterior view (c) transparent view. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

In addition to producing useful energy, the reactions in heart cells produce wastes such as water and carbon dioxide, which are removed from the heart by blood in the coronary capillaries and veins. These wastes are finally eliminated by the lungs and kidneys.

If myocardial cells do not get enough oxygen for their energy requirements, they malfunction and the heart cannot pump blood adequately. People in this condition get out of breath easily. They feel weak and lethargic, tire quickly, may become dizzy and faint, and can suffer heart attacks. Therefore, the coronary arteries must deliver plenty of oxygen‑rich blood to the myocardium.

Cardiac Adaptability

Recall that as the rate of activity of body cells changes, the amount of blood flow around the cells must also change to provide for their varying needs. This is especially important when levels of physical activity change because active muscles use materials and produce wastes much faster than resting muscles do. One way in which blood flow is adjusted is an alteration in cardiac output caused by changes in stroke volume or heart rate. Since alterations in CO must be made to maintain homeostasis, the heart is controlled by negative feedback systems. The nervous system detects changes in internal body conditions when exercise begins or ends. Cardiac output is then adjusted through changes in the nerve impulses sent to control the heart. Levels of hormones that influence the heart are also adjusted. Finally, the heart has intrinsic mechanisms to increase or decrease its own stroke volume as needed. As a result, the parts of the body receive the right amount of blood.