About the size of a fist, the heart lies in the thoracic cavity in the mediastinum.
The heart tilts toward the left: two thirds of it extends to the left of the body’s midline.
The broadest part of the heart (the base) is at the upper right; the pointed end (the apex) is at the lower left.
The point of maximum impulse (where the strongest beat can be felt or heard) is located at the apex.
The great vessels enter and leave the heart at the base.
A double-walled sac called the pericardium surrounds the heart; it is anchored to surrounding structures by ligaments and tissue.
The pericardium has two layers: the outermost is the fibrous pericardium (a loose-fitting sac of strong connective tissue); the serous pericardium covers the heart’s surface.
The serous pericardium also has two layers. At the heart’s base, it folds back on itself to form the parietal layer (which lines the inside of the fibrous pericardium) and the visceral layer (which covers the heart’s surface).
The pericardial cavity exists between these two layers; it contains a small amount of serous fluid that helps prevent friction as the heart beats.
The heart wall has three layers:
The endocardium lines the chambers, covers the valves, and continues into the vessels; it consists of a thin layer of squamous epithelial cells.
The myocardium, composed of cardiac muscle, forms the middle layer. It is the thickest of the three layers and performs the work of the heart.
The epicardium, which consists of a thin layer of squamous epithelial cells, covers the heart’s surface. The epicardium is the visceral layer of the serous pericardium.
The heart contains four hollow chambers: atria (upper chambers) and ventricles (lower chambers).
The atria serve as reservoirs, receiving blood from the body or lungs; they are separated by the interatrial septum.
The walls of the atria are not as thick as the walls of the ventricles because the atria move blood only a short distance (from the atria to the ventricles).
The ventricles receive blood from the atria and pump it to the lungs (right ventricle) or the body (left ventricle); they are separated by the interventricular septum.
Because the ventricles pump blood, they must generate more force than the atria. Therefore, the walls of ventricles are thicker than those of the atria. (Because the left ventricle must generate force to push blood throughout the body, its walls are thicker than those of the right ventricle.)
The great vessels (superior and inferior vena, the pulmonary artery, four pulmonary veins, and aorta) transport blood to and from the heart.
Strands of fibrous connective tissue (chordae tendineae) extend from conical papillary muscles on the floor of the ventricle to the valve cusps. Papillary muscles contract with the ventricles, pulling on the chordae tendineae and anchoring the valve cusps in the proper position.
Each of the heart’s four valves are formed by two or three flaps of tissue called cusps or leaflets.
Two atrioventricular (AV) valves regulate flow between atria and ventricles; two semilunar valves regulate flow between ventricles and great arteries.
The right AV valve—also called the tricuspid valve (because it has three leaflets)—prevents backflow from the right ventricle to the right atria.
The left AV valve—also called the bicuspid valve (because it has two leaflets) or the mitral valve—prevents backflow from the left ventricle to the left atria.
Semilunar valves are the pulmonary and aortic valves: The pulmonary valve prevents backflow from the pulmonary artery to the right ventricle; the aortic valve prevents backflow from the aorta to the left ventricle.
A semi-rigid, fibrous, connective tissue called the skeleton of the heart encircles each valve.
The skeleton supports the heart and keeps the valves from stretching; it also provides insulation between the atria and the ventricles, preventing electrical impulses from reaching the ventricles other than through a normal conduction pathway.
When the heart valves close, they produce vibrations that can be heard with a stethoscope on the body surface. By listening in key areas, the sounds made by the individual valves can be identified.
Aortic area: Second intercostal space; right sternal border
Pulmonic area: Second intercostal space; left sternal border
Tricuspid area: Fourth (or fifth) intercostal space; left sternal border
Mistral area (apex): Fifth intercostal space; left midclavicular line
No connection exists between the right and left sides of the heart. The two sides work together to ensure that the organs and tissues of the body receive an adequate supply of oxygenated blood.
Both the right and left atria contract at the same time, as do both ventricles.
The right atrium receives deoxygenated blood returning from the body through the superior and inferior vena cavae.
Once the right atrium is full, it contracts. This forces the tricuspid valve open and blood flows into the right ventricle. When the right ventricle is full, the tricuspid valve snaps closed to prevent blood from flowing backward into the atrium.
3. After filling, the right ventricle contracts, forcing the pulmonary valve open. Blood is pumped into the right and left pulmonary arteries and on to the lungs. After the right ventricle empties, the pulmonary valve closes to prevent the blood from flowing backward into the ventricle.
4. After replenishing its supply of oxygen (and cleansing itself of carbon dioxide) in the lungs, the blood enters the pulmonary veins and returns to the heart through the left atrium.
5. When the left atrium is full, it contracts. This forces the mitral, or bicuspid, valve open and blood is pumped into the left ventricle.
6. When the left ventricle is full, the mitral valve closes to prevent backflow. The ventricle then contracts, forcing the aortic valve to open, allowing blood to flow into the aorta. From there, oxygenated blood is distributed to every organ in the body.
The heart has its own vascular system (coronary circulation) to keep it supplied with oxygenated blood.
Coronary arteries deliver oxygenated blood to the myocardium; cardiac veins collect deoxygenated blood.
The right and left coronary arteries arise from the ascending aorta.
The right coronary artery supplies blood to the right atrium, part of the left atrium, most of the right ventricle, and the inferior part of the left ventricle.
The left coronary artery supplies blood to the left atrium, most of the left ventricle, and most of the interventricular septum; this artery branches into the anterior descending and circumflex arteries.
Most cardiac veins empty into the coronary sinus, which returns the blood to the right atrium. (Except for the anterior cardiac veins, which empty directly into the right atrium.)
Coronary arteries receive blood when ventricles relax.
Because the left ventricle performs most of the heart’s work, it receives the greatest amount of blood.
The heart contains special pacemaker cells that allow it to contract spontaneously (automaticity). Because the heart beats regularly, it is said to have rhythmicity.
The electrical impulses generated by the heart follow a specific route through the myocardium:
Normal cardiac impulses arise in the sinoatrial (SA) node.
An interatrial bundle of conducting fibers rapidly conducts the impulses to the left atrium, and both atria begin to contract.
When the impulse enters the atrioventricular (AV) node, it slows considerably to allow the atria time to contract completely and the ventricles to fill with blood. The heart’s skeleton insulates the ventricles, ensuring that only impulses passing through the AV node can enter.
After passing through the AV node, the impulse picks up speed. It then travels down the bundle of His (also called the AV bundle).
The AV bundle branches into right and left bundle branches.
Purkinje fibers conduct the impulses throughout the muscle of both ventricles, causing them to contract almost simultaneously.
The SA node (the primary pacemaker) fires at 60 to 80 beats per minute. If it fails, the AV node takes over with a firing rate of 40 to 60 beats per minute. If it fails, the Purkinje fibers can initiate impulses with a firing rate of 20 to 40 beats per minute.
Cardiac impulses generate electrical currents that spread through surrounding tissue and can be detected by electrodes on the body’s surface. The record of these signals is called an electrocardiogram (ECG).
An ECG records the electrical activity or impulses; it does not record the heart’s contractions.
An ECG that appears normal is called normal sinus rhythm (meaning that the impulse originates in the sinoatrial [SA] node). An irregular heartbeat is called an arrhythmia.
The P wave represents atrial depolarization—the transmission of electrical impulses from the SA node through the atria. This occurs right before the atria contract.
The PR interval represents the time it takes for the cardiac impulse to travel from the atria to the ventricles.
The QRS complex represents ventricular depolarization—the spread of electrical impulses throughout the ventricles.
The ST segment represents the end of ventricular depolarization and the beginning of ventricular repolarization.
The T wave represents ventricular repolarization.
The prefix “iso” means equal; “volumetric” refers to volume. Therefore, isovolumetric refers to something having the same or equal volume.
Volume of blood remains unchanged; pressure rises because ventricles are beginning to contract.
Semilunar valves open when pressure in the ventricles exceeds pressure in the pulmonary artery and aorta.
Blood spurts out of ventricles rapidly at first and then slows as pressure drops.
The volume of blood in the ventricles remains unchanged; pressure falls because ventricles are relaxing.
The right and left ventricles receive about 70% of their blood passively: It simply flows from the right and left atria after the mitral and tricuspid valves open. Late in the filling process, both atria contract (atrial kick) to supply the ventricles with the remaining 30% of the blood.
The ventricles must actively pump all their blood to the arteries.
Cardiac output refers to the amount of blood the heart pumps in 1 minute.
To determine cardiac output, multiply the heart rate (the number of times the heart beats in 1 minute) by the stroke volume (the amount of blood ejected with each heartbeat).
The average resting cardiac output is between 5 and 6 liters per minute. (Cardiac output increases with activity.)
The only ways to affect cardiac output are to change the heart rate or change the stroke volume.
Heart rate (pulse) is the number of times the heart beats each minute.
Newborns have heart rates of about 120 beats per minute. Young adult women have heart rates of 72 to 80 beats per minute; young adult men have heart rates of 64 to 72 beats per minute.
A persistent pulse rate slower than 60 beats per minute is called bradycardia; a persistent resting heart rate greater than 100 beats per minute is called tachycardia.
Although the heart generates and maintains its own beat, the nervous system can alter the heart’s rhythm and force of contractions.
The cardiac center in the medulla receives input from multiple sources.
Proprioceptors signal changes in physical activity, allowing the heart to increase output before muscles demand more blood flow.
Baroreceptors (pressoreceptors) are pressure sensors; they detect changes in blood pressure. If blood pressure falls, cardiac output decreases; the cardioacceleratory center then stimulates the heart to beat faster and maintain cardiac output. (If blood pressure suddenly rises, impulses will be sent to slow the heart rate, decreasing cardiac output and lowering blood pressure.)
Chemoreceptors detect increases in carbon dioxide, decreases in oxygen, and decreases in pH. In response, the sympathetic nervous system increases heart rate and stroke volume to circulate more oxygen.
Example: Think of shooting a rubber band. The tension, or stretch, placed on the rubber band before you shoot it is its preload.
The more the ventricle is stretched (within limits), the more forcefully it will contract. This is Starling’s law of the heart.
The amount of blood in the ventricle at the end of diastole determines how much the ventricle is stretched; so, the more blood returned to the heart each minute, the more forcefully it will contract. (Too much blood, however, can overstretch the heart’s muscle, causing it to lose elasticity.)
Example: The more you stretch the rubber band, the more it will contract when it is released, and the farther it will fly. (Likewise, an old rubber band that has been overstretched is not as elastic as a new one and will not fly as far when released.)
The heart must work against the pressure of the blood in the arteries.
An increase in afterload (such as high blood pressure) opposes the ejection of blood from the ventricles, leading to decreased stroke volume.
Example: If you try to shoot a rubber band underwater, the pressure of the water (the afterload) will resist the forward movement of the rubber band.
Prolonged high blood pressure or incompetent heart valves force the heart to work harder and can weaken the ventricles.
Chronic lung disease strains on the right ventricle because diseased lungs make it more difficult for the right ventricle to pump blood into pulmonary circulation.
Symptoms of congestive heart failure vary according to the side of the heart affected.
Failure of one ventricle places an added strain on the other ventricle. Eventually, both ventricles fail.
