1. The heart and blood vessels make up the cardiovascular system, which transports blood throughout the body.

2. The cardiovascular system has two major divisions: a pulmonary circuit, which serves the lungs, and a systemic circuit, which supplies blood to the remainder of the body. The right side of the heart sends blood to the pulmonary circuit. The left side of the heart sends blood to the systemic circuit.

3. The major arteries and veins close to the heart are called the great vessels because of their relatively large diameters.

1. The heart lies at the center of the thoracic cavity in the mediastinum. Its broad, superior portion (the base) is the point of attachment for the great vessels.

2. The heart weighs about 300 g (10 oz).

1. The heart is enclosed in a double-walled sac called the pericardium.

2. The parietal pericardium consists of a tough, fibrous layer of dense connective tissue with a thin, smooth, moist serous layer.

3. The serous layer turns in at the base of the heart, forming the visceral pericardium covering the heart surface.

4. Between the parietal and visceral pericardia is a space called the pericardial cavity. It contains pericardial fluid that lubricates the membranes and allows the heart to beat almost without friction.

1. The heart consists of three layers: the epicardium, myocardium, and endocardium.

2. The epicardium is a serous membrane overlying a thin layer of areolar tissue. In many areas, it has thick deposits of adipose tissue.

3. The myocardium is composed of cardiac muscle and forms the bulk of the heart. It performs the work of the heart. Muscle fibers are bound together by a fibrous skeleton that serves to provide support for the heart, gives the muscle something to pull against, and limits the routes by which electrical activity can travel through the heart.

4. The endocardium is made up of a layer of endothelium overlying a thin layer of areolar tissue. It forms the smooth inner lining of the chambers and valves and is continuous with the endothelium of the blood vessels.

1. The heart has four chambers.

2. The right and left atria receive blood flowing to the heart. Each atrium has a small ear-like extension called an auricle that slightly increases its volume.

3. The two inferior chambers, the left and right ventricles, pump the blood into the arteries for distribution elsewhere.

4. Other features of the heart include the atrioventricular sulcus, the anterior and posterior interventricular sulci, the interatrial septum, and the interventricular septum.

1. Valves prevent the back flow of blood into the heart. The pulmonary valve guards the opening from the right ventricle to the pulmonary trunk. The aortic valve guards the opening from the left ventricle to the aorta. Both these valves are called semilunar valves because of the moonlike shape of their three cusps.

2. An atrioventricular (AV) valve guards the opening between each atrium and ventricle. The right AV valve is also known as the tricuspid valve, the left is also called the bicuspid or mitral valve. String-like chordae tendineae attach the valve cusps to papillary muscles.

3. The opening a closing of the heart valves is the result of pressure gradients from one side of the valve cusps to the other.

1. The path that blood takes on its journey through the heart is depicted in Fig. 19.8, p. 681; Transp. 364.

1. Although the heart accounts for a tiny fraction of the body weight, it uses 5% of its output to supply its own needs. The myocardium has an extensive network of coronary arteries to ensure an adequate blood supply to itself.

2. Arterial Supply (p. 681; Fig. 19.9; Transp. 365)

3. Venous Drainage (p. 682)

4. Perfusion in Relation to the Cardiac Cycle (p. 682)

1. A four-chambered heart with complete separation of the pulmonary and systemic circuits is achieved only in birds, mammals, and a few reptiles. This ensures that the organs of the systemic circuit receive only "fresh" fully oxygenated blood has not mixed with the "stale" deoxygenated blood.

2. The four chambers of the human heart are essential for the support of our high metabolic rate.

3. The evolutionary history of the human heart is somewhat replayed in its embryonic development.

1. Cardiac muscle cells (myocytes) are different from skeletal muscle fibers in that they are relatively short, thick, branching cells that have one nucleus. The sarcoplasmic reticulum (SR) is less developed, lacks terminal cisternae, but T tubules are larger than in skeletal muscle.

2. Each myocyte is surrounded by a connective tissue endomysium, which allows access to blood capillaries.

3. The myocytes are joined end to end by intercalated disks, which have three distinct features: the plasma membranes of the adjacent cells display interdigitating folds; the cells are tightly joined by desmosomes; and there are gap junctions between the cells.

4. The muscle of a given heart chamber is sometimes described as a functional syncytium.

1. At rest, the heart gets 60% of its energy from fatty acids, 35% from glucose, and 5% from other fuels. Cardiac muscle is more vulnerable to oxygen deficiency than it is to a lack of fuel.

2. Heart muscle is not prone to fatigue because it makes little use of anaerobic fermentation or the oxygen debt mechanism.

1. Vertebrate hearts are myogenic because the pacemaker is in the heart itself, and the autonomic nervous system can only modify heart rate.

2. Cardiac myocytes are autorhythmic. Some of them lose their ability to contract and become specialized for generating and transmitting action potentials. These make up the cardiac conduction system.

3. The cardiac conduction system begins with the sinoatrial (SA) node, which serves as the pacemaker. Signals from the SA node spread throughout the atria and on to the atrioventricular (AV) node near the lower interatrial septum. The AV node is an electrical gateway to the ventricles. The AV bundle leaves the AV node and divides into right and left bundle branches leading into the interventricular septum. Next, the impulse is spread to the Purkinje fibers.

4. In addition to setting heart rate, the cardiac conduction system controls the route and timing of electrical conduction. The four chambers must contract in a coordinated fashion.

5. Failure of any part of the cardiac conduction system to transmit signals is called a heart block.

1. The normal heartbeat generated by the SA node is called the sinus rhythm, and shows a resting rate of 70-80 bpm. The SA node tends to speed up, and the vagus nerve keeps a brake of heart rate.

2. Sometimes regions other than the SA node fire spontaneously; these are called ectopic foci.

3. An abnormal cardiac rhythm is called arrhythmia.

1. The autorhythmic cells of the SA node have a resting potential that starts at -60mV and then drifts spontaneously upward. This gradual depolarization is called the pacemaker potential. The reason for it is uncertain.

2. When the pacemaker potential reaches -40mV (threshold), fast calcium channels open and calcium rushes in. This produces a rising phase of action potential.

3. Next, potassium channels open, potassium rushes out of the cell, and the cytosol becomes more negative, creating a falling phase of action potential.

4. When repolarization is complete, potassium channels close again and the pacemaker potential begins anew. One depolarization of the SA node sets off one heartbeat, and also triggers depolarization in the remainder of the cardiac conduction system.

1. Firing of the SA node stimulates the two atria to contract together almost immediately.

2. Next, impulses travel to the AV node where thinner myocytes slow the impulse momentarily. This gives the ventricles time to fill with blood before contracting.

3. The signal reaches the AV bundle and spreads rapidly through fast Purkinje fibers, causing the entire ventricular myocardium to depolarize and ventricles to contract in unison.

4. The impulse reaches the papillary muscle slightly before it spreads throughout the ventricles, allowing the papillary muscles to tighten chordae tendineae on the AV valves first.

1. Contractile myocytes exhibit action potentials that are different than neurons or skeletal muscle. They have a stable RMP of -90 mV and depolarize only when stimulated. The influx of sodium ions and depolarization of the myocytes is similar to that seen in neurons.

2. Depolarization of a myocyte, with its sparse SR, causes release of supplemental calcium ions from the ECF. Unlike cardiac muscle, the SR of skeletal muscle contains sufficient calcium.

3. In skeletal muscle and neurons, an action potential falls back to RMP within 2 msec, but in cardiac muscle, slow calcium channels remain open longer (after sodium channels close) prolonging the depolarization of the cell. As long as the action potential is in its plateau and calcium is entering the myocytes, the myocytes contract. These plateaus are more pronounced in the ventricles.

4. Cardiac muscle has an absolute refractory period of 250 msec, compared with 1-2 msec of skeletal muscle.

1. Electrical currents generated in the heart can be detected by recording electrodes on the skin. An electrocardiograph amplifies these signals and produces an electrocardiogram (ECG).

2. An ECG shows three principal deflections: the P wave, the QRS complex, and the T wave.

1. Measurement of Pressure (p. 691)

2. Pressure Gradients and Flow (p. 691; Fig. 19.18; Transp. 372)

1. Heart sounds, as heard through a stethoscope, result from the closing of the valve and the turbulence of the blood against the inner heart wall.

2. Heart sounds are described as a "lubb-dupp", or first and second heart sounds (S1 and S2). S1 is louder and longer, while S2 is softer and sharper.

1. Refer to Fig. 19.19, p. 693 (Transp. 373) to examine the phases of the cardiac cycle, pressure changes, and how the pressure changes and valves govern the flow of blood.

2. The cycle begins with the quiescent period in which the heart is at rest, blood is flowing into the atria, and AV valves are open, allowing blood to passively flow through to the ventricles.

3. The SA nodes fire, and atrial systole results.

4. Third, isovolumetric contraction of the ventricles occurs after the ventricles depolarize. The AV valves close as ventricular blood surges back against the cusps. This phase is called isovolumetric because the ventricles do not yet eject blood and there is no change in their volume even though they are contracting.

5. Next comes ventricular ejection, in which the amount known as stroke volume passes into the major vessels on top of the heart.

6. Isovolumetric relaxation and ventricular filling follow, completing one cardiac cycle. Pressure changes that occur coincident with these events are best viewed in Fig. 19.19; Transp. 373.

1. Viewing the balance sheet, p. 694, it is evident that the ventricle pumps out as much blood as it received during diastole. The two ventricles eject the same amount of blood, but pressure is less in the right ventricle than it is in the left. The left ventricle contracts with greater force to overcome the arterial pressure of the systemic circuit.

1. Cardiac output (CO) is the volume pumped by each ventricle per minute. A person's cardiac reserve is the difference between maximum and resting cardiac output. Agents that raise and lower the heart rate are called positive and negative chronotropic agents; agents that alter stroke volume of the ventricles are inotropic agents.

2. The resting heart rate of an adult is 64-72 in males, and 72-80 in females. In infants, it is 120 bpm or more.

3. Tachycardia is a persistent, resting heart rate above 100 bpm. This can be caused by anxiety, drugs, heart disease, or elevated body temperature. The opposite condition, bradycardia, is a persistent resting adult heart rate of less than 60 bpm. This can be seen in well-conditioned athletes, but can also indicate hypothermia and occurs with immersion of the face in water.

4. Chronotropic Effects of the Autonomic Nervous System (p. 696)

5. Chronotropic Effects of Chemicals (p. 697)

1. Preload (p. 697)

2. Contractility (p. 698; Table 19.2)

3. Afterload (p. 698)

1. The act of exercise increases cardiac output because proprioceptors signify the cardiac center that the muscles are active and need more oxygen delivery and carbon dioxide removal. During exercise, venous return to the heart increases, which increases preload on the heart.

2. A sustained program of exercise causes hypertrophy of the ventricles, which increases their stroke volume, and lowers heart rate. Athletes have greater cardiac reserve, so they can tolerate more exertion than a sedentary person can.