DETAILED LECTURE OUTLINE

Fundamentals of Anatomy and Physiology, 7th edition, ©2006 by Frederic H. Martini

Prepared by Professor Albia Dugger, Miami-Dade College, Miami, Florida

Please note:

  • References to textbook headings, figures and tables appear in italics.
  • “100 Keys” are designated by Key
  • Important vocabulary terms are underlined.

Chapter 20: The Heart

I. The Organization of the Cardiovascular System, p. 670

Objective:

  1. Describe the organization of the cardiovascular system and of the heart.

Figure 20-1

  • The heart pumps blood through 2 separate circuits of blood vessels:
  • The pulmonary circuit:

- carries blood to and from the gas exchange surfaces of the lungs

  1. The systemic circuit:

- carries blood to and from the rest of the body

  • Circulating blood alternates between the systemic and pulmonary systems: Blood returning to the heart from the systemic circulation must pass through the pulmonary circuit before returning to the systemic circuit.
  • There are 3 types of blood vessels:
  • Arteries carry blood away from the heart
  • Veins carry blood to the heart
  • Capillaries are networks of small, thin-walled vessels between arteries and veins.
  • Capillaries are called exchange vessels because they are the location where dissolved gases, nutrients, and wastes are exchanged between the blood and the surrounding tissues.
  • The heart has 4 chambers, 2 for each circuit:
  • The right atrium collects blood from the systemic circuit.
  • The right ventricle pumps blood to the pulmonary circuit.
  • The left atrium collects blood from the pulmonary circuit.
  • The left ventricle pumps blood to the systemic circuit.

II. Anatomy of the Heart, p. 670

Objectives:

  1. Describe the location and general features of the heart.
  2. Describe the structure of the pericardium and explain its functions.
  3. Trace the flow of blood through the heart, identifying the major blood vessels, chambers and heart valves.
  4. Identify the layers of the heart wall.
  5. Describe the vascular supply to the heart.

Figure 20-2a

  • The heart is located directly posterior to the sternum. The great veins and arteries attach at the top (the base). The pointed tip is the apex.

Figure 20-2b

  • The heart, surrounded by the pericardial sac, sits in the mediastinum (between the 2 pleural cavities) which also holds the great vessels, thymus, esophagus and trachea.

The Pericardium, p. 671

Figure20-2c

  • The pericardial cavity has a double lining called the pericardium.
  • The outer layer is the parietal pericardium. The parietal pericardium forms the inner layer of the fibrous pericardial sac which surrounds and stabilizes the heart.
  • The inner layer is the visceral pericardium. The space between the 2 layers is the pericardial cavity, which contains a small amount of lubricating pericardial fluid.
  • An infection of the pericardium is called pericarditis.

Superficial Anatomy of the Heart, p. 672

Figure 20-3

  • The 4 cardiac chambers can be seen in the superficial view of the heart.
  • The atria are thin-walled, with an expandable outer portion called the auricle. A coronary sulcus divides the atria and the ventricles. The left and right ventricles are separated along the anterior and posterior interventricular sulci, which also contain the blood vessels that supply the cardiac muscle.

The Heart Wall, p. 673

Figure 20-4

  • The heart wall has 3 layers:
  • the outer epicardium
  • the middle myocardium
  • the inner endocardium
  • The epicardium is the visceral pericardium that covers the heart.
  • The myocardium is the muscular wall of the heart, consisting of concentric layers of cardiac muscle tissue. The atrial myocardium wraps around the great vessels. Superficial ventricular muscles surround the ventricles. Deep ventricular muscles spiral around and between the ventricles.

- Cardiac Muscle Tissue

Figure 20-5

  • Cardiac muscle cells are interconnected by intercalated discs which are held together by desmosomes and linked by gap junctions. Intercalated discs convey the force of contraction from cell to cell and propagate action potentials.
  • The characteristics of cardiac muscle cells include:
  • small size
  • single, central nucleus
  • branching interconnections between cells
  • intercalated discs

Table 20-1 summarizes differences between cardiac cells and skeletal fibers.

Internal Anatomy and Organization, p. 674

Figure20-6a

  • The right atrium opens to the right ventricle, and the left atrium opens to the left ventricle. Atrioventricular (AV) valves permit blood to flow in only 1 direction: from the atria to the ventricles.
  • The atria are separated by the interatrial septum, and the ventricles by the interventricular septum.
  • The right atrium receives blood from the systemic circuit through the superior vena cava (head, neck, upper limbs and chest) and the inferior vena cava (trunk, viscera and lower limbs).
  • The cardiac veins of the heart return blood to the coronary sinus, which opens into the right atrium. Before birth, an opening through the interatrial septum (the foramen ovale) connects the 2 atria. At birth, the foramen ovale seals off, leaving a small depression called the fossa ovalis.
  • The anterior atrial wall and inner surfaces of the right auricle have prominent muscular ridges called pectinate muscles.
  • The opening from the right atrium to the right ventricle has 3 fibrous flaps or cusps, which are part of the right atrioventricular (AV) valve (tricuspid valve). The free edge of each cusp is attached to connective tissue fibers (chordae tendineae) which originate at the papillary muscles of the right ventricle and prevent the AV valve from opening backward.
  • The internal surface of the right ventricle has a series of muscular ridges (the trabeculae carneae) which includes the moderator band, a ridge that contains part of the conducting system that coordinates the contractions of cardiac muscle cells.
  • The superior part of the right ventricle (the conus arteriosus) leads to the pulmonary trunk, which divides into the left and right pulmonary arteries of the pulmonary circuit. Blood flows from the right ventricle to the pulmonary trunk through the pulmonary valve, which has 3 semilunar cusps that prevent backflow.
  • After the blood passes through the pulmonary circuit, it is gathered into the left and right pulmonary veins, which deliver blood into the left atrium. Blood passes from the left atrium to the left ventricle through the left atrioventricular (AV) valve, a 2-cusp or bicuspid valve, also known as the mitral valve.
  • The left ventricle holds the same amount of blood as the right ventricle, but is larger because its muscle is thicker and more powerful. Internally, the left ventricle is similar to the right ventricle, but has no moderator band.
  • Blood heading for the systemic circulation leaves the left ventricle through the aortic valve into the ascending aorta. (The aortic valve is similar in structure to the pulmonary valve.) The ascending aorta turns at the aortic arch and becomes the descending aorta.

Figure 20-7

  • The left and right ventricles have significant structural differences:

-The wall of the right ventricle is relatively thin and develops less pressure than the left ventricle.

-The right ventricle is pouch-shaped, the left ventricle round.

- The Heart Valves

Figure 20-8

  • The heart has a series of one-way valves that prevent backflow during contraction.
  • The atrioventricular (AV) valves between the atria and the ventricles are controlled by the chordae tendineae and the papillary muscles. When the ventricles contract, the pressure of the blood swings the cusps together, closing the valves to the atria. At the same time, the papillary muscles tense the chordae tendineae and prevent the valves from swinging into the atria. Failure of the valves causes backflow or regurgitation of blood into the atria.
  • The pulmonary and aortic semilunar valves prevent backflow from the pulmonary trunk and aorta into the ventricles. Semilunar valves have no muscular support; the 3 cusps of the valve support each other like a tripod.
  • The aortic sinuses at the base of the ascending aorta prevent the valve cusps from sticking to the aorta. The right and left coronary arteries originate at the aortic sinuses.
  • An inflammation of the heart (carditis) can result in valvular heart disease (VHD) such as rheumatic fever.

Key

  • The heart has 4 chambers, 2 associated with the pulmonary circuit (right atrium and right ventricle) and 2 with the systemic circuit (left atrium and left ventricle).
  • The left ventricle has a greater workload and is much more massive than the right ventricle, but the two chambers pump equal amounts of blood.
  • AV valves prevent backflow from the ventricles into the atria, and semilunar valves prevent backflow from the aortic and pulmonary trunks into the ventricles.

Connective Tissues and Fibrous Skeleton, p. 680

  • Connective tissue fibers of the heart have several functions:
  • they provide physical support for cardiac muscle fibers
  • they distribute the forces of contraction
  • they add strength and prevent overexpansion of the heart
  • elastic fibers help return the heart to its original shape after a contraction
  • The fibrous skeleton of the heart (4 bands around the heart valves and the bases of the pulmonary trunk and aorta) stabilize the valves and electrically insulate ventricular cells from atrial cells.

The Blood Supply to the Heart, p. 680

Figure 20-9

  • The coronary circulation, consisting of coronary arteries and cardiac veins, supplies blood to the muscle tissue of the heart.
  • The left and right coronary arteries originate at the aortic sinuses. High blood pressure and elastic rebound force blood through the coronary arteries between contractions.
  • The right coronary artery supplies blood to the right atrium, portions of both ventricles, and cells of the sinoatrial (SA) and atrioventricular nodes that regulate the heartbeat. The right coronary artery supplies marginal arteries on the surface of the right ventricle, and the posterior interventricular artery.
  • The left coronary artery supplies blood to the left ventricle, left atrium and interventricular septum. The 2 main branches of the left coronary artery are the circumflex artery (on the left around the coronary sulcus) and the anterior interventricular artery.
  • The anterior and posterior interventricular arteries are interconnected by arterial anastomoses, which stabilize the blood supply to the cardiac muscle.
  • The great cardiac vein drains blood from the region of the anterior interventricular artery into the coronary sinus. Other cardiac veins that empty into the great cardiac vein or coronary sinus include the posterior cardiac vein, the middle cardiac vein, and the small cardiac vein. The anterior cardiac vein empties into the right atrium.

III The Heartbeat, p. 684

Objectives:

  1. Describe the events of an action potential in cardiac muscle and explain the importance of calcium ions to the contractile process.
  2. Discuss the differences between nodal cells and conducting cells, and describe the components and functions of the conducting system of the heart.
  3. Identify the electrical events associated with a normal electrocardiogram.
  4. Explain the events of the cardiac cycle, including atrial and ventricular systole and diastole, and relate the heart sounds to specific events in the cycle.

Cardiac Physiology, p. 684

Figure 20-11

  • In a single contraction or heartbeat, the entire heart contracts in series: first the atria, then the ventricles.
  • Two types of cardiac muscle cells are involved in the heartbeat:
  • the conducting system:

-controls and coordinates the heartbeat

  1. the contractile cells:

-produce contractions

  • The cardiac cycle begins with an action potential at the SA node, which is transmitted through the conducting system. This produces action potentials in the cardiac muscle cells (contractile cells) which cause the contraction.
  • The electrical events in the cardiac cycle can be recorded on an electrocardiogram (ECG).

The Conducting System, p. 684

Figure 20-12

  • Cardiac muscle tissue contracts automatically (automaticity). A system of specialized cardiac muscle cells (the conducting system) initiates and distributes the electrical impulses that stimulate contraction.
  • The conducting system includes:

-the sinoatrial (SA) node

-the atrioventricular (AV) node

-conducting cells

  • Conducting cells interconnect the SA and AV nodes and distribute the contractile stimulus through the heart. In the atria, conducting cells are found in internodal pathways that distribute the impulse to atrial cells and the AV node. Ventricular conducting cells are found in the AV bundle, bundle branches and Purkinje fibers.
  • Conducting cells of the SA and AV nodes cannot maintain a stable resting potential; after repolarization they gradually drift toward threshold (prepotential or pacemaker potential). The SA node depolarizes first, establishing a heart rate of about 80-100 beats per minute. Normal heart rate is slowed by parasympathetic stimulation. The AV node generates 40-60 action potentials per minute

Figure 20-13

- Impulse Conduction Through the Heart

  1. The Sinoatrial (SA) Node

- in posterior wall of right atrium

- contains pacemaker cells

- connected to AV node by internodal pathways

- begins atrial activation (Step 1)

  1. The Atrioventricular (AV) Node

- in floor of right atrium

- receives impulse from SA node (Step 2)

- delays impulse (Step 3)

- atrial contraction begins

  1. The AV Bundle

- in the septum

- carries impulse to left and right bundle branches:

- which conduct to Purkinje fibers (Step 4)

- and to the moderator band:

- which conducts to papillary muscles

  1. The Purkinje Fibers

- distribute impulse through ventricles (Step 5)

- atrial contraction is completed

- ventricular contraction begins

  • Abnormal pacemaker function changes the heart rate:

-bradycardia is an abnormally slow heart rate.

-tachycardia is an abnormally fast heart rate.

  • If an abnormal conducting cell or ventricular muscle cell begins generating a high rate of action potentials, they can override the SA and AV nodes and bypass the conducting system, disrupting ventricular contractions. The origin of these abnormal impulses is called an ectopic pacemaker.

The Electrocardiogram, p. 687

Figure 20-14a

  • The electrocardiogram (ECG or EKG) is a recording of electrical events in the heart, obtained by placing electrodes at specific locations on the body. Abnormal ECG patterns are used to diagnose damage to specific nodal, conducting and contractile components of the heart.
  • The basic features of an ECG include:
  • The P wave: A small wave produced when the atria depolarize.
  • The QRS complex: A complex signal produced when the ventricles depolarize. The ventricles begin contracting just after the peak of the R wave.
  • The T wave: A small wave produced when the ventricles repolarize.
  • The time intervals between waves are also important. Time intervals commonly used in clinical diagnosis include:
  • P-R interval: The time from the start of atrial depolarization to the start of the QRS complex.
  • Q-T interval: The time from ventricular depolarization to ventricular repolarization.
  • Abnormal patterns of cardiac electrical activity are called cardiac arrhythmias.

Key

  • The heart rate is normally established by cells of the SA node, but that rate can be modified by autonomic activity, hormones and other factors.
  • From the SA node the stimulus is conducted to the AV node, the AV bundle, the bundle branches and Purkinje fibers before reaching the ventricular muscle cells.
  • The electrical events associated with the heartbeat can be monitored in an electrocardiogram (ECG).

Contractile Cells, p. 688

  • Purkinje fibers distribute the stimulus to the contractile cells, which make up most of the muscle cells in the heart.

Figure 20-15

  • Action potentials in cardiac muscle are different than action potentials in skeletal muscle. The resting potential of a ventricular cell is about -90mV, an atrial cell about -80 mV.
  • Once threshold is reached, the action potential proceeds in 3 steps:
  • Rapid depolarization: Voltage-regulated sodium channels (fast channels) open.
  • As sodium channels close, voltage-regulated calcium channels (slow channels) open, balancing the Na+ ions being pumped out and holding the membrane at a 0 mV plateau.
  • Repolarization: As the plateau continues, slow calcium channels close, and slow potassium channels open, resulting in rapid repolarization that restores the resting potential.
  • Cardiac muscle cells then undergo an absolute refractory period, when they cannot respond, followed by a shorter relative refractory period. The total time for a cardiac action potential in a ventricular cell is 250-300 msecs -- 30 times longer than a skeletal muscle fiber. The long refractory period prevents summation and tetany in cardiac tissues.
  • Contraction of a cardiac muscle cell is produced by an increase in calcium ion concentration around the myofibrils, which occurs in 2 steps:
  • 20% of the calcium ions required for a contraction enter the cell membrane during the plateau phase.
  • The arrival of extracellular Ca++ triggers the release of calcium ion reserves from the sarcoplasmic reticulum.
  • As the slow calcium channels close, intracellular Ca++ is absorbed by the SR or pumped out of the cell. Cardiac muscle tissue is very sensitive to extracellular Ca++ concentrations.

The Cardiac Cycle, p. 690

  • The period between the start of one heartbeat and the beginning of the next is one cardiac cycle, including both contraction and relaxation.
  • Within any one chamber, the cycle is divided into 2 phases: systole (contraction) and diastole (relaxation). Pressure in the chamber rises during systole and falls during diastole. Blood flows from high pressure to low pressure, controlled by the timing of contractions and directed by one-way valves.

Figure 20-16