Circulation and the Heart

A Look Back

• Cardiac muscle is one of the three types of muscles.
• The heart propels blood through the body.
• Gradients and flow provide a foundation for understanding heart function.
• The nervous and endocrine systems influence the heart to maintain adequate blood pressure.
• Homeostasis and negative feedback, and communication are important.
• The relationship between structure and function and the importance of adaptation are relevant.

Introduction

• The blood delivers oxygen and nutrients to the cells and carries away the waste products of cellular metabolism.
• The continuous one-way circuit of blood through the blood vessels is known as circulation.
• The heart propels blood throughout the body.
• The heart contracts about 72 times per minute.

Heart Structure

• The heart is slightly bigger than a person's fist.
• It is located between the lungs in the center and a bit to the left of the body's midline.
• It occupies most of the mediastinum, the central region of the thorax.
• The heart's apex, the pointed, inferior portion, is directed toward the left.
• The broad, superior base, directed toward the right, is the area of attachment for the large vessels carrying blood into and out of the heart.

Tissue Layers of the Heart Wall

• The heart is a hollow organ, with walls formed of three different layers:
  • Endocardium: A thin, smooth layer of epithelial cells that lines the heart's interior, providing a smooth surface for easy flow as blood travels through the heart. Extensions of this membrane cover the flaps (cusps) of the heart valves.
  • Myocardium: The heart muscle, is the thickest layer and pumps blood through the vessels.
  • Epicardium: A serous membrane that forms the thin, outermost layer of the heart wall. It is also considered the visceral layer of the pericardium.

The Pericardium

• The pericardium is the sac that encloses the heart.
• The outermost and heaviest layer is the fibrous pericardium, a connective tissue membrane.
• Additional connective tissue anchors this pericardial layer to the diaphragm, located inferiorly; to the sternum, located anteriorly; and to other structures surrounding the heart, thus holding the heart in place.
• A serous membrane forms the inner layer of the pericardium.
• This membrane, known as the serous pericardium, consists of an outer, parietal layer that lines the fibrous pericardium and an inner, visceral layer (the epicardium) that covers the myocardium.
• A thin film of fluid between these two layers reduces friction as the heart moves within the pericardium.
• The pericardial cavity is the region between the visceral and parietal layers where fluid may accumulate under certain disease conditions.

Special Features of the Myocardium

• Cardiac muscle cells are lightly striated based on alternating actin and myosin filaments.
• Cardiac muscle cells have a single nucleus instead of multiple nuclei.
• Cardiac muscle tissue is involuntarily controlled.
• Intercalated disks are specialized partitions between cardiac muscle cells that are tightly joined together by specialized membrane proteins.
• Other membrane proteins within the disks permit electric impulses to travel between adjacent cells.
• Electrical synapses provide rapid and coordinated communication between cells.
• Another feature of cardiac muscle tissue is the branching of the muscle fibers (cells).
• These branched fibers are interwoven so that the stimulation that causes the contraction of one fiber results in the contraction of a whole group.
• The intercalated disks between the fibers and the branching cellular networks allow cardiac muscle cells to contract in a coordinated manner for effective pumping.

Heart Divisions

• The right heart receives blood low in oxygen content that has already passed through the body and pumps it to the lungs through the pulmonary circuit.
• The left side receives highly oxygenated blood from the lungs and pumps it throughout the body via the systemic circuit.
• Each side of the heart is divided into two chambers.
• Four Chambers:
  • The upper chambers on the right and left sides, the atria, are mainly blood-receiving chambers.
  • The lower chambers on the right and left sides, the ventricles, are forceful pumps.
  • The thickness of the wall in each chamber indicates the force it can generate.
  • The atria have the thinnest walls and the weakest contractions.
  • The stronger ventricles have thicker walls, with the left ventricular wall being the thickest of all.
• The chambers, listed in the order in which blood originating in the body tissues flows through them, are:
  1. The right atrium is a thin-walled chamber that receives the blood returning from the body tissues. This blood, which is comparatively low in oxygen, is carried in veins, the blood vessels leading back to the heart. The superior vena cava brings blood from the head, chest, and arms; the inferior vena cava delivers blood from the trunk and legs. A third vessel that opens into the right atrium brings blood from the heart muscle itself.
  2. The right ventricle receives blood from the right atrium and pumps it to the lungs. Blood passes from the right ventricle to a large pulmonary trunk, which then divides into right and left pulmonary arteries. Branches of these arteries carry blood to the lungs.
  3. The left atrium receives oxygen-rich blood as it returns from the lungs in pulmonary veins.
  4. The left ventricle, the chamber with the thickest wall, pumps highly oxygenated blood to all parts of the body, including the lung tissues. This blood goes first into the aorta, the largest artery, and then into the branching systemic arteries that take blood to the tissues. The heart's apex, the lower pointed region, is formed by the wall of the left ventricle.
• The heart's right and left chambers are completely separated from each other by partitions, each of which is called a septum.
  • The interatrial septum separates the two atria.
  • The interventricular septum separates the two ventricles.
  • The septa, like the heart wall, consist largely of myocardium.
• Four Valves:
  • One-way valves that direct blood flow through the heart are located at the entrance and exit of each ventricle. The entrance valves are the atrioventricular (AV) valves, so named because they are between the atria and ventricles. The exit valves are the semilunar valves, so named because each flap of these valves resembles a half-moon.
  • Blood flow through the heart is governed by pressure gradients. The heart valves provide resistance when the pressure gradient would promote flow in the wrong direction.
    • The atrioventricular valves open when the pressure is greater in the atria than in the ventricles, and flow occurs. However, they close when ventricular pressure exceeds atrial pressure, preventing flow.
    • The semilunar valves similarly govern flow between the ventricles and great vessels.
  • Each valve has a specific name:
    • The right atrioventricular (AV) valve is also known as the tricuspid valve because it has three cusps, or flaps, that open and close. When this valve is open, blood flows freely from the right atrium into the right ventricle. When the right ventricle begins to contract, however, the valve is closed by blood pressing against the cusps. With the valve closed, blood cannot return to the right atrium but must flow forward into the pulmonary trunk.
    • The left atrioventricular (AV) valve is the bicuspid valve, but it is commonly referred to as the mitral valve. It has two heavy cusps that permit blood to flow freely from the left atrium to the left ventricle. The cusps close when the left ventricle begins to contract; this closure prevents blood from returning to the left atrium and ensures the forward flow of blood into the aorta. Both the right and left AV valves are attached by means of thin fibrous threads to papillary muscles arising from the walls of the ventricles. The function of these threads, called the chordae tendineae, is to stabilize the valve flaps when the ventricles contract so that the blood's force will not push the valves up into the atria. In this manner, they help prevent a backflow of blood when the heart beats.
    • The pulmonary valve is a semilunar valve located between the right ventricle and the pulmonary trunk that leads to the lungs. When the right ventricle relaxes, pressure in that chamber drops. The higher pressure in the pulmonary artery, described as back pressure, closes the valve and prevents blood from returning to the ventricle.
    • The aortic valve is a semilunar valve located between the left ventricle and the aorta. When the left ventricle relaxes, back pressure closes the aortic valve and prevents the backflow of blood from the aorta to the ventricle.
• Blood passes through the heart twice in making a trip from the heart's right side through the pulmonary circuit to the lungs and back to the heart's left side to start on its way through the systemic circuit.
• The heart's two sides function in unison to pump identical volumes of blood through both circuits at the same time.

Blood Supply to the Myocardium

• Only the endocardium comes into contact with the blood that flows through the heart chambers.
• The myocardium must have its own blood vessels to provide oxygen and nourishment and to remove waste products.
• Together, these blood vessels form the coronary circulation.
• The main arteries that supply blood to the heart muscle are the right and left coronary arteries, named because they encircle the heart like a crown.
• These arteries, which are the first to branch off the aorta, arise just above the cusps of the aortic valve and branch to all regions of the heart muscle.
• They receive blood only when the ventricles relax because the aortic valve must be closed to expose the entrance to these vessels.
• The left coronary artery (LCA) branches into the circumflex artery and the left anterior descending (LAD) artery.
• The right coronary artery (RCA) snakes around the heart just inferior to the right atrium, giving off a major branch called the posterior descending artery.
• After passing through the capillaries in the myocardium, blood drains into a system of cardiac veins that brings blood back toward the right atrium.
• Blood finally collects in the coronary sinus, a dilated vein that opens into the right atrium near the inferior vena cava.

Function

• The heart's right and left sides work together.
• A heart muscle contraction begins in the thin-walled upper chambers, the atria, and is followed by a contraction of the thick muscle of the lower chambers, the ventricles.
• The active phase, called systole, is followed by a resting phase known as diastole.
• One complete sequence of heart contraction and relaxation is called the cardiac cycle.
• Each cardiac cycle represents a single heartbeat.
• At rest, one cycle takes an average of 0.8 seconds.
• The cardiac cycle begins with contraction of both atria, which forces blood through the AV valves into the ventricles.
• The atrial walls are thin, and their contractions are not very powerful.
• Atrial contraction ends before ventricular contraction begins.
• Atrial diastole begins at the same time ventricular systole begins.
• While the ventricles are contracting, forcing blood through the semilunar valves, the atria are relaxed and again are filling with blood.
• After the ventricles have contracted, all the chambers are relaxed for a short period.
• During this period of complete relaxation, blood enters the atria from the great veins and passively drains into the ventricles.
• The semilunar valves are closed during atrial systole to prevent blood from flowing backward from the great vessels to the ventricles.
• Although both upper and lower chambers have a systolic and diastolic phase in each cardiac cycle, discussions of heart function usually refer to these phases as they occur in the ventricles, because these chambers contract more forcefully and drive blood into the arteries.

The Heart's Conduction System

• The heart muscle is stimulated to contract by a wave of electric energy that passes along the cells.
• This action potential is generated by specialized tissue within the heart and spreads over structures that form the heart's conduction system.
• Two of these structures are tissue masses called nodes, and the remainder consists of specialized fibers that branch through the myocardium.
• The sinoatrial (SA) node is located in the upper wall of the right atrium in a small depression described as a sinus.
• This node initiates the heartbeats by generating an action potential at regular intervals.
• Because the SA node sets the rate of heart contractions, it is commonly called the pacemaker.
• The second node, located in the interatrial septum at the bottom of the right atrium, is called the atrioventricular (AV) node.
• The atrioventricular (AV) bundle, also known as the bundle of His, is located at the top of the interventricular septum.
• Fibers travel first down both sides of the interventricular septum in groups called the right and left bundle branches.
• Smaller Purkinje fibers then travel in a branching network throughout the myocardium of the ventricles.
• Intercalated disks allow the rapid flow of impulses throughout the heart muscle.
• The order in which impulses travel through the heart is as follows:
  1. The SA generates the electric impulse that begins the heartbeat.
  2. The excitation wave travels throughout the myocardium of each atrium, causing the atria to contract. At the same time, impulses also travel directly to the AV node by means of fibers in the wall of the atrium that make up the internodal pathways.
  3. The atrioventricular node is stimulated. A relatively slower rate of conduction through the AV node allows time for the atria to contract and complete the filling of the ventricles before the ventricles contract.
  4. The excitation wave rapidly travels through the AV bundle and then throughout the ventricular walls by means of the bundle branches and Purkinje fibers. The entire ventricular musculature contracts in a wave, beginning at the apex and squeezing the blood upward toward the aorta and pulmonary artery.
• A normal heart rhythm originating at the SA node is termed a sinus rhythm.
• A region of the conduction system other than the SA node can generate a heartbeat if the SA node fails, but it does so at a slower rate.

Cardiac Output

• Mass Balance: Output Balances Input
  • The amount of material in a system depends on both inputs and outputs.
  • With few exceptions, the adult body keeps storage pools of most substances relatively constant by matching input and output.
  • For instance, increased fluid consumption results in increased urine production, thus keeping the amount of "stored" body water constant.
• Determinants of Cardiac Output
  • A unique property of heart muscle is its ability to adjust the ventricles' output to their input, thereby keeping ventricular blood volume at the end of systole relatively constant.
  • When the heart chamber is filled and the wall stretched (within limits), the contraction is strong. As less blood enters the heart, contractions become less forceful.
  • The heart's ability to pump out all of the blood it receives prevents blood from pooling in the chambers.
  • The volume of blood pumped by each ventricle in one minute is termed the cardiac output (CO).
  • Cardiac output is the product of the stroke volume (SV) - the volume of blood ejected from the ventricle with each beat - and the heart rate (HR), the number of times the heart beats per minute.
  • $CO = HR \times SV$
  • Based on a heart rate of 75 bpm and a stroke volume of 70 mL/beat, the average cardiac output for an adult at rest is about 5 L/min.
  • The cardiac reserve is a measure of how many times more than resting output the heart can produce when needed.
  • During mild exercise, cardiac output might double. During strenuous exercise, it might double again.
• Variations in Heart Rate
  • The resting heart rate varies significantly among individuals.
  • Generally speaking, the heart rate is generally faster in small people than large people and slightly faster in women than in men.
  • Resting heart rate also declines with age, from 120 to 140 bpm in a newborn to 60 to 80 bpm in adults.
  • Trained athletes usually have a low resting heart rate.
  • Specific Terms:
    • Bradycardia is a relatively slow heart rate of less than 60 bpm.
    • Tachycardia refers to a heart rate of more than 100 bpm. Tachycardia is normal during exercise or stress or with excessive caffeine intake but may also occur with certain disorders.
    • Sinus arrhythmia is a regular variation in heart rate caused by changes in the rate and depth of breathing. It is a normal phenomenon.
    • Premature ventricular contraction (PVC), also called ventricular extrasystole, is a ventricular contraction initiated by the Purkinje fibers rather than the SA node. It can be experienced as a palpitation between normal heartbeats or as a skipped beat. PVCs may be initiated by caffeine, nicotine, or psychological stresses. They are also common in people with heart disease.
• Control of Cardiac Output
  • The heart rate and stroke volume (and thus cardiac output) can be influenced by the nervous system, hormones, drugs, and ions.
  • Increasing the cardiac output increases blood pressure. When blood pressure decreases, heart rate and stroke volume increase in order to restore normal blood pressure.
  • The autonomic nervous system carries the signals in the negative feedback loop that controls the cardiac output.
  • Sympathetic fibers increase the heart rate by stimulating the SA and AV nodes. They also increase the contraction force and thus the stroke volume by acting directly on the fibers of the myocardium.
  • Parasympathetic stimulation decreases cardiac output by lowering the heart rate. The parasympathetic nerve that supplies the heart is the vagus nerve (cranial nerve X). It slows the heart rate by acting on the SA and AV nodes but does not influence the stroke volume.
  • Epinephrine, a hormone released from the adrenal gland, also increases cardiac output.

Normal and Abnormal Heart Sounds

• The normal heart sounds are usually described by the syllables "lub" and "dup."
  • The first heart sound (S1), the "lub," is a longer, lower-pitched sound that occurs at the start of ventricular systole. It is caused by a combination of events, mainly closure of the AV valves. This action causes vibrations in the blood passing through the valves and in the tissue surrounding the valves.
  • The second heart sound (S2), the "dup," is shorter and sharper. It occurs at the beginning of ventricular relaxation and is caused largely by sudden closure of the semilunar valves.
• An abnormal sound is called a murmur.
• Anything that disrupts the smooth flow of blood through the heart will cause a murmur.
• An abnormal sound caused by any structural change in the heart or the vessels connected with the heart is called an organic murmur.