chapter 5 The Cardiovascular System and Its Control

Learning Objectives Review the structure and function of the heart, vascular system, and blood Explore the role of the cardiovascular system in delivering oxygen and nutrients to active body tissues Discover how the cardiovascular system removes metabolic waste from tissues

Major Cardiovascular Functions Delivery of oxygen and other nutrients Removal of carbon dioxide and other metabolic waste Transport of hormones Thermoregulation Maintenance of acid–base balance and overall body fluid balance Immune function

The Anatomy of the Human Heart

Myocardium—The Cardiac Muscle Thickness varies according to the stress placed on chamber walls Left ventricle is the most powerful of chambers and the largest chamber With exercise training the size of the left ventricle increases Intercalated disks in the myocardium allow impulses travel quickly in cardiac muscle and allow it to act as one large muscle fiber; all fibers in the heart contract together as one unit

Mechanism of Cardiac Muscle Contraction Courtesy of Dr. Donna Korzick, Pennsylvania State University.

Structural and Functional Characteristics of Skeletal and Cardiac Muscle Adapted, by permission, from K.L. Moore and A.F. Dalley, 1999, Clinically oriented anatomy, 4th ed. (Baltimore, MD: Lippincott, Williams, and Wilkins), 27.

Coronary Circulation

Cardiac Conduction System Sinoatrial (SA) node Atrioventricular (AV) node AV bundle (bundle of His) Purkinje fibers

Intrinsic Conduction System of the Heart

Extrinsic Control of the Heart PNS acts through the vagus nerve, releasing ACh to decrease heart rate and force of cardiac contraction Hyperpolarization of the conduction cells Absence of vagal tone, HR = 100 beats/min Maximal vagal tone, HR = 20-30 beats/min SNS increases rate of impulse generation and conduction speed, increasing heart rate and force of cardiac contraction Maximal sympathetic stimulation, HR = 250 beats/min Epinephrine and norepinephrine—released from the adrenal medulla as a result of sympathetic stimulation—increase heart rate

Relative Contribution of Sympathetic and Parasympathetic Nervous Systems to the Rise in Heart Rate During Exercise Adapted from L.B. Rowell, Human Cardiovascular Control. Oxford University Press, 1993.

Heart Rate and Endurance Training Resting heart rates in adults tend to be between 60 and 85 beats per min. However, extended endurance training can lower resting heart rate to 35 beats or lower. This lower heart rate is thought to be due to increased parasympathetic stimulation.

Electrocardiogram (ECG) An ECG provides a graphical record of the electrical activity of the heart and can be used to aid clinical diagnoses

Cardiac Anatomy, Conduction, and Control Key Points The atria receive blood from the veins; the ventricles eject blood from the heart The left ventricular myocardium is larger because it must produce more force than the other ventricles to pump blood to the systemic circulation Cardiac tissue is capable of spontaneous rhythmicity and has its own conduction system The SA node normally establishes heart rate Heart rate and contractility can be altered by the PNS, SNS, and the endocrine system The ECG is a recording of the heart’s electrical activity

Cardiac Arrhythmias Bradycardia: resting heart rate below 60 bpm Tachycardia: resting heart rate above 100 bpm Premature ventricular contractions (PVCs): skipped or extra beats from impulses originating outside the SA node Ventricular tachycardia: three or more consecutive PVCs Ventricular fibrillation: contraction of the ventricular tissue is uncoordinated and can result in cardiac death

Endurance Training vs. Pathological Bradycardia The decrease in resting heart rate that occurs as an adaptation to endurance training is different from pathological bradycardia, an abnormal disturbance in the resting heart rate.

Cardiac Cycle Defined as the mechanical and electrical events that occur during one heart beat (systole to systole) Diastole is the relaxation phase during which the chambers fill with blood (T wave to QRS) Systole is the contraction phase during which the chambers expel blood (QRS to T wave)

Wiggers Diagram Figure 14.27, p. 433 from Human Physiology, 2nd ed. By Dee Unglaub Silverthorn. Copyright © 2001 Prentice-Hall, Inc. Reprinted by permission of Pearson Education, Inc.

Stroke Volume and Cardiac Output Stroke Volume (SV) Volume of blood pumped per contraction End-diastolic volume (EDV)—volume of blood in ventricle just before contraction End-systolic volume (ESV)—volume of blood in ventricle just after contraction SV = EDV – ESV Cardiac Output (Q) Total volume of blood pumped by the ventricle per minute Q = HR  SV . .

Ejection Fraction (EF) Proportion of blood pumped out of the left ventricle with each beat EF = SV / EDV Averages 60% at rest

Calculation of Stroke Volume, Ejection Fraction, and Cardiac Output

The Vascular System Arteries Arterioles Capillaries Venules Veins

Blood Pressure Systolic blood pressure (SBP) is the highest pressure within the vascular system generated during cardiac contraction Diastolic blood pressure (DBP) is the lowest pressure within the vascular system when the heart is relaxed Mean arterial pressure is the average pressure MAP = 2/3 DBP + 1/3 SBP MAP = DPB + [0.33 (SBP - DBP)]

General Hemodynamics Blood flows from a region within the vessel of high pressure to a region within the vessel with lower pressure (pressure gradient) Pressure gradient across the entire cardiovascular system = 100 mmHg Blood vessels and the blood itself provide resistance to blood flow Resistance to blood flow = [ηL / r4], η = viscosity of the blood L = length of the vessel r4 = radius of the vessel to the 4th power

Blood Flow = Δpressure / resistance Changing Blood Flow Blood Flow = Δpressure / resistance Blood flow can change by either changing pressure or resistance or a combination of the two Changing resistance has a larger effect on blood flow because of the fourth power mathematical relationship between vascular resistance and vessel radius Vasoconstriction: radius of the vessel decreases, decreasing blood flow Vasodilation: radius of the vessel increases, increasing blood flow

Pressure Changes Across the Systemic Circulation

Distribution of Cardiac Output at Rest and During Heavy Exercise Reprinted, by permission, from P.O. Åstrand et al., 2003, Textbook of work physiology: Physiological bases of exercise, 4th ed. (Champaign, IL: Human Kinetics), 143.

Intrinsic Control of Blood Flow Stimuli to increase local blood flow Metabolic factors increased oxygen demand increases in metabolic by-products inflammatory chemicals Endothelium released factors Nitric oxide Prostaglandins Endothelium-derived hyperpolarization factors (EDHF) Myogenic responses

Intrinsic Control of Blood Flow Figure courtesy of Dr. Donna H. Korzick, Pennsylvania State University.

Extrinsic Neural Control Accomplished by the sympathetic nervous system through vasoconstriction

Blood Distribution Within the Vasculature When the Body Is at Rest

Integrative Control of Blood Pressure Blood pressure is maintained and controlled by the autonomic nervous system Receptors that modify blood pressure control through the cardiovascular control centers: Baroreceptors: stretch receptors in the aortic arch and carotid arteries that are sensitive to changes in blood pressure Chemoreceptors: chemical receptors that relay information about the chemical environment Mechanoreceptors: receptors that sense changes in muscle length and tension

Return of Blood to the Heart Valves in the veins The muscle pump The respiratory pump

Return of Blood to the Heart: The Muscle Pump

Control of Blood Flow Key Points Blood is distributed throughout the body based on the needs of individual tissues Redistribution of blood is controlled locally by the release of chemical substances, which cause vasodilation Extrinsic neural control of blood flow distribution is accomplished through vasoconstriction through the sympathetic nervous system Blood returns to the heart through veins, assisted by valves, the muscle pump, and respiratory pump

Functions of the Blood Important to Exercise Transportation Temperature regulation Acid–base (pH) balance

Composition of Whole Blood

Hematocrit Ratio of formed elements to the total blood volume White blood cells are involved in the immune response Blood platelets are cell fragments that help blood coagulation Red blood cells carry oxygen to tissues with the help of hemoglobin

Blood Viscosity Thickness of the blood The more viscous, the more resistant to flow Higher hematocrits result in higher blood viscosity

Blood Key Points Blood is about 55% to 60% plasma and 40% to 45% formed elements Oxygen is transported primarily by binding to hemoglobin in red blood cells As blood viscosity increases, so does resistance to blood flow