5

The Cardiovascular System and Its Control

chapter

Major Cardiovascular Functions

• Delivery• Removal• Transport• Thermoregulation• Immune function

The Anatomy of the Human Heart

Myocardium—The Cardiac Muscle

• Thickness varies directly with stress placed on chamber walls.• Left ventricle is the most powerful of chambers and, thus, the largest.• With vigorous exercise, the size of the left ventricle increases.• Due to intercalated disks, impulses travel quickly in cardiac muscle and

allow it to act as one large muscle fiber; all fibers contract together.

Coronary Circulation

Extrinsic Control of the Heart

• PNS acts through the vagus nerve to decrease heart rate and force contraction.

• SNS is stimulated by stress to increase heart rate and force of contraction.

• Epinephrine and norepinephrine—released due to sympathetic stimulation—increase heart rate.

Did You Know . . . ?

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 decreased intrinsic heart rate and increased parasympathetic stimulation.

Intrinsic Conduction System of the Heart

Cardiac Arrhythmias

Bradycardia is a resting heart rate below 60 bpm.

Tachycardia is a resting heart rate above 100 bpm.

Premature ventricular contractions (PVCs) feel like skipped or extra beats.

Ventricular tachycardia involves three or more consecutive PVCs that can lead to ventricular fibrillation in which contraction of the ventricular tissue is uncoordinated.

Did You Know . . . ?

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.

Electrocardiogram

• Records the heart’s electrical activity and monitors cardiac changes.

• The P wave represents atrial depolarization.• The QRS complex represents ventricular depolarization

and atrial repolarization.• The T wave represents ventricular repolarization.

Graphic Illustration of the Various Phases of the Resting Electrocardiogram

Key Points

Structure and Function of the Cardiovascular System• The two atria receive blood into the heart; the two

ventricles send blood from the heart to the rest of the body.

• The left ventricle has a thicker myocardium due to hypertrophy resulting from the force with which it must contract.

• Cardiac tissue has its own conduction system through which it initiates its own pulse without neural control.

(continued)

Key Points (continued)

Structure and Function of the Cardiovascular System• The pacemaker of the heart is the SA node; it establishes

the pulse and coordinates conduction.• The autonomic nervous system or the endocrine system

can alter heart rate and contraction strength.• ECG records the heart’s electrical function and can be

used in diagnosing cardiac disorders.

Cardiac Cycle

• Defined as events that occur between two consecutive heartbeats (systole to systole).

• Diastole is the relaxation phase during which the chambers fill with blood (T waves to QRS).

• Systole is the contraction phase during which the chambers expel blood (QRS to T wave).

Stroke Volume and Cardiac Output

Stroke Volume (SV)• Volume of blood pumped per contraction• End-diastolic volume (EDV)—volume of blood in

ventricle before contraction• End-systolic volume (ESV)—volume of blood in

ventricle 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 in each beat

• EF = SV / EDV• Averages 60% at rest

CALCULATIONS OF SV, EF, AND Q

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The Vascular System

• Arteries• Arterioles • Capillaries• Venules• Veins

Did You Know . . . ?

Arteries always carry blood away from the heart; veins always carry blood back to the heart with the help of breathing, the muscle pump, and valves.

Blood Distribution Within the Vasculature When the Body Is at Rest

The Muscle Pump

Blood Distribution

• Matched to overall metabolic demands• Autoregulation—arterioles with organs or tissues dilate

or constrict• Extrinsic neural control—sympathetic nerves within

walls of vessels are stimulated• Determined by the balance between mean arterial

pressure and total peripheral resistance

RELATIVE DISTRIBUTION OF Q DURING EXERCISE

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ABSOLUTE DISTRIBUTION OF Q DURING EXERCISE

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Blood Pressure

• Mean arterial pressure (MAP) is the average pressure exerted by the blood as it travels through arteries.

• MAP = DBP + [0.333 (SBP) – (DBP)]• Blood vessel constriction increases blood pressure;

dilation reduces blood pressure.

Key PointsThe Vascular System• Blood returns to the heart with the help of

breathing, the muscle pump, and valves in the veins.

• Blood is distributed throughout the body based on the needs of tissues; the most active tissues receive the most blood.

• Autoregulation controls blood flow by vasodilation in response to local chemical changes in an area.

(continued)

Key Points (continued)The Vascular System• Extrinsic neural factors control blood flow primarily

by vasoconstriction.• Systolic blood pressure (SBP) is the highest

pressure within the vascular system; diastolic blood pressure (DBP) is the lowest.

• Mean arterial pressure (MAP) is the average pressure on the arterial walls.

Functions of the Blood

• Transports gas, nutrients, and wastes• Regulates temperature• Buffers and balances acids and bases

Composition of Whole Blood

HEMOCONCENTRATION

Hematocrit

Ratio of formed elements to the total blood volume• White blood cells protect body from disease

organisms.• 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

Key PointsBlood• Blood and lymph transport materials to and from

body tissues.• Blood is about 55% to 60% plasma and 40% to

45% formed elements (white and red blood cells and blood platelets).

• Oxygen travels through the body by binding to hemoglobin in red blood cells.

• An increase in blood viscosity results in resistance to flow.