10.1 Anatomy and Physiology of the Cardiovascular System

FIGURE 10.1 Overview of the Cardiovascular System

The cardiovascular system includes the heart, arteries, veins, and capil-laries.

veins

heart

arteries

capillaries

Pharmacy Technician Series © Paradigm Education SolutionsPharmacology for Technicians, Seventh Edition: Chapter 10, Section 10.1

10.1 Anatomy and Physiology of the Cardiovascular System

The cardiovascular system circulates blood throughout the body (see Figure 10.1) and is responsible for bringing oxygen and nutrients to tissues and for car-rying away carbon dioxide and toxic by-products. The cardiovascular system, also known as the circulatory system, is composed of the heart and blood vessels (arter-ies, veins, and capillaries). Without a properly functioning cardiovascular system, human life is not sustainable. The heart pumps blood to the body through arter-ies and veins. An artery carries blood away from the heart to the body. A veincarries blood from tissues back to the heart.

FIGURE 10.2 Blood Flow through the Cardiovascular System

Oxygenated blood is depicted in red, whereas blood returning from the body, in need of oxygen, is shown in blue.

lung

systemic capillaries

pulmonary capillaries

circulation to headand upper body tissues

circulation to lower body tissues

lung

left ventricle

rightventricle

left atrium

mitralvalve

aortic semilunar valve

pulmonary semilunar valve

right atrium

superiorvena cava

inferiorvena cava

CO2

CO2 CO2

O2

CO2 O2

O2 O2tricuspidvalve

aorta

main pulmonary artery

pulmonary veins


The path that the blood takes through the body ensures that it carries deoxygen-ated blood to the lungs (for gas exchange) and then sends that reoxygenated blood to other tissues in the body (see Figure 10.2). A capillary is a tiny blood vessel. In the cap-illaries, critical fluids, gases, and nutrients are exchanged between the blood and body tissues. For additional information on gas exchange, see Chapter 11.

The heart is a hollow organ that has three functional parts: the cardiac muscle (also called the myocardium or heart muscle), which contracts to pump blood out of the heart and maintain blood flow through the body; the conduction system, which consists of cardiac muscle cells and conducting fibers that coordinate contraction; and blood supply, which is pumped in and out of each chamber.

The heart is made of specialized cardiac muscle fibers and is divided into four chambers. These chambers, each named for its location within the heart, are the right atrium, the left atrium, the right ventricle, and the left ventricle (see Figure 10.3). The atria receive the blood that is brought to the heart from the veins, and the ventricles pump the blood out through the arteries. The arteries direct blood either to the lungs or other body tissues.

The heart both contracts (effectively pumping blood) and relaxes (fills with blood). Systole refers to the period during which the heart is contracting and actively pumping blood. Diastole describes the period when the cardiac muscle relaxes, allowing blood to passively flow into the heart and fill the heart’s chambers. Valves within the heart prevent blood from flowing in the wrong direction.

Cardiac ContractilityA normal heartbeat is the result of a coordinated series of electrical events in the con-duction system, a group of cardiac muscle cells that send signals to the heart muscle. It begins in the membranes of myocardial conducting cells in the sinoatrial (SA) node, often called the heart’s natural pacemaker. Between beats, these cells are polarized; that is, the inside of each cell is at a negative voltage relative to the outside. The beat origi-nates when ion channels in a cell membrane open to allow positively charged sodium and calcium ions to flow into the cell, making the voltage positive instead of negative (depolarization). Other channels then open to allow positively charged potassium ions to flow out of the cell, making the voltage negative again (repolarization). The result-ing action potential (the electrical signal that causes a muscle to contract) spreads through the conduction system to the other muscle cells of the myocardium, called myocardial contractile cells. When the action potential arrives at a myocardial contractile cell, which is also polarized, the cell depolarizes with a rapid inflow of sodium and a slower inflow of calcium. Then the cell repolarizes with an outflow of potassium.

If the depolarizing and repolarizing flows were the only ion flows, the cell would run out of potassium and accumulate huge amounts of sodium and calcium. Other proteins in the cell membrane continually restore the balance by using energy to pump sodium and calcium out and potassium in simultaneously.

The electrical signal from the SA node is carried through the atria and at the same time down to the atrioventricular (AV) node. After a delay, the signal travels through the bundle of His (pronounced “hiss”) and the bundle branches to the heart’s apex. At this point, the electrical signal branches into the Purkinje fibers. A Purkinje fiber is one of the fibers that stretch from the apex up into the ventricles to contract the lower—and largest—part of the heart. This conduction system is shown in Figure 10.4.

The typical rate at which the SA node fires to initiate each heartbeat is 60 to 100 times per minute. This heart rate (HR) is typically reported in beats per minute (BPM) and is measured by taking a person’s pulse. Common places to take a pulse include the carotid artery on the neck (this pulse is called a carotid pulse) and the thumb side of the wrist (this pulse is called a radial pulse).

Name Exchange

Nexterone and

Pacerone are

brand names

for the generic

drug amiodarone.

Amiodarone,

which treats heart

arrhythmias, works

by blocking potas-

sium channels.


pulmonarysemilunar valve

tricuspid valve

inferior vena cava left ventricle

apex

interventricular septum

aorta

left pulmonary artery

left pulmonary veins

right atrium

right pulmonary veins

left atrium

right ventricle

mitral valve

chordae tendineae

aortic semilunar valve

superiorvena cava

right pulmonaryartery

FIGURE 10.3 Internal Structuresof the Heart

The heart is an organ with four chambers: the upper chambers are the right atrium and left atrium, and the lower chambers are the right ventricle and left ventricle.

Blood PressureBlood pressure (BP) describes the force of blood in the circulatory system and is main-tained by various feedback mechanisms. In simple terms, blood pressure is a function of capacitance, cardiac output, peripheral vascular resistance, and the renin-angiotensin system:

• Capacitance: the amount of blood held in the veins and venules (small veinsthat collect blood from capillaries)

• Cardiac output (CO): the force and volume of blood coming from the heart

• Peripheral vascular resistance (PVR): the degree to which the blood vessels,particularly small vessels called arterioles, are constricted or relaxed

• Renin-angiotensin system: the feedback mechanism that is regulated by thekidneys and balances fluid volume and vessel constriction

The sympathetic nervous system, part of the central nervous system, also plays a role in blood pressure regulation by influencing peripheral vascular resistance and the renin-angiotensin system. Figure 10.5 illustrates the complex feedback mechanisms that work together to balance blood pressure.

PracticeTip

BP is a commonly

used abbreviation

for blood pressure.

capacitancevenules

cardiac output (CO)heart

peripheral vascular resistancearterioles

CNSsympathetic

nervesrenin-angiotensin system

kidneys

FIGURE 10.5 Maintaining Blood Pressure

Blood pressure is maintained by a variety of mechanisms.


right and leftbundle branches

Purkinje fibers

SA node(pacemaker)

AV node

1

2

bundle of His(AV bundle)

3

4

5

FIGURE 10.4 Conduction System of the Heart

Heartbeat is regulated by the conduction system, which initiates impulses and conducts them throughout the heart. The numbers in this figure represent the order in which the electri-cal signals travel through the heart.

Blood pressure is expressed as the systolic blood pressure reading, which measures the pressure when the heart contracts and ejects blood (systole), and the dia-stolic blood pressure reading, which measures the pressure when the heart relaxes and fills (diastole). Cardiac output is the major determinant of systolic pressure, which is the product of cardiac output and peripheral vascular resistance. Diastolic pressure is related to the volume of venous blood return. Both readings are in millime-ters of mercury (abbreviated mm Hg)—that is, the height of a mercury column whose weight offsets the pressure. The reading is stated as systolic over diastolic pressure; measurements of 120 mm Hg systolic and 80 mm Hg diastolic are written 120/80 and read 120 over 80. A blood pressure of 120/80 is often considered normal.


If blood vessels are constricted, causing increased PVR, the heart has to work harder to maintain the same cardiac output to keep blood pressure stable. Therefore, constriction (narrowing) or dilation (widening) of blood vessels raises or lowers blood pressure, respectively. High blood pressure is often caused by elevated PVR. Stress on the heart from high blood pressure may cause cardiac disease. High blood pressure also affects vital organs and can eventually result in kidney failure and stroke. Sympathetic nerves in the autonomic nervous system regulate that multifac-eted system. Alterations in any one of the compensatory checks and balances in the feedback mechanisms that maintain blood pressure can cause hypertension. Losing large amounts of blood lowers blood pressure to dangerous levels. If blood pressure falls low enough, the patient may go into shock, a condition during which vital organs are not supplied with blood and begin to shut down.

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10.1 Anatomy and Physiology of the Cardiovascular System