Circular System

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The Circulatory SystemPart I

Anatomy and Physiology

The circulatory system is responsible for the transport of water and dissolved materialsthroughout the body, including oxygen, carbon dioxide, nutrients, and waste. The circulatory

system transports oxygen from the lungs and nutrients from the digestive tract to every cell in thebody, allowing for the continuation of cell metabolism. The circulatory system also transports

the waste products of cell metabolism to the lungs and kidneys where they can be expelled from

the body. Without this important function toxic substances would quickly build up in the body.

Anatomy of the Circulatory System

The human circulatory system is organized into two major circulations. Each has its

own pump with both pumps being incorporated into a single organthe heart. The

two sides of the human heart are separated by partitions, the interatrial septum andthe interventricular septum. Both septa are complete so that the two sides are

anatomically and functionally separate pumping units. The right side of the heart

pumps blood through the pulmonary circulation (the lungs) while the left side of the

heart pumps blood through the systemic circulation (the body).

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The human heart is a specialized, four-chambered muscle that maintains the blood flow in the

circulatory system. It lies immediately behind the sternum, or breastbone, and between the lungs.The apex, or bottom of the heart, is tilted to the left side. At rest, the heart pumps about 59 cc (2

oz) of blood per beat and 5 l (5 qt) per minute. During exercise it pumps 120-220 cc (4-7.3 oz) of

blood per beat and 20-30 l (21-32 qt) per minute. The adult human heart is about the size of a fistand weighs about 250-350 gm (9 oz).

The human heart begins beating early in fetal life and continues regular beating throughout the

life span of the individual. If the heart stops beating for more than 3 or 4 minutes permanent

brain damage may occur. Blood flow to the heart muscle itself also depends on the continuedbeating of the heart and if this flow is stopped for more than a few minutes, as in a heart attack,

the heart muscle may be damaged to such a great extent that it may be irreversibly stopped.

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The heart is made up of two muscle masses. One of these forms the two atria (the upper

chambers) of the heart, and the other forms the two ventricles (the lower chambers). Both atria

contract or relax at the same time, as do both ventricles.

An electrical impulse called an action potential is generated at regular intervals in a specialized

region of the right atrium called the sinoauricular (or sinoatrial, or SA) node. Since the two atriaform a single muscular unit, the action potential will spread over the atria. A fraction of a second

later, having been triggered by the action potential, the atrial muscle contracts.

The ventricles form a single muscle mass separate from the atria. When the atrial action potential

reaches the juncture of the atria and the ventricles, the atrioventricular or AV node (another

specialized region for conduction) conducts the impulse. After a slight delay, the impulse ispassed by way of yet another bundle of muscle fibers (the Bundle of His and the Purkinje

system.) Contraction of the ventricle quickly follows the onset of its action potential. From this

pattern it can be seen that both atria will contract simultaneously and that both ventricles will

contract simultaneously, with a brief delay between the contraction of the two parts of the heart.

The electrical stimulus that leads to contraction of the heart muscle thus originates in the heart

itself, in the sinoatrial node (SA node), which is also known as the heart's pacemaker. This

node, which lies just in front of the opening of the superior vena cava, measures no more than a

few millimeters. It consists of heart cells that emit regular impulses. Because of this spontaneousdischarge of the sinoatrial node, the heart muscle is automated. A completely isolated heart can

contract on its own as long as its metabolic processes remain intact.

The rate at which the cells of the SA node discharge is externally influenced through theautonomic nervous system, which sends nerve branches to the heart. Through their stimulatory

and inhibitory influences they determine the resultant heart rate. In adults at rest this is between

60 and 74 beats a minute. In infants and young children it may be between 100 and 120 beats aminute. Tension, exertion, or fever may cause the rate of the heart to vary between 55 and 200beats a minute.

The heart's pacemaker is the. . .

Sinoatrial (SA) Node.

Autonomic Nervous System.

Atrioventricular (AV) node.

The heart's pace maker is the Sinoatrial (SA) Node.

The Heart Sounds

The closure of the heart valves and the contraction of the heart muscle produce sounds that can

be heard through the thoracic wall by the unaided ear, although they can be heard better when

amplified by a stethoscope. The sounds of the heart may be represented as lubb-dubb-pause-

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lubb-dubb-pause. The lubb sound indicates the closing of the valves between the atria and

ventricles and the contracting ventricles; the dubb sound indicates the closing of the semilunar

valves. In addition, there may also be cardiac murmurs, especially when the valves are abnormal.Some heart murmurs, however, may also occur in healthy persons, mainly during rapid or

pronounced cardiac action. The study of heart sounds and murmurs furnishes valuable

information to physicians regarding the condition of the heart muscle and valves.

Coronary Circulation

The coronary arteries supply blood to the heart muscle. These vessels originate from the aorta

immediately after the aortic valve and branch out through the heart muscle. The coronary veins

transport the deoxygenated blood from the heart muscle to the right atrium. The heart's energysupply is almost completely dependent on these coronary vessels. When the coronary vessels

become blocked, as inarteriosclerosisor hardening of the arteries, blood flow to the cardiac

muscle is compromised. This is when the common "bypass surgery" is performed where thecoronary arteries are "bypassed" by replacing them with, for example, a vein from the leg. A

"double bypass" is when two coronary arteries are bypassed. A "triple bypass" is when three arebypassed, etc.

The Heartbeat

The heart muscle pumps the blood through the body by means of rhythmical contractions

(systole) and relaxations or dilations (diastole). The heart's left and right halves work almost

synchronously. When the ventricles contract (systole), the valves between the atria and theventricles close as the result of increasing pressure, and the valves to the pulmonary artery and

the aorta open. When the ventricles become flaccid during diastole, and the pressure decreases,

the reverse process takes place.

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The Pulmonary Circulation

From the right atrium the blood passes to the right ventricle through the tricuspid valve, whichconsists of three flaps (or cusps) of tissue. The tricuspid valve remains open during diastole, or

ventricular filling. When the ventricle contracts, the valve closes, sealing the opening and

preventing backflow into the right atrium. Five cords attached to small muscles, called papillarymuscles, on the ventricles' inner surface prevent the valves' flaps from being forced backward.

From the right ventricle blood is pumped through the pulmonary or semilunar valve, which has

three half-moon-shaped flaps, into the pulmonary artery. This valve prevents backflow from the

artery into the right ventricle. From the pulmonary artery blood is pumped to the lungs where itreleases carbon dioxide and picks up oxygen.

The Systemic Circulation

From the lungs, the blood is returned to the heart through pulmonary veins, two from each lung.From the pulmonary veins the blood enters the left atrium and then passes through the mitral

valve to the left ventricle. As the ventricles contract, the mitral valve prevents backflow of blood

into the left atrium, and blood is driven through the aortic valve into the aorta, the major artery

that supplies blood to the entire body. The aortic valve, like the pulmonary valve, has asemilunar shape.

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The aorta has many branches, which carry the blood to various parts of the body. Each of these

branches in turn has branches, and these branches divide, and so on until there are literally

millions of small blood vessels. The smallest of these on the arterial side of the circulation arecalled arterioles. They contain a great deal of smooth muscle, and because of their ability to

constrict or dilate, they play a major role in regulating blood flow through the tissues.

The major artery that supplies blood to the body is the ________.

superior vena cava

inferior vena cava

aorta

femoral

Pulmonary Artery

This image shows the pulmonary artery extending from the right ventricle of the heart.

Image Courtesy of MedValet

Arteries are vessels that carry blood away from theheart. The pulmonary artery transports blood

from the heart to the lungs. While most major arteries branch off from theaorta, the main

pulmonary artery extends from the right ventricleof the heart and branches into left and rightpulmonary arteries. The left and right pulmonary arteries extend to the left lung and right lungs.

The pulmonary arteries are unique in that unlike most arteries which carry oxygenated blood to

other parts of the body, the pulmonary arteries carry de-oxygenated blood to the lungs. After

picking up oxygen, the oxygen rich blood is returned to the heart via thepulmonary veins.

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What is the heart?The heart is the organ that supplies blood and oxygen to all parts of the body. It is

about the size of a clenched fist, weighs about 10.5 ounces and is shaped like a

cone. The heart is located in the chest cavity just posterior to the breastbone,

between the lungs and superior to the diaphragm. The heart is surrounded by a

fluid filled sac called the pericardium. Blood is pumped away from the heart through

arteries and returns to the heart through veins. The major artery of the body is the

aorta and the major veins of the body are the vena cavae.

Chambers of the Heart

The heart is divided by a partition or septum into two halves. The halves are in turn

divided into chambers. The upper two chambers of the heart are called atria and

the lower two chambers are called ventricles. The atria receive blood returning to

the heart from the body and the ventricles pump blood from the heart to the body.

Valves allow blood to flow in one direction between the chambers of the heart.

The Heart Wall

The heart is composed ofcardiac muscle which enable the heart to contract and

allow the synchronization of the heart beat. The heart wall is divided into three

layers: the epicardium, myocardium, and endocardium.

Epicardium - outer protective layer of the heart. Myocardium - muscular middle layer wall of the heart.

Endocardium - inner layer of the heart that is continuous with the inner liningofblood vessels.

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The Main Features

A liquid, blood, to transporto nutrients

o wastes

o oxygen and carbon dioxide

o hormones

Two pumps (in a single heart)

o one to pump deoxygenated blood to the lungs;

o

the other to pump oxygenated blood to all the other organs andtissues of the body.

A system ofblood vessels to distribute blood throughout the body

Specialized organs for exchange of materials between the blood and theexternal environment; for example

o organs like the lungs and intestine that add materials to the blood and

o organs like the lungs and kidneys that remove materials from theblood and deposit them back in the external environment.

The heart and pulmonary system

The heart is located roughly in the center of the chest cavity. It is covered by a

protective membrane, the pericardium.

Deoxygenated blood from the body enters the right atrium. It flows through the tricuspid valve into the right ventricle. The term

tricuspid refers to the three flaps of tissue that make up the valve.

Contraction of the ventricle then closes the tricuspid valve and forces openthe pulmonary valve.

Blood flows into the pulmonary artery.

This branches immediately, carrying blood to the right and left lungs.

Here the blood gives up carbon dioxide and takes on a fresh supply of oxygen[More].

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The capillary beds of the lungs are drained by venules that are the tributariesof the pulmonary veins.

Four pulmonary veins, two draining each lung, carry oxygenated blood to theleft atrium of the heart

The coronary system

From the left atrium,

Blood flows through the mitral valve (also known as the bicuspid valve) intothe left ventricle.

Contraction of the ventricle closes the mitral valve and opens the aorticvalve at the entrance to the aorta.

The first branches from the aorta occur just beyond the aortic valve stillwithin the heart.

Two openings lead to the right and left coronary arteries, which supplyblood to the heart itself.

Although the coronary arteries arise within the heart, they pass directly out to the surfaceof the heart and extend down across it. They supply blood to the network of capillaries

that penetrate every portion of the heart.

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The capillaries drain into two coronary veins that empty into the rightatrium.

Arteriosclerosis

The coronary arteries arise at the point of maximum blood pressure in the circulatory system.Over the course of time, the arterial walls are apt to lose elasticity, which limits the amount of

blood that can surge through them and hence limits the supply of oxygen to the heart. This

condition is known as arteriosclerosis.

Atherosclerosis

Fatty deposits, called plaque, may accumulate on the interior surface of the coronary arteries.

This is particularly common in people who have high levels ofcholesterolin their blood. Plaquedeposits reduce the bore of the coronary arteries and thus the amount of blood they can carry.

Atherosclerosis (usually along with arteriosclerosis) may

so limit the blood supply to the heart that during times of stress the heartmuscle is so deprived of oxygen that the pain ofangina is created.

trigger the formation of a clot causing a coronary thrombosis. This stopsthe flow of blood through the vessel and the capillary network it suppliescausing a heart attack. The portion of the heart muscle deprived of oxygendies quickly of oxygen starvation. If the area is not too large, the undamagedpart of the heart can, in time, compensate for the damage.

Coronary bypass surgery uses segments of leg veins to bypass the clogged portions of thecoronary arteries.

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The Systemic Circulation

The remainder of the system is known as the systemic circulation. The graphic shows the major

arteries (in bright red) and veins (dark red) of the system.

Blood from the aorta passes into a branching system of arteries that lead to all parts of the body.

It then flows into a system of capillaries where its exchange functions take place.

Blood from the capillaries flows into venules which are drained by veins.

Veins draining the upper portion of the body lead to the superior venacava.

Veins draining the lower part of the body lead to the inferior vena cava.

Both empty into the right atrium.

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