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CHAPTER OUTLINE
12.1 The Blood Vessels
The cardiovascular system has three types of blood vessels: the arteries, the capillaries, and the
veins.
The Arteries
An arterial wall has three layers including an innermost layer of endothelium, a middle
layer of smooth muscle that can contract to regulate blood flow and blood pressure, and
an outer layer of fibrous connective tissue that becomes loose at its periphery. The aorta
is the largest artery in the human body and smaller arteries branch off of it into a number
of arterioles.
The Capillaries
Capillaries join arterioles to venules. They are extremely narrow and have thin walls.
The exchange of substances takes place in the capillaries, an important role in
homeostasis.
The Veins
Veins and venules take blood from the capillary beds to the heart. Veins often have
valves that prevent blood from flowing backward when closed.
12.2 Blood
Blood has transport functions, regulatory functions, and protective functions. It consists of
plasma, the liquid portion of the blood, and the formed elements.
Plasma
Plasma contains a variety of inorganic and organic substances dissolved or suspended in
water.
The Red Blood Cells
Mature red blood cells don’t have a nucleus and are biconcave disks. Their shape
increases their flexibility for moving through capillary beds and their surface area for
diffusion of gases. Red blood cells carry oxygen because they contain hemoglobin, the
respiratory pigment.
The White Blood Cells
White blood cells differ from red blood cells in that they are usually larger, have a
nucleus, lack hemoglobin, and without staining appear translucent. They fight infection
and play a role in the development of immunity.
The Platelets and Blood Clotting
Platelets are fragments of certain large cells and are involved in the process of blood
clotting.
Blood Clotting
There are several steps involved in blood clotting. First, platelets clump at the
site of the puncture and partially seal the leak. Platelets and damaged tissue
activate several factors that result in a cascade of enzymatic reactions, eventually
resulting in a fibrin clot, which is temporary until the blood vessel is repaired
and restores the fluidity of the plasma.
Hemophilia
Hemophilia refers to a group of inherited clotting disorders caused by a
deficiency in a clotting factor.
Bone Marrow Stem Cells
A stem cell is a cell that is ever capable of dividing and producing new cells that go on to
differentiate into particular types of cells. Bone marrow contains multipotent stem cells,
which have the potential to give rise to other stem cells. The possibility exists that a
patient’s own bone marrow stem cells could be used for curing certain conditions that
might develop.
Capillary Exchange
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Two forces primarily control movement of fluid through the capillary wall: osmotic
pressure and blood pressure.
12.3 The Human Heart
The heart is a cone-shaped, muscular organ about the size of a fist. The major portion of the
heart, called the myocardium, consists largely of cardiac muscle tissue. Internally, the heart has
four chambers: two upper, thin-walled atria, and two lower, thick-walled ventricles.
Path of Blood through the Heart
There is a specific pathway whereby blood flows through the heart such that oxygen-poor
blood never mixes with oxygen-rich blood. The pumping of the heart sends blood out to
the rest of the body under pressure.
The Heartbeat
Each heartbeat is called a cardiac cycle. When the heart beats, first the two atria contract
at the same time; then the two ventricles contract at the same time. Then all the chambers
relax. The word systole refers to contraction of heart muscle, and the word diastole refers
to relaxation of heart muscle.
Intrinsic Control of Heartbeat
The rhythmic contraction of the atria and ventricles is due to the intrinsic
conduction system of the heart, made possible by nodal tissue, which has both
muscular and nervous characteristics.
Extrinsic Control of Heartbeat
The body also has extrinsic ways to regulate the heartbeat. A cardiac control
center exists in the medulla oblongata of the brain and can alter the beat of the
heart. Hormones can also stimulate the heart.
The Electrocardiogram
An electrocardiogram (ECG) is a recording of the electrical changes that occur in the
myocardium during a cardiac cycle. Body fluids contain ions that conduct electrical
currents; electrical changes in the myocardium can be detected on the skin’s surface.
12.4 The Vascular Pathways
The cardiovascular system includes the pulmonary and systemic circuits.
The Pulmonary Circuit
The pulmonary arteries take oxygen-poor blood from the right ventricle to the lungs and
the pulmonary veins return oxygen-rich blood to the left atrium.
The Systemic Circuit
The systemic circuit includes all the arteries and veins that feed the head, chest, arms, and
lower body regions. The aorta and the venae cavae serve as the major pathways for
blood in the systemic circuit.
Blood Pressure
Systolic pressure results from blood being forced into the arteries during
ventricular systole, and diastolic pressure is the pressure in the arteries during
ventricular diastole. A blood pressure reading consists of these two numbers: for
example, 120/80 (systolic/diastolic).
12.5 Cardiovascular Disorders
Cardiovascular disease is the leading cause of untimely death in Western countries.
Atherosclerosis
Atherosclerosis is an accumulation of soft masses of fatty materials, particularly
cholesterol, beneath the inner lining of arteries.
Hypertension
Hypertension, or high blood pressure, is sometimes called “the silent killer” because it
may not be detected until a stroke or heart attack occurs. It most often occurs secondary
to a narrowing of a person’s arteries from atherosclerosis.
Heart Valve Disease
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Heart valves can be repaired or replaced due to being malformed at birth, but more
commonly they degenerate due to age or infections to a point where they no longer
prevent the backflow of blood. A narrowing of the aortic valve opening is the most
common heart valve malfunction.
Stroke, Heart Attack, and Aneurysm
A stroke results when an arteriole in the brain bursts or is blocked by an embolus. Lack
of oxygen causes a portion of the brain to die, and paralysis or death can result. If a
coronary artery becomes completely blocked, a portion of the heart muscle dies due to
lack of oxygen and a heart attack occurs. An aneurysm is a ballooning of a blood
vessel; if a major vessel bursts about half of victims die before reaching a hospital.
Coronary Bypass Operations
Coronary bypass surgery is a common way to treat an obstructed coronary artery.
During this operation, a surgeon usually takes a segment from another blood
vessel and stitches one end to the aorta and the other end to a coronary artery past
the point of obstruction.
Clearing Clogged Arteries
In angioplasty, a cardiologist uses a catheter to open a clogged artery.
Dissolving Blood Clots
Medical treatment for thromboembolism includes the use of tissue plasminogen
activator, which converts plasminogen into plasmin, an enzyme that dissolves
blood clots.
Heart Transplants and Artificial Hearts
Although heart transplants are usually successful, the need for hearts is greater
than the supply. Only a few patients have received a total artificial heart.
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