Functions and Components of the Circulatory System 366

I. The blood transports oxygen and nutrients to all the cells of the body and removes waste products from the tissues. It also serves a regulatory function through its transport of hormones.

A. Oxygen is carried by red blood cells, or erythrocytes.

B. White blood cells, or leukocytes, serve to protect the body from disease.

II. The circulatory system consists of the cardiovascular system (heart and blood vessels) and the lymphatic system.

Composition of the Blood 367

I. Plasma is the fluid part of the blood, containing dissolved ions and various organic molecules.

A. Hormones are found in the plasma portion of the blood.

B. Plasma proteins include albumins; globulins (alpha, beta, and gamma); and fibrinogen.

II. The formed elements of the blood include erythrocytes, leukocytes, and platelets.

A. Erythrocytes, or red blood cells, contain hemoglobin and transport oxygen.

B. Leukocytes may be granular (also called polymorphonuclear) or agranular. They function in immunity.

C. Platelets, or thrombocytes, are required for blood clotting.

III. The production of red blood cells is stimulated by the hormone erythropoietin, and the development of different kinds of white blood cells is controlled by chemicals called lymphokines.

IV. The major blood typing groups are the ABO system and the Rh system.

A. Blood type refers to the kind of antigens found on the surface of red blood cells.

B. When different types of blood are mixed, antibodies against the red blood cell antigens cause the red blood cells to agglutinate.

V. When a blood vessel is damaged, platelets adhere to the exposed subendothelial collagen proteins.

A. Platelets that stick to collagen undergo a release reaction in which they secrete ADP, serotonin, and thromboxane A2.

B. Serotonin and thromboxane A2 cause vasoconstriction. ADP and thromboxane A2 attract other platelets and cause them to stick to the growing mass of platelets that are stuck to the collagen in the broken vessel.

VI. In the formation of a blood clot, a soluble protein called fibrinogen is converted into insoluble threads of fibrin.

A. This reaction is catalyzed by the enzyme thrombin.

B. Thrombin is derived from prothrombin, its inactive precursor, by either an intrinsic or an extrinsic pathway.

1. The intrinsic pathway, the longer of the two, requires the activation of more clotting factors.

2. The shorter extrinsic pathway is initiated by the secretion of tissue thromboplastin.

C. The clotting sequence requires Ca2+ as a cofactor and phospholipids present in the platelet cell membranes.

VII. Dissolution of the clot eventually occurs by the action of plasmin, which cleaves fibrin into split products.

Acid-Base Balance of the Blood 377

I. The normal pH of arterial blood is 7.40, with a range of 7.35 to 7.45. A. Carbonic acid is formed from carbon dioxide and contributes to the blood pH. It is referred to as a volatile acid because it can be eliminated in the exhaled breath. B. Nonvolatile acids, such as lactic acid and the ketone bodies, are buffered by bicarbonate.

II. The blood pH is maintained by a proper ratio of carbon dioxide to bicarbonate.

A. The lungs maintain the correct carbon dioxide concentration. An increase in carbon dioxide, due to inadequate ventilation, produces respiratory acidosis.

B. The kidneys maintain the free-bicarbonate concentration. An abnormally low plasma bicarbonate concentration produces metabolic acidosis.

Structure of the Heart 378

I. The right and left sides of the heart pump blood through the pulmonary and systemic circulations.

A. The right ventricle pumps blood to the lungs. This blood then returns to the left atrium.

B. The left ventricle pumps blood into the aorta and systemic arteries. This blood then returns to the right atrium.

II. The heart contains two pairs of oneway valves.

A. The atrioventricular valves allow blood to flow from the atria to the ventricles, but not in the reverse direction.

B. The semilunar valves allow blood to leave the ventricles and enter the pulmonary and systemic circulations, but they prevent blood from returning from the arteries to the ventricles.

III. The electrical impulse begins in the sinoatrial node and spreads through both atria by electrical conduction from one myocardial cell to another.

A. The impulse then excites the atrioventricular node, from which it is conducted by the bundle of His into the ventricles.

B. The Purkinje fibers transmit the impulse into the ventricular muscle and cause it to contract.

Cardiac Cycle and Heart Sounds 381

I. The heart is a two-step pump. The atria contract first, and then the ventricles.

A. During diastole, first the atria and then the ventricles fill with blood.

B. The ventricles are about 80% filled before the atria contract and add the final 20% to the end-diastolic volume.

C. Contraction of the ventricles ejects about two-thirds of their blood, leaving about one-third as the end-systolic volume.

II. When the ventricles contract at systole, the pressure within them first rises sufficiently to close the AV valves and then rises sufficiently to open the semilunar valves.

A. Blood is ejected from the ventricles until the pressure within them falls below the pressure in the arteries. At this point, the semilunar valves close and the ventricles begin relaxation.

B. When the pressure in the ventricles falls below the pressure in the atria, a phase of rapid filling of the ventricles occurs, followed by the final filling caused by contraction of the atria.

III. Closing of the AV valves produces the first heart sound, or "lub," at systole. Closing of the semilunar valves produces the second heart sound, or "dub," at diastole. Abnormal valves can cause abnormal sounds called murmurs.

Electrical Activity of the Heart and the Electrocardiogram 383

I. In the normal heart, action potentials originate in the SA node as a result of spontaneous depolarization called the pacemaker potential.

A. When this spontaneous depolarization reaches a threshold value, opening of the voltage-regulated Na+ gates and fast Ca2+ channels produces action potential.

B. Repolarization is produced by the outward diffusion of K+, but a stable resting membrane potential is not attained because spontaneous depolarization once again occurs.

C. Other myocardial cells are capable of spontaneous activity, but the SA node is the normal pacemaker because its rate of spontaneous depolarization is the fastest.

D. When the action potential produced by the SA node reaches

Heart and Circulation 403

other myocardial cells, they produce action potentials with a long plateau phase because of the slow inward diffusion of Ca2+. E. The long action potential and long refractory period of myocardial cells allows the entire mass of cells to be in a refractory period while it contracts. This prevents the myocardium from being stimulated again until after it relaxes.

II. The regular pattern of conduction in the heart produces a changing pattern of potential differences between two points on the body surface.

A. The recording of this changing pattern caused by the heart's electrical activity is called an electrocardiogram (ECG).

B. The P wave is caused by depolarization of the atria; the QRS wave is caused by depolarization of the ventricles; and the T wave is produced by repolarization of the ventricles.

Blood Vessels 390

I. Arteries contain three layers, or tunics: the interna, media, and externa.

A. The tunica interna consists of a layer of endothelium, which is separated from the tunica media by a band of elastin fibers.

B. The tunica media consists of smooth muscle.

C. The tunica externa is the outermost layer.

D. Large arteries, containing many layers of elastin, can expand and recoil with rising and falling blood pressure. Medium and small arteries and arterioles are less distensible, and thus provide greater resistance to blood flow.

II. Capillaries are the narrowest but the most numerous of the blood vessels.

A. Capillary walls consist of just one layer of endothelial cells. They provide for the exchange of molecules between the blood and the surrounding tissues.

B. The flow of blood from arterioles to capillaries is regulated by precapillary sphincter muscles.

C. The capillary wall may be continuous, fenestrated, or discontinuous.

III. Veins have the same three tunics as arteries, but they generally have a thinner muscular layer than comparably sized arteries.

A. Veins are more distensible than arteries and can expand to hold a larger quantity of blood.

B. Many veins have venous valves that ensure a one-way flow of blood to the heart.

C. The flow of blood back to the heart is aided by contraction of the skeletal muscles that surround veins. The effect of this action is called the skeletal muscle pump.

Atherosclerosis and Cardiac Arrhythmias 395

I. Atherosclerosis of arteries can occlude blood flow to the heart and brain and is a causative factor in about 50% of all deaths in the United States, Europe, and Japan.

A. Atherosclerosis begins with injury to the endothelium, the movement of monocytes and lymphocytes into the tunica interna, and the conversion of monocytes into macrophages that engulf lipids. Smooth muscle cells then proliferate and secrete extracellular matrix.

B. Atherosclerosis is promoted by such risk factors as smoking, hypertension, and high plasma cholesterol concentration. Low-density lipoproteins (LDLs), which carry cholesterol into the artery wall, are oxidized by the endothelium and are a major contributor to atherosclerosis.

II. Occlusion of blood flow in the coronary arteries by atherosclerosis may produce ischemia of the heart muscle and angina pectoris, which may lead to myocardial infarction.

III. The ECG can be used to detect abnormal cardiac rates, abnormal conduction between the atria and ventricles, and other abnormal patterns of electrical conduction in the heart.

Lymphatic System 399

I. Lymphatic capillaries are blind-ended but highly permeable. They drain excess tissue fluid into lymph ducts.

II. Lymph passes through lymph nodes and is returned by way of the lymph ducts to the venous blood.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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