Chapter Summary

Functions and Components of the Circulatory System

1. 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.
1. Oxygen is carried by red blood cells, or erythrocytes.
2. White blood cells, or leukocytes, sever to protect the body from disease.
2. The circulatory system consists of the cardiovascular system (heart and blood vessels) and the lymphatic system.

Composition of the Blood

1. Plasma is the fluid part of the blood, containing dissolved ions and various organic molecules.
1. Hormones are found in the plasma portion of the blood.
2. Plasma proteins include albumins; globulins (alpha, beta and gamma); and fibrinogen.
2. The formed elements of the blood include erythrocytes, leukocytes, and platelets.
1. Erythrocytes, or red blood cells, contain hemoglobin and transport oxygen.
2. Leukocytes may be granular (also called polymorphonuclear) or agranular. They function in immunity.
3. Platelets or thrombocytes are required for blood clotting.
3. 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.
4. The major blood typing groups are the ABO system and the Rh system.
1. Blood type refers to the kind of antigens found on the surface of the red blood cells.
2. When different types of blood are mixed, antibodies against the red blood cell antigens cause the red blood cells to agglutinate.
5. When a blood vessel is damaged, platelets adhere to the exposed subendothelial collagen proteins.
1. Platelets that stick to collagen undergo a release reaction, in which they secrete ADP, serotonin, and thromboxane A₂.
2. Serotonin and thromboxane A₂ cause vasoconstriction. ADP and thromboxane A₂ attract other platelets and make them stick to the growing mass of platelets that are stuck to the collagen in the broken vessel.
6. In the formation of a blood clot, a soluble protein called fibrinogen is converted into insoluble threads of fibrin.
1. This reaction is catalyzed by the enzyme thrombin.
2. Thrombin is derived from prothrombin, its inactive precursors, 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.
3. The clotting sequence requires Ca²⁺ as a cofactor and phospholipids present in the platelet cell membranes.
7. Dissolution of the clot eventually occurs by the action of plasmin, which cleaves fibrin into split products.

Acid-Base Balance of the Blood

1. The normal pH of arterial blood is 7.40, with a range of 7.35 to 7.45.
1. 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.
2. Nonvolatile acids, such as lactic acid and the ketone bodies, are buffered by bicarbonate.
2. The blood pH is maintained by a proper ratio of carbon dioxide to bicarbonate.
1. The lungs maintain the correct carbon dioxide concentration. An increase in carbon dioxide, due to inadequate ventilation, produces respiratory acidosis, for example.
2. The kidneys maintain the free-bicarbonate concentration. An abnormally low plasma bicarbonate concentration produces metabolic acidosis, for example.

Structure of the Heart

1. The right and left sides of the heart pump blood through the pulmonary and systemic circulations.
1. The right ventricle pumps blood to the lungs. This blood then returns to the left atrium.
2. The left ventricle pumps blood into the aorta and systemic arteries. This blood then returns to the right atrium.
2. The heart contains two pairs of one-way valves.
1. The atrioventricular valves allow blood to flow from the atria to the ventricles, but not in the reverse direction.
2. The semilunar valves allow blood to leave the ventricles and enter the pulmonary and systemic circulations, but these valves prevent blood from returning from the arteries to the ventricles.
3. The electrical impulse begins in the sinoatrial node and spreads through both atria by electrical conduction from one myocardial cell to another.
1. The impulse then excites the atrioventricular node, from which it is conducted by the bundle of His into the ventricles.
2. The Purkinje fibers transmit the impulse into the ventricular muscle and cause it to contract.

Cardiac Cycle and Heart Sounds

1. The heart is a two-step pump. The atria contract first, and then the ventricles.
1. During diastole, first the atria and then the ventricles fill with blood.
2. The ventricles are about 80% filled before the atria contract and add the final 20% to the end-diastolic volume.
3. Contraction of the ventricles ejects about two-thirds of their blood, leaving about one-third as the end-systolic volume.
2. 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.
1. Blood is ejected from the ventricles until the pressure within the falls below the pressure in the arteries. At this point, the semilunar valves close and the ventricles begin relaxation.
2. 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.
3. 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

1. In the normal heart the impulse originates in the SA node, due to a spontaneous depolarization called the pacemaker potential.
1. When this spontaneous depolarization reaches a threshold value, opening of the voltage-regulated Na⁺ gates and fast Ca²⁺ channels produces an action potential.
2. Repolarization is produced by the outward diffusion of K⁺, but a stable resting membrane potential is not attained because spontaneous depolarization once more occurs.
3. 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.
4. When the action potential produced by the SA node reaches other myocardial cells, they produce action potentials with a long plateau phase because of the slow, inward diffusion of Ca²⁺.
5. 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.
2. The regular pattern of conduction in the heart produces a changing pattern of potential differences between two points on the body surface.
1. The recording of this changing pattern caused by the heart's electrical activity of the heart is called an electrocardiogram (ECG).
2. 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

1. Arteries contain three layers, or tunics: the interna, media, and externa.
1. The tunica interna consists of a layer of endothelium, which is separated from the tunica media by a band of elastin fibers.
2. The tunica media consists of smooth muscle.
3. The tunica externa is the outermost layer.
4. 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.
2. Capillaries are the narrowest but the most numerous of the blood vessels.
1. Capillary walls consist of just one layer of endothelial cells. They provide for the exchange of molecules between the blood and the surrounding tissues.
2. The flow of blood from arterioles to capillaries is regulated by precapillary sphincter muscles.
3. The capillary wall may be continuous, fenestrated, or discontinuous.
3. Veins have the same three tunics as arteries, but they generally have a thinner muscular layer than comparably sized arteries.
1. Veins are more distensible than arteries and can expand to hold a larger quantity of blood.
2. Many veins have venous valves that ensure a one-way flow of blood to the heart.
3. The flow of blood back to the heart is aided by contraction of the skeletal muscles that surround veins. The effects of this action is called the skeletal muscle pump.

Atherosclerosis and Cardiac Arrhythmias

1. Atherosclerosis of arteries can occlude blood flow to the heart and brain, causing up to 50% of all mortality in the United States, Europe, and Japan.
1. 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.
2. Atherosclerosis is promoted by smoking, hypertension, and high plasma cholesterol concentration, among other risk factors; low-density lipoproteins (LDL), which carry cholesterol into the artery wall, is oxidized by the endothelium and is a major contributor to atherosclerosis.
2. 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.
3. 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

1. Lymphatic capillaries are blind-ended but highly permeable. They drain excess tissue fluid into lymph ducts.
2. Lymph passes through lymph nodes and is returned by way of the lymph ducts to the venous blood.

After studying this chapter, students should be able to . . .

1. describe the general functions of the major components of the circulatory system.
2. describe the composition of blood plasma and the physical characteristics and functions of the formed elements of the blood.
3. identify the chemical regulators of blood cell production and describe the process of erythorpoiesis.
4. describe the ABO system of red blood cell antigens and explain the significance of the blood types.
5. explain how a blood clot is formed and how it is ultimately destroyed.
6. explain how the acid-base balance of the blood is affected by carbon dioxide and bicarbonate, and describe the roles of the lungs and kidneys in maintaining acid-base balance.
7. describe the path of the blood through the heart and the function of the atrioventricular and semilunar valves.
8. describe the structures and pathways of the pulmonary and systemic circulations.
9. describe the structures and pathways of electrical impulse conduction in the heart.
10. describe the electrical activity in the sinoatrial node and explain why this tissue functions as the heart’s normal pacemaker.
11. relate the time involved in the production of an action potential to the time involved in the contraction of myocardial cells and explain the significance of this relationship.
12. describe the pressure changes that occur in the vetricles during the cardiac cycle and relate these changes to the action of the valves and the flow of blood.
13. explain the origin of the heart sounds and state when in the cardiac cycle these sounds are produced.
14. explain the cause of each wave in an electrocardiogram and relate these waves to other events in the cardiac cycle.
15. compare the structure of an artery and vein, and explain how the structure of each type of vessel relates to its function.
16. describe the structure of capillaries and explain the physiological significance of this structure.
17. explain how atherosclerosis may develop and comment on the significance of this condition.
18. define ischemia and discuss the possible causes of myocardial ischemia.
19. describe some common arrhythmias that can be detected with an ECG.
20. describe the components and functions of the lymphatic system.