The Circulation of the Blood Chapter 14 The Circulation of the Blood

Objectives Describe the structure and function of each major type of blood vessel: artery, vein, and capillary List the major disorders of blood vessels and explain how they develop Trace the path of blood through the systemic, pulmonary, portal, and fetal circulations

Objectives Identify and discuss the factors involved in the generation of blood pressure and how they relate to each other Define pulse and locate the major pulse points on the body Explain what is meant by the term circulatory shock and describe the major types

Blood Vessels Types Arteries—carry blood away from the heart and toward capillaries Veins—carry blood toward the heart and away from capillaries Capillaries—carry blood from the arterioles to the venules What is the largest artery in the body? Aorta What is the difference between the functions of arteries and veins? Arteries and arterioles distribute blood from the heart to capillaries in all parts of the body; they also help maintain arterial blood pressure by constricting or dilating. Venules and veins collect blood from capillaries and return it to the heart; they also serve as blood reservoirs. Where does the exchange of nutrients and respiratory gases between blood and tissue fluid take place? Capillary bed

Blood Vessels Structure Arteries Capillaries—microscopic vessels Tunica intima—inner layer of endothelial cells Tunica media—smooth muscle, thick in arteries; important in blood pressure regulation Tunica externa—thin outer layer of elastic tissue Capillaries—microscopic vessels Only one layer—the tunica intima Why is the muscle layer thicker in arteries than in veins? The thicker muscle layer is able to resist great pressures generated by ventricular systole. This ability is critical to maintaining blood pressure and controlling blood distribution. What feature is present in veins but not in arteries? Valves What is the function of a precapillary sphincter? It guards the entrance to the capillary and determines how much blood will flow into each capillary bed. What vessels distribute blood from the heart to capillaries? Arteries and arterioles

Blood Vessels Structure Veins Tunica intima—inner layer; valves prevent retrograde movement of blood Tunica media—smooth muscle; thin in veins Tunica externa—heavy layer of fibrous connective tissue in many veins How do arterioles help keep arterial blood pressure at a normal level? The muscle layer, tunica media, constricts or dilates as necessary.

Artery and vein. Schematic drawings of an artery and a vein show comparative thicknesses of the three layers: the outer layer or tunica externa, the muscle layer or tunica media, and the tunica intima made of endothelium. Note that the muscle and outer layers are much thinner in veins than in arteries and that veins have valves.

Blood Vessels Functions Arteries—distribution of nutrients, gases, etc., with movement of blood under high pressure; assist in maintaining the arterial blood pressure Capillaries—serve as exchange vessels for nutrients, wastes, and fluids Veins—collect blood for return to the heart; low-pressure vessels Why do venules and veins serve as blood reservoirs? Venules and veins carry blood under lower pressure than arteries and can expand to hold a larger volume of blood or constrict to hold a much smaller amount. Why are capillaries known as exchange vessels? Nutrients and gases pass through the capillaries into and out of the cells. What are some of the major arteries? See Figure 14-2 and Table 14-1.

Principal arteries of the body.

Principal veins of the body.

Disorders of Blood Vessels Disorders of arteries—arteries must withstand high pressure and remain free of blockage Arteriosclerosis—hardening of arteries Reduces flow of blood, possibly causing ischemia that may progress to necrosis (or gangrene) Atherosclerosis: lipids and other matter block arteries May be corrected by vasodilators (vessel-relaxing drugs) or angioplasty (mechanical widening of vessels), or surgical replacement What is the difference between arteriosclerosis and atherosclerosis? Arteriosclerosis is calcification in the arterial walls; atherosclerosis is blockage by lipids and other matter. How do vasodilators treat arteriosclerosis? Vasodilators trigger the smooth muscles of the arterial walls to relax and the arteries to dilate.

Partial blockage of an artery in atherosclerosis Partial blockage of an artery in atherosclerosis. Atherosclerotic plaque develops from the deposition of fats and other substances in the wall of the artery. The inset is a photograph showing a cross section of an artery partially blocked by plaque.

Balloon angioplasty. A, A catheter is inserted into the vessel until it reaches the affected region. B, A probe with a metal tip is pushed out the end of the catheter into the blocked region of the vessel. C, The balloon is inflated, pushing the walls of the vessel outward. Sometimes metal coils or tubes (stents) are inserted to keep the vessel open.

Disorders of Blood Vessels Disorders of arteries Aneurysm—abnormal widening of arterial wall Aneurysms promote formation of thrombi that may obstruct vital tissues Aneurysms may burst, resulting in life-threatening hemorrhaging Cerebrovascular accident (CVA) or stroke—ischemia of brain tissue caused by embolism or hemorrhage What is the danger of an aneurysm? It could burst, causing hemorrhage and death.

Disorders of Blood Vessels Disorders of veins—veins are low-pressure vessels Varicose veins (varices)—enlarged veins in which blood pools Hemorrhoids are varicose veins in the rectum Treatments include supporting affected veins and surgical removal Thrombophlebitis—vein inflammation (phlebitis) accompanied by clot (thrombus) formation; may result in fatal pulmonary embolism What are some nonsurgical methods of treating varicose veins? Avoid standing for long periods, wear support stockings, and rest periodically with legs slightly elevated. What is the danger of thrombophlebitis? Clots can dislodge and become an embolism circulating in the blood.

Varicose veins. A, Veins near the surface of the body—especially in the legs—may bulge and cause venous valves to leak. B, Photograph showing varicose veins on the surface of the leg.

Circulation of Blood Systemic and pulmonary circulation Blood circulation—refers to the flow of blood through all the vessels, which are arranged in a complete circuit or circular pattern Systemic circulation Carries blood throughout the body Path goes from left ventricle through aorta, smaller arteries, arterioles, capillaries, venules, venae cavae, to right atrium Is the plan of systemic circulation similar in all tissues and organs? In what parts of the body do capillaries host the exchange of nutrients and respiratory gases? The alveoli

Diagram of blood flow in the circulatory system Diagram of blood flow in the circulatory system. Blood leaves the heart through arteries, and then travels through arterioles, capillaries, venules, and veins before returning to the opposite side of the heart. AV, Atrioventricular; SL, semilunar.

Circulation of Blood Pulmonary circulation Hepatic portal circulation Carries blood to and from the lungs Arteries deliver deoxygenated blood to the lungs for gas exchange Path goes from right ventricle through pulmonary arteries, lungs, pulmonary veins, to left atrium Hepatic portal circulation Unique blood route through the liver Vein (hepatic portal vein) exists between two capillary beds Assists with homeostasis of blood glucose levels What is the function of liver cells in hepatic portal circulation? Liver cells remove excess glucose after a meal and store it as glycogen; liver cells also remove and detoxify various poisonous substances that may be present in the blood. Why is it important to understand the roles of blood pressure and blood pressure gradient in circulation? The blood pressure gradient is vitally important to keep the blood flowing. If there were no blood pressure gradient between arteries and arterioles, the blood would stop circulation and life would cease. What is the function of pulmonary circulation? Oxygenation of blood and removal of carbon dioxide from the blood What occurs in hepatic portal circulation? Storage of excess glucose as glycogen, detoxification of poisonous substances in the blood

Hepatic portal circulation Hepatic portal circulation. In this very unusual circulation, a vein is located between two capillary beds. The hepatic portal vein collects blood from capillaries in visceral structures located in the abdomen and empties it into the liver. Hepatic veins return blood to the inferior vena cava. (Note that organs are not drawn to scale in this illustration and portion of pancreas has been removed to show veins.)

Circulation of Blood Fetal circulation Refers to circulation before birth Modifications required for fetus to efficiently secure oxygen and nutrients from the maternal blood Unique structures include the placenta, umbilical arteries and vein, ductus venosus, ductus arteriosus, and foramen ovale What must happen before the exchange of nutrients and oxygen between fetal and maternal blood can occur? Specialized blood vessels must carry fetal blood to the placenta where exchange occurs and then return it to the fetal body. What is the role of the ductus venosus? It is a continuation of the umbilical vein that serves as a shunt to allow most of the blood returning from the placenta to bypass the immature liver and empty directly into the inferior vena cava.

The fetal circulation.

Blood Pressure Defining blood pressure “Push” or force of blood in the blood vessels Exists in all blood vessels—highest in arteries, lowest in veins Blood pressure gradient—causes blood to circulate; liquids can flow only from the area where pressure is higher to where it is lower Low or nonexistent blood pressure gradient is fatal if not reversed quickly Hypertension (high blood pressure) can cause a blood vessel to rupture How is the blood pressure gradient for the entire systemic circulation measured? The mean blood pressure in the aorta (Figure 14-11) is 100 mmHg, and the pressure at the termination of the vena cava is 0. Therefore, with these normal typical figures, the systemic blood pressure gradient is 100 mmHg. Why can high blood pressure become a problem in circulation? If blood pressure becomes too high, it may cause rupture of one or more blood vessels. Why can very low blood pressure become a problem in circulation? If arterial pressure falls low enough, circulation and life would cease. Massive hemorrhage could cause this.

Pressure gradients in blood flow Pressure gradients in blood flow. Blood flows down a “blood pressure hill” from arteries, where blood pressure is highest, into arterioles, where it is somewhat lower, into capillaries, where it is lower still, and so on. All numbers on the graph indicate blood pressure measured in millimeters of mercury. The broken line, starting at 100 mm, represents the average pressure in each part of the circulatory system.

Blood Pressure Factors that influence blood pressure Blood volume—the larger the volume, the more pressure is exerted on vessel walls Strength of heart contractions—affect cardiac output; stronger heartbeat increases pressure; weaker beat decreases it Heart rate—increased rate increases pressure; decreased rate decreases pressure

Blood Pressure Factors that influence blood pressure Blood viscosity (thickness)—less than normal viscosity decreases pressure; more than normal viscosity increases pressure Resistance to blood flow (peripheral resistance)—affected by many factors, including the vasomotor mechanism (vessel muscle contraction/relaxation) Why is the diameter of the arterioles important? The diameter of the arterioles plays an important role in determining how much blood drains out of arteries into arterioles. What is the relationship between the strength of a heartbeat and blood pressure? The stronger the contraction, the more blood that is pumped, the higher the pressure. What happens if blood is less viscous than normal? Blood pressure decreases What device is used to take blood pressure readings? Sphygmomanometer

Vasomotor mechanism. Changes in smooth muscle tension in the wall of an arteriole influence the resistance of the vessel to blood flow. Relaxation of muscle results in decreased resistance; contraction of muscle results in increased resistance.

Blood Pressure Fluctuations in blood pressure Blood pressure varies within normal range from time to time Central venous pressure—influences pressure in large peripheral veins Venous return of blood to the heart depends on five mechanisms A strongly beating heart An adequate arterial blood pressure Valves in the veins Pumping action of skeletal muscles as they contract Changing pressures in the chest cavity caused by breathing

Pulse Definition—alternate expansion and recoil of the blood vessel wall Nine major pulse points named after arteries over which they are felt What information can the pulse provide about the heartbeat? The pulse provides information about the rate, strength, and rhythmicity of the heartbeat. How many major pulse points are there? here are nine (9) major pulse points. See Figure 14-13. What are pulse points named after?The artery that lies near the surface of the body and over a bone or other firm base.

Circulatory Shock Circulatory shock—failure of the circulatory system to deliver oxygen to the tissues adequately, resulting in cell impairment When the cause is known, shock can be classified by this scheme: Cardiogenic shock—caused by heart failure

Pulse points. Each pulse point is named after the artery with which it is associated.

Hypertension (HTN) Occurs when blood pressure exceeds 140/90 mmHg (Figure 14-13) 90% of HTN cases are primary-essential (idiopathic); secondary HTN can be caused by kidney disease or other causes Many risk factors for HTN, including genetics, age, stress, obesity, and more Untreated HTN may contribute to heart disease, kidney failure, and stroke

Classification of hypertension Classification of hypertension. This chart is adapted from the National High Blood Pressure Education Program. After age 50, the systolic pressure becomes more significant than diastolic pressure in assessing high blood pressure and associated risk of cardiovascular and renal disease.

Circulatory Shock Circulatory shock—failure of the circulatory system to deliver oxygen to the tissues adequately, resulting in cell impairment When the cause is known, shock can be classified as follows: Cardiogenic shock—caused by heart failure

Circulatory Shock Hypovolemic shock—drop in blood volume that causes blood pressure (and blood flow) to drop Neurogenic shock—caused by nerve condition that relaxes (dilates) blood vessels and thus reduces blood flow Anaphylactic shock—caused by a severe allergic reaction characterized by blood vessel dilation Septic shock—results from complications of septicemia (toxins in blood from infection) Give a cause for each of the types of circulatory shock listed. A severe allergic reaction can result in anaphylactic shock. What are some of the symptoms that occur when someone has anaphylactic shock? Symptoms appear rapidly: difficulty in breathing, rapid or weak pulse, nausea and vomiting, rash, wheezing, anxiety, confusion, dizziness, and abdominal cramping.