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Chapter

4 Vasculature

♦ Overview -The term vasculature is a collective term that refers to all of the blood vessels of the body (Figure 4.1, Derrickson). -The blood vessels that comprise the vasculature transport blood from the heart to the cells of the body and then back to the heart: heart arteries arterioles capillaries venules veins -Altogether, blood travels through about 60,000 miles of blood vessels!!!!!!!!!!

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♦Organization of Blood Vessels in the Body -The structural organization of a blood vessel can vary, depending on the type of blood vessel that is being considered (Figure 4.2, Derrickson). -All blood vessels contain at least an inner layer of endothelium surrounded by a basement membrane. -Except for capillaries, blood vessels also contain layers of connective tissue and smooth muscle that surround the basement membrane. -Here are more details about the different layers of a blood vessel: ■ endothelium -The endothelium is a layer of epithelial cells that is closest to the lumen (interior space) of the blood vessel.

-Because of its location, the endothelium is in direct contact with the blood that passes through the blood vessel.

■ basement membrane -The basement membrane is a thin layer of extracellular material that surrounds and provides support to the endothelium. ■ smooth muscle -A layer of smooth muscle may be present toward the middle of the blood vessel wall. -The smooth muscle can contract or relax, thereby altering the diameter of the blood vessel. ■ connective tissue -Two types of connective tissue may be present in the wall of a blood vessel: elastic connective tissue and fibrous connective tissue. -Elastic connective tissue contains elastic fibers that allow the blood vessel to stretch. -Fibrous connective tissue contains collagen fibers that resist stretch, thereby preventing the blood vessel from overstretching. -Fibrous connective tissue usually forms the outer layer of a blood vessel, whereas elastic connective tissue is typically found deeper within the wall of a blood vessel. -Blood vessels vary with respect to the amounts of elastic and fibrous connective tissues that they contain. -Arteries and veins both have all of the above layers (fibrous connective tissue, elastic connective tissue, smooth muscle, basement membrane, and endothelium). As arteries give rise to arterioles and arterioles branch into capillaries, the outer and middle layers gradually disappear until all that is left is a basement membrane and endothelium that comprise the structure of a given capillary. As capillaries turn into larger venules and then venules give rise to even larger veins, the outer and middle layers gradually reappear.

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– Of all of the blood vessels, capillaries have the thinnest wall (consisting of only an endothelium and basement membrane) and they are porous. -The thin wall and pores allow capillaries to serve as the blood vessels that undergo gas, nutrient, and waste exchange with cells in the surrounding tissues. -Arteries, arterioles, venules, and veins are too thick (because they have additional layers) and lack pores; as a result, gases, nutrients, and wastes cannot move across the walls of these blood vessels. ♦ Functions of the Different Types of Blood Vessels in the Body -Each type of blood vessel has a particular function: ● Arteries carry blood away from the heart to arterioles. ● Arterioles deliver blood to capillaries. ● Capillaries function as the exchange vessels: They permit the exchange of gases, nutrients, and wastes between blood and the cells of the body. ● Venules drain capillary blood and deliver the blood to veins. ● Veins carry blood back to the heart.

♦ Exchange Across Capillary Walls -Most cells in the body are located near at least one capillary; this is due to the fact that capillaries are the most numerous type of blood vessel: about 10 billion of them exist in the body. -A capillary is a microscopic blood vessel that consists of 2 layers: an endothelium and a basement membrane (Figures 4.2 and 4.3, Derrickson): 1. endothelium -single layer of epithelial cells -The endothelium of a typical capillary contains pores (holes) in the form of intercellular clefts and fenestrations. intercellular clefts -pores that exist between the endothelial cells of a capillary fenestrations -pores that run through individual endothelial cells of a capillary 2. basement membrane – a thin layer of extracellular material that surrounds the endothelium -The basement membrane of a capillary is also porous, but these pores have no specific names.

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-Capillaries are the only blood vessels that permit the exchange of gases, nutrients, and wastes between the blood and the cells of the body, transforming oxygenated blood into deoxygenated blood. -Only relatively small molecules (such as gases, H2O, ions, glucose, amino acids, and hormones) are able to move across capillary walls (Figure 4.3, Derrickson). ● Lipid-soluble (hydrophobic) substances, such as O2, CO2, and steroid hormones, typically diffuse across capillary walls directly through the lipid bilayer of endothelial cell plasma membranes. ● Most small, water-soluble (hydrophilic) substances, which include ions and polar molecules such as glucose and amino acids, pass across capillary walls through pores (intercellular clefts and fenestrations). ● Water can diffuse across capillary walls in all possible ways: by moving through the pores of the capillary wall or by moving through endothelial cell plasma membranes. -Recall that although water is a hydrophilic molecule, it is small and has no charge. -Most blood cells and plasma proteins (albumins, fibrinogen, etc.) are too large to diffuse through the pores of capillary walls and are therefore confined to the blood. – Protein hormones, however, are able to move across the capillary wall via transcytosis (endocytosis followed by exocytosis), allowing protein hormones to leave the bloodstream and then bind to specific receptors on their target cells. -Note that any H2O that diffuses out of the blood into the interstitium (the spaces between and around body cells) is called interstitial fluid (ISF). -Therefore, interstitial fluid is a type of extracellular fluid located between and around body cells, while blood is a type of extracellular fluid located within blood vessels. -The H2O in the interstitial fluid can diffuse into body cells if needed. -If there is too much H2O in the interstitium, the excess interstitial fluid drains into a nearby lymphatic capillary, which is a type of lymphatic vessel. – Once the interstitial fluid enters a lymphatic vessel, it is called lymph. -Lymph, therefore, is extracellular fluid that is located within a lymphatic vessel. -The lymphatic vessels, which are part of lymphatic system, eventually transport the lymph to the bloodstream, but only after the lymph is filtered at lymph nodes that are found periodically between larger lymphatic vessels.

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♦ Venous Valves, Skeletal Muscle Pump, and Venous Return -Venous return refers to the process by which veins return blood back to the heart. -Veins have a much lower blood pressure than arteries, and when a person is standing, veins have to move blood against gravity in order to get blood back to the heart. -These two issues would reduce or completely stop venous return if it were not for the help of venous valves and the skeletal muscle pump. 1. venous valves -Many veins of the limbs contain valves. -A venous valve consists of folds of endothelium of the vein that function to prevent backflow of blood (Figure 21.9, Tortora). 2. skeletal muscle pump -The skeletal muscle pump refers to contractions of skeletal muscles surrounding veins that help push blood up the veins (Figure 21.9, Tortora). -Any time that you have to stand for a long period of time (members of a wedding party, a soldier standing guard, etc.) it is advisable to periodically contract the muscles in your legs in order to make sure that blood goes back to the heart; otherwise, blood may pool in the veins and you may pass out since the heart is not able to pump enough blood to the brain. ♦Alteration of Blood Vessel Diameter -Most blood vessels (arteries, arterioles, venules, and veins) can change their diameters by contracting or relaxing the smooth muscle in their walls (Figure 4.4, Derrickson). -When a blood vessel undergoes vasoconstriction, the smooth muscle within its wall contracts. -Consequently, the blood vessel lumen becomes smaller (Figure 4.4, Derrickson). -Vasoconstriction causes a decrease in blood flow and an increase in blood pressure. -The decrease in blood flow that occurs in response to vasoconstriction is due to the fact that there is less room for blood to pass through when the lumen of the blood vessel is smaller. -The increase in blood pressure that occurs in response to vasoconstriction is due to the fact that there is more interaction between blood and the blood vessel wall when the lumen of the blood vessel is smaller. -When a blood vessel undergoes vasodilation, the smooth muscle within its wall relaxes. -Consequently, the blood vessel lumen becomes wider and larger (Figure 4.4, Derrickson). -Vasodilation causes an increase in blood flow and a decrease in blood pressure. -The increase in blood flow that occurs in response to vasodilation is due to the fact that there is more room for blood to pass through when the lumen of the blood vessel is larger. -The decrease in blood pressure that occurs in response to vasodilation is due to the fact that there is less interaction between blood and the blood vessel wall when the lumen of the blood vessel is smaller. -Capillaries are the only blood vessels that do not vasoconstrict or vasodilate; this is due to the fact that capillaries lack smooth muscle in their walls.

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♦ Blood Distribution Cardiovascular Component % Blood Volume heart 7 systemic arteries and arterioles 13 systemic capillaries 7 systemic veins and venules 64 pulmonary blood vessels 9 -As is evident from the above table, most of the blood volume at any given time is found in the systemic veins and venules; consequently, systemic veins and venules are called the blood reservoirs of the body. -In cases of a blood shortage, blood can be can be mobilized from the veins and venules and sent to the other parts of the body that need it. ♦ Vascular Compliance -Compliance -the ability of an object to stretch -Examples ● rubber band -has a high compliance because it can be easily stretched ● a shower rod -has a low compliance because it is not easily stretched -Blood vessels also exhibit varying degrees of compliance (Figure 4.5, Derrickson). -Veins have a high compliance because they contain thin walls that are easily stretched; consequently, veins are called the capacitance elements. -Because veins are so highly compliant, an increase in the blood volume of veins simply stretches their walls, allowing blood to pool in the veins; this is why veins contain the majority of the blood volume of the body. -In addition, the high compliance of veins also means that an increase in venous volume is converted to stretch rather than a significant increase in venous pressure. -So, even though veins contain the majority of blood in the body, venous pressure is very low (typically near 0 mm Hg in the large great veins like the inferior vena cava and superior vena cava). -Arteries have a low compliance compared to veins because arteries contain thick walls that are not as easy to stretch. -Hence, an increase in the blood volume of arteries causes an increase in arterial pressure because the arteries cannot stretch as much as the veins. -Consequently, arterial pressures are typically high, varying between 110 mm Hg and 70 mm Hg.

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♦Blood Pressure blood pressure -the pressure that blood exerts on the walls of a blood vessel (Figure 4.6, Derrickson) -Blood pressure is usually measured in the brachial artery using a device called the sphygmomanometer (Figure 4.7, Derrickson). -Blood pressure in any major artery of the body varies between the systolic pressure and the diastolic pressure (Figure 21.8, Tortora) systolic pressure -the maximum pressure exerted by blood on arterial walls during ventricular systole -normally equals about 110 mm Hg diastolic pressure -the minimum pressure exerted by blood on arterial walls during ventricular diastole -normally equals about 70 mm Hg -Blood pressure is generally written as follows: systolic pressure blood pressure = ______________ diastolic pressure -Under normal circumstances, blood pressure is 110/70 mm Hg. -Factors That Affect Blood Pressure -Figure 4.8 (Derrickson) illustrates that blood pressure is affected by changes in stroke volume, heart rate, cardiac output, and blood vessel diameter (vasoconstriction or vasodilation). • An increase in stroke volume and/or heart rate causes an increase in cardiac output. ⇒ The increase in cardiac output means that more blood is ejected from the heart, which causes an increase in blood pressure due to the excess blood pushing on the walls of the blood vessels of the body. • Vasoconstriction also causes an increase in blood pressure since blood pushes more on blood vessel walls due to the smaller volume of the blood vessels. • A decrease in stroke volume and/or heart rate causes a decrease in cardiac output. ⇒ The decrease in cardiac output means that less blood is ejected from the heart, which causes a decrease in blood pressure due to less blood pushing on blood vessel walls. • Vasodilation also causes a decrease in blood pressure since blood does not push as much on blood vessel walls due to the larger volume of the blood vessels.

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-Neural Regulation of Blood Pressure -Blood pressure is regulated via a negative feedback system that involves the cardiovascular center. cardiovascular center -located in the medulla, which is part of the brainstem (Figure 4.9, Derrickson). -Output • Part of the cardiovascular center gives rise to the parasympathetic (Vagus) nerves that innervate the heart. –Recall that stimulation of these nerves causes a decrease in heart rate. • The cardiovascular center also makes connections with the cardiac accelerator nerves, which are sympathetic nerves that innervate the heart. -Recall that stimulation of these nerves causes an increase in heart rate and an increase in ventricular contraction (stroke volume). • In addition, the cardiovascular center has connections with the vasomotor nerves, which are sympathetic nerves that innervate the smooth muscle in the walls of the blood vessels of the body (except the capillaries). -Stimulation of these nerves causes vasoconstriction of most blood vessels in the circulation. -Input -The cardiovascular center receives input via baroreceptors. baroreceptors -sensory receptors that respond to changes in blood pressure -2 major types: 1. aortic arch baroreceptors -located in the wall of the aortic arch -connect with the cardiovascular center via the Vagus (X) nerves 2. carotid sinus baroreceptors -located in the walls of the carotid sinuses -The carotid sinuses are dilated regions of the internal carotid arteries just above the area where these arteries branch off of the common carotid arteries -connect with the cardiovascular center via the glossopharyngeal (IX) nerves

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-Mechanisms -Through baroreceptor reflexes, the cardiovascular center regulates blood pressure in the following ways: (i) baroreceptor reflex in response to a decrease in blood pressure (Figure 4.10, Derrickson). A stimulus causes a decrease in blood pressure. The decrease in blood pressure causes the baroreceptors to stretch less than normal. Consequently, the baroreceptors send fewer action potentials to the cardiovascular center. The cardiovascular center responds by activating the cardiac accelerator nerves that supply the heart, which causes an increase in stroke volume and heart rate, and therefore an increase in cardiac output. In addition, the cardiovascular center activates the vasomotor nerves that supply most of the blood vessels of the body, which causes vasoconstriction. The increase in cardiac output and the vasoconstriction cause an increase in blood pressure back to normal. (ii) baroreceptor reflex in response to an increase in blood pressure (Figure 4.11, Derrickson). A stimulus causes an increase in blood pressure. The increase in blood pressure causes the baroreceptors to stretch more than normal. Consequently, the baroreceptors send a large number of action potentials to the cardiovascular center. The cardiovascular center responds by activating the parasympathetic (Vagus) nerves that supply the heart, which causes the heart rate to decrease. A decrease in heart rate causes a decrease in cardiac output. In addition, the lack of sympathetic stimulation by the cardiovascular center results in vasodilation of blood vessels. The decrease in cardiac output and the vasodilation result in a decrease in blood pressure back to normal.

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♦Clinical Applications and Disorders -Look up the following clinical applications and disorders in Tortora: 1. varicose veins and spider veins p. 779 2. edema p. 783 3. syncope p. 786 4. carotid sinus massage and carotid sinus syncope p. 790 5. checking circulation p. 792-793 6. shock p. 793-796 7. hypertension p. 839, 840, and 842 8. aneurysm p. 842 9. carotid endarterectomy p. 842 10. deep vein thrombosis p. 842 11. hypotension p. 842 12. normotensive p. 842 13. occlusion p. 842 14. orthostatic hypotension p. 842

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15. phlebitis p. 842 16. thrombectomy p. 842

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Figure 4.1 Overview of the Vasculature

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Figure 4.2 Organization of Blood Vessels

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Figure 4.3 Capillary Exchange

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Figure 4.4 Alteration of Blood Vessel Diameter

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Figure 4.5 Vascular Compliance

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Figure 4.6 Blood Pressure

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Figure 4.7 Measuring Blood Pressure

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Figure 4.8 Factors That Affect Blood Pressure

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Figure 4.9 Neural Regulation of Blood Pressure

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Figure 4.10 Baroreceptor Reflex in Response to a Decrease in Blood

Pressure

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Figure 4.11 Baroreceptor Reflex in Response to an Increase in Blood

Pressure

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Chapter

5 Lymphatic System ♦Overview -The lymphatic system, also known as the lymphoid system, consists of the following components (Figure 5.1, Derrickson): (i) lymphatic vessels, which contain a fluid called lymph, (ii) lymphatic organs, which include the lymph nodes, tonsils, spleen, and the thymus gland; and (iii) lymphocytes, which include B cells and T cells. ♦Functions -The lymphatic system has 2 major functions (Figure 5.2, Derrickson): 1. It drains the tissues of excess interstitial fluid. -Recall that some H2O moves from the blood into the interstitium as cells undergo gas, nutrient, and waste exchange with the capillaries. -The H2O in the interstitial fluid can diffuse into body cells if needed. -If there is too much H2O in the interstitium, the excess interstitial fluid drains into a nearby lymphatic vessel (usually a lymphatic capillary). -Note that if this excess interstitial fluid were to remain in the interstitium, the tissues would swell (edema), which causes tissue damage. -Once inside of a lymphatic capillary, the interstitial fluid is called lymph. -Thus, lymph is any excess interstitial fluid found within the lymphatic vessels of the body. -The excess interstitial fluid will eventually make it back to the blood because lymphatic vessels ultimately empty into veins. -Any pathogens (such as bacteria and viruses) that happen to be in the interstitium trying to invade body cells will also be swept into the lymphatic capillaries as a component of lymph (just like a swimmer may be swept away from the shore by a tidal wave). -Figures 5.3 and 5.4 (Derrickson) illustrate the components of a bacterial cell and a virus, respectively. 2. It participates in immunity. immunity -resistance to disease -This function is achieved by the leukocytes (white blood cells) of the body. -Because of its role in immunity, parts of the lymphatic system can also function as the immune system.

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♦Lymphatic Vessels -Lymphatic Capillaries -The lymphatic system begins with the lymphatic capillaries, which are the smallest lymphatic vessels (Figures 5.5 and 5.6, Derrickson). -Like a blood capillary, a lymphatic capillary consists of endothelial cells. –However, a lymphatic capillary differs from a blood capillary