Chapter 10 The Blood Vessels and Blood Pressure

Blood Flow Blood is transported to all parts of the body through a system of vessels. Reconditioning organs (digestive organs, kidneys, skin) receive a disproportionally high blood flow. This allows them replenish nutrient supplies and remove metabolic wastes to help maintain homeostasis. Blood flow to other organs can be adjusted according to metabolic needs. Brain function requires constant blood flow Maintaining adequate flow to the brain is a priority of the circulatory system.

100% Lungs Right side of heart Left side of heart 21% 6% Liver 20% Digestive system (Hepatic portal system) 6% Liver 20% Kidneys 9% Skin 13% Brain Figure 10.1: Distribution of cardiac output at rest. The lungs receive all the blood pumped out by the right side of the heart, whereas the systemic organs each receive some of the blood pumped out by the left side of the heart. The percentage of pumped blood received by the various organs under resting conditions is indicated. This distribution of cardiac output can be adjusted as needed. 3% Heart muscle 15% Skeletal muscle 5% Bone 8% Other Fig. 10-1, p. 344

Flow Rate directly proportional to the pressure gradient Flow rate through a vessel (volume of blood passing through per unit of time): directly proportional to the pressure gradient inversely proportional to vascular resistance F = ΔP R F = flow rate of blood through a vessel ΔP = pressure gradient R = resistance of blood vessels Pressure gradient is the difference in pressure between the beginning and end of a vessel. Blood flows from an area of higher pressure to an area of lower pressure down a pressure gradient. The greater the pressure gradient the greater the flow rate through that vessel

Resistance Resistance is the hindrance to blood flow through a vessel. Factors affecting resistance include: blood viscosity-friction between molecules of a fluid during flow vessel length- the longer the vessel the greater the resistance to flow vessel radius-the smaller the radius the greater the resistance Major determinant of resistance to flow is vessel radius. Small change in radius produces significant change in blood flow. R is proportional to 1/r4 Doubling the radius reduces resistance to 1/16th original value and increases flow 16-fold.

(a) Comparison of contact of a given volume of blood with the 10 ml 10 ml FIGURE 10.3 Relationship of resistance and fl ow to the vessel radius. (a) The smaller-radius vessel off ers more resistance to blood flow, because the blood “rubs” against a larger surface area. (b) Doubling the radius decreases the resistance to 1/16th and increases the flow 16 times, because the resistance is inversely proportional to the fourth power of the radius. (a) Comparison of contact of a given volume of blood with the surface area of a small-radius vessel and a large-radius vessel Fig. 10-3, p. 346

Radius in vessel 2 = 2 times that of vessel 1 Same pressure gradient Vessel 2 Radius in vessel 2 = 2 times that of vessel 1 Resistance in vessel 2 = 1/16 that of vessel 1 FIGURE 10.3 Relationship of resistance and fl ow to the vessel radius. (a) The smaller-radius vessel off ers more resistance to blood flow, because the blood “rubs” against a larger surface area. (b) Doubling the radius decreases the resistance to 1/16th and increases the flow 16 times, because the resistance is inversely proportional to the fourth power of the radius. Flow in vessel 2 = 16 times that of vessel 1 Relationship of resistance and flow to the vessel radius. The smaller-radius vessel offers more resistance to blood flow, because the blood “rubs” against a larger surface area. Doubling the radius decreases the resistance to 1/16th and increases the flow 16 times, because the resistance is inversely proportional to the fourth power of the radius. Fig. 10-3, p. 346

Vascular Tree Blood flows in a closed loop between the heart and the organs. Arteries transport blood from the heart throughout the body. Arterioles control the amount of blood that flows through each organ. Capillaries are vessels where materials are exchanged between blood and surrounding tissue cells. Veins return blood from the tissue back to the heart.

PULMONARY CIRCULATION SYSTEMIC CIRCULATION Airway Lungs Air sac Pulmonary capillaries Blood flows in a closed loop between the heart and the organs. Arteries transport blood from the heart throughout the body. Arterioles control the amount of blood that flows through each organ. Capillaries are vessels where materials are exchanged between blood and surrounding tissue cells. Veins return blood from the tissue back to the heart. Arterioles Venules Pulmonary artery PULMONARY CIRCULATION Pulmonary veins Aorta (major systemic artery) Systemic veins Figure 10.4: Basic organization of the cardiovascular system. Arteries progressively branch as they carry blood from the heart to the organs. A separate small arterial branch delivers blood to each of the various organs. As a small artery enters the organ it is supplying, it branches into arterioles, which further branch into an extensive network of capillaries. The capillaries rejoin to form venules, which further unite to form small veins that leave the organ. The small veins progressively merge as they carry blood back to the heart. SYSTEMIC CIRCULATION Systemic capillaries Tissues Venules Arterioles For simplicity, only two capillary beds within two organs are illustrated. Smaller arteries branching off to supply various tissues Fig. 10-4, p. 349

Red and white blood cells within an arteriole. Arteries branch into arterioles within organs and deliver blood to the capillaries. SEM X6130. Artery and Vein. A distributing artery (right) and a medium-sized vein (left) surrounded by connective tissue. Arteries and veins are part of the extensive network of vessels that make up the vascular system. SEM X305. Credit: © Dr. Richard Kessel & Dr. Randy Kardon/Tissues & Organs/Visuals Unlimited 900019

Connective tisssue coat (mostly collagen fibers) Endothelium Venous valve Elastin fibers Endothelium Smooth muscle Smooth muscle; elastin fibers Elastin fibers Connective tisssue coat (mostly collagen fibers) Connective tisssue coat (mostly collagen fibers) Capillary Large artery Arteriole Large vein Relative Thickness of Layers in Wall Endothelium Elastin fibers Smooth muscle Collagen fibers Table 10-1, p. 348

Arteries - large radius, rapid transit, low resistance, elastic pressure vessels Arterioles To capillaries From veins (a) Heart contracting and emptying Arteries Figure 10-6 Arteries as a pressure reservoir. Because of their elasticity, arteries act as a pressure reservoir. (a) The elastic arteries distend during cardiac systole as more blood is ejected into them than drains off into the narrow, high-resistance arterioles downstream. (b) The elastic recoil of arteries during cardiac diastole continues driving the blood forward when the heart is not pumping. Arterioles To capillaries From veins Force exerted by blood against a vessel wall. Pressure depends on: the volume of blood contained within the vessel compliance of the vessel walls; how easily the vessel distends when it fills with blood (b) Heart relaxing and filling

Blood Pressure Systolic pressure: Diastolic pressure: peak pressure exerted by ejected blood against vessel walls during cardiac systole averages 120 mm Hg Diastolic pressure: minimum pressure in arteries when blood is draining off into vessels downstream averages 80 mm Hg Pulse Pressure: the difference between systolic and diastolic pressures Mean Arterial Pressure (MAP): the average driving pressure throughout the cardiac cycle= diastolic pressure + 1/3 (pulse pressure) Mean arterial pressure is monitored and regulated by blood pressure reflexes

Arterial pressure (mm Hg) Notch caused by closure of aortic valve 120 Systolic pressure Pulse pressure Arterial pressure (mm Hg) Mean pressure 93 Figure 10-7 Arterial blood pressure. The systolic pressure is the peak pressure exerted in the arteries when blood is pumped into them during ventricular systole. The diastolic pressure is the lowest pressure exerted in the arteries when blood is draining off into the vessels downstream during ventricular diastole. The pulse pressure is the difference between systolic and diastolic pressure. The mean pressure is the average pressure throughout the cardiac cycle. 80 Diastolic pressure Time (msec) Fig. 10-7, p. 350

Blood Pressure Can be measured indirectly using sphygmomanometer and stethoscope Sounds heard when determining blood pressure Sounds are distinct from heart sounds associated with valve closure

Systolic pressure 120 110 100 90 80 70 60 50 40 30 20 10 Mean pressure Mean pressure Diastolic pressure Pressure (mm Hg) Figure 10.9: Pressures throughout the systemic circulation. Left ventricular pressure swings between a low pressure of 0 mm Hg during diastole to a high pressure of 120 mm Hg during systole. Arterial blood pressure, which fluctuates between a peak systolic pressure of 120 mm Hg and a low diastolic pressure of 80 mm Hg each cardiac cycle, is of the same magnitude throughout the large arteries. Because of the arterioles’ high resistance, the pressure drops precipitously and the systolic-to-diastolic swings in pressure are converted to a nonpulsatile pressure when blood flows through the arterioles. The pressure continues to decline but at a slower rate as blood flows through the capillaries and venous system. Left ventricle Large arteries Arterioles Capillaries Venules and veins Fig. 10-9, p. 352

Arterioles Major resistance vessels. High resistance produces a large drop in mean pressure between the arteries and capillaries. This decline enhances blood flow by contributing to the pressure gradient between the heart and organs. Have a thick layer of circular smooth muscle. The radius of arterioles can be adjusted to accomplish two functions: to variably distribute cardiac output among the organs depending on body needs to help regulate arterial blood pressure Video

Vascular Tone Arteriolar vasodilation decreases resistance and increases blood flow through the vessel. Arteriole vasoconstriction increases resistance and decreases flow.

Vascular tone is the state of partial constriction that establishes a baseline of arteriolar resistance. SEM of arteriole Shows smooth muscle Cross section of arteriole Figure 10.10: Arteriolar vasoconstriction and vasodilation. (b) Normal arteriolar tone Fig. 10-10, p. 353

Arteriolar tone is controlled by local (intrinsic) controls and extrinsic controls. Video Caused by: Myogenic activity O2 CO2 and other metabolites Nitric oxide Sympathetic stimulation Histamine release Heat Caused by: Myogenic activity Oxygen (O2) Carbon dioxide (CO2) And other metabolites Endothelin Sympathetic stimulation Vasopression; angiotensin II Cold Figure 10.10: Arteriolar vasoconstriction and vasodilation. (d) Vasodilation (decreased contraction of circular smooth muscle in the arteriolar wall, which leads to decreased resistance and increased flow through the vessel) (c) Vasoconstriction (increased contraction of circular smooth muscle in the arteriolar wall, which leads to increased resistance and decreased flow through the vessel) Fig. 10-10, p. 353

Intrinsic Control Extrinsic Control Extrinsic sympathetic control of arteriolar radius is important in regulating blood pressure. Increased sympathetic activity produces generalized arteriolar vasoconstriction. Decreased sympathetic activity leads to generalized arteriolar vasodilation. Changes in arteriolar resistance bring about changes in mean arterial pressure. Local chemical changes associated with changes in the level of metabolic activity affect arteriole resistance. Increased blood flow in response to enhanced tissue activity is called active hyperemia.

Total peripheral resistance Arteriolar radius Blood viscosity Number of red blood cells Local (intrinsic) control (local changes acting on arteriolar smooth muscle in the vicinity) Extrinsic control (important in regulation of blood pressure) Heat, cold application (therapeutic use) Response to shear stress (compensates for changes in longitudinal force of blood flow) Vasopressin (hormone important in fluid balance; exerts vasoconstrictor effect) Myogenic responses to stretch (important in autoregulation) Angiotensin II (hormone important in fluid balance; exerts vasoconstrictor effect) Figure 10.14: Factors affecting total peripheral resistance. The primary determinant of total peripheral resistance is the adjustable arteriolar radius. Two major categories of factors influence arteriolar radius: (1) local (intrinsic) control, which is primarily important in matching blood flow through a tissue with the tissue’s metabolic needs and is mediated by local factors acting on the arteriolar smooth muscle; and (2) extrinsic control, which is important in regulating blood pressure and is mediated primarily by sympathetic influence on arteriolar smooth muscle. Epinephrine and nor-epinephrine (hormones that generally reinforce sympathetic nervous system) Histamine release (involved with injuries and allergic responses) Local metabolic changes In O2 and other Metabolites (important in matching blood flow with metabolic needs) Sympathetic activity (exerts generalized vasoconstrictor effect) Major factors affecting arteriolar radius Fig. 10-14, p. 360

Capillaries Capillaries are thin-walled, small-radius, extensively branched vessels. Surface area for exchange is maximized. Diffusion distance is minimized. Large cross-sectional area results in slow blood velocity to maximize time for exchange.

Endothelial cell nucleus Capillary lumen Figure 10-15 Capillary anatomy. (a) Electron micrograph showing that the capillary wall consists of a single layer of endothelial cells. The nucleus of one of these cells is shown. (b) The capillaries are so narrow that red blood cells must pass through the capillary bed in single file. (a) Cross section of a capillary Fig. 10-15, p. 361

Red blood cell Capillary Figure 10-15 Capillary anatomy. (a) Electron micrograph showing that the capillary wall consists of a single layer of endothelial cells. The nucleus of one of these cells is shown. (b) The capillaries are so narrow that red blood cells must pass through the capillary bed in single file. Fig. 10-15, p. 361

Total cross-sectional area (cm2) 5 Blood flow rate (liters/min) 3000 Total cross-sectional area (cm2) 4.0 Anatomical distribution Figure 10.16: Comparison of blood flow rate and velocity of flow in relation to total cross-sectional area. The blood flow rate (red curve) is identical through all levels of the circulatory system and is equal to the cardiac output (5 liters/min at rest). The velocity of flow (purple curve) varies throughout the vascular tree and is inversely proportional to the total cross-sectional area (green curve) of all the vessels at a given level. Note that the velocity of flow is slowest in the capillaries, which have the largest total cross-sectional area. 200 Velocity of flow (mm/sec) 0.3 Aorta Arteries Veins Venae cavae Arterioles Venules Capillaries Fig. 10-16, p. 362

precapillary sphincters Arteriolar vasodilation Tissue metabolic activity O2, CO2 and other metabolites Relaxation of precapillary sphincters Arteriolar vasodilation Capillary blood flow Number of open capillaries Delivery of O2, more rapid removal of CO2 and other metabolites Figure 10-20 Complementary action of precapillary sphincters and arterioles in adjusting blood flow through a tissue in response to changing metabolic needs. Concentration gradient for these materials between blood and tissue cells Capillary surface area available for exchange Diffusion distance from cell to open capillary Exchange between blood and tissue to support increased metabolic activity Fig. 10-20, p. 365

Capillary Exchange Exchanges between blood and tissues across the capillary are accomplished in two ways: passive diffusion down concentration gradients is the primary mechanism for exchanging solutes bulk flow determines the distribution of the ECF volume between the vascular and the interstitial fluid compartments

Capillary Exchange Individual solutes are exchanged primarily by diffusion down concentration gradients. Lipid-soluble substances pass directly through endothelial cells lining a capillary. Water-soluble substances pass through water-filled pores between the endothelial cells. Plasma proteins generally do not escape.

Endothelial cell Pores (a) Typical capillary Figure 10.18: Exchanges across a continuous capillary wall, the most common type of capillary. (a) Slitlike gaps between adjacent endothelial cells form pores within the capillary wall. (b) As depicted in this cross section of a capillary wall, small water-soluble substances are exchanged between the plasma and the interstitial fluid by passing through the water-filled pores, whereas lipid-soluble substances are exchanged across the capillary wall by passing through the endothelial cells. Proteins to be moved across are exchanged by vesicular transport. Plasma proteins generally cannot escape from the plasma across the capillary wall. Pores (a) Typical capillary Fig. 10-18, p. 363

(b) Transport across a typical capillary wall Interstitial fluid Endothelial cell Water-filled pore Plasma proteins generally cannot cross the capillary wall Plasma Plasma proteins Plasma membrane Lipid-soluble substances pass through the endothelial cells typical Cytoplasm O2, CO2 Exchangeable proteins Na+, K+, glucose, amino acids Figure 10.18: Exchanges across a continuous capillary wall, the most common type of capillary. (a) Slitlike gaps between adjacent endothelial cells form pores within the capillary wall. (b) As depicted in this cross section of a capillary wall, small water-soluble substances are exchanged between the plasma and the interstitial fluid by passing through the water-filled pores, whereas lipid-soluble substances are exchanged across the capillary wall by passing through the endothelial cells. Proteins to be moved across are exchanged by vesicular transport. Plasma proteins generally cannot escape from the plasma across the capillary wall. Exchangeable proteins are moved across by vesicular transport Small water-soluble substances pass through the pores (b) Transport across a typical capillary wall Fig. 10-18, p. 363

Glucose O2 CO2 Plasma Interstitial fluid Facilitated diffusion by carrier Figure 10.21: Independent exchange of individual solutes down their own concentration gradients across the capillary wall. Glucose + O2 CO2 + H2O + ATP Tissue cell Fig. 10-21, p. 366

Bulk Flow Bulk flow occurs when protein-free plasma filters out of the capillary, mixes with the interstitial fluid and then is reabsorbed. Important in regulating the distribution of ECF between plasma and interstitial fluid to help maintain arterial blood pressure. Depends on two processes: ultrafiltration reabsorption

Bulk Flow Ultrafiltration occurs when pressure inside the capillary exceeds pressure outside and fluid is pushed out through the pores. Reabsorption occurs when inward-driving pressures exceed outward pressures and net movement of fluid back into the capillaries occurs.

Bulk Flow Forces Bulk flow occurs because of differences in the: hydrostatic pressure PC (capillary) PIF (interstitial fluid) colloid osmotic pressures πP (capillary) π I (interstitial fluid) between plasma and interstitial fluid. Capillary hydrostatic pressure pushes fluid out of the capillary bed. Plasma colloid osmotic pressure draws fluid back into the capillary bed.

All values are given in mm Hg. Blood capillary FORCES AT ARTERIOLAR END OF CAPILLARY FORCES AT VENULAR END OF CAPILLARY Initial lymphatic vessel • Outward pressure • Outward pressure Interstitial fluid 11 mm Hg (ultrafiltration) 9 mm Hg (reabsorption) • Inward pressure • Inward pressure Net outward pressure of 11 mm Hg = Ultrafiltration pressure Net inward pressure of 9 mm Hg = Reabsorption pressure Figure 10.22: Bulk flow across the capillary wall. Ultrafiltration occurs at the arteriolar end and reabsorption occurs at the venule end of the capillary as a result of imbalances in the physical forces acting across the capillary wall. From arteriole To venule All values are given in mm Hg. Blood capillary Fig. 10-22, p. 367

Bulk Flow Forces Slightly more fluid is filtered out of the capillaries into the interstitial fluid than is reabsorbed from the interstitial fluid back into the plasma. Excess fluid is picked up by the lymphatic system and returned to general circulation.

(a) Relationship between initial lymphatics and blood capillaries To venous system Arteriole Tissue cells Interstitial fluid Venule Figure 10.24: Initial lymphatics. (a) Blind-ended initial lymphatics pick up excess fluid filtered by blood capillaries and return it to the venous system in the chest. (b) Note that the overlapping edges of the endothelial cells create valvelike openings in the vessel wall. Blood capillary Initial lymphatic (a) Relationship between initial lymphatics and blood capillaries Fig. 10-24 p. 368

Fluid pressure on the outside of the vessel pushes the endothelial cell's free edge inward, permitting entrance of interstitial fluid (now lymph). Interstitial fluid Lymph Overlapping endothelial cells Figure 10.24: Initial lymphatics. (a) Blind-ended initial lymphatics pick up excess fluid filtered by blood capillaries and return it to the venous system in the chest. (b) Note that the overlapping edges of the endothelial cells create valvelike openings in the vessel wall. Fluid pressure on the inside of the vessel forces the overlapping edges together so that lymph cannot escape. (b) Arrangement of endothelial cells in an initial lymphatic Fig. 10-24 p. 368

Lymphatic System The extra fluid, any leaked proteins, and bacteria in the tissue are picked up by the lymphatic system. Bacteria are destroyed as lymph passes through lymph nodes on the way to being returned to the venous system. Transports absorbed digestive fats.

(a) Relationship of lymphatic system to circulatory system Systemic circulation Lymph node Pulmonary circulation Initial lymphatics Lymph vessel Valve Blood capillaries Arteries Veins Heart Figure 10.25: Lymphatic system. (a) Lymph empties into the venous system near its entrance to the right atrium. (b) Lymph flow averages 3 liters per day, whereas blood flow averages 7200 liters per day. Lymph node Blood capillaries Initial lymphatics (a) Relationship of lymphatic system to circulatory system Fig. 10-25, p. 369

Veins Venules communicate chemically with nearby arterioles to control capillary flow. Veins serve as a blood reservoir containing 60% of blood volume. Veins return blood back to the heart.

Pulmonary Systemic vessels arteries 9% 13% Systemic arterioles 2% Heart 7% Systemic capillaries 5% Systemic veins 64% Figure 10.27: Percentage of total blood volume in different parts of the circulatory system. Fig. 10-27, p. 371

(b) Action of venous valves, permitting flow of blood toward Open venous valve permits flow of blood toward heart Vein Contracted skeletal muscle Closed venous valve prevents backflow of blood Figure 10-32 Function of venous valves. (b) Action of venous valves, permitting flow of blood toward heart and preventing backflow of blood Fig. 10-32, p. 374