The Blood Vessels and Blood Pressure Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Outline Vessel types and flow Arteries Blood pressure Arterioles Capillaries Lymphatic system Veins

Outline Vessel types and flow Vessel anatomy Flow, flow rate, pressure gradients Blood distribution, Poiselle’s law

Credit: © Biodisc/Visuals Unlimited Human Coronary Artery. LM X6. Human artery showing atherosclerosis. LM X3. Human artery cross-section, elastic tissue stain. X80. Credit: © Biodisc/Visuals Unlimited 213940

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

Blood Flow Blood is constantly reconditioned so composition remains relatively constant Reconditioning organs receive more blood than needed for metabolic needs Digestive organs, kidneys, skin Adjust extra blood to achieve homeostasis Blood flow to other organs can be adjusted according to metabolic needs Brain can least tolerate disrupted supply

Distribution of Cardiac Output at Rest

Blood Flow Flow rate through a vessel (volume of blood passing through per unit of time) is directly proportional to the pressure gradient and inversely proportional to vascular resistance F = ΔP R F = flow rate of blood through a vessel ΔP = pressure gradient R = resistance of blood vessels

Blood Flow Pressure gradient is pressure difference between beginning and end of a vessel Blood flows from area of higher pressure to area of lower pressure Resistance is measure of opposition of blood flow through a vessel Depends on three things Blood viscosity, vessel length, vessel radius Major determinant of resistance to flow is vessel’s radius Slight change in radius produces significant change in blood flow R is proportional to 1 r4

Relationship of Resistance and Flow to Vessel Radius

Vascular Tree Closed system of vessels Consists of Arteries Arterioles Carry blood away from heart to tissues Arterioles Smaller branches of arteries Capillaries Smaller branches of arterioles Smallest of vessels across which all exchanges are made with surrounding cells Venules Formed when capillaries rejoin Return blood to heart Veins Formed when venules merge

Basic Organization of the Cardiovascular System

Outline Arteries “pressure reservoir” anatomy

Arteries Specialized to Serve as rapid-transit passageways for blood from heart to organs Due to large radius, arteries offer little resistance to blood flow Act as pressure reservoir to provide driving force for blood when heart is relaxing Arterial connective tissue contains Collagen fibers Provide tensile strength Elastin fibers Provide elasticity to arterial walls

Arteries as a Pressure Reservoir

Outline Blood pressure Systolic/diastolic Measurements Pressure and pulse Mean arterial pressure Pressure abnormalities

Blood Pressure Force exerted by blood against a vessel wall Depends on Volume of blood contained within vessel Compliance of vessel walls Systolic 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

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

Blood Pressure

Pulse Pressure Pressure difference between systolic and diastolic pressure Example If blood pressure is 120/80, pulse pressure is 40 mm Hg (120mm Hg – 80mm Hg) Pulse that can be felt in artery lying close to surface of skin is due to pulse pressure

Mean Arterial Pressure Average pressure driving blood forward into tissues throughout cardiac cycle Formula for approximating mean arterial pressure Mean arterial pressure = diastolic pressure + ⅓ pulse pressure At 120/80, mean arterial pressure = 80 mm Hg + ⅓ (40 mm Hg) = 93 mm Hg

Outline Arterioles Resistance vessels Vasoconstriction and vasodilation Regulation of arteiolar diameter Local factors Endothelial cells, nitric oxide

Arterioles Major resistance vessels Radius supplying individual organs can be adjusted independently to Distribute cardiac output among systemic organs, depending on body’s momentary needs Help regulate arterial blood pressure

Arterioles Mechanisms involved in adjusting arteriolar resistance Vasoconstriction Refers to narrowing of a vessel Vasodilation Refers to enlargement in circumference and radius of vessel Results from relaxation of smooth muscle layer Leads to decreased resistance and increased flow through that vessel

Arteriolar Vasoconstriction and Vasodilation

Arterioles Only blood supply to brain remains constant Changes within other organs alter radius of vessels and adjust blood flow to organ Local chemical influences on arteriolar radius Local metabolic changes Histamine release Local physical influences on arteriolar radius Local application of heat or cold Chemical response to shear stress Myogenic response to stretch

Magnitude and Distribution Of the Cardiac Output at Rest and During Moderate Exercise

Arterioles Specific local chemical factors that produce relaxation of arteriolar smooth muscle Decreased O2 Increased CO2 Increased acid Increased K+ Increased osmolarity Adenosine release Prostaglandin release

Arterioles Local vasoactive mediators Endothelial cells Release chemical mediators that play key role in locally regulating arteriolar caliber Release locally acting chemical messengers in response to chemical changes in their environment Among best studied local vasoactive mediators is nitric oxide (NO)

Functions of Endothelial Cells

Functions of Nitric Oxide (NO)

Arterioles Extrinsic control Accomplished primarily by sympathetic nerve influence Accomplished to lesser extent by hormonal influence over arteriolar smooth muscle

Arterioles Cardiovascular control center In medulla of brain stem Integrating center for blood pressure regulation Other brain regions also influence blood distribution Hypothalamus Controls blood flow to skin to adjust heat loss to environment Hormones that influence arteriolar radius Adrenal medullary hormones Epinephrine and norepinephrine Generally reinforce sympathetic nervous system in most organs Vasopressin and angiotensin II Important in controlling fluid balance

Outline Capillaries Exchange and anatomy Pressures driving diffusion

Capillaries Thin-walled, small-radius, extensively branched Sites of exchange between blood and surrounding tissue cells Maximized surface area and minimized diffusion distance Velocity of blood flow through capillaries is relatively slow Provides adequate exchange time Two types of passive exchanges Diffusion Bulk flow

Factors affecting diffusion Ficks law of diffusion Slow blood velocity Permeability Metarterioles and precapillary sphincters Exchanges Blood to interstitial fluid to cell Passive Active Bulk Flow Ultrafiltration reabsorption

Table 3-1, p. 62

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. Fig. 10-16, p. 355

Capillary blood pressure Plasma colloid osmotic pressure Figure 10.22: Bulk flow across the capillary wall. Schematic representation of ultrafiltration and reabsorption as a result of imbalances in the physical forces acting across the capillary wall. Capillary blood pressure Plasma colloid osmotic pressure Interstitial fluid hydrostatic pressure Interstitial fluid colloid osmotic pressure Fig. 10-22, p. 359

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. Fig. 10-9, p. 345

Myogenic tone Stopcock Local metabolic changes Arteriole Smooth muscles Precapillary sphincter Myogenic tone Stopcock Local metabolic changes Metarteriole Capillary Venule Fig. 10-19, p. 357

Capillaries Narrow, water-filled gaps (pores) lie at junctions between cells Permit passage of water-soluble substances Lipid soluble substances readily pass through endothelial cells by dissolving in lipid bilayer barrier Size of pores varies from organ to organ

Capillaries Under resting conditions many capillaries are not open Capillaries surrounded by precapillary sphincters Contraction of sphincters reduces blood flowing into capillaries in an organ Relaxation of sphincters has opposite effect Metarteriole Runs between an arteriole and a venule

Outline Lymphatic system Drainage of tissue Immune function Edema

Lymphatic System

Lymphatic System Extensive network of one-way vessels Provides accessory route by which fluid can be returned from interstitial to the blood Initial lymphatics Small, blind-ended terminal lymph vessels Permeate almost every tissue of the body Lymph Interstitial fluid that enters a lymphatic vessel Lymph vessels Formed from convergence of initial lymphatics Eventually empty into venous system near where blood enters right atrium One way valves spaced at intervals direct flow of lymph toward venous outlet in chest

Lymphatic System Functions Return of excess filtered fluid Defense against disease Lymph nodes have phagocytes which destroy bacteria filtered from interstitial fluid Transport of absorbed fat Return of filtered protein

To venous system Arteriole Interstitial fluid Tissue cells Venule Blood capillary Initial lymphatic Fig. 10-24, p. 362

Fluid pressure on the outside of the vessel pushes the endothelial cell’s free edge inward, permitting entrance of interstitial fluid (now lymph). Overlapping endothelial cell Interstitial fluid Lymph Fluid pressure on the inside of the vessel forces the overlapping edges together so that lymph cannot escape. Fig. 10-24, p. 362

Edema Swelling of tissues Occurs when too much interstitial fluid accumulates Causes of edema Reduced concentration of plasma proteins Increased permeability of capillary wall Increased venous pressure Blockage of lymph vessels

Outline Veins Capacitance vessels Regulation of pressure and flow Venous return

Veins Venous system transports blood back to heart Capillaries drain into venules Venules converge to form small veins that exit organs Smaller veins merge to form larger vessels Veins Large radius offers little resistance to blood flow Also serve as blood reservoir

Capacitance! Pulmonary vessels Systemic arteries 9% 13% Systemic arterioles 2% Heart 7% Systemic capillaries 5% Systemic veins 64% Capacitance! Fig. 10-27, p. 364

Veins Factors which enhance venous return Driving pressure from cardiac contraction Sympathetically induced venous vasoconstriction Skeletal muscle activity Effect of venous valves Respiratory activity Effect of cardiac suction

Skeletal muscle pump Fig. 10-29, p. 366 Figure 10.29: Skeletal muscle pump enhancing venous return. Fig. 10-29, p. 366

Effects of Gravity Fig. 10-30, p. 367 Figure 10.30: Effect of gravity on venous pressure. In an upright adult, the blood in the vessels extending between the heart and foot is equivalent to a 1.5 m column of blood. The pressure exerted by this column of blood as a result of the effect of gravity is 90 mm Hg. The pressure imparted to the blood by the heart has declined to about 10 mm Hg in the lower-leg veins because of frictional losses in preceding vessels. Together these pressures produce a venous pressure of 100 mm Hg in the ankle and foot veins. Similarly, the capillaries in the region are subjected to these same gravitational effects. Fig. 10-30, p. 367

Standing Walking Heart Thigh 150 cm Calf 34 cm Foot 100 mm Hg Venous pressure in foot 27 mm Hg Foot vein supporting column of blood 1.5 m (150 cm) in height Foot vein supporting column of blood 34 cm in height Fig. 10-31, p. 367

Vein Open venous valve permits flow of blood toward heart Contracted skeletal muscle Closed venous valve prevents backflow of blood Stepped art Fig. 10-32, p. 368

Respiratory activity and venous return 5 mm Hg less than atmospheric pressure Atmospheric pressure Atmospheric pressure 5 mm Hg less than atmospheric pressure Cardiac suction also Fig. 10-33, p. 368

Factors that Influence Venous Return summary

Blood pressure Mean Arterial Pressure Blood pressure that is monitored and regulated in the body Regulation is important: Provides adequate driving pressure (brain and organs) Prevents extra work Primary determinants Cardiac output Total peripheral resistance Mean arterial pressure = cardiac output x total peripheral resistance

Determinants of Mean Arterial Pressure

Mean Arterial Pressure Constantly monitored by baroreceptors (pressure sensors) within circulatory system Short-term control adjustments Occur within seconds Adjustments made by alterations in cardiac output and total peripheral resistance Mediated by means of autonomic nervous system influences on heart, veins, and arterioles Long-term control adjustments Require minutes to days Involve adjusting total blood volume by restoring normal salt and water balance through mechanisms that regulate urine output and thirst

Figure 10.11: Flow rate as a function of resistance. Fig. 10-11, p. 347

Carotid sinus baroreceptor Neural signals to cardiovascular control center in medulla Common carotid arteries (Blood to the brain) Aortic arch baroreceptor Aorta (Blood to rest of body) Fig. 10-35, p. 371

Cardiac output Heart rate Heart Heart rate Cardiac output Heart Stroke Parasympathetic stimulation Heart rate Blood pressure Heart Heart rate Sympathetic stimulation Cardiac output Blood pressure Heart Contractile strength of heart Stroke volume Total peripheral resistance Blood pressure Arterioles Vasoconstriction Venous return Stroke volume Cardiac output Blood pressure Veins Vasoconstriction Fig. 10-37, p. 372

Baroreceptor Reflexes to Restore Blood Pressure to Normal

Normal Increased Decreased Arterial pressure (mm Hg) 120 80 Firing rate in afferent neuron arising from carotid sinus baroreceptor Time Fig. 10-36, p. 371

Blood Pressure Additional reflexes and responses that influence blood pressure Left atrial receptors and hypothalamic osmoreceptors affect long-term regulation of blood pressure by controlling plasma volume Chemoreceptors in carotid and aortic arteries are sensitive to low O2 or high acid levels in blood – reflexly increase respiratory activity Associated with certain behaviors and emotions mediated through cerebral-hypothalamic pathway Exercise modifies cardiac responses Hypothalamus controls skin arterioles for temperature regulation Vasoactive substances released from endothelial cells play role

Blood Pressure Abnormalities Hypertension Blood pressure above 140/90 mm Hg Two broad classes Primary hypertension Secondary hypertension Hypotension Blood pressure below 100/60 mm Hg

Hypertension Most common of blood pressure abnormalities Primary hypertension Catchall category for blood pressure elevated by variety of unknown causes rather than by a single disease entity Potential causes being investigated Defects in salt management by the kidneys Excessive salt intake Diets low in K+ and Ca2+ Plasma membrane abnormalities such as defective Na+-K+ pumps Variation in gene that encodes for angiotensinogen Endogenous digitalis-like substances Abnormalities in NO, endothelin, or other locally acting vasoactive chemicals Excess vasopressin

Hypertension Secondary hypertension Accounts for about 10% of hypertension cases Occurs secondary to another known primary problem Examples of secondary hypertension Renal hypertension Endocrine hypertension Neurogenic hypertension

Hypertension Complication of hypertension Congestive heart failure Stroke Heart attack Spontaneous hemorrhage Renal failure Retinal damage

Hypotension Low blood pressure Occurs when There is too little blood to fill the vessels Heart is too weak to drive the blood Orthostatic (postural) hypotension Transient hypotensive condition resulting from insufficient compensatory responses to gravitational shifts in blood when person moves from horizontal to vertical position

Hypotension Circulatory shock Occurs when blood pressure falls so low that adequate blood flow to the tissues can no longer be maintained Four main types Hypovolemic (“low volume”) shock Cardiogenic (“heart produced”) shock Vasogenic (“vessel produced”) shock Neurogenic (“nerve produced”) shock

( mean arterial pressure) Circulatory shock ( mean arterial pressure) Cardiac output Cardiac output Total peripheral resistance Widespread vasodilation Loss of blood volume Vasodilator substances released from bacteria Histamine released in severe allergic reaction Loss of fluids derived from plasma Loss of vascular tone Excessive vomiting, diarrhea, urinary losses, etc. Severe hemorrhage Weakened heart Septic shock Anaphylactic shock Sympathetic nerve activity Hypovolemic shock Cardiogenic shock Cardiogenic shock Neurogenic shock Fig. 10-39, p. 377

Figure 10.40: Consequences and compensations of hemorrhage. The reduction in blood volume resulting from hemorrhage leads to a fall in arterial pressure. (Note the blue boxes, representing consequences of hemorrhage.) A series of compensations ensue (light pink boxes) that ultimately restore plasma volume, arterial pressure, and the number of red blood cells toward normal (dark pink boxes). Refer to the text (pp. 376–377) for an explanation of the circled numbers and a detailed discussion of the compensations. Fig. 10-40, p. 378