The Blood Vessels and Blood Pressure

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

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

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

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

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

Basic Organization of the Cardiovascular System

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

Arteries as a Pressure Reservoir

Blood flow is the volume of blood that flows through any tissue in a given time period ( in mL /min). Total blood flow is cardiac output (CO), volume of blood that circulates through systemic ( or pulmonary) blood vessels each minute. Two main factors affect the distribution of cardiac output: 1) pressure difference that drives the blood flow through a tissue, 2) The resistance to blood flow in specific blood vessels. Blood flows from regions of higher pressure to regions of lower pressure; the greater the pressure difference, the greater the blood flow. But the higher the resistance, the smaller the blood flow. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

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

Blood Pressure Can be measured indirectly using sphygmomanometer
Korotkoff sounds Sounds heard when determining blood pressure Sounds are distinct from heart sounds associated with valve closure Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

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

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

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

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

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

Magnitude and Distribution Of the Cardiac Output at Rest
and During Moderate Exercise Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

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

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

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

Vascular resistance is the opposition to blood flow due to friction between blood and the walls of the blood vessels. Vascular Resistance depends on: 1)size of lumen,2) blood viscosity and 3) total vessel length. 1) size of lumen; the smaller the lumen of the blood vessels the greater its resistance to blood flow. Resistance is inversely proportional to the fourth power of the diameter (d) of the blood vessels lumen (R α 1/d4) Vasoconstriction narrows the lumen, vasodilation widens it. Normally, moment –to –moment fluctuations in blood flow through a given tissue are due to vasoconstriction and vasodilation of the tissue’s arterioles. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

As arterioles dilate, resistance decreases, and blood pressure falls
As arterioles dilate, resistance decreases, and blood pressure falls. As arterioles constricts, resistance increases, and blood pressure rises. 2) Blood viscosity: The viscosity (thickness) of blood depends mostly on the ratio of red blood cells to plasma (fluid) volume, and to smaller extent on the concentration of proteins in plasma. The higher the blood viscosity, the higher the resistance. 3)Total blood vessel length; Resistance to blood flow through a vessel is directly proportional to the length of the blood vessel. The longer a blood vessel, the greater the resistance. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

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

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

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

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

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

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

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

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

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

Factors that Influence Venous Return

Blood pressure that is monitored and regulated in the body Primary determinants Cardiac output Total peripheral resistance Mean arterial pressure = cardiac output x total peripheral resistance Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

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

Baroreceptor Reflexes to Restore Blood Pressure to Normal

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

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

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

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

Hypertension Complication of hypertension Congestive heart failure
Stroke Heart attack Spontaneous hemorrhage Renal failure Retinal damage Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

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

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

Control of blood pressure and blood flow.
1)Role of cardiovascular center 2)Neural regulation of blood pressure 3)Hormonal regulation of blood pressure. 4)Autoregulation of blood pressure Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Neural Regulation Of Blood Pressure
The nervous system regulates blood pressure via negative feedback loops that occurs as two types of reflexes: baroreceptor reflexes and chemoreceptor reflexes. Baroreceptor reflexes: Baroreceptor pressure –sensitive sensory receptors are located in the aorta, internal carotid arteries and other arteries in the neck and chest. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Two most important baroreceptor reflexes are the carotid sinus reflexes and aortic reflexes.
Baroreceptors in the wall of the carotid sinuses initiate the carotid sinus reflexes, which help regulate blood pressure in the brain The carotid sinuses are small widenings of the right and left internal carotid arteries just above the where they branch from the common carotid arteries. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Blood pressure stretches the wall of the carotid sinus, which stimulates the baroreceptors. Nerve impulses propagate from the carotid sinus baroreceptors over sensory axons in the glossopharygeal (IX) nerves to the cardiovascular center in the medulla oblongata. Baroreceptors in the walls of the ascending aorta and arch of the aorta initiate the aortic reflex, which regulates systemic blood pressure. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Nerve impulse from aortic baroreceptors reach the cardiovascular center via sensory axons of the vagus (X) nerves. Chemoreceptor reflexes: chemoreceptor, sensory receptors that monitor the chemical composition of blood, are located close to the baroreceptors of the carotid sinus and arch of the aorta in small structure called carotid bodies and aortic bodies respectively. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

These chemoreceptor detect changes in blood level of O2, CO2, and H+
These chemoreceptor detect changes in blood level of O2, CO2, and H+. Hypoxia (lowered O2 availability), acidosis (an increase in H+ concentration), or hypercapnia (excess CO2) stimulates the chemoreceptor to send impulses to the cardiovascular centre. In response, the CV centre increases sympathetic stimulation to arterioles and veins, producing vasoconstriction and increase in blood pressure. These chemoreceptor also provide respiratory centre in the brain stem to adjust the rate of breathing. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Hormonal regulation of blood pressure.
Renin – angiotensin - aldosterone (RAA) system Epinephrine and norepinepherine Antidiuretic hormone (ADH) Atrial naturiuretic peptides Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Renin – angiotensin-aldosterone (RAA) system.
When blood volume falls or blood flow to the kidney decreases, juxtaglomerular cells in the kidneys secrete renin in the blood-stream. In sequence, renin and angiotensin converting enzyme (ACE) act on their substrate to produce the active hormone angiotensin II, which raise blood pressure in two ways: First angiotensin II is a potent vasoconstrictor; it raises blood pressure by increasing systemic vascular resistance Secondly it stimulates secretion of aldosterone, which increases reabsorption of sodium ions (Na+) and water by the kidneys. The water reabsorption increases the total blood volume, which increases blood pressure. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Epinepherine and norepinepherine
In response to sympathetic stimulation, the adrenal medulla releases epinepherine and norepinepherine. These hormones increase cardiac output by increasing the rate and force of heart contractions. They also cause vasoconstriction of arterioles and veins in the skin and abdominal organs and vasodilation of arterioles in cardiac and skeletal muscle, which helps increase blood flow to muscle during exercise. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Antidiuretic hormones (ADH)
ADH is produced by the hypothalamus and released from the posterior pituitary in response to dehydration or decreased blood volume. Among other actions, ADH causes vasoconstriction, which increases blood pressure. For this reason ADH is called a vasopressin. Atrial natriuretic peptide (ANP) ANP released by cells in the atria of the heart, ANP lowers blood pressure by causing vasodilation and by promoting the loss of salt and water in the urine, which reduces blood volume. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Autoregulation of blood pressure
In each capillary bed, local changes can regulate vasomotion. The ability of the tissue to automatically adjust its blood flow to match its metabolic demand is called autoregulation. Two important general types of stimuli causes autoregulatary changes in the blood. 1) Physical changes : Warming promotes vasodilation, and cooling causes vasoconstriction. Smooth muscle in arteriole walls exhibits a myogenic response- it contracts more forcefully when it is stretched and relaxes when stretching lessens. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

2) Vasodilating and vasoconstricting chemicals
2) Vasodilating and vasoconstricting chemicals. Several types of cells – including white blood cells, platelets, smooth muscle fibers, macrophages, and endothelial cells – release a wide variety of chemicals that alter blood vessel diameter. Vasodilating chemicals released by metabolically active tissue cells include K+, H+, lactic acid (lactate), and adenosine (from ATP). Another important vasodilators released by the endothelial cells is nitric oxide (NO). Tissue trauma or inflammation causes release of vasodilating kinins and histamine. Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Vasoconstrictors include thromboxane A2, superoxide radicals, serotonin also known as 5HT(from platelets), and endothelins ( from endothelial cells). Chapter 10 The Blood Vessels and Blood Pressure Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

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