Chapter 21 - Peripheral Circulation and Regulation

Blood Vessels

• Blood is carried in a closed system of vessels that begins and ends at the heart

• The three major types of vessels are arteries, capillaries, and veins

• Arteries carry blood away from the heart, veins carry blood toward the heart

• Capillaries contact tissue cells and directly serve cellular needs

Generalized Structure of Blood Vessels

• Arteries and veins are composed of three tunics – tunica interna, tunica media, and tunica externa

• Capillaries are composed of endothelium with sparse basal lamina

• Lumen – central blood-containing space surrounded by tunics

Blood Vessel Structure

• Arteries

• Elastic, muscular, arterioles

• Veins

• Venules, small veins, medium or large veins

• Capillaries

• Blood flows from arterioles to capillaries

• Most of exchange between blood and interstitial spaces occurs across the walls

• Blood flows from capillaries to venous system

Tunics

• Tunica interna (tunica intima)

• Endothelial layer that lines the lumen of all vessels

• In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present

• Tunica media

• Smooth muscle and elastic fiber layer, regulated by the sympathetic nervous system

• Controls vasoconstriction/vasodilation of vessels

• Tunica externa (tunica adventitia)

• Collagen fibers that protect and reinforce vessels

• Larger vessels contain vasa vasorum

Structure of Arteries and Veins

• Three layers except for capillaries and venules

• Tunica intima

• Endothelium

• Tunica media

• Vasoconstriction

• Vasodilation

• Tunica adventitia

• Merges with connective tissue surrounding blood vessels

Structure of Arteries

• Elastic or conducting arteries

• Largest diameters, pressure high and fluctuates

• Muscular or medium arteries

• Smooth muscle allows vessels to regulate blood supply by constricting or dilating

• Arterioles

• Transport blood from small arteries to capillaries

Elastic (Conducting) Arteries

• Thick-walled arteries near the heart; the aorta and its major branches

• Large lumen allow low-resistance conduction of blood

• Contain elastin in all three tunics

• Withstand and smooth out large blood pressure fluctuations

• Allow blood to flow fairly continuously through the body

Muscular Arteries and Arterioles

• Muscular arteries – distal to elastic arteries; deliver blood to body organs

• Have thick tunica media with more smooth muscle and less elastic tissue

• Active in vasoconstriction

• Arterioles – smallest arteries; lead to capillary beds

• Control flow into capillary beds via vasodilation and constriction

Venous System: Venules

• Are formed when capillary beds unite

• Allow fluids and WBCs to pass from the bloodstream to tissues

• Postcapillary venules – smallest venules, composed of endothelium and a few pericytes

• Large venules have one or two layers of smooth muscle (tunica media)

Venous System: Veins

• Veins are:

• Formed when venules converge

• Composed of three tunics, with a thin tunica media and a thick tunica externa consisting of collagen fibers and elastic networks

• Capacitance vessels (blood reservoirs) that contain 65% of the blood supply

• Veins have much lower blood pressure and thinner walls than arteries

• To return blood to the heart, veins have special adaptations

• Large-diameter lumens, which offer little resistance to flow

• Valves (resembling semilunar heart valves), which prevent backflow of blood

• Venous sinuses – specialized, flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain)

Structure of Veins

• Venules and small veins

• Tubes of endothelium on delicate basement membrane

• Medium and large veins

• Valves

• Allow blood to flow toward heart but not in opposite direction

• Arteriovenous anastomoses

• Allow blood to flow from arterioles to small veins without passing through capillaries

Vascular Anastomoses

• Merging blood vessels, more common in veins than arteries

• Arterial anastomoses provide alternate pathways (collateral channels) for blood to reach a given body region

• If one branch is blocked, the collateral channel can supply the area with adequate blood supply

• Thoroughfare channels are examples of arteriovenous anastomoses

Aging of the Arteries

• Arteriosclerosis

• General term for degeneration changes in arteries making them less elastic

• Atherosclerosis

• Deposition of plaque on walls

Capillaries

• Capillaries are the smallest blood vessels

• Walls consisting of a thin tunica interna, one cell thick

• Allow only a single RBC to pass at a time

• Pericytes on the outer surface stabilize their walls

• There are three structural types of capillaries: continuous, fenestrated, and sinusoids

• Capillary wall consists mostly of endothelial cells

• Types classified by diameter/permeability

• Continuous

• Do not have fenestrae

• Fenestrated

• Have pores

• Sinusoidal

• Large diameter with large fenestrae

Continuous Capillaries

• Continuous capillaries are abundant in the skin and muscles, and have:

• Endothelial cells that provide an uninterrupted lining

• Adjacent cells that are held together with tight junctions

• Intercellular clefts of unjoined membranes that allow the passage of fluids

• Continuous capillaries of the brain:

• Have tight junctions completely around the endothelium

• Constitute the blood-brain barrier

Fenestrated Capillaries

• Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys)

• Characterized by:

• An endothelium riddled with pores (fenestrations)

• Greater permeability to solutes and fluids than other capillaries

Sinusoids

• Highly modified, leaky, fenestrated capillaries with large lumens

• Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs

• Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues

• Blood flows sluggishly, allowing for modification in various ways

Capillary Network

• Blood flows from arterioles through metarterioles, then through capillary network

• Venules drain network

• Smooth muscle in arterioles, metarterioles, precapillary sphincters regulates blood flow

Capillary Beds

• A microcirculation of interwoven networks of capillaries, consisting of:

• Vascular shunts – metarteriole–thoroughfare channel connecting an arteriole directly with a postcapillary venule

• True capillaries – 10 to 100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed

Blood Flow Through Capillary Beds

• Precapillary sphincter

• Cuff of smooth muscle that surrounds each true capillary

• Regulates blood flow into the capillary

• Blood flow is regulated by vasomotor nerves and local chemical conditions, so it can either bypass or flood the capillary bed

Peripheral Circulatory System

• Systemic vessels

• Transport blood through most all body parts from left ventricle and back to right atrium

• Pulmonary vessels

• Transport blood from right ventricle through lungs and back to left atrium

• Blood vessels and heart regulated to ensure blood pressure is high enough for blood flow to meet metabolic needs of tissues

Physiology of Systemic Circulation

• Determined by

• Anatomy of circulatory system

• Dynamics of blood flow

• Regulatory mechanisms that control heart and blood vessels

• Blood volume

• Most in the veins

• Smaller volumes in arteries and capillaries

Systemic Circulation: Arteries

• Aorta

• From which all arteries are derived either directly or indirectly

• Parts

• Ascending, descending, thoracic, abdominal

• Coronary arteries

• Supply the heart

Systemic Circulation: Veins

• Return blood from body to right atrium

• Major veins

• Coronary sinus (heart)

• Superior vena cava (head, neck, thorax, upper limbs)

• Inferior vena cava (abdomen, pelvis, lower limbs)

• Types of veins

• Superficial, deep, sinuses

Pulmonary Circulation

• Moves blood to and from the lungs

• Pulmonary trunk

• Arises from right ventricle

• Pulmonary arteries

• Branches of pulmonary trunk which project to lungs

• Pulmonary veins

• Exit each lung and enter left atrium

Dynamics of Blood Circulation

• Interrelationships between

• Pressure

• Flow

• Resistance

• Control mechanisms that regulate blood pressure

• Blood flow through vessels

Blood Pressure (BP)

• Force per unit area exerted on the wall of a blood vessel by its contained blood

• Expressed in terms of millimeters of mercury (mm Hg)

• Measured in reference to systemic arterial BP in large arteries near the heart

• The differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas

Blood Pressure

• Measure of force exerted by blood against the wall

• Blood moves through vessels because of blood pressure

• Measured by listening for Korotkoff sounds produced by turbulent flow in arteries as pressure released from blood pressure cuff

Measuring Blood Pressure

• Systemic arterial BP is measured indirectly with the auscultatory method

• A sphygmomanometer is placed on the arm superior to the elbow

• Pressure is increased in the cuff until it is greater than systolic pressure in the brachial artery

• Pressure is released slowly and the examiner listens with a stethoscope

• The first sounds heard is recorded as the systolic pressure

• The pressure when sound disappears is recorded as the diastolic pressure

Systemic Blood Pressure

• The pumping action of the heart generates blood flow through the vessels along a pressure gradient, always moving from higher- to lower-pressure areas

• Pressure results when flow is opposed by resistance

• Systemic pressure:

• Is highest in the aorta

• Declines throughout the length of the pathway

• Is 0 mm Hg in the right atrium

• The steepest change in blood pressure occurs in the arterioles

Arterial Blood Pressure

• Arterial BP reflects two factors of the arteries close to the heart

• Their elasticity (compliance, or distensibility)

• The amount of blood forced into them at any given time

• Blood pressure in elastic arteries near the heart is pulsatile (BP rises and falls)

• Systolic pressure – pressure exerted on arterial walls during ventricular contraction

• Diastolic pressure – lowest level of arterial pressure during a ventricular cycle

• Pulse pressure – the difference between systolic and diastolic pressure

• Mean arterial pressure (MAP) – pressure that propels the blood to the tissues

• MAP = diastolic pressure + 1/3 pulse pressure

Capillary Blood Pressure

• Capillary BP ranges from 20 to 40 mm Hg

• Low capillary pressure is desirable because high BP would rupture fragile, thin-walled capillaries

• Low BP is sufficient to force filtrate out into interstitial space and distribute nutrients, gases, and hormones between blood and tissues

Venous Blood Pressure

• Venous BP is steady and changes little during the cardiac cycle

• The pressure gradient in the venous system is only about 20 mm Hg

• A cut vein has even blood flow; a lacerated artery flows in spurts

Factors Aiding Venous Return

• Venous BP alone is too low to promote adequate blood return and is aided by the:

• Respiratory pump – pressure changes created during breathing suck blood toward the heart by squeezing local veins

• Muscular pump – contraction of skeletal muscles “milk” blood toward the heart

• Valves prevent backflow during venous return

Blood Pressure

• Maintaining blood pressure requires:

• Cooperation of the heart, blood vessels, and kidneys

• Supervision of the brain

• The main factors influencing blood pressure are:

• Cardiac output (CO)

• Peripheral resistance (PR)

• Blood volume

• Blood pressure = CO x PR

• Blood pressure varies directly with CO, PR, and blood volume

Cardiac Output (CO)

• Cardiac output is determined by venous return and neural and hormonal controls

• Resting heart rate is controlled by the cardioinhibitory center via the vagus nerves

• Stroke volume is controlled by venous return (end diastolic volume, or EDV)

• Under stress, the cardioacceleratory center increases heart rate and stroke volume

• The end systolic volume (ESV) decreases and MAP increases

Pulse Pressure

• Difference between systolic and diastolic pressures

• Increases when stroke volume increases or vascular compliance decreases

• Pulse pressure can be used to take a pulse to determine heart rate and rhythmicity

Blood Flow

• Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period is:

• Measured in ml per min

• Equivalent to cardiac output (CO), considering the entire vascular system

• Relatively constant when at rest

• Varies widely through individual organs, according to immediate needs

Blood Flow through Tissues

• Blood flow, or tissue perfusion, is involved in:

• Delivery of oxygen and nutrients to, and removal of wastes from, tissue cells

• Gas exchange in the lungs

• Absorption of nutrients from the digestive tract

• Urine formation by the kidneys

• Blood flow is precisely the right amount to provide proper tissue function

Laminar and Turbulent Flow

• Laminar flow

• Streamlined

• Outermost layer moving slowest and center moving fastest

• Turbulent flow

• Interrupted

• Rate of flow exceeds critical velocity

• Fluid passes a constriction, sharp turn, rough surface

Velocity of Blood Flow

• Blood velocity:

• Changes as it travels through the systemic circulation

• Is inversely proportional to the cross-sectional area

• Slow capillary flow allows adequate time for exchange between blood and tissues

Resistance

• Resistance – opposition to flow

• Measure of the amount of friction blood encounters as it passes through vessels

• Generally encountered in the systemic circulation

• Referred to as peripheral resistance (PR)

• The three important sources of resistance are blood viscosity, total blood vessel length, and blood vessel diameter

Resistance Factors: Viscosity and Vessel Length

• Resistance factors that remain relatively constant are:

• Blood viscosity – thickness or “stickiness” of the blood

• Blood vessel length – the longer the vessel, the greater the resistance encountered

Blood Flow, Poiseuille’s Law, and Viscosity

• Blood flow

• Amount of blood moving through a vessel in a given time period

• Directly proportional to pressure differences, inversely proportional to resistance

• Poiseuille’s Law

• Flow decreases when resistance increases

• Flow resistance decreases when vessel diameter increases

• Viscosity

• Measure of resistance of liquid to flow

• As viscosity increases, pressure required to flow increases

Resistance Factors: Blood Vessel Diameter

• Changes in vessel diameter are frequent and significantly alter peripheral resistance

• Resistance varies inversely with the fourth power of vessel radius (one-half the diameter)

• For example, if the radius is doubled, the resistance is 1/16 as much

• Small-diameter arterioles are the major determinants of peripheral resistance

• Fatty plaques from atherosclerosis:

• Cause turbulent blood flow

• Dramatically increase resistance due to turbulence

Cross-Sectional Area

• As diameter of vessels decreases, the total cross-sectional area increases and velocity of blood flow decreases (aorta = 5cm² vs. capillaries = 2500cm²)

• Much like a stream that flows rapidly through a narrow gorge but flows slowly through a broad plane

Critical Closing Pressure, Laplace’s Law and Compliance

Critical closing pressure

• Pressure at which a blood vessel collapses and blood flow stops

Laplace’s Law

• Force acting on blood vessel wall is proportional to diameter of the vessel times blood pressure

Vascular compliance

• Tendency for blood vessel volume to increase as blood pressure increases

• More easily the vessel wall stretches, the greater its compliance

• Venous system has a large compliance and acts as a blood reservoir

Blood Flow, Blood Pressure, and Resistance

• Blood flow (F) is directly proportional to the difference in blood pressure (DP) between two points in the circulation

• If DP increases, blood flow speeds up; if DP decreases, blood flow declines

• Blood flow is inversely proportional to resistance (R)

• If R increases, blood flow decreases

• R is more important than DP in influencing local blood pressure

Pressure and Resistance

• Blood pressure averages 100 mm Hg in aorta and drops to 0 mm Hg in the right atrium

• Greatest drop in pressure occurs in arterioles which regulate blood flow through tissues

• No large fluctuations in capillaries and veins

Capillary Exchange and Interstitial Fluid Volume Regulation

• Blood pressure, capillary permeability, and osmosis affect movement of fluid from capillaries

• A net movement of fluid occurs from blood into tissues. Fluid gained by tissues is removed by lymphatic system.

Capillary Exchange of Respiratory Gases and Nutrients

• Oxygen, carbon dioxide, nutrients, and metabolic wastes diffuse between the blood and interstitial fluid along concentration gradients

• Oxygen and nutrients pass from the blood to tissues

• Carbon dioxide and metabolic wastes pass from tissues to the blood

• Water-soluble solutes pass through clefts and fenestrations

• Lipid-soluble molecules diffuse directly through endothelial membranes

Capillary Exchange: Fluid Movements

• Direction of movement depends upon the difference between:

• Net capillary hydrostatic pressure (HP_c) +33 mm Hg

• Net capillary colloid osmotic pressure (OP_c) -20mm Hg

• HP_c – pressure of blood against the capillary walls:

• Tends to force fluids through the capillary walls

• Is greater at the arterial end of a bed than at the venule end

• OP_c– created by nondiffusible plasma proteins, which draw water toward themselves

Net Filtration Pressure (NFP)

• NFP – considers all the forces acting on a capillary bed

• NFP = (HP_c – HP_if) – (OP_c – OP_if)

• At the arterial end of a bed, hydrostatic forces dominate (fluids flow out)

• At the venous end of a bed, osmotic forces dominate (fluids flow in)

• More fluids enter the tissue beds than return to the blood and the excess fluid is returned to the blood via the lymphatic system

Control of Blood Flow by Tissues

• Local control

• In most tissues, blood flow is proportional to metabolic needs of tissues

• Nervous System

• Responsible for routing blood flow and maintaining blood pressure

• Hormonal Control

• Sympathetic action potentials stimulate epinephrine and norepinephrine

Local Control of Blood Flow by Tissues

• Blood flow can increase 7-8 times as a result of vasodilation of metarterioles and precapillary sphincters in response to increased rate of metabolism

• Vasodilator substances produced as metabolism increases

• Vasomotion is periodic contraction and relaxation of precapillary sphincters

Local Regulation of Blood Flow

• Autoregulation – automatic adjustment of blood flow to each tissue in proportion to its requirements at any given point in time

• Blood flow through an individual organ is intrinsically controlled by modifying the diameter of local arterioles feeding its capillaries

• MAP remains constant, while local demands regulate the amount of blood delivered to various areas according to need

Circulatory Shock

• Circulatory shock – any condition in which blood vessels are inadequately filled and blood cannot circulate normally

• Results in inadequate blood flow to meet tissue needs

• Three types include:

• Hypovolemic shock – results from large-scale blood loss

• Vascular shock – poor circulation resulting from extreme vasodilation

• Cardiogenic shock – the heart cannot sustain adequate circulation

• Inadequate blood flow throughout body

• Three stages

• Compensated: Blood pressure decreases only a moderate amount and mechanisms able to reestablish normal blood pressure and flow

• Progressive: Compensatory mechanisms inadequate and positive feedback cycle develops; cycle proceeds to next stage or medical treatment reestablishes adequate blood flow to tissues

• Irreversible: Leads to death, regardless of medical treatment

Controls of Blood Pressure

• Short-term controls:

• Are mediated by the nervous system and bloodborne chemicals

• Counteract moment-to-moment fluctuations in blood pressure by altering peripheral resistance

• Long-term controls regulate blood volume

Short-Term Regulation of Blood Pressure

• Baroreceptor reflexes

• Change peripheral resistance, heart rate, and stroke volume in response to changes in blood pressure

• Chemoreceptor reflexes

• Sensory receptors sensitive to oxygen, carbon dioxide, and pH levels of blood

• Central nervous system ischemic response

• Results from high carbon dioxide or low pH levels in medulla and increases peripheral resistance

Short-Term Mechanisms: Neural Controls

• Neural controls of peripheral resistance:

• Alter blood distribution to respond to specific demands

• Maintain MAP by altering blood vessel diameter

• Neural controls operate via reflex arcs, involving:

• Baroreceptors

• Vasomotor centers of the medulla and vasomotor fibers

• Vascular smooth muscle

Short-Term Mechanisms: Vasomotor Center

• Vasomotor center – a cluster of sympathetic neurons in the medulla that oversees changes in blood vessel diameter

• Maintains blood vessel tone by innervating smooth muscles of blood vessels, especially arterioles

• Cardiovascular center – vasomotor center plus the cardiac centers that integrate blood pressure control by altering cardiac output and blood vessel diameter

Short-Term Mechanisms: Vasomotor Activity

• Sympathetic activity causes:

• Vasoconstriction and a rise in blood pressure if increased

• Blood pressure to decline to basal levels if decreased

• Vasomotor activity is modified by:

• Baroreceptors (pressure-sensitive), chemoreceptors (O₂, CO₂, and H⁺ sensitive), higher brain centers, bloodborne chemicals, and hormones

Short-Term Mechanisms: Baroreceptor-Initiated Reflex

• Increased blood pressure stimulates the cardioinhibitory center to:

• Increase vessel diameter

• Decrease heart rate, cardiac output, peripheral resistance, and blood pressure

• Declining blood pressure stimulates the cardioacceleratory center to:

• Increase cardiac output and peripheral resistance

• Low blood pressure also stimulates the vasomotor center to constrict blood vessels

Short-Term Mechanisms: Chemical Controls

• Blood pressure is regulated by chemoreceptor reflexes sensitive to oxygen and carbon dioxide

• Prominent chemoreceptors are the carotid and aortic bodies

• Reflexes that regulate blood pressure are integrated in the medulla

• Higher brain centers (cortex and hypothalamus) can modify BP via relays to medullary centers

Long-Term Regulation of Blood Pressure

• Renin-angiotensin-aldosterone mechanism

• Vasopressin (ADH) mechanism

• Atrial natriuretic mechanism

• Fluid shift mechanism

• Stress-relaxation response

Long-Term Mechanisms: Renal Regulation

• Baroreceptors adapt to chronic high or low blood pressure

• Kidneys maintain long-term BP by regulating blood volume

• Increased BP stimulates the kidneys to eliminate water, thus reducing BP

• Decreased BP stimulates the kidneys to increase blood volume and BP

Kidney Action and Blood Pressure

• Kidneys act directly and indirectly to maintain long-term blood pressure

• Direct renal mechanism alters blood volume

• Indirect renal mechanism involves the renin-angiotensin mechanism

• Declining BP causes the release of renin, which triggers the release of angiotensin II

• Angiotensin II is a potent vasoconstrictor that stimulates aldosterone secretion

• Aldosterone enhances renal reabsorption and stimulates ADH release

Long Term Mechanisms

• Atrial natriuretic

• Hormone released from cardiac muscle cells when atrial blood pressure increases, simulating an increase in urinary production, causing a decrease in blood volume and blood pressure

• Fluid shift

• Movement of fluid from interstitial spaces into capillaries in response to decrease in blood pressure to maintain blood volume

• Stress-relaxation

• Adjustment of blood vessel smooth muscle to respond to change in blood volume

Chemicals that Increase Blood Pressure

• Adrenal medulla hormones – norepinephrine and epinephrine increase blood pressure

• Antidiuretic hormone (ADH) – causes intense vasoconstriction in cases of extremely low BP

• Angiotensin II – causes intense vasoconstriction when renal profusion is inadequate

• Endothelium-derived factors – endothelin and prostaglandin-derived growth factor (PDGF) are both vasoconstrictors

Chemicals that Decrease Blood Pressure

• Atrial natriuretic peptide (ANP) – causes blood volume and pressure to decline

• Nitric oxide (NO) – has brief but potent vasodilator effects

• Inflammatory chemicals – histamine, prostacyclin, and kinins are potent vasodilators

• Alcohol – causes BP to drop by inhibiting ADH

Monitoring Circulatory Efficiency

• Efficiency of the circulation can be assessed by taking pulse and blood pressure measurements

• Vital signs – pulse and blood pressure, along with respiratory rate and body temperature

• Pulse – pressure wave caused by the expansion and recoil of elastic arteries

• Radial pulse (taken on the radial artery at the wrist) is routinely used

• Varies with health, body position, and activity

Alterations in Blood Pressure

• Hypotension – low BP in which systolic pressure is below 100 mm Hg

• Hypertension – condition of sustained elevated arterial pressure of 140/90 or higher

• Transient elevations are normal and can be caused by fever, physical exertion, and emotional upset

• Chronic elevation is a major cause of heart failure, vascular disease, renal failure, and stroke

Hypotension

• Orthostatic hypotension – temporary low BP and dizziness when suddenly rising from a sitting or reclining position

• Chronic hypotension – hint of poor nutrition and warning sign for Addison’s disease

• Acute hypotension – important sign of circulatory shock

• Threat to patients undergoing surgery and those in intensive care units

Hypertension

• Hypertension – sustained BP of 140/90 or higher:

• Is the major cause of heart failure, vascular disease, renal failure, and stroke

• Weakens the heart and ravages the blood vessels

• Causes tears in vessel endothelium that accelerate atherosclerosis

• Elevated diastolic pressure is more significant than systolic

• It indicates progressive occlusion and/or hardening of the arterial tree

• Primary or essential hypertension – risk factors in primary hypertension include diet, obesity, age, race, heredity, stress, and smoking

• Secondary hypertension – due to identifiable disorders, including excessive renin secretion, arteriosclerosis, and endocrine disorders

Intrinsic Control of Blood Flow: Metabolic

• Declining tissue nutrient and oxygen levels are stimuli for autoregulation

• Hemoglobin delivers nitric oxide (NO) as well as oxygen to tissues

• Nitric oxide induces vasodilation at the capillaries to help get oxygen to tissue cells

• Other autoregulatory substances include: potassium and hydrogen ions, adenosine, lactic acid, histamines, kinins, and prostaglandins

Intrinsic Control of Blood Flow: Myogenic

• Inadequate blood perfusion or excessively high arterial pressure:

• Are autoregulatory

• Provoke myogenic responses – stimulation of vascular smooth muscle

• Vascular muscle responds directly to:

• Increased vascular pressure with increased tone, which causes vasoconstriction

• Reduced stretch with vasodilation, which promotes increased blood flow to the tissue

Long-Term Autoregulation

• Is evoked when short-term autoregulation cannot meet tissue nutrient requirements

• May evolve over weeks or months to enrich local blood flow

• Angiogenesis takes place:

• As the number of vessels to a region increases

• When existing vessels enlarge

• When a heart vessel becomes partly occluded

• Routinely to people in high altitudes, where oxygen content of the air is low

Blood Flow: Skeletal Muscles

• Resting muscle blood flow is regulated by myogenic and general neural mechanisms in response to oxygen and carbon dioxide levels

• When muscles become active, hyperemia is directly proportional to greater metabolic activity of the muscle (active or exercise hyperemia)

• Arterioles in muscles have cholinergic, and alpha (a) and beta (b) adrenergic receptors

• a and b adrenergic receptors bind to epinephrine

Blood Flow: Skeletal Muscle Regulation

• Muscle blood flow can increase tenfold or more during physical activity as vasodilation occurs

• Low levels of epinephrine bind to b receptors

• Cholinergic receptors are occupied

• Intense exercise or sympathetic nervous system activation result in high levels of epinephrine

• High levels of epinephrine bind to a receptors and cause vasoconstriction

• This is a protective response to prevent muscle oxygen demands from exceeding cardiac pumping ability

Blood Flow: Brain

• Blood flow to the brain is constant, as neurons are intolerant of ischemia

• Metabolic controls – brain tissue is extremely sensitive to declines in pH, and increased carbon dioxide causes marked vasodilation

• Myogenic controls protect the brain from damaging changes in blood pressure

• Decreases in MAP cause cerebral vessels to dilate to insure adequate perfusion

• Increases in MAP cause cerebral vessels to constrict

• The brain can regulate is own blood flow in certain circumstances, such as ischemia caused by a tumor

• The brain is vulnerable under extreme systemic pressure changes

• MAP below 60mm Hg can cause syncope (fainting)

• MAP above 160 can result in cerebral edema

Blood Flow: Skin

• Blood flow through the skin:

• Supplies nutrients to cells in response to oxygen need

• Aids in body temperature regulation and provides a blood reservoir

• Blood flow to venous plexuses below the skin surface:

• Varies from 50 ml/min to 2500 ml/min, depending upon body temperature

• Is controlled by sympathetic nervous system reflexes initiated by temperature receptors and the central nervous system

Temperature Regulation

• As temperature rises (e.g., heat exposure, fever, vigorous exercise):

• Hypothalamic signals reduce vasomotor stimulation of the skin vessels

• Heat radiates from the skin

• Sweat also causes vasodilation via bradykinin in perspiration

• Bradykinin stimulates the release of NO

• As temperature decreases, blood is shunted to deeper, more vital organs

Blood Flow: Lungs

• Blood flow in the pulmonary circulation is unusual in that:

• The pathway is short

• Arteries/arterioles are more like veins/venules (thin-walled, with large lumens)

• They have a much lower arterial pressure (24/8 mm Hg versus 120/80 mm Hg)

• The autoregulatory mechanism is exactly opposite of that in most tissues

• Low oxygen levels cause vasoconstriction; high levels promote vasodilation

• This allows for proper oxygen loading in the lungs

Blood Flow: Heart

• Small vessel coronary circulation is influenced by:

• Aortic pressure

• The pumping activity of the ventricles

• During ventricular systole:

• Coronary vessels compress

• Myocardial blood flow ceases

• Stored myoglobin supplies sufficient oxygen

• During ventricular diastole, oxygen and nutrients are carried to the heart

Review Outline - The Cardiovascular System: Blood Vessels

I. Overview of Blood Vessel Structure and Function

A. Structure of Blood Vessel Walls

B. Arterial System

1. Elastic (Conducting) Arteries

2. Muscular Arteries

3. Arterioles

C. Capillaries

1. Types of Capillaries

a. Continuous Capillaries

b. Fenestrated Capillaries

c. Sinusoids

2. Capillary Beds

D. Venous System

1. Venules

2. Veins

3. Varicose Veins

E. Vascular Anastomoses

1. Arterial Anastomoses

2. Arteriovenous Anastomoses

3. Venous Anastomoses

II. Physiology of Circulation

A. Introduction to Blood Flow, Blood Pressure, and Resistance

1. Blood Flow

2. Blood Pressure

3. Peripheral Resistance

a. Blood Viscosity

b. Blood Vessel Length

c. Blood Vessel Diameter

4. Relationship Between Blood Flow, Blood Pressure, and Resistance

B. Systemic Blood Pressure

1. Arterial Blood Pressure

2. Capillary Blood Pressure

3. Venous Blood Pressure

a. Factors Aiding Venous Return

1. Respiratory Pump

2. Muscular Pump

C. Maintaining Blood Pressure

1. Cardiac Output

2. Peripheral Resistance

3. Blood Volume

4. Short-Term Mechanisms: Neural Controls

a. Role of the Vasomotor Center

1. Vasomotor Fibers

b. Baroreceptor-Initiated Reflexes

c. Chemoreceptor-Initiated Reflexes

d. Higher Brain Centers

5. Short-Term Mechanisms: Chemical Controls

a. Adrenal Medulla Hormones

b. Atrial Natriuretic Peptide (ANP)

c. Antidiuretic Hormone

d. Inflammatory Chemicals

e. Alcohol

6. Long-Term Mechanisms: Renal Regulation

7. Monitoring Circulatory Efficiency

a. Taking a Pulse

b. Measuring Blood Pressure

8. Alterations in Blood Pressure

a. Hypotension

b. Hypertension

D. Blood Flow through Body Tissues

1. Velocity of Blood Flow

2. Autoregulation: Local Regulation of Blood Flow

a. Metabolic Controls

b. Myogenic Controls

c. Long-Term Autoregulation

3. Blood Flow in Special Areas

a. The Skin

b. The Lungs

c. The Heart

4. Blood Flow through the Capillaries and Capillary Dynamics

a. Capillary Exchanges of Respiratory Gases and Nutrients

b. Fluid Movements

1. Hydrostatic Pressures

2. ColloidOsmotic Pressures

3. Hydrostatic-Osmotic Pressure Interactions

5. Circulatory Shock

a. Hypovolemic Shock

b. Vascular Shock

c. Cardiogenic Shock

III. Circulatory Pathways: Blood Vessels of the Body

A. Pulmonary Circulation

B. Systemic Circulation

1. Systemic Arteries

a. Aorta

b. Arteries of the Head and Neck

c. Arteries of the Upper Limbs and Thorax

d. Arteries of the Abdomen

e. Arteries of the Pelvis and Lower Limbs

2. Systemic Veins

a. Venae Cavae and Their Major Tributaries

b. Major Veins of the Systemic Circulation

c. Veins of the Head and Neck

d. Veins of the Upper Limbs and Thorax

e. Veins of the Abdomen

f. Veins of the Pelvis and Lower Limbs