chapter 10 the blood vessels and blood pressure human physiology by lauralee sherwood ©2010...

Chapter 10The Blood Vessels and Blood Pressure

Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning

Chapter 10 The Blood Vessels and Blood PressureHuman Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage 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


Distribution of Cardiac Output at Rest


Blood Flow

• 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


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 3 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


Relationship of Resistance and Flow to Vessel Radius

Animation: Measuring blood pressure

Animation: Arteries as pressure reservoir


Vascular Tree

• Closed system of vessels• Consists of

– Arteries• 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• Return blood to heart


Basic Organization of the Cardiovascular System

Animation: Flow velocity

Animation: Vein function


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


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

• Korotkoff sounds

– 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


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 DistributionOf the Cardiac Output at Restand 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


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

– 2 types of passive exchanges• Diffusion

• Bulk flow


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 increases blood flow

• Metarteriole

– Runs between an arteriole and a venule


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


Lymphatic System


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


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


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


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


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


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


Blood Pressure Abnormalities

• Hypertension

– Blood pressure above 140/90 mm Hg

– 2 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

– 4 main types• Hypovolemic (“low volume”) shock

• Cardiogenic (“heart produced”) shock

• Vasogenic (“vessel produced”) shock

• Neurogenic (“nerve produced”) shock

Animation: Pattern of blood circulation

Animation: Lymph vessels and lymph nodes

Animation: Blood vessel anatomy

chapter 10 the blood vessels and blood pressure human physiology by lauralee sherwood ©2010...

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