blood flow

Copyright © 2010 Pearson Education, Inc.

Blood Flow• The purpose of cardiovascular regulation is to maintain

adequate blood flow through the capillaries to the tissues

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

• Is measured in ml/min.

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

• Is relatively constant when at rest

• Varies widely through individual organs


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

Copyright © 2010 Pearson Education, Inc. Figure 19.13

Brain

Heart

Skeletalmuscles

Skin

Kidney

Abdomen

Other

Total blood flow during strenuousexercise 17,500 ml/min

Total bloodflow at rest5800 ml/min


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 or dehydration

• Vascular shock – normal blood volume but too much accumulate in the limbs (long period of standing/sitting)

• Cardiogenic shock – the heart cannot sustain adequate circulation


Blood flow

• Capillary blood flow is determined by the interplay between:

• Pressure

• Resistance

• The heart must generate sufficient pressure to overcome resistance


Physiology of Circulation: Blood Pressure (BP)

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

• Expressed in 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

• Absolute pressure is less important than pressure gradient


Physiology of Circulation: Resistance

• Opposition to flow

• Measure of the amount of friction blood encounters

• Generally encountered in the peripheral systemic circulation

• Because the resistance of the venous system is very low (why?) usually the focus is on the resistance of the arterial system:

• Peripheral resistance (PR) is the resistance of the arterial system


Physiology of Circulation: Resistance

• Three important sources of resistance

• Blood viscosity

• Total blood vessel length

• Blood vessel diameter


Resistance – constant factors

• Blood viscosity

• The “stickiness” of the blood due to formed elements and plasma proteins

• Blood vessel length

• The longer the vessel, the greater the resistance encountered


Resistance – frequently changed factors

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

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

• Frequent changes alter peripheral resistance

• Fatty plaques from atherosclerosis:

• Cause turbulent blood flow

• Dramatically increase resistance due to turbulence


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 between arteries to arterioles


Arterial Blood Pressure

• Arterial pressure is important because it maintains blood flow through capillaries

• Blood pressure near the heart is pulsatile

• Systolic pressure: pressure exerted during ventricular contraction

• Diastolic pressure: lowest level of arterial pressure


Arterial Blood Pressure

• A pulse is rhythmic pressure oscillation that accompanies every heartbeat

• Pulse pressure = 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

• Pulse pressure and MAP both decline with increasing distance from the heart

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


Measuring BP

• Systemic arterial BP is measured indirectly

• The first sound heard is recorded as the systolic pressure

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

• http://www.youtube.com/watch?v=bNApnYMR16w

http://www.youtube.com/watch?v=bNApnYMR16w


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


Capillary Blood Pressure

• Ranges from 15 to 35 mm Hg

• Low capillary pressure is desirable

• 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

• Capillary blood pressure is one of the driving forces for transport through the capillary wall (filtration)


Passage through capillary - diffusion

1. Water, ions, and small molecules such as glucose

• Diffuse between adjacent endothelial cells

• Or through fenestrated capillaries

2. Some ions (Na, K, Ca2, Cl)

• Diffuse through channels in plasma membranes


Passage through capillary - diffusion

3. Large, water-soluble compounds

• Pass through fenestrated capillaries

4. Lipids and lipid-soluble materials such as O2 and

CO2

• Diffuse through endothelial plasma membranes

5. Plasma proteins

• Cross endothelial lining in sinusoids


Passage through capillary - filtration

• Driven by hydrostatic pressure

• Water and small solutes forced through capillary

wall

• Leaves larger solutes in bloodstream


Passage through capillary - reabsorption

• The result of osmotic pressure (OP)

• Blood colloid osmotic pressure (BCOP)

• Equals pressure required to prevent osmosis

• Caused by blood proteins that are too large to

cross capillary walls


Hydrostatic Pressures

• Capillary hydrostatic pressure (HPc) (capillary blood pressure)

• Tends to force fluids through the capillary walls

• Is greater at the arterial end (35 mm Hg) of a bed than at the venule end (17 mm Hg)

• Interstitial fluid hydrostatic pressure (HPif)

• Usually assumed to be zero because of lymphatic vessels


Colloid Osmotic Pressures

• Capillary colloid osmotic pressure (oncotic pressure) (OPc)

• Created by non-diffusible plasma proteins, which draw water toward themselves

• ~26 mm Hg

• Interstitial fluid osmotic pressure (OPif)

• Low (~1 mm Hg) due to low protein content


Net Filtration Pressure (NFP)

• NFP—comprises all the forces acting on a capillary bed

• NFP = (HPc—HPif)—(OPc—OPif)

• At the arterial end of a bed, hydrostatic forces dominate

• At the venous end, osmotic forces dominate

• Excess fluid is returned to the blood via the lymphatic system


HP = hydrostatic pressure• Due to fluid pressing against a wall• “Pushes”• In capillary (HPc) • Pushes fluid out of capillary • 35 mm Hg at arterial end and 17 mm Hg at venous end of capillary in this example• In interstitial fluid (HPif) • Pushes fluid into capillary • 0 mm Hg in this example

OP = osmotic pressure• Due to presence of nondiffusible solutes (e.g., plasma proteins)• “Sucks”• In capillary (OPc) • Pulls fluid into capillary • 26 mm Hg in this example• In interstitial fluid (OPif) • Pulls fluid out of capillary • 1 mm Hg in this example

Arteriole

Capillary

Interstitial fluid

Net HP—Net OP(35—0)—(26—1)

Net HP—Net OP(17—0)—(26—1)

Venule

NFP (net filtration pressure)is 10 mm Hg; fluid moves out

NFP is ~8 mm Hg;fluid moves in

NetHP35mm

NetOP25mm

NetHP17mm

NetOP25mm

CHP – 35-17 mmHgIHP – 0 mmHgBCOP – 25 mmHgICOP – 0 mmHg


Fluid Exchange at a Capillary

• Hydrostatic pressure and osmotic pressure regulate bulk flow

Figure 15-19a


Venous Blood Pressure

• Changes little during the cardiac cycle

• Small pressure gradient, about 15 mm Hg


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


Controls of Blood Pressure

• Short-term controls:

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

• Long-term controls regulate blood volume


Central Regulation

Stimulation ofreceptors sensitiveto changes insystemic bloodpressure orchemistry

Endocrine mechanisms

HOMEOSTASISRESTORED

Neuralmechanisms

Activation ofcardiovascularcenters

Stimulationof endocrineresponse

Long-term increasein blood volumeand blood pressure

Short-term elevation of blood pressure bysympatheticstimulation of theheart and peripheralvasoconstriction

Central regulation involves neuroendocrine mechanisms that control the total systemic circulation. This regulation involves both the cardiovascular centers and the vasomotor centers.

If autoregulation is ineffective


Factors that Influence Mean Arterial Pressure

Figure 15-10


Controls of Blood Pressure

• Autoregulation

• Causes immediate, localized homeostatic adjustments

• Neural mechanisms

• Respond quickly to changes at specific sites

• Endocrine mechanisms

• Direct long-term changes


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

• Local vasodilators accelerate blood flow in response to:

• Decreased tissue O2 levels or increased CO2 levels

• Generation of lactic acid

• Rising K+ or H+ concentrations in interstitial fluid

• Local inflammation

• Elevated temperature

• Vasoconstrictors:

• Injured vessels constrict strongly (why?)

• Drop in tissue temperature (why?)

Autoregulation of blood flow within tissues


Myogenic (myo =muscle; gen=origin) Controls

• Inadequate blood perfusion (tissue might die) or excessively high arterial pressure (rupture of vessels) may interfere with the function of the tissue

• Vascular smooth muscle can prevent these problems by responding directly to passive stretch (increased intravascular pressure)

• The muscle response is by resist to the stretch and that results in vasoconstriction

• The opposite happens when there is a reduced stretch

• The myogenic mechanisms keeps the tissue perforation relatively constant.


Metaboliccontrols

pH Sympathetic

a Receptors

b ReceptorsEpinephrine,norepinephrine

Angiotensin II

Antidiuretichormone (ADH)

Atrialnatriureticpeptide (ANP)

Dilates

Constricts

Prostaglandins

Adenosine

Nitric oxide

Endothelins

Stretch

O2

CO2

K+

Amounts of:

Amounts of:

Nerves

Hormones

Myogeniccontrols

Intrinsic mechanisms(autoregulation)

• Distribute blood flow to individual organs and tissues as needed

Extrinsic mechanisms

• Maintain mean arterial pressure (MAP)• Redistribute blood during exercise and thermoregulation


Short-Term Mechanisms: Neural Controls

• Neural controls of peripheral resistance:

• Alter blood distribution in response to demands

• Maintain MAP by altering blood vessel diameter

• Neural controls operate via reflex arcs involving:

• Vasomotor centers and vasomotor fibers

• Baroreceptors

• 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

• Vasomotor activity is modified by:

• Baroreceptors (pressure-sensitive)

• chemoreceptors (O2, CO2, and H+ sensitive)

• bloodborne chemicals

• hormones


Short-Term Mechanisms: Baroreceptor-Initiated Reflexes

• Baroreceptors are located in

• Carotid sinuses

• Aortic arch

• Walls of large arteries of the neck and thorax

http://archive.student.bmj.com/back_issues/0599/graphics/0599ed2_1.gif


Short-Term Mechanisms: Baroreceptor-Initiated Reflexes

• 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


Medullarycardiovascularcontrol center

Carotid and aorticbaroreceptors

Change inblood

pressure

Parasympatheticneurons

Sympatheticneurons

Veins

Arterioles

Ventricles

SA node

Integrating center

Stimulus

Efferent path

Effector

Sensory receptor

KEY

Blood Pressure

Figure 15-22 (10 of 10)


Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes

• Chemoreceptors are located in the

• Carotid sinus

• Aortic arch

• Large arteries of the neck

• Chemoreceptors respond to rise in CO2, drop in pH or O2

• Increase blood pressure via the vasomotor center and the cardioacceleratory center


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 – kidney release of renin generates angiotensin II, which causes vasoconstriction

• 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) – is a brief but potent vasodilator

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

• Alcohol – causes BP to drop by inhibiting ADH


Long-Term Mechanisms: Renal Regulation

• Long-term mechanisms control BP by altering blood volume

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

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

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

• Direct renal mechanism alters blood volume (changes in urine volume)

• Indirect renal mechanism involves the renin-angiotensin mechanism


Indirect Mechanism: renin-angiotensin

• The renin-angiotensin mechanism

• Arterial blood pressure release of renin

• Renin production of angiotensin II

• Angiotensin II is a potent vasoconstrictor

• Angiotensin II aldosterone secretion

• Aldosterone renal reabsorption of Na+ and urine formation

• Angiotensin II stimulates ADH release

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