C h a p t e r 21 Blood Vessels and Circulation Chapter 21

C h a p t e r

Blood Vessels and Circulation

Chapter 21

Pressure and Resistance

Total capillary blood flow directly related to

cardiac output

Is determined by:

pressure and resistance in the cardiovascular

system

Figure 21–8 An Overview of Cardiovascular Physiology

Pressure (P)

The heart generates P to overcome resistance (R)

Absolute pressure less important than pressure gradient

The Pressure Gradient (P) is the difference in P’s

Circulatory pressure has pressure gradient

It is the difference between:

Pressure at the heart

Pressure at peripheral capillary beds

Flow (F)

Is proportional to the pressure gradient (P)

divided by R

F=P / R

Measuring Pressure

Blood pressure (BP)

Arterial pressure (mm Hg)

Capillary hydrostatic pressure (CHP)

Pressure within the capillary beds

Venous pressure

Pressure in the venous system

Circulatory Pressure:

∆P across the systemic circuit (about 100 mm Hg)

Circulatory pressure must overcome total

peripheral R

R of entire cardiovascular system

Total Peripheral “R” affected by 3 factors:

1. Vascular R

Due to friction between blood and vessel walls

Depends on vessel length and vessel diameter:

– adult vessel length is constant

– vessel diameter varies by vasodilation/vasoconstriction...

(R increases exponentially as vessel diameter ________)

2. Viscosity

R caused by molecules and suspended

materials in a liquid

Whole blood viscosity is about 4 x water

3. Turbulence

Swirling action that disturbs smooth flow of

liquid

Occurs in heart chambers and great vessels

Atherosclerotic plaques cause abnormal

turbulence

Cardiovascular Pressures:

Systolic pressure

Peak arterial pressure during ventricular systole

Diastolic pressure

Minimum arterial pressure during diastole

Pulse pressure

Difference between systolic pressure and diastolic pressure

Mean arterial pressure (MAP)

MAP = diastolic pressure + 1/3 pulse pressure

Normal/Average BP= 120/80

Abnormal BP:

Hypertension - Abnormally high BP

–greater than 140/90

Hypotension - Abnormally low BP

Larger arterial walls have elastic rebound

Stretch during systole

Recoil to original shape during diastole

Keep blood moving during diastole

Figure 21–9

Pressures in Small Arteries and Arterioles

affected by distance

MAP and pulse pressure decrease with distance

from heart

Blood pressure decreases as multiple branches arise

Figure 21–10 Pressures within the Systemic Circuit

Venous Pressure and Venous Return Low pressure exists in venous system 2 actions improve venous return:

Muscular compression of peripheral veins:– compression of skeletal muscles pushes blood toward

heart (through one-way valves) The respiratory pump in thoracic cavity:

– inhaling decreases thoracic pressure (draws air/blood toward lungs)

– exhaling raises thoracic pressure (forces air/blood away from lungs)

Capillary Pressures and Exchange

Vital to homeostasis

Moves materials across capillary walls by

Diffusion (from concentration gradient)

Filtration (from hydrostatic pressure)

Reabsorption (from osmosis)

Figure 21–11 Capillary Filtration

Interplay between Filtration and Reabsorption:

Hydrostatic pressure

Forces water out of solution

Osmotic pressure

Pulls water into solution

Both control filtration and reabsorption through

capillaries

Net Hydrostatic Pressure:

Is the difference between

Capillary hydrostatic pressure (CHP)

And Interstitial fluid hydrostatic pressure (IHP)

Pushes water and solutes...

From __________ to ____________

Net Colloid Osmotic Pressure:

Is the difference between

Blood colloid osmotic pressure (BCOP)

And interstitial fluid colloid osmotic pressure

(ICOP)

Pulls water and solutes…

From __________ into ____________

Net Filtration Pressure (NFP):

The difference between:

Net Hydrostatic pressure

And Net Osmotic pressure

NFP = (CHP – IHP) – (BCOP – ICOP)

Capillary Exchange At arterial end of capillary

Fluid moves from capillary

Into interstitial fluid

At venous end of capillary Fluid moves into capillary

From interstitial fluid

Transition point between filtration and reabsorption Is closer to venous end than arterial end

So… capillaries usually filter more than they reabsorb

Figure 21–12 Forces Acting across Capillary Walls

Water continuously moves out of capillaries and

back into the bloodstream (Fluid Recycling ) via

the lymphatic system1. Ensures constant plasma and interstitial fluid

communication

2. Accelerates distribution of nutrients, hormones, and

dissolved gases through tissues

3. Transports insoluble lipids and tissue proteins that

cannot cross capillary walls

4. Flushes bacterial toxins and chemicals to immune

system tissues

Capillary Dynamics Hemorrhaging

Reduces CHP and NFP

Increase _____________of interstitial fluid (recall of fluids)

Dehydration Increases BCOP

Increases _____________ of interstitial fluid

High BP Elevated CHP

Increases _____________ into the interstitial fluid

Fluid builds up in peripheral tissues (Edema)

Cardiovascular Regulation

Tissue Perfusion is the blood flow through tissues

Carries O2 and nutrients to tissues and organs

Carries CO2 and wastes away

Is affected by

Cardiac output

Peripheral resistance

Blood Pressure

Cardiovascular regulation changes blood

flow to a specific area

At an appropriate time

In the right area

Without changing blood pressure and blood flow

to vital organs (Ex: ______________)

Figure 21–13 Short-Term and Long-Term Cardiovascular Responses

Controlling Cardiac Output and Blood Pressure

Autoregulation

Causes immediate, localized homeostatic adjustments

Neural mechanisms

Respond quickly to changes at specific sites

Endocrine mechanisms

Direct long-term changes

Autoregulation of Blood Flow within Tissues- Adjusted by Peripheral Resistance while CO stays the

same Can cause local vasoconstriction or vasodialation

Triggered by prostaglandins and other “local” factors

Local vasoconstrictors can reduce blood flow by:

1. Constricting precapillary sphincters

2. Affecting a single capillary bed

Autoregulation of Blood Flow within Tissues-Local vasodilators can accelerate blood flow at

tissue level if:» low O2 or high CO2 levels

» low pH (acids)

» nitric oxide (NO) levels high

» high K+ or H+ concentrations

» chemicals released by inflammation (histamine)

» elevated local temperature

Neural Mechanisms Cardiovascular (CV) centers of the Medulla

Oblongata Cardiac centers:

– cardioacceleratory center: increases cardiac output– cardioinhibitory center: reduces cardiac output

Vasomotor center:– vasoconstriction

» controlled by adrenergic nerves (NE)

» stimulates smooth muscle contraction in arteriole walls

– vasodilation:» controlled by cholinergic nerves (ACh)

» relaxes smooth muscle

Vasomotor Tone

Maintained by constant action of sympathetic

vasoconstrictor nerves

Keeps your blood pressure high enough to get

good capillary exchange!

Reflex Control of Cardiovascular Function

Cardiovascular centers monitor arterial blood

Baroreceptor reflexes:

– respond to changes in blood pressure

Chemoreceptor reflexes:

– respond to changes in chemical composition, particularly

pH and dissolved gases

Baroreceptor Reflexes Stretch receptors in walls of

Carotid sinuses: maintain blood flow to brain

Aortic sinuses: monitor start of systemic circuit

Right atrium: monitors end of systemic circuit

When blood pressure rises, CV centers Decrease cardiac output

Cause peripheral vasodilation

When blood pressure falls, CV centers Increase cardiac output

Cause peripheral vasoconstriction

Figure 21–14 Baroreceptor Reflexes of the Carotid and Aortic Sinuses

CNS Activities and the Cardiovascular

Centers

Thought processes and emotional states can

elevate blood pressure by cardiac stimulation

and vasoconstriction

Hormones and Cardiovascular Regulation

Hormones have short-term and long-term

effects on cardiovascular regulation

E and NE from suprarenal medullae stimulate

cardiac output and peripheral vasoconstriction

Antidiuretic Hormone (ADH)

Released by neurohypophysis (posterior lobe of

pituitary)

Elevates blood pressure

Reduces water loss at kidneys

ADH responds to:

Low blood volume

High plasma osmotic concentration

Circulating angiotensin II

Angiotensin II

Responds to fall in renal blood pressure

Stimulates

Aldosterone production

ADH production

Thirst

Cardiac output

Peripheral vasoconstriction

Erythropoietin (EPO)

Released at kidneys

Responds to low blood pressure, low O2

content in blood

Stimulates red blood cell production

Natriuretic Peptides

Atrial natriuretic peptide (ANP)

Produced by cells in right atrium

Brain natriuretic peptide (BNP)

Produced by ventricular muscle cells

Respond to excessive diastolic stretching

Will lower blood volume and blood pressure

And reduce stress on heart

Figure 21–16a The Hormonal Regulation of BP and BV

Figure 21–16b The Hormonal Regulation of BP and BV

Cardiovascular Adaptation

Blood, heart, and cardiovascular system

Work together as unit

Respond to physical and physiological

changes (for example, exercise, blood loss)

Maintains homeostasis

The Cardiovascular Response to Exercise

Light exercise Extensive vasodilation occurs:

– increasing circulation

Venous return increases:

– with muscle contractions

Cardiac output rises:

– due to rise in venous return (Frank–Starling principle)

and atrial stretching

The Cardiovascular Response to Exercise

Heavy exercise Activates sympathetic nervous system

Cardiac output increases to maximum: – about 4 x resting level

Restricts blood flow to “nonessential” organs (e.g., digestive system)

Redirects blood flow to skeletal muscles, lungs, and heart, skin

Blood supply to brain is unaffected

Exercise, Cardiovascular Fitness, and Health

Regular moderate exercise

______ total blood cholesterol levels

Intense exercise

Can cause severe physiological stress

The Cardiovascular Response to Hemorrhaging

Entire cardiovascular system adjusts to

Maintain blood pressure

Restore blood volume

Short-term responses compensate after blood

losses of up to 20% of total blood volume

Failure to restore blood pressure results in

Organ systems start to fail

Vascular Supply to Special Regions

Blood Flow to the Brain is top priority due to

high oxygen demand

When peripheral vessels constrict, cerebral

vessels dilate, normalizing blood flow to brain

Stroke

Also called cerebrovascular accident (CVA)

Blockage or rupture in a cerebral artery

Maybe due to aneurism rupture

Stops blood flow to brain

Blood Flow to the Heart Through coronary arteries

Oxygen demand increases with activity

Rising Lactic acid and low O2 levels

Dilate coronary vessels

Increase coronary blood flow

Heart Attack (Myocardial Infarction/MI)

A blockage of coronary blood flow

Can cause:

Angina (chest pain)

Tissue damage

Heart failure

Blood Flow to the Lungs

Regulated by O2 levels in alveoli

High O2 content in alveolus

Vessels dilate, _______ cap exchange

Low O2 content in alveolus

Vessels constrict, _______cap exchange

– Shunts blood to other O2rich alveoli

Aging and the Cardiovascular System

Cardiovascular capabilities decline with

increasing age

Age-related changes occur in:

Blood vessels

3 Age-Related Changes in Blood

1. Decreased hematocrit

2. Peripheral blockage by blood clot (thrombus)

3. Pooling of blood in legs

Due to venous valve deterioration

5 Age-Related Changes in the Heart

1. Reduced maximum cardiac output

2. Changes in nodal and conducting cells

3. Reduced elasticity of cardiac (fibrous) skeleton

4. Progressive atherosclerosis

5. Replacement of damaged cardiac muscle cells by

scar tissue

3 Age-Related Changes in Blood Vessels

1. Arteries become less elastic

Pressure change can cause aneurysm

2. Calcium deposits on vessel walls

Can cause stroke or infarction

3. Thrombi can form

At atherosclerotic plaques

C h a p t e r 21 Blood Vessels and Circulation Chapter 21

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