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Lect 17, Chapter 21

Peripheral Circulation and Regulation

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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

• Skeletal muscle pump

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Blood Vessel Structure

• Arteries– Elastic, muscular, arterioles

• 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

• Veins– Venules, small veins, medium or large veins

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Capillaries• Capillary wall consists

mostly of endothelial cells

• Types classified by diameter/permeability – Continuous

• Do not have fenestrae many tissues

– Fenestrated• Have pores (20-100nm)

found in endocrines

– Sinusoidal• Large diameter with large

fenestrae

• In the liver, bone marrow, spleen

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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

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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

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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

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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

• Atriovenous anastomoses– Allow blood to flow from arterioles to small

veins without passing through capillaries

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Blood Vessel Comparison

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Aging of the Arteries

• Arteriosclerosis– General term for

degeneration changes in arteries making them less elastic

• Atherosclerosis– Deposition of plaque

on walls

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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

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Fig. 21-8 Cardiovascular physiology

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Systemic Circulation: Arteries

• Aorta– From which all arteries are derived either

directly or indirectly– Parts

• Ascending, descending, thoracic, abdominal

• Coronary arteries– Supply the heart

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Dynamics of Blood Circulation

• Interrelationships between– Pressure– Flow– Resistance– Control mechanisms that regulate blood

pressure– Blood flow through vessels

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2. Physics and Physiology of Circulation

Human's circulatory system is a closed circulatory system. Thus the blood pressure and the flow resistance become dependent on each other.

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What is meant by 760 mmHg? 1 atmosphere is 760 mm of mercury (760 torr) and is the barometric pressure at the sea level and at 0˚C, and decreases in response to the altitude. At Detroit the barometric pressure is about 740 torr.

1 A.P. = 14.7 lb/in2 = 1,033 gm/cm 2

1 lb/in2 = 70.31 gm/cm2

Tire pressure of 30 lb/in2 = 2.04 A.P.

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a. Pressure and blood flow

Pressure is the driving force of the blood flow.

When blood vessels are connected, the blood flows from the higher pressure site to the lower pressure site and the rate of flow is proportional to the pressure difference.

FLOW RATE = PRESSUREDIFFERENCE/RESISTANCE

The overall pressure difference is between the ascending aorta and the entrance to the right atrium - the circulatory pressure (about 100 mmHg or torr). (Fig. 21.30 and 31 of Seeley) Note the overall cross-sectional areas.

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POISEUILLE’S LAW

FLOW RATE = PRESSURE DIFFERENCE/Resistance

=(PRESSURE DIFFERENCE) / (8VL/ R4)

Where V is the viscosity of the blood, L the length of the blood vessel and R the radius of the blood vessel..

How can you obtain a faster flow rate?

Apply a large entrance pressure -- P

Keep the diameter large -- R

Use low viscosity fluid -- V

Use a short tube -- L

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Critical closing pressure and Laplace's law.

Blood vessels collapse below a critical closing pressure.

At this point, the blood flow stops.

Thus, if the blood pressure is reduced by shock or loss of blood, it may lead to a collapsed blood vessel.

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Laplace's law states:

F = D * P

where, F is the force applied to the vessel, D diameter of the vessel and P the blood pressure.

If the blood pressure remains the same, but a disease caused to enlarge the diameter of the blood vessel, the force to the blood vessel will increase and could even burst the blood vessel, if its wall is not strong enough!!

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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

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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

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Blood Pressure Measurement

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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

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b. Resistance

Resistance to circulation is dominated at the site of the arterial system, inclusive of capillaries. The peripheral resistance, and resistance of the venous system is low.

Resistance is dependent on the viscosity of blood, the diameter and length of small blood vessels.

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i. Vascular resistance

It is not difficult to imagine that vascular resistance becomes large when the blood attempts to pass through thin capillary blood vessels.

It is the resistance between the blood matters and the wall of the capillaries.

The size of capillaries may also be regulated with the contraction and relaxation of muscle tissues, arterioles and constriction of precapillary sphincter muscles.

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ii. Viscosity

We have already seen that the viscosity of blood is about five times higher than that of water.

Under pathological conditions blood viscosity may change due to the change of erythrocytes or plasma protein contents.

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iii. Turbulence

When a solution passes through a smooth tubing, the flow will be fastest at the center of the tube and, due to the adhesion to the wall, the slowest near at the wall forming multiple layers of liquid with different velocities - LAMINAR flow.

Turbulence of flow may be created when the LAMINAR flow is disturbed by adding a restrictive substance in the flow, bending or changing he diameter of the vessels.

When there is a sudden increase in the diameter of the vessel, turbulence will be created causing the flow rate to decrease and decrease the pressure.

Turbulent blood flow becomes the source of sound, especially in the heart and can be heard with the stethoscope leading observation of Korotkoff sounds.

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c. Circulatory pressures

The change of pressure in the circulatory system is shown in Fig. 21-9, 10

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i. Arterial blood pressure

Note that the systolic and diastolic pressures (pulse pressure) of the ventricle are distinctive in the arteries. The pressure drops gradually as the blood passes through arterioles, then drops more in the pressure prior to the capillary sphincter, and in the veins.

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ii. Capillary pressures and capillary dynamics

Up to 40 mmHg of pressure difference exist between the entrance and exit of a capillary and is an important difference.

The exchanges of blood substances across the capillary epithelia are via (a) passing through the cellular gaps, (b) solubilizing through the membranes (c) fenestrae, and (d) through vesicles. Fig. 21-12

Also note that in the brain, the matters cross through the capillary epithelia only through mediated transport.

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At least, several driving forces must be considered: (a) Blood pressure difference between the lumen of

capillary and interstitial fluid, (b) The concentration gradient (c) Osmotic pressure.

Recall that the osmotic pressure drives transport of water from the region of a low concentration of solute to that of a higher concentration across a semi-permeable membrane.

Thus, at the entrance to the capillary, high blood pressure pushes water out from the capillary, but a higher osmotic pressure in the capillary due to its high content of plasma proteins, brings the water back into the capillary. The balance of the two is to push water out of the capillary.(Fig. 21-13)

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Fluid Exchange Across Capillary Walls

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The blood pressure decreases at the other side of the capillary, but the osmostic pressure (proteins inside the capillaries vs. proteins in the tissue fluid) remains about the same. The balance will be that to bring water back into the capillary.

Water and solubilized substances, such as oxygen, glucose, nutrients, may get into the interstitial fluids along with their concentration gradients at the start of the capillary.

90% of water (the rest goes to the lymphatic circulatory system) and the waste products will come back to the capillary at the other end.

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Vein Characteristics andEffect of Gravity on Blood

PressureVein Characteristics• Venous return to heart

increases due to increase in blood volume, venous tone, and arteriole dilation

Effect of Gravity• In a standing position,

hydrostatic pressure caused by gravity increases blood pressure below the heart and decreases pressure above the heart

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iii. Venous pressure

The venous pressure is low and it gets lower as the blood vessels become larger.

It is only 2 torr at the right atrium, or about 16 torr in the vein.

Blood flow through the venous of skeletal muscles may be maintained through muscular compression and the valves.

The calves are the second heart.

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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

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3. Cardiovascular regulation (Fig. 21-14)

The homeostatic mechanism to maintain the steady flow of blood relies on (a) cardiac output, (b) peripheral resistance, and (c) blood pressure.

They are under the control of neuronal and endocrine systems.

Throughout the body, most of the cells are relatively close to the capillaries. Within 134 um.

This is important, since after leaving capillaries, nutrients and oxygen must reach the target sites mostly by diffusion.

In addition to the neuronal and endocrine factors, the regulation of cardiovascular function may be maintained through "local factors"

Local factors imply the shift of blood flows within the capillary bed to direct the flow to the needed site - auto-regulation.

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a. Autoregulation of blood flow

Precapillary sphincter muscle responds to the tissue oxygen or carbon dioxide level and regulates the direction of blood flow.

Release of histamine, bacterial toxin and prostaglandins cause a relaxation of precapillary sphincters and vasodilation occurs at the injury site - vasodilators vs. vasoconstrictors. (Fig. 21.33 of Seeley)

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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

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b. Neuronal control of blood pressure and blood flow FIG. 21.34 of Seeley.

The nervous system regulates the cardiac output and peripheral resistance.

We have already learnt that the cardioacceleratory center in the medulla oblongata to increase cardiac output through sympathetic innervation and the cardioinhibitory center to reduce cardiac output through parasympathetic innervation.

The regulation of peripheral resistance at the region of the arterioles is primarily dependent on the vasomotor center of the medulla oblongata. Through the sympathetic motor neurons, it provides tonic stimulation demonstrating vasomotor tone that partially constricts the blood vessels. This peripheral resistance results in a lower cardiac output.

Another region of the vasomotor center is initiatory to the above and results in vasodilation, thus an increased cardiac output.

The cardiovascular center of the medulla oblongata responds to the signals from the baroreceptors and chemoreceptors in response to the blood pressure change, pH and dissolved gases.

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Nervous Regulation of Blood Vessels

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Short-Term Regulation ofBlood 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

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Baroreceptor Reflex Control

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Chemoreceptor Reflex Control

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Long-Term Regulation of Blood Pressure

• Renin-angiotensin-aldosterone mechanism

• Vasopressin (ADH) mechanism

• Atrial natriuretic mechanism

• Fluid shift mechanism

• Stress-relaxation response

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Renin-Angiotensin-AldosteroneMechanism

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Vasopressin (ADH) Mechanism

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iii. Erythropoietin

Loss of blood pressure or the oxygen content, releases erythropoietin from the kidneys.

Increased erythrocytes.

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iv. Atrial natriuretic peptide

Released when blood pressure is increased.

When the atrial wall is stretched this hormone is made.

Reduces blood volume and pressure by (a) loss of Na+ and water in the kidneys, (b) increasing water losses at the kidneys by blocking ADH and aldosterone, (c) blocking the release of epinephrine and norepinephrine, (d) peripheral vasodilation.

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Shock

• 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

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Fig. 21-35 Fetal circulation