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Learning Objectives:

Explain how capillary perfusion is

regulated

Eplain how the elasticity of arties

affects diastolic pressure

Describe the theory of

sphygmomanometry

Calculate mean arterial pressure

and pulse pressure

QUIZ/TEST REVIEW NOTES

SECTION 1 BLOOD FLOW/CONTROL OF BLOOD PRESSURE

[VASCULATURE] CHAPTER 15

I. STRUCTURE OF VESSELS

ARTERY VEIN

Walls Thick, Low Compliance (not

very collapsible), High Recoil

(very stretchable)

Thin, High Compliance (very

collapsible), Low Recoil (not

stretchable)

Inner Diameter Small Larger

Pulse Pressure Greatest Pressure Least Pressure

Valves None Yes

Miscellaneous 65% of blood volume is held in

the veins except during exercise

a. General Characteristics of blood vessels a. Compliance and Recoil

Compliance: How easily the wall can collapse

Recoil: How easily the wall can stretch Arteries No Compliance/Yes Recoil

Veins Yes Compliance/No Recoil

Blood leaving the left side of the heart (oxygenated) enters systemic arteries where there is

expandable elastic regions; where pressure produced contraction of left ventricle is stored in

the elastic walls of arteries and slowly released through elastic recoil

Artery Arteriole Capillary Venule Vein

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This mechanism maintains continuous driving pressure for blood flow during the time when

ventricles are relaxing

Blood flow through any level of circulation is equal to CO (If cardiac output is 5L/min by

heart, it will be 5L/min in all other systemic capillaries)

>Blood Vessels Contain Vascular Smooth Muscle<

- Smooth muscle in vessels is known as vascular smooth muscle

- Vasoconstriction: Narrows the diameter of vessels lumen

- Vasodilatation: Widens the diameter of vessels lumen

b. Exchange Between the Blood and Interstitial Fluid Takes Place in the Capillaries a. General characteristics

Smallest vessels in the cardiovascular system

Site of exchange between blood and interstitial fluid

Site of exchange also for nutrients, waste, and water

Lack smooth muscle and elastic/fibrous tissue reinforcement

Walls contain only flat layer of endothelium one cell thick supported on basal lamina

(acellular matrix/basement membrane)

- Pericytes are contractile cells that surround the capillaries forming

mesh like outer layer between endothelium of capillaries and interstitial fluid

- Pericytes contribute to tightness of capillary permeability; more

perictyes the less leaky the capillary

2 MAIN TYPES OF CAPILLARIES:

(1) Continuous Capillaries: Endothelia cells are joined to one another with leaky

junctions;

(2) Fenestrated Capillaries: Large pores that allow high volumes of fluid to pass

rapidly; found primary in kidney/intestine

b. Microcirculation through capillary beds

- Capillary density is directly related to the metabolic activity of a tissue

cell (Muscles/Glands have highest density)

- Blood cells pass single file; one at a time

- Cell junctions between endothelia cells account for leakiness of

capillary

- 3 Liters of fluid pushed out of capillaries every day

- HYDROSTATIC PRESSURE

(1) Lateral pressure component of blood flow that pushes fluid

out through the capillary pores

(2) Decreases along the length of the capillary as energy is lost to friction

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o Bulk Flow: Mass movements of fluid between blood and

interstitial fluid

o Absorption: If direction of bulk flow is into the capillary

o Filtration: If the direction of bulk flow is out of the

capillary

II. MEASURING BLOOD PRESSURE

a. Blood Pressure - Pressure created by ventricular contraction is the driving force for

blood flow throughout the cardiovascular system

- By sustaining the driving pressure for blood flow during ventricular

relaxation (diastole) the arteries create continuous blood flow through

the blood vessels

- Four factors affecting resistance

Radius blood vessels Viscosity of blood

Length of system

Friction between blood vessels

b. Systemic Blood pressure is Highest in Arteries and Lowest in veins

Blood pressure is highest in arteries and decreases through circulatory system Decrease occurs because energy is lost as a result of flow resistance

a. VENTRICLE Systolic [WORKING] pressure

- Maximal pressure created by ventricular systole causing a pressure

wave that moves ten times faster then the blood itself

- Highest pressure occurrence in the Aorta because of left ventricle

systole [120mm Hg]

b. VENTRICLE Diastolic [RESTING] pressure

- Arterial pressure during ventricular diastole

- Ventricle pressure falls near [0mm Hg] but diastolic pressure in large

arteries remains relativity high because of their ability to capture and

store energy in their elastic walls

c. Pulse pressure

- Rapid pressure increase that occurs when left ventricle pushes blood

into aorta is called the PULSE (pressure wave)

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- Pressure wave deceases over distance because of friction and

eventually disappears at the capillaries

- Pulse Pressure: Measure of the strength of the pressure wave; defined

as systolic pressure minus diastolic pressure

PP = Systolic – Diastolic

Areas where pulse pressure is present Left Ventricle Artery

Arteriole

Areas where there is no pulse pressure present Capillaries

Venules

Right Ventricle

- Holding your arm straight down for a couple minutes

causes the veins in the back of your hand to stand out as

they fill with blood

- More evident in older people whose subcutaneous C.T.

has lost elasticity

- Then raising your hand above your head so gravity

assists venous flow causes bulging veins to disappear - This is example of venous return to the heart aided by

skeletal muscle pump and respiratory pump; which

contain internal one-way valves to ensure blood passing

the valve cannot flow backward

d. Sphygmomanometry - The measuring of arterial blood pressure in the radial artery of an arm

using an instrument consisting of an inflatable cuff and a pressure

gauge (sphygmus, pulse + manometer, instrument for measuring

pressure of fluid) - Cuff is inflated to higher then systolic pressure to cut off blow flow - Pressure gradually released until pumping noise of systolic flow can be

heard as thumping sound from radial artery, called Korotkoff Sound First Korotkoff sound is the systolic pressure

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Diasoltic Pressure

1/3 (Systolic Pressure -Diasotlic Pressure)

MAP

Point where Korotkoff sound disappears is the

diastolic pressure

c. Arterial Blood Pressure Reflects the Driving Pressure for Blood Flow (MAP:

Mean Arterial Pressure) a. MAP

- Ventricular pressure is difficult to measure so it is customary to assume

that arterial blood pressure reflects ventricular pressure

- MAP represents driving pressure, “blood pressure” created by the pumping action of the heart

- MAP is closer to diastolic pressure than to systolic pressure because diastole lasts twice as long as systole

HYPOTENSION: Blood pressure falls to low,

driving force of blood will be unable to

overcome gravity (dizzy/faint)

HYPERTENSION: Chronically elevated

pressure; vessel walls may weaken and rupture

CEREBRAL HEMORRHAGE: If rupture occurs

in brain; “stroke”

b. Cardiac Output and Peripheral Resistance Determine MAP

- Arterial pressure is a balance between flow into the arteries and blood flow out of the arteries

More blood in then out: Increase MAP (blood collects in arteries)

More blood out then in: Decrease MAP (no blood in arteries)

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- Peripheral Resistance: Influences blood flow out of the arterioles;

Definition: Resistance to flow offered by the arterioles; how much

resistance blood has to fight against to move through arterioles

- Most cases of hypertension are believed to be caused by increased

peripheral resistance without changes in C.O

c. Changes In Blood Volume Affect Blood Pressure

- If blood volume increases blood pressure increases

- If blood volume decreases blood pressure decreases

- Adjustments for increased blood volume are primarily the

responsibility of the kidneys;

o If blood volume increases the kidneys excrete excess

water in urine

o If blood volume decreases the kidneys cannot restore lost

fluid

III. Resistance in the Arterioles

- Arterioles are the main site of variable resistance in the systemic

circulation and contribute more then 60% of total resistance - Resistance in arterioles is variable because of large amounts of smooth

muscle

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- Arteriolar resistance in influence by both systemic and local control

mechanisms 1. Sympathetic Reflexes: Mediated by C.N.S. – maintain

MAP and govern blood distribution for homeostatic needs

2. Local Control Of Arteriolar Resistance: Matches

tissue blood flow to metabolic needs of the tissue;

3. Hormones: Regular salt and water excretion by

Kidneys – influence blood pressure by acting directly

on arterioles and altering autonomic reflex control

- BELOW: Table that lists chemicals that mediate arteriolar resistance by

producing vasoconstriction or vasodilatation that influence blood flow at tissue level

CHEMICAL PHYSIOLOGICAL ROLE SOURCE TYPE

Vasoconstrictors

Norepinephrine (α

receptors)

Baroreceptor reflex

Sympathetic neurons Neurotransmitter

Vasopressin Increase blood pressure in

hemorrhage

Posterior pituitary Neurohormone

Angiotensin II Increase blood pressure Plasma hormone

Vasodilators

Epinephrine (β2

receptors)

Increase blood flow to

skeletal muscle, heart,

liver

Adrenal Medulla Neurohormone

Alpha Antagonists - - -

Nitrates (NO) Paracrine mediator Endothelium Paracrine

Adenosine Increase blood flow to

match metabolism

Hypoxic cells Paracrine

↓O2↑CO2↑ H+↑

K

+ Increase blood flow to

match metabolism

Cell metabolism Paracrine

Histamine Increase blood flow Mast Cells Paracrine

b. Myogenic Autoregulation Automatically Adjusts Blood Flow - Vascular smooth muscle has ability to regulate its own state of

contraction, process called Myogenic Autoregulation

- Absence of Autoregulation an increase in blood pressure increases

blood flow through arteriole

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- When smooth muscle fibers in wall arteriole wall stretch because of

increased blood pressure the arteriole constricts

- Vasoconstriction increases resistance offered by arteriole, automatically

decreasing blood flow through the vessel

Mechanism responsible for intrinsic response of

vascular smooth muscle is stretch that opens gated Ca2+ channels

c. Paracrines Alter Vascular Smooth Muscle Contraction - Active Hyperemia: Process in which an increase in blood flow

accompanies an increase in metabolic activity

If blood flow to tissue is occluded (closed) O2 levels fall and metabolically

produced Paracrines such as CO2/H+ accumulate

Local hypoxia (low oxygen) causes synthesis of vasodilator NO causing significant vasodilatation

- Reactive Hyperemia: An increase in tissue blood flow following a

period of low perfusion

d. Sympathetic Branch Controls Vascular Smooth Muscle - Most systemic arterioles are innervated by sympathetic neurons

(exception erection reflex penis/clitoris which indirectly by

parasympathetic)

- Tonically controlled and always actively regulating and maintain Myogenic tone of arterioles

- Norepinephrine binding to α-Receptors on vascular smooth muscles

causes vasoconstriction

If sympathetic release of Norepinephrine

decrease the arterioles dilate

Overall:

Metabolic Paracrines (adenosine, histamine, ATP) induce

vasodilatation

Blood flow (perfusion) is matched to metabolic activity)

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Flight or Fight Response Correlations to Sympathetic Control

β2-Receptors respond to epinephine by vasodilating; enhancing blood flow to the heart, skeletal msucles and liver, tissues that are active during flight-fight repsonse

α-Receptor causes vasoconstriction; increased sympathetic activity that

diverts blood from nonessential organs such as

G.I. tract to skeletal muscles, liver and heart

If sympathetic release of Norepinephrine

increases the arterioles constrict

- Epinephrine from adrenal medulla binds with α-Receptors reinforcing

vasoconstriction

Also binds to β2-Receptors on vascular smooth

muscle on heart/liver/skeletal muscle arterioles These sites are not innervated and respond only

to excess/present epinephrine freely flowing in

circulation

Activation β2-Receptors causes vasodilatation

IV. Blood Distribution to Tissue - Distribution of system blood varies according to metabolic needs of

individual organs

Ex: Non-exercise Skeletal Muscles: 20%

Ex: Exercise Skeletal Muscles: 85%

- All arterioles receive blood at the same time from the aorta

Flow through individual arterioles depends on

their resistance

Blood is diverted from high-resistance arterioles

to lower-resistance arterioles

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Total blood flow through all the arterioles of the body always equals

the C.O.

V. CAPILLARY EXCHANGE

a. Overview - Capillary density in tissue is directly related to metabolic activity of

cells tissue

- Tissues with high metabolic activity and high need for oxygen/nutrients and contain more capillaries per unit area

- Most capillaries in muscles and gland

- Have thinnest walls of all blood vessels composed of single endothelial

cell on a basal lamina

- Diameter allows for blood to line up single filed to diffuse

- Continuous Capillaries: Most common; joined to one another by leaky

junctions found in muscle, C.T., neural tissue (blood-brain barrier)

- Fenestrated Capillaries: Large pores to allow for high volumes of fluid

to pass between plasma and interstitial fluid; primary in kidney and

intestine

b. Velocity Blood Flow is Lowest in Capillaries - Determinant for velocity of flow through capillaries is not diameter but

total cross-sectional area

- Least resistance of all arterial blood vessels

- Opening of capillary beds decreases PR and MAP

- Velocity of flow is inversely proportional to cross-sectional area

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Hydrostatic Pressure

• Lateral pressure component of blood flow that pushses fluid out through capilary pores

• Capillary Hydrostatic Pressure decreases along the lenght of capillary as energy is lost to driction

•Water movement due to hydrostatic pressure will always be directed out of capillary due to low hdrostatic pressure in interstitial fluid

Osmotic Pressure

•Determined by a solute concentration of a compartment

•Main solute difference between plasma and interstitial fluid is due to proteins

• Pressure created by protein presence is known as colloid osmotic pressure

c. Diffusion and Transcytosis Exchange - Diffusion rate is determined by concentration gradient between plasma

and interstitial fluid

- Transcytosis: Transport of large molecules including selected proteins

across endothelium

d. Filtration and Absorption by Bulk Flow - Bulk Flow: Mass movement of fluid between blood and interstitial

fluid as result of hydrostatic or osmotic pressure gradients

Movement into the capillary: absorption

Movement out of the capillary: filtration which is caused by

hydrostatic pressure that forces fluid out of capillary through

cell junctions]

- Most capillaries show net filtration from arterial end to net absorption

at venous end

STARLING FORCES the regulators of bulk flow through capillaries

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• Carotid Artery

• Aortic Arch

Baroreceptors

•Medullary Cardiovascular Control Center

• Integrating Center

Medulla Oblongata •Autnomic Neurons

• Sympathetic

• Parasympathetic

Efferent Pathway

VI. REGULATION OF BLOOD PRESSURE a. Baroreceptor Reflex Primary Homeostatic Control for Blood Pressure

- CNS coordinates reflex control of blood pressure

- Main integrating center is the medulla oblongata

- Stretch-sensitive mechanoreceptors known as baroreceptors are located

in walls of carotid arteries and aorta where they measure blood flow to

the brain (carotid baroreceptors) and body (aorta baroreceptors)

[carotid and aorta = tonically regulated] (always firing)

- Baroreceptor reflex

Primary reflex pathway for homeostatic control of blood pressure

Increased blood pressure in arteries stretches baroreceptors

membrane, firing rate of receptor then increases

Decreases blood pressure in arteries decreases firing rate

Changes in CO and Peripheral resistance occur within two

heartbeats

EFFECTORS Sympathetic EFFECTORS Parasympathetic

SA/AV Node Increase HR; Adrenergic β1

Receptors

SA/AV Node Decrease HR: Cholergenic

[mACH] receptors;

Ventricles Increases Contraction; β1

Receptors

Arterioles and Veinules Vasoconstriction;

increase peripheral resistance; α-receptors

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b. Orthostatic Hypotension Triggers Baroreceptor Reflex

- When you are lying flat gravitation forces are disturbed evenly up and down your body and blood is distributed evenly

- When you stand up gravity causes blood to pool in lower extremities

causing an instantaneous decrease in venous return C.O falls from 5L/min to 3L/min causes decrease in blood pressure; this

decrease due to standing is known as orthostatic hypotension Orthostatic hypotension triggers the Baroreceptor reflex

Carotid and aortic baroreceptors respond to the fall by decreasing their firing rate; with diminished input into the cardiovascular

control center sympathetic activity increases and parasympathetic

decreases

RESULT: Heart rate and force of contraction increases while arterioles and veins constrict

RESULT: Increased C.O. and increased Peripheral Resistance increase MAP and restore it to normal within 2 heart beats

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Problem

•CAD (Coronary Artery Disease) Inflamatory Disease

•Stenosis/Ischemia: Increased blood pressure

Rapid Response

•AP Baroreceptors (Increase signals to brain)

•Sympathetic Stimulation (β1 SA/β1 Contractile/α Ssytemic Arterioles/β2 of Arterioles and skeletal muscle, liver, and coronary artiers)

• Parasympathetic: Slow down heart rate

Slow Response

•1. Elevated MAP stretches heart

•2. Artrial natriuretic peptide (ANP) hormone

•3. Enhanecd water loss at kidneys

•4. Decreases MAP

Receptor Adapation

• FAILURE of Homeostasis

•a. Tonic barorecepotrs slowly adapt

•b. Reduce AP freuqency to CVCC

•c. High MAP is can be misread as normal by CVCC

Case Study: Hypertension

Problem

•CHF (Chronic Heart Failure)

• Insufficient CO to maintain flow; not sufficient MAP

Role of Chonric Hypertension

•Results from Uncontrolled Hypertension

•1. Failure of Left Ventricle (resulting in Pulmonary Edema; excess blood and fluid in lungs)

•2. Increased Afterload

•3. Increased preload stretch causing enlargement of the ventricles (leads to incompentence of valves)

Treatments

•1. Diuretics and low salt diet

•2. Positve inotropic agents (to increase beat/more force)

•LVAD

•Heart replacement

Case Study: Congestive Heart Failure

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Case Study: Hypervolemia1. Shock 2. Overhydration

• Fainting

• Tachycardia

• Cold clamy skin

• Low urine output