Los Angeles Mission College



Lecture: Circulatory Physiology

I. Factors Involved in Blood Circulation

A. Blood Flow - the actual VOLUME of blood moving through a particular site (vessel or organ) over a certain TIME period (liter/hour, ml/min)

1. Relatively constant when at rest

2. Varies widely to body organs based on need

B. Blood Pressure - the FORCE exerted on the wall of a blood vessel by the blood contained within (millimeters of Mercury; mm Hg)

blood pressure = the systemic arterial pressure of large vessels of the body (mm Hg)

C. Resistance to Flow (Peripheral Resistance) – opposition to flow; the FORCE resisting the flow of blood through a vessel (usually from friction)

1. viscosity - a measure of the "thickness" or "stickiness" of a fluid flowing through a pipe

a. V water < V blood < V toothpaste

b. water flows easier than blood

2. tube length - the longer the vessel, the greater the drop in pressure due to friction

3. tube diameter – greatest influence on resistance

a. smaller diameter = greater friction and greater force opposing flow

b. frequent changes alter peripheral resistance

c. small diameter arterioles major influence on resistance

d. abrupt changes in diameter or arteriosclerosis dramatically increase resistance

e.

D. Relation Between Blood Flow, Pressure, Resistance

difference in blood pressure ( P)

Blood Flow (F) = --------------------------------------------

peripheral resistance (R)

a. increased P -> increased flow

b. decreased P -> decreased flow

b. increased R (vasoconstriction) -> DECREASED flow

c. decreased R (vasodilation) -> INCREASED flow

II. Systemic Blood Pressure

A. Blood Pressure Near the Heart

1. HEART produces blood pressure by pumping the blood

2. Blood pressure decreases with distance from Heart

3. systolic arterial blood pressure - pressure in aorta (& major arteries) in middle of ventricular contraction (120 mm Hg in healthy adult)

4. diastolic arterial blood pressure - pressure in aorta (& major arteries) during ventricular diastole, when semilunar valves are closed (80 mm Hg in healthy adult)

5. mean arterial pressure (MAP) - the "average" blood pressure produced by the heart (93 mm Hg in healthy adult)

mean arterial pressure = diastolic pressure + 1/3 pulse pressure

** pulse pressure = systolic pressure - diastolic pressure

6. blood pressure decreases throughout system

L ventricle --> 120 mm Hg

arteries --> 120 - 60 mm Hg

arterioles --> 60 - 40 mm Hg

capillaries --> 40 - 20 mm Hg

venous --> 20 - 10 mm Hg

R atrium --> 10 - 0 mm Hg

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7. venous return - venous blood pressure is so low, other factors contribute to venous blood flow

a. respiratory pump - breathing action of thorax "squeezes" blood back toward the heart

b. muscular pump - contraction/relaxation of skeletal muscles "milk" blood up veins to heart

c. venoconstriction – under sympathetic control pushes blood to the heart

d. deep vein thrombosis (DVT) – clot that forms in vein; often from sitting to long (e.g. flight on an airplane)

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III. Factors Affecting Blood Pressure

A. Cardiac Output ( = stroke volume X heart rate)

CO = SV (ml/beat) x HR (beats/min)

= 75 ml/beat x 70 beats/min = 5250 ml/min = 5.25 L/min

1. increased cardiac output -> increased blood pressure

2. increased stroke volume -> increased blood pressure

3. increased heart rate -> increased blood pressure

B. Peripheral Resistance

1. arteriole constriction ---> increased blood pressure

2. resistance inversely proportional to the "fourth power" of the radius change of the vessel

- if radius doubles the resistance will be 1/16 of original value

C. Blood Volume

1. hemorrhage - decrease in blood pressure

2. salt/fluid - increase in blood pressure

3. polycythemia - increase in blood viscosity

4. RBC anemia - decrease in blood viscosity

IV. Regulation of Blood Pressure

A. Nervous System Control

1. control of arteriole diameter

2. directs blood flow to proper organs and tissues that need it

3. REFLEX PATHWAY:

baroreceptors/chemoreceptors/brain -->

afferent nerve fibers -->

medulla (vasomotor center) -->

vasomotor (efferent) nerve fibers -->

smooth muscle of arterioles

B. Vasomotor Fibers to Smooth Muscle of Arterioles

1. sympathetic fibers that release norepinephrine (NE); cause vasoconstriction of arterioles

C. Vasomotor Center of the Medulla

1. sympathetic neuron cell bodies in the medulla

2. receive input from baroreceptors, chemoreceptors, and brain

3. vasomotor tone - general constricted state of arterioles set by vasomotor center

D. Baroreceptors

1. blood pressure receptors large arteries (carotid sinuses, aortic arch, neck/thorax arteries)

2. send blood pressure information to vasomotor center of medulla

increased pressure --> decreased pressure -->

inhibits vasomotor center -> stimulates vasomotor center ->

vasodilation vasoconstriction

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E. Chemoreceptors

1. located in aortic arch and carotid arteries

a. carotid and aortic bodies

2. monitor increase of Carbon Dioxide or decrease of OXYGEN and pH levels of the blood

low OXYGEN or pH ------------> cause increase blood pressure via signals to

or higher CO2

F. Higher Brain Centers Control on BP

1. hypothalamus & cortex also effect vasomotor area of the medulla

2. hypothalamus increases blood pressure during stress; mediates redistribution of blood flow during exercise and changes in body temperature

G. Chemical Controls of Blood Pressure

1. hormones of adrenal medulla - "fight-or-flight" response to fear; release of norepinephrine and epinephrine from adrenal medulla; causes vasoconstriction and increased BP and CO

2. atrial natriuretic factor (ANF) - secreted by the atria of the heart, promotes general decline in blood pressure kidney releasing more Na+ and water, reducing fluid volume

3. antidiuretic hormone (ADH) - released by the hypothalamus, causes increase in blood pressure by getting the kidneys to conserve water in the body; eg. during hypotensive situations

4. endothelium derived factors

a. endothelin - strong vasoconstrictor

b. endothelium derived relaxing factor - vasodilation

5. alcohol - causes vasodilation

H. Renal (Kidney) Regulation

1. direct regulation - fluid loss through urine

a. low pressure/volume --> conserve water

b. high pressure/volume --> release more water

2. renin-angiotensin mechanism

low blood pressure -->

release of renin -->

formation of angiotensin II --> vasoconstriction

release of aldosterone --> Na+/water reabsorption (by kidney)

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V. Variations in Blood Pressure

A. Measuring Blood Pressure

1. vital signs - blood pressure, pulse, respiratory rate, and body temperature

2. auscultory method of blood pressure measurement

a. "sphygmomanometer" wrapped around upper arm

b. inflate above systolic pressure of brachial a.

c. pressure released, first sounds of Korotkoff - systolic blood pressure

d. disappearance of sounds of Korotkoff - diastolic blood pressure

3. blood pressure changes – changes in posture, physical exertion, emotional upset, fever, age, sex, race

B. Hypotension (below normal blood pressure, < 100/60)

1. factors - age, physical conditioning, illness

2. orthostatic hypotension - generally in elderly, drop in blood pressure during postural changes

3. chronic hypotension - ongoing low blood pressure

a. low blood protein levels (nutrition)

b. Addison's disease (adrenal cortex malfunction)

c. hypothyroidism

d. also sign of various types of cancer

C. Hypertension (above normal blood pressure at rest, > 140/90)

1. factors - weight, exercise, emotions, stress

2. prehypertension – if values elevated but not yet in that range; may be transient adaptations due to fever, physical exertion, emotional upset

3. chronic hypertension - ongoing high blood pressure

a. prevalent in obese and elderly

b. leads to heart disease, renal failure, stroke

c. heart must work harder > myocardium enlarges, weakens, becomes flabby

c. also leads to more arteriosclerosis

d. primary hypertension – 90% of hypertensive conditions

i. no underlying cause identified

ii. risk factors include: heredity, diet, obesity, age, diabetes, stress, smoking

iii. high Na+, cholesterol, fat levels

iv. diuretics - promote water removal

v. NE blockers - slow vasoconstriction

e. secondary hypertension – due to identifiable disorder

i. kidney disorders

ii. endocrine (hormone) disorders

iii. arteriosclerosis

VI. Blood Flow in the Body

A. General Features

1. delivery of oxygen and removal of carbon dioxide

2. gas exchange in the lungs

3. absorption and delivery of nutrients from GI tract

4. processing/waste removal in the kidneys

5. normal blood flow at rest (tissue perfusion)

abdominal organs 24%

skeletal muscle 20%

kidneys 20%

brain 13%

heart 4%

other 15%

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B. Velocity of Blood Flow

1. velocity directly related to the TOTAL cross-sectional area of the vessel(s)

FASTEST aorta 40-50 cm/s

arteries 20-40 cm/s

arterioles 1-20 cm/s

SLOWEST capillaries 0.1-1 cm/s

C. Local Regulation of Blood Flow

1. autoregulation - regulation of blood flow by altering arteriole diameter; based on need of tissue

a. metabolic controls

b. myogenic controls

2. metabolic controls - vasodilation of arterioles and relaxation of precapillary sphincters

a. declining tissue O2; substances from metabolically active tissues (H+, K+, adenosine, and prostaglandins) and inflammatory chemical messengers

b. effects – relaxation of vascular smooth muscle; release of NO (nitrous oxide - powerful vasodilator) by endothelial cells; endothelin – released from endothelium (powerful vasoconstrictor);

c. NO and endothelins are balanced unless blood flow is inadequate then NO dominates

3. myogenic response - change in flow through arteriole in response to stretch of smooth muscle; keeps tissue perfusion constant despite fluctuations in systemic pressure.

a. passive stretch – promotes increased tone and vasoconstriction

b. reduced stretch – promotes vasodilation and increased blood flow to the area

4. reactive hyperemia - increase in blood flow to area where an occlusion has occurred

5. angiogenesis (increased vasculature) - results from prolonged lack of oxygen/nutrients to an area (eg. heart and people living at high altitude)

D. Blood Flow to Skeletal Muscles

1. active (exercise) hyperemia - increased blood flow to muscles during heavy activity

a. decreased oxygen and increased lactic acid

b. visceral organ blood flow is decreased

E. Blood Flow to The Brain

1. MUST maintain constant blood flow (750 ml/min)

2. sensitive to low pH and high carbon dioxide – blood flow will increase

3. blood pressure tightly regulated in the brain

a. fainting (syncope) -> below 60 mm Hg

b. edema (brain swelling) -> above 180 mm Hg

F. Blood Flow to The Skin

1. intimately involved in temperature regulation

increased body temperature ->

hypothalamic inhibition of vasomotor area ->

vasodilation of vessels in skin ->

increased blood flow ->

sweating -> (bradykinin -> more vasodilation)

G. Blood Flow to the Lungs

1. short pathway from heart, less pressure required

2. low oxygen level --> vasoconstriction

H. Blood Flow to the Heart

1. blood to coronary arteries during diastole

2. vasodilation from ADP and carbon dioxide

VII. Blood Flow in the Capillaries

A. Exchange of Gases and Nutrients

1. diffusion - all molecules move DOWN the concentration gradient (from HIGH to LOW) into or out of the blood

2. oxygen/nutrients (blood ------> body cells)

carbon dioxide/ wastes (body cells ------> blood)

3. lipid soluble molecules pass through membranes of endothelial cells

4. water soluble solutes pass through clefts and fenestrations

5. larger molecules such as proteins, are actively transported in pinocytic vesicles

B. Fluid Movements

1. hydrostatic pressure - force of the capillary wall on the blood itself

a. filtration pressure - the pressure forcing fluid and solutes through capillary clefts

2. osmotic pressure - force driving fluid in the direction of HIGHER solute concentration

3. movement out: Hydrostatic pressure > Osmotic difference

movement in : Hydrostatic pressure < Osmotic difference

4. normal fluid movement - 1.5 ml/min in the entire body

[pic]

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C. Circulatory Shock

1. circulatory shock - blood pressure gets so low that blood will not flow adequately

2. hypovolemic shock - circulatory shock resulting from loss of fluid (bleeding, diarrhea, burn)

a. heart rate increases rapidly

b. general vasoconstriction of vessels

3. vascular shock - extreme vasodilation causes sudden drop in blood pressure

a. snake and spider bites with NE blockers

b. septicemia - bacterial infection

4. cardiogenic shock - heart is unable to provide sufficient blood pressure

5. anaphylactic shock – in response to allergies

VIII. Lymphatic System

A. interacts with the cardiovascular system, digestive system and immune system

1. returns interstitial fluid to the cardiovascular system

2. picks up fat absorbed at the small intestine and transfers it to the circulation

3. serving as a filtering system to help capture and destroy foreign agents (e.g. bacteria)

4. edema – build-up of extracellular fluid in extracellular (interstitial) space

a. high blood pressure

b. leakage of plasma proteins into interstitial space

c. decreased plasma protein concentration

d. obstruction of lymphatic drainage

Man with elephantiasis

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5

2

1

4a

3

2

1

5

4b

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4a

Baroreceptors in carotid sinuses and aortic arch are stimulated.

Stimulus:

Blood pressure (arterial blood pressure rises above normal range).

CO and R return blood pressure to homeostatic range.

Sympathetic impulses to heart cause HR,

contractility, and

CO.

Impulses from baroreceptors activate cardioacceleratory center (and inhibit cardioinhibitory center) and stimulate vasomotor center.

Baroreceptors in carotid sinuses and aortic arch are inhibited.

Stimulus:

Blood pressure (arterial blood pressure falls below normal range).

Vasomotor fibers stimulate vasoconstriction, causing R.

IMBALANCE

Homeostasis: Blood pressure in normal range

CO and R return blood pressure to homeostatic range.

Sympathetic impulses to heart

cause HR,

Impulses from baroreceptors stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center.

Rate of vasomotor impulses allows vasodilation, causing R.

IMBALANCE

contractility, and

CO.

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