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Lecture 9

Physiology 2130 Lecture 9: Module 9 Study guide

Course Code
John Bell

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Physiology 2130 Week 12
Module 8 The Circulatory System II
- Circulatory System
Closed system of tubes [blood vessels]
Filled with fluid [blood]
Has central pump [heart]
- Large arteries Smaller arteries Smaller arterioles Smaller vessels capillaries small venules
- Capillaries = smallest of all the blood vessels, functional units of the circulatory system
Pulmonary Circulation
Begins on the right side of the heart and returns to the left side of the heart
Returns to the heart with oxygenated blood
Blood sent to the lungs by pulmonary arteries
Systemic Circulation
Begins on the left side of the heart and returns to the right side of the heart
Returns to the heart with deoxygenated blood
- Total Blood Volume (TBV) of an average human = 5L [1.3 Gallons]
70% of the TBV = veins [“capacitance vessels”]
10% of the TBV = arteries
15% of the TBV = heart and lungs
5% of the TBV = capillaries
- Blood Velocity and Cross-Sectional Area of Vessels
Arteries = Highest blood pressure & velocity
Very LOW cross-sectional area
FAST distribution of blood throughout the body
Arterioles = Lower blood pressure & velocity than arteries
Higher cross-sectional area than arteries
Site of highest resistance in circulation
Blood flow regulation to organs
Capillaries = Slowest blood velocity
HIGHEST cross-sectional area To maximize exchange of substances across the blood vessels
Veins & Venules = Decrease in blood pressure & Increase in blood velocity
Return the blood back to the heart
- Forces that move the blood through circulatory system = Pressure Gradient
Pressure drops from arteries to veins causes the blood to flow through both the pulmonary and
systemic circulation
Higher the resistance => Slower the flow
Blood flow through a vessel laminar [streamlined] flow
Flow is slower at the edges & faster in the center
𝐹𝑙𝑜𝑤 = (P1P2)
Resistance (R)
P1-P2 = Pressure Gradient
Resistance to Blood Flow
Thickness or viscosity of the fluid
Thicker the fluid = Higher the resistance
Blood viscosity generally DO NOT change
Length of the vessel
Longer the blood vessel = Higher the resistance
Vessels are constant, therefore it is NOT a major factor
Diameter of the blood vessel
Smaller the inside diameter = Higher the resistance
***most important factor in determining blood flow resistance***
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- Blood flow resistance calculation needs the viscosity of
the blood and the length and radius of the blood vessel
- Equation 2 can be ignored as vessel length and blood
viscosity remain relatively constant
- Equation 4 = Changing either pressure gradient or radius
of the blood vessel Change in blood flow
- Blood Flow Control
Blood pressure is kept relatively constant
Best way to change the blood flow is to change the
radius of the vessel
Reducing the radius of the vessel Increase in
resistance Decrease in blood flow to organs
Arterioles = vessels that control blood flow in organs
Blood flow to an organ depends on the needs of that organ for oxygen or nutrients
Diverting the blood is done by changing the radius of arterioles
Vasodilation [making vessels wider]
Vasoconstriction [making vessels narrower]
- Korotkoff’s sound = specific tapping sounds [used when measuring blood pressure]
Sounds produced by as blood flow becomes turbulent when it “squeezes” through blood vessels
pinched off by the pressure cuff
First sound = systolic pressure
Sound disappearing = diastolic pressure
- Pressure in the aorta and the large arteries is pulsatile
Systolic pressure = 120 mmHg in healthy individual
Diastolic pressure = 80 mmHg in healthy individual
Aorta and larger arteries are very elastic and have a large radius very little resistance to the blood
- Greatest drop in blood pressure in the arterioles due to very large resistance
Decreases from ~ 80 mmHg to ~ 30 mmHg
Pressure continues to drop in the capillary from ~ 30 mmHg to ~ 10 mmHg
Pressure continues to drop in the veins from ~ 10 mmHg to ~ 5 mmHg
By the time blood reaches the Right Atrium, the pressure is almost 0 mmHg
- Structure of the Blood Vessels
Arteries and veins have 3 layers in their walls
Outermost layer = tunica externa [mostly of fibrous connective tissue]
Middle layer = tunica media [smooth muscle and elastic tissue]
Innermost layer = tunica interna [endothelial cells]
Veins also contain valves to ensure blood flows in one direction [back to the heart]
Capillaries = entirely composed of a single layer of endothelial cells
Thin walls permit the diffusion of substances INTO and OUT of the blood
Arteries larger proportion of elastic tissue to tolerate large pulsatile pressure
Arterioles mostly smooth muscle; can constrict/dilate to redirect blood
Veins thinner walls than arteries
Some smooth muscle and a little elastic tissue more flexible and distensible
Therefore, can contain 70% of Total blood volume
Venules No smooth muscle or elastic tissue since blood pressure is very LOW
Capillaries Entirely endothelial cells
Movement of substances is enhanced by clefts and fenestrations in the capillary
Movement occurs by diffusion, filtration and reabsorption
O2 and CO2 are lipid soluble, and diffuse through the capillary endothelium
Filtration = Fluid moving FROM the capillary OUT to the interstitial space
Reabsorption = Fluid moving FROM the interstitial space INTO the capillary
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