Flow of blood through a vessel
Flow (F)= Change in pressure (delta P) / Resistance (R)
What is a pressure gradient and how does it influence blood flow?
- As blood travels through vessels, friction between blood and vessel walls causes pressure to decrease with distance from the heart, creating a pressure gradient
- A larger pressure gradient results in a higher flow rate when vessel diameter is the same
How does pressure gradient affect blood flow when resistance is constant?
- Blood flow is proportional to the pressure gradient
- If the pressure gradient doubles, flow rate doubles
- Ex: Vessel 2 with twice the ΔP has twice the flow of Vessel 1
How do vessels with different absolute pressures but the same pressure gradient compare in blood flow?
- Flow is determined by the pressure gradient (ΔP), not the absolute pressures at each end
- If vessels have the same ΔP, they have the same flow rate
What happens when resistance increases?
- A greater pressure gradient is needed to keep blood flowing
- The heart has to work harder
Causes of resistance - Viscosity of the blood
- Viscosity is the state of being thick, or semifluid in consistency, due to internal friction
- The thicker the blood, the higher the frictional forces and therefore the higher the resistance
Causes of resistance - Vessel length
- At a constant radius, a longer vessel has higher resistance
- Vessel length in the body is constant so its not an important variable in determining blood flow
Causes of resistance - Vessel radius
- Most important factor in determining resistance
- A small change in vessel radius will have a very large influence on its resistance to flow
Resistance and vessel radius formula
R∝1/r^4
Poiseuille’s Law
Pressure gradient (ΔP), vessel radius (r), vessel length (L), and blood viscosity (η) come together to determine blood flow rate
Poiseuille’s law equation
Flow rate (Q) = πΔPr^4/8ηL
Vascular tree - Step 1 (Systemic arteries)
- Blood leaves the left ventricle and enters the aorta
- The aorta divides into major arteries, which further divide into smaller vessels until they are a small artery that enters an organ
Vascular tree - Step 2 (Arterioles)
- Once this artery enters the organ, it branches into further arterioles
- These arterioles branch further into the smallest of blood vessels, the capillaries
Vascular tree - Step 3 (Capillaries)
- Capillaries are important for nutrient and waste exchange
- Most important function of the circulatory system
Vascular tree - Step 4 (Venules)
Capillaries then rejoin to form small venules, which further come together to form the veins leaving organs
Vascular tree - Step 5 (Systemic veins)
The veins from individual organs continue to come together to form larger and larger veins until they return to the heart
Is cardiac output distributed equally to all tissues? Why or why not?
- No, cardiac output is not distributed equally
- Blood flow to each organ varies and can be adjusted based on need so tissues receive the appropriate amount
- The pulmonary system receives 100% of cardiac output because all blood pumped by the right heart must pass through the lungs
What are blood vessel walls made up of?
- Connective tissue
- Smooth muscle cells
- Endothelial cells
Inner layer of blood vessel
Comprised of a single layer of endothelial cells surrounded by a basement membrane of connective tissue
Middle layer of blood vessel
Made up of circular arrangements of smooth muscle cells and some connective tissue
Outer layer of blood vessel
Made up of collagen-rich connective tissues
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Module 1: Section 1 + 256
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Module 1: Section 316
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Module 1: Section 413
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Module 1: Section 531
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Module 2: Section 120
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Module 2: Section 216
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Module 2: Section 332
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Module 2: Section 451
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Module 2: Section 511
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Module 4: Section 127
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Module 4: Section 235
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Module 4: Section 333
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Module 4: Section 426
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Module 4: Section 59
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Module 4: Section 628
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Module 5: Section 115
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Module 5: Section 234
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Module 5: Section 352
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Module 5: Section 47
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Module 5: Section 533
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Module 5: Section 67
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Module 6: Section 121
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Module 6: Section 223
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Module 6: Section 344
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Module 6: Section 428
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Module 6: Section 512
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Module 6: Section 613
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Module 6: Section 743
