Capillaries
Form an extensive network through all tissues to ensure that all cells of the body are in close proximity to them
Capillaries structure
- Capillary walls are made up of a single layer of endothelial cells
- There are no smooth muscle cells
How does exchange across capillaries occur in all parts of the body except the brain?
Occurs by diffusion
How does exchange across capillaries occur in the brain?
Because of the blood brain barrier, exchange occurs by carrier-mediated transport
3 factors that make capillaries ideal for diffusion
- Short distance of diffusion
- Large surface area
- Slow velocity of blood
Short distance of diffusion - factor that makes capillaries ideal for diffusion
- Capillary walls are made of a single, thin layer of endothelial cells, ~1um in thickness
- Vessels are also very narrow with a lumen diameter of 7um
- Surrounding cells are no farther than 0.01 cm away from a capillary
Large surface area - factor that makes capillaries ideal for diffusion
Large surface area provides many sites for gas and nutrient exchange
Slow velocity of blood - factor that makes capillaries ideal for diffusion
- Since the vascular system is a closed loop, the same cardiac output flows through all vessels
- When total cross-sectional area increases (like in capillaries), blood velocity decreases, allowing more time for exchange
Capillary beds
- Branching networks of capillaries
- Made to reach as many cells as possible
Where do capillaries branch from?
An arteriole or a metarteriole
What is the purpose of metarterioles?
- Provide a bypass between the arterioles and the venues
Precapillary sphincters
- Smooth muscle cells in metarterioles located where capillaries branch off
- They control the flow of blood through a capillary bed
- Depending on metabolic activity, they contract and relax
Precapillary sphincters - Relaxed
Blood flows through the mtarteriole and the entire capillary bed
Precapillary sphincters - Contracted
Blood flows only through the metarteriole, bypassing the capillary bed
Pores in the capillary walls
- Capillaries contain pores
- Pores are of variable size throughout the body
Capillary pores in the brain
The brain has no pores due to the blood brain barrier
Capillary pores in the liver
- Liver capillaries have very large spaces between endothelial cells
- Large enough for some protein exchange to occur
What passes through endothelial cells?
- Lipid soluble particles
- Includes gases
What usually cannot pass through endothelial cell walls?
- Plasma proteins
- When it is possible, they pass by vesicular transport
Bulk flow across capillary walls
- Movement of protein-free plasma from the capillary lumen, through the pores and into the interstitial space
- Also transports small waster-soluble molecules
- Most of the fluid is reabsorbed back into the capillary lumen
4 forces contributing to bulk flow
- Capillary blood pressure
- Interstitial fluid colloid osmotic pressure
- Interstitial fluid hydrostatic pressure
- Plasma colloid osmotic pressure
Capillary blood pressure (Pc) - force contributing to bulk flow
- It’s the hydrostatic pressure exerted by the blood on the capillary walls that tries to force fluid out of the capillaries
- When blood enter the capillaries the average pressure is ~37 mmHg
- By the end of the capillaries the hydrostatic pressure decrease to ~17 mmHg
Interstitial fluid colloid osmotic pressure (πIF) - force contributing to bulk flow
- It’s the osmotic pressure created by the presence of proteins in the interstitial fluid
- This force wants to move fluid out of the capillary
- Under normal conditions, this pressure is 0 mmHg as there is no free protein in the interstitial fluid
Interstitial fluid hydrostatic pressure (PIF) - force contributing to bulk flow
- It’s the hydrostatic pressure exerted on the outside of the capillary by fluids in the interstitial space
- This force wants to move fluid into the capillary
- Pressure is ~1 mmHg
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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
