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What are the four primary forces that determine fluid movement through the capillary membrane?
- Capillary Hydrostatic Pressure
- Interstitial fluid Colloid Osmotic Pressure
- Interstitial fluid Hydrostatic Pressure
- Capillary Colloid Osmotic Pressure
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What does colloid mean?
A colloid is a substance microscopically dispersed evenly throughout another substance.
Colloids are mixtures whose particles are larger than the size of a molecule but smaller than particles that can be seen with the naked eye.
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Capillary Hydrostatic Pressure
Tends to force fluid outward through the capillary membrane.
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Interstitial fluid colloid osmotic pressure
- Causes osmosis out of capillary
- Osmotic pressure determined primarily by protein concentration
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Interstitial fluid hydrostatic pressure
Normally, close to zero. This is because the interstitial fluid normally doesn't have much pressure compared to the blood flowing through a capillary.
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Capillary colloid osmotic pressure
- Causes osmosis into capillary
- Osmotic pressure primarily determined by protein concentration
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What type of filtration is favored at the arterial side of the capillaries?
At arterial side of capillaries, net filtration is favored.
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What type of filtration is favored at the venous side of the capillaries?
At venous side of capillaries, net reabsorption is favored.
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Why is there a difference in filtration between the arterial and venous end of the capillary?
Primary reason for this difference is that there is a difference of 15 to 25 mm Hg in capillary hydrostatic pressure between the arterial end and venous end.
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Analysis of forces causing filtration at arterial end of capillary
- Forces moving fluid outward: = 36 mm Hg
- - Capillary hydrostatic pressure = 35
- - Interstitial fluid colloid osmotic pressure = 1
- Forces moving fluid inward: =26 mm Hg
- - Capillary colloid osmotic pressure = 26
- - Interstitial fluid hydrostatic pressure = 0
- Summation of forces
- 36-26 = 10 mm Hg net outward flow of filtration
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Analysis of reabsorption at the venous end of the capillary
- Forces moving fluid outward = 17 mm Hg
- - Capillary hydrostatic pressure = 16
- - Interstitial fluid colloid osmotic pressure = 1
- Forces moving fluid inward = 26 mm Hg
- - Capillary colloid osmotic pressure = 26
- - Interstitial fluid hydrostatic pressure = 0
- Summation of forces
- 17- 26 = -9 (therefore, net flow is INTO capillary)
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Why doesn’t fluid accumulate in interstitial space if filtration pressure exceeds reabsorption in this case (under normal conditions)?
9/10 of fluid that is filtered out of capillary is reabsorbed.
1/10 of fluid that is filtered out of capillary is returned to circulation via lymphatic system.
The normal rate of net filtration in whole body = 2 ml/min or 2-3 L/day therefore this is the amount that is returned to circulation via lymphatic system each day.
Overall, the amount of fluid exiting capillary is removed out of interstitial space and returned to circulation.
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How much fluid does the lymphatic system return to circulation per day?
2-3 Liters/day
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What happens to proteins that leak out of capillaries?
Most importantly, proteins that leak out of capillaries are removed from interstitial space and put back into circulation.
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Why is it important that proteins be put back into circulation rather than linger in the interstitial fluid?
Recall that interstitial colloid oncotic pressure is what is sucking fluids out of capillaries. Therefore, by striving to keep proteins in the capillary, we keep interstitial fluid colloid osmotic pressure low, and therefore decrease the amount of fluid that is sucked out of capillary.
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What is the only way proteins can be removed from the interstitial space?
The Lymphatic system is the only way proteins in interstitial space can be removed (and then subsequently put back into circulation).
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What force pushes out of a capillary?
- Blood capillary hydrostatic pressure: pushes OUT
- Interstitial colloid oncotic pressure: sucks OUT
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What are the factors increasing lymph flow?
- Interstitial fluid pressure
- - As interstitial fluid pressure rises, lymph flow increases
- Activity of lymphatic pump
- - Examples: contraction of muscles surrounding lymph vessels and movement of body parts will increase lymph flow
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What is edema?
Edema is accumulation of fluid in the interstitial spaces. It is not good because the extra fluid will interfere with gas exchange in the tissues.
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Why would liver failure cause peripheral edema?
- A person with liver failure
- cannot synthesize normal amount of plasma proteins. Therefore when the liver fails, then the liver is not producing plasma proteins. If there are no plasma proteins, the capillary pressures are going to be pushed out of the capillary more than they will be sucked back in. Therefore, edema will result.
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Why would infusion of albumin treat edema?
Because albumin, a very large plasma protein, will increase the osmolarity of the blood, and therefore cause osmosis into the capillary, thus relieving edema.
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If a patient has varicose veins and hydrostatic pressure rises in capillaries in lower extremities, how will this affect capillary filtration and reabsorption?
Varicose veins are swollen, twisted, and sometimes painful veins that have filled with an abnormal collection of blood. Therefore, because there is excess fluid in the veins, the osmolarity will start to pull fluid out of the capillaries because of the rising hydrostatic pressure, thus pushing fluid into the tissues and causing edema. The filtration system won't be able to keep up with the added pressures, and this just adds to the edema.
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