Fluid Exchange Determinants of Fluid Movement across the Capillary Wall

Transcription

1 Fluid Exchange Determinants of Fluid Movement across the Capillary Wall Model #1: Hydrostatic Pressure Model 1 shows a 90 foot long soaker hose. The hose is attached to a faucet at its left end and has an adjustable end cap at its right end, which affects the amount of flow out of the right end of the hose (the end cap acts functionally like a valve). The hose is porous it has small holes in it (as shown) for water to leak across the wall of the hose. Under normal conditions, water enters the hose at 60mmHg pressure (standard plumbing pressure in a home) and a small amount of water leaks out the right end of the hose while the rest leaks across the wall of the hose. The average pressure in each region is indicated. The pressure outside of the hose is 0 mmhg. All pressures are relative to atmospheric; thus 0 mmhg is equal to atmospheric pressure and 60 mmhg is 60 mmhg above atmospheric. Questions: 1. Which region contains the highest average pressure? Region A 2. Which region contains the lowest average pressure? Region C 3. Predict the pressure at the end cap. Approximately 20 mmhg. The value students choose should be less than 30 mmhg, which is the average pressure in region C. 4. Based on your answers, explain how direction of flow along the length of the hose is related to pressure along the length of the hose. You might consider using a graph to show the relationship. Before moving on, please check your answer to this question with your instructor. Fluid flows from high pressure to low pressure, so pressure must be highest in A and lowest in C. If a graph is used, it should have an X-axis of distance and a Y-axis of pressure. Pressure should start at 60 mmhg and linearly decrease over the length of the hose, down to about 20 mmhg. Note that there is a very slight increase in pressure right at the end cap. This is caused by the velocity of the fluid decelerating as it hits the cap (i.e. it is the conversion of kinetic energy to pressure), but this increase is minimal in these systems and tends not to have a significant effect on the pressure in region C.

2 5. Pressure gradients across a wall of the hose are also affected by the pressure inside of the hose, but in this case they are compared to the pressuer on the outside of the hose. Complete the following table as they relate to pressures across the wall of the hose in the regions A, B and C. Segment Average pressure inside the hose Average pressure outside the hose Pressure across the wall of the hose (inside outside) A B C Draw relevant sized arrows on the model indicating the direction and magnitude of water flow across the wall (not along the inside) of the hose in each region. Fluid flow across the wall will then be highest in A and lowest in C. Students should draw arrows to indicate this. 7. Water flows across the wall of the hose because of hydrostatic pressure (HP). Have each person write their own definition of hydrostatic pressure. Answers may vary some, but see #7 for the answer. 8. As a team, write your best definition of hydrostatic pressure. Hydrostatic pressure is the outward pressure that a fluid exerts on the wall of a hose/vessel. 9. If a person loosens the end cap of the hose (allowing a greater amount of water to flow out of the end of the hose), predict what will happen to the water flow across each region of the wall of the hose. Explain your reasoning based on hydrostatic pressure changes. In all regions, the flow across the wall will decrease. This is because when the end cap is loosened, the pressure at that end of the hose decreases providing a pressure distribution similar to the one shown in the following graph: where the entrance pressure would be the same, but exit pressure would be lower, decreasing the average pressure in each segment (note that the linear lines in the graph are more accurate because the linear distances are not the same between the end and the middle of a region compared to the distance between the middle of two neighboring regions). This would lead to decreased hydrostatic pressure in each region and corresponding decreases in flow across the wall. Note the flow would increase out the end of the hose.

3 Model 2: Colloid Osmotic Pressure Model 2 shows a fenestrated (porous) capillary. The large molecules (indicated by rounded squares) are dissolved in the plasma (non-cellular portion of blood) and are too large to pass through the fenestrations. The small molecules (triangles) are dissolved in the plasma, but can easily pass through the fenestrations. This model shows the capillary prior to any molecules moving through the fenestrations. 10. Which class of molecules cannot pass through the fenestrations? Based on prior knowledge list specific molecules that would be included in this class. Plasma proteins such as albumin and fibrinogen 11. What class of molecules can move through the fenestrations? Based on your prior knowledge, list molecules that could be included in this class. Ions (such as sodium and chloride), glucose, waste products (urea, etc.) 12. Based on what you already know, write a working definition of osmosis. The movement of water from a region of low solute concentration to a region of high solute concentration across a semi permeable membrane. Before moving on, please check your answer to this question with your instructor. 13. Sketch the capillary in Model 2 after several minutes pass, indicating how the molecules would distribute. Students should draw the capillary with all large molecules still inside and the small molecules distributed both inside and outside the capillary.

4 14. Soluble molecules that cannot freely diffuse between compartments contribute to osmotic gradients (water-attracting) between these compartments. Which molecule(s) (small, large, neither or both) would act to create an osmotic gradient across the capillary wall? Only large molecules would contribute because you need a solute gradient for osmosis. 15. Draw an arrow on the model showing the direction of the net flow of water across the capillary wall. They should draw an arrow into the capillary (across the wall) toward the high concentration of protein A. 16. The force that allows the capillary to attract water from the interstitial space is colloid osmotic pressure (COP). Write an individual definition of colloid osmotic pressure. See answer to next question. 17. As a team, write your best definition of colloid osmotic pressure. Colloid osmotic pressure is the form of osmotic pressure caused by large plasma proteins that attract fluid from the interstitium and into the plasma. 18. If the concentration of large molecules increases in the capillary, how would this affect the magnitude of the colloid osmotic pressure? Why? Colloid osmotic pressure would increase because of the greater concentration of impermeable solutes. Model 3: Movement of fluid across the capillary wall is determined by pressure gradients. Model 3 shows a capillary with blood flowing through it. The large molecules (indicated by the rounded squares) are too large to pass through the fenestrations. The small molecules (triangles) are freely permeable through the fenestrations and their concentration inside the vessel is the same as the concentration outside of the vessel.

5 19. Look at the image of the capillary in Model 3. The open arrow at the left indicates the location where blood enters this capillary. Label the capillary to indicate the arterial and venous ends of the vessel. Arterial is near the open arrow, venous is at the other (right) end. 20. Two types of pressure were introduced in Models 1 and 2. Restate them and indicate the direction that each tries to move fluid across the capillary wall by adding arrows and labels to model 3. Hydrostatic pressure forces fluid out of the capillary. Colloid osmotic pressure attracts fluid into the capillary. 21. Blood pressure is the more common name for the hydrostatic pressure in blood vessels. Explain what happens to the magnitude of this pressure along the length of the capillary. It decreases along the length of the capillary 22. The total pressure for fluid movement across the capillary wall is called Net Filtration Pressure (NFP). Write an equation for determining Net Filtration Pressure. NFP = Blood hydrostatic Pressure Blood Colloid Osmotic Pressure 23. Fill in the following table, which represent values of a standard systemic capillary in the body. Region of Capillary Arterial End Middle Venous End Hydrostatic (blood) Pressure (mm Hg) Colloid Osmotic Pressure (mm Hg) Net Filtration Pressure (mm Hg) Direction of Fluid Movement (into, none, or out of capillary?) Out None Into 24. The magnitude of the net filtration pressure at each point helps determine how much fluid moves across the wall of the capillary. Based on the values in the above table, is there a net fluid of flow into, out of, or no net change across the capillary wall? Explain. There is a small amount of net outward flow. This is because the net filtration pressure pushing fluid out of the arterial end of the capillary is greater in magnitude than the net force causing reabsorption at the venous end.

6 25. Predict how fluid movement across the capillary wall would be affected by each of the following clinical conditions: A. Hypoalbuminemia: Colloid osmotic pressure is decreased in this condition, so net filtration pressure is increased and fluid movement across the capillary wall is increased. B. Increased venous pressure (i.e. venous congestion): Venous pressure causes hydrostatic pressure increase, so fluid movement across the capillary wall is increased. 26. Some conditions, such as allergies and inflammation, can make capillaries permeable to large molecules as well. How would this affect Net Filtration Pressure and the amount of fluid moving across the capillary wall? Because large molecules (proteins) can leak out under this condition, this will add a colloid osmotic pressure to the interstitial fluid surrounding the capillary and decrease the colloid osmotic inside the capillary. This can lead to a significant increase in fluid leaving the capillary, such as is seen when an ankle swells significantly after injury.