Chapter 20 - Section 20.3 - Capillary Exchange - Before You Go On - Page 762: 14

Capillary exchange involves the movement of fluid and solutes across the capillary walls. Two primary opposing forces influence this process: filtration forces and reabsorption forces. These forces are responsible for the movement of fluid and substances into and out of the capillaries, respectively. **Forces Favoring Capillary Filtration:** 1. **Hydrostatic Pressure (Blood Pressure):** This is the pressure exerted by the blood against the capillary walls. It tends to push fluid and solutes out of the capillaries and into the interstitial space. This force is highest at the arterial end of the capillary due to the pressure generated by the heart's pumping action. 2. **Interstitial Osmotic Pressure:** This pressure is due to the presence of solutes in the interstitial fluid. While it's generally lower than the blood's osmotic pressure, it still contributes to the net outward movement of fluid from the capillaries. **Forces Favoring Capillary Reabsorption:** 1. **Osmotic Pressure (Oncotic Pressure):** This pressure is mainly due to the presence of proteins, particularly albumin, in the blood plasma. Proteins are too large to easily cross the capillary walls, so they remain in the bloodstream. Their presence creates an osmotic gradient that draws fluid back into the capillaries, helping to counteract the outward movement caused by hydrostatic pressure. 2. **Hydrostatic Pressure in the Interstitial Fluid:** While this pressure is often smaller than the blood's hydrostatic pressure, it still contributes to reabsorption by pushing fluid back into the capillaries. It's generated by the fluid outside the capillaries pressing against their walls. In summary, capillary filtration is favored by the hydrostatic pressure of blood and the interstitial osmotic pressure, which push fluid and solutes out of the capillaries. On the other hand, capillary reabsorption is favored by the osmotic pressure of proteins in the blood plasma and the hydrostatic pressure in the interstitial fluid, which help draw fluid back into the capillaries. These opposing forces work together to maintain a delicate balance between the filtration of nutrients and waste products out of the blood and the reabsorption of essential fluid and solutes back into the bloodstream, ensuring proper tissue function and overall fluid balance.

Capillary exchange involves the movement of fluid and solutes across the capillary walls. Two primary opposing forces influence this process: filtration forces and reabsorption forces. These forces are responsible for the movement of fluid and substances into and out of the capillaries, respectively. **Forces Favoring Capillary Filtration:** 1. **Hydrostatic Pressure (Blood Pressure):** This is the pressure exerted by the blood against the capillary walls. It tends to push fluid and solutes out of the capillaries and into the interstitial space. This force is highest at the arterial end of the capillary due to the pressure generated by the heart's pumping action. 2. **Interstitial Osmotic Pressure:** This pressure is due to the presence of solutes in the interstitial fluid. While it's generally lower than the blood's osmotic pressure, it still contributes to the net outward movement of fluid from the capillaries. **Forces Favoring Capillary Reabsorption:** 1. **Osmotic Pressure (Oncotic Pressure):** This pressure is mainly due to the presence of proteins, particularly albumin, in the blood plasma. Proteins are too large to easily cross the capillary walls, so they remain in the bloodstream. Their presence creates an osmotic gradient that draws fluid back into the capillaries, helping to counteract the outward movement caused by hydrostatic pressure. 2. **Hydrostatic Pressure in the Interstitial Fluid:** While this pressure is often smaller than the blood's hydrostatic pressure, it still contributes to reabsorption by pushing fluid back into the capillaries. It's generated by the fluid outside the capillaries pressing against their walls. In summary, capillary filtration is favored by the hydrostatic pressure of blood and the interstitial osmotic pressure, which push fluid and solutes out of the capillaries. On the other hand, capillary reabsorption is favored by the osmotic pressure of proteins in the blood plasma and the hydrostatic pressure in the interstitial fluid, which help draw fluid back into the capillaries. These opposing forces work together to maintain a delicate balance between the filtration of nutrients and waste products out of the blood and the reabsorption of essential fluid and solutes back into the bloodstream, ensuring proper tissue function and overall fluid balance.