Question

The net filtration across relaxed skeletal muscle capillaries is about 0.005 mL/min per 100 g of tissue. Assume the following values: the density of muscle is 1.08 g/cm3; the length of the capillaries is 500 μm and their cross-sectional density is 250 capillaries/mm2. Their average radius is 4 μm. Pressure at the arteriolar end of the capillary is 40 mm Hg and it decays linearly to 15 mm Hg at the venule end. The oncotic pressure of plasma is 25 mm Hg. Interstitial fluid pressure in relaxed muscle is -1 mm Hg and intersitial fluid oncotic pressure is 5 mm Hg.

Does the capillary filter fluid along its entire length, or does it reabsorb fluid at the venule end? Support your answer with calculations based on the given information.

Answer 1

To understand the above scenario, it is essential to comprehend the hemodynamics happening within capillaries present inside the tissues, in this case, the muscles.

Blood and Lymph are the fluid transport systems by means of which molecules are carried to different parts of the body. The blood flows through arteries, arterioles and capillaries, feeding the tissues with essential elements and exits out of the tissues as venules and veins.

For effective movement of particles across the capillaries, different forces act in concordance. Due to the volume of blood within the capillaries, the intra-capillary area is a region of high pressure (especially near the arteriolar end). In contrast, the interstitial area, which means the region within the muscle, where there is not much of fluid volume, is an area of low pressure. When movement of particles happens from an area of high pressure to a low pressure area, the process is called filtration.

In case, the pressure in the interstitial zone is high and the pressure in the capillaries is low (as is usually the case near the venular end), the process of movement of molecules from the high pressure area to the low pressure area is termed reabsorption.

The pressure exerted by the blood on the capillaries is termed capillary hydrostatic pressure. Conversely, pressure exerted by interstitial fluid (within the muscle or other tissues) is termed interstitial fluid hydrostatic pressure. As blood flows from the arteriolar end of the capillary, it exerts a high hydrostatic pressure resulting in filtration of molecules. When the blood enters the venular end, the capillary hydrostatic pressure comes down due to movement of fluid into the capillary continuously and reaches a minimum resulting in reabsorption predominantly at the venular end.

Oncotic/Osmotic pressure is the pressure exerted by proteins present in the blood and other fluids. It works in a way which is just the opposite of what hydrostatic pressure does. When hydrostatic pressure drives fluid out, oncotic pressure tends to draw it in.

In the problem stated above, it is obvious that reabsorption of fluid happens at the venular end due to the differences in hydrostatic and oncotic pressures at the capillary side and the interstitial side.

According to a simplified version of the Starling's equation, the net filtration force is given by (Pc-Pi) - σ(πp-πi) where σ is a constant, Pc is capillary hydrostatic pressure, Pi is interstitial fluid hydrostatic pressure, πp is plasma protein oncotic pressure and πi is interstitial fluid oncotic pressure.

Omitting the constant, to calculate the net driving force at the arteriolar end, by substituting the given values in the equation, we get (40-(-1)-(25-5) = 21 mmHg. Hence due to the high pressure at the arteriolar end, filtration into the tissues occurs. Likewise at the venular end, we get (15-(-1)-(25-5) = 4 mmHg, which results in reabsorption of the fluid at the venular end.

Also, the capillary does not filter fluid along its entire length. Filtration is maximum at the arteriolar end. Since the pressures cancel out or are equal at mid-section, there is no movement of fluid here. Again as the pressure goes down near the venular end, there is reabsorption. Hence, the movement of fluid through the capillary length is not constant and varies according to differences in the hydrostatic and oncotic pressures of the capillary blood and the interstitial fluid.