Small molecules can cross into and out of capillaries via simple or facilitated diffusion. Some large molecules can cross in vesicles or through clefts, fenestrations, or gaps between cells in capillary walls. However, the bulk flow of capillary and tissue fluid occurs via filtration and reabsorption. Filtration, the movement of fluid out of the capillaries, is driven by the CHP. Reabsorption, the influx of tissue fluid into the capillaries, is driven by the BCOP. Filtration predominates in the arterial end of the capillary; in the middle section, the opposing pressures are virtually identical so there is no net exchange, whereas reabsorption predominates at the venule end of the capillary. The hydrostatic and colloid osmotic pressures in the interstitial fluid are negligible in healthy circumstances.
blood colloidal osmotic pressure (BCOP)
pressure exerted by colloids suspended in blood within a vessel; a primary determinant is the presence of plasma proteins
blood hydrostatic pressure
force blood exerts against the walls of a blood vessel or heart chamber
capillary hydrostatic pressure (CHP)
force blood exerts against a capillary
filtration
in the cardiovascular system, the movement of material from a capillary into the interstitial fluid, moving from an area of higher pressure to lower pressure
interstitial fluid colloidal osmotic pressure (IFCOP)
pressure exerted by the colloids within the interstitial fluid
interstitial fluid hydrostatic pressure (IFHP)
force exerted by the fluid in the tissue spaces
net filtration pressure (NFP)
force driving fluid out of the capillary and into the tissue spaces; equal to the difference of the capillary hydrostatic pressure and the blood colloidal osmotic pressure
reabsorption
in the cardiovascular system, the movement of material from the interstitial fluid into the capillaries
Watch this video to explore capillaries and how they function in the body. Capillaries are never more than 100 micrometers away. What is the main component of interstitial fluid?
Water.
1. Hydrostatic pressure is ________.
A) greater than colloid osmotic pressure at the venous end of the capillary bed
B) the pressure exerted by fluid in an enclosed space
C) about zero at the midpoint of a capillary bed
D) all of the above
B
2. Net filtration pressure is calculated by ________.
A) adding the capillary hydrostatic pressure to the interstitial fluid hydrostatic pressure
B) subtracting the fluid drained by the lymphatic vessels from the total fluid in the interstitial fluid
C) adding the blood colloid osmotic pressure to the capillary hydrostatic pressure
D) subtracting the blood colloid osmotic pressure from the capillary hydrostatic pressure
D
3. Which of the following statements is true?
A) In one day, more fluid exits the capillary through filtration than enters through reabsorption.
B) In one day, approximately 35 mm of blood are filtered and 7 mm are reabsorbed.
C) In one day, the capillaries of the lymphatic system absorb about 20.4 liters of fluid.
D) None of the above are true.
A
1. A patient arrives at the emergency department with dangerously low blood pressure. The patient’s blood colloid osmotic pressure is normal. How would you expect this situation to affect the patient’s net filtration pressure?
The patient’s blood would flow more sluggishly from the arteriole into the capillary bed. Thus, the patient’s capillary hydrostatic pressure would be below the normal 35 mm Hg at the arterial end. At the same time, the patient’s blood colloidal osmotic pressure is normal—about 25 mm Hg. Thus, even at the arterial end of the capillary bed, the net filtration pressure would be below 10 mm Hg, and an abnormally reduced level of filtration would occur. In fact, reabsorption might begin to occur by the midpoint of the capillary bed.
2. True or false? The plasma proteins suspended in blood cross the capillary cell membrane and enter the tissue fluid via facilitated diffusion. Explain your thinking.
False. The plasma proteins suspended in blood cannot cross the semipermeable capillary cell membrane, and so they remain in the plasma within the vessel, where they account for the blood colloid osmotic pressure.