John W. Ledbetter, Jr. and Harry D. Jones
Medical College of South Carolina Charleston 29401
Demonstrating Osmotic and Hydrostatic Pressures in Blood Capillaries
T h e following demonstration was designed to demonstrate the various pressures which exist within a blood capillary and to emphasize the role of physical chemistry in the human body. The pressures generally may be classified as either osmotic or hydrostatic. When blood enters the capillary from the artery it is forced through by the hydrostatic pressure exerted by the pumping heart. I t then follows t,hat the hydrostatic pressure must be greater at t,he arterial end of the capillary. This hydrostatic pressure is opposed, however, by an osmotic pressure due to the substances in the blood to which the capillary wall acts as a semipermeable membrane. The result is that the hydrostatic pressure tends to force outside those substances permeable to the capillary wall while the osmotic pressure causes a flow into the capillary. If the hydrostatic pressure exceeds the osmotir pressure there will be a net flow outward. This is t,hought to be the case at the arterial end of the capillary. Toward the venous end of t,he capillary the hydrostatic pressure decreases to the extent that the osmotic pressure is greater. When this occurs there will be a net flow of permeable substances into the capillary. By this type of process the blood delivers its nutrients to the cells and collect,^ the waste mat,erials. The capillary is demonstrated by the use of a dialysis membrane. Chosen here is a cellophane cylinder 2.7 cm in diameter and 1.5 cm in length. One end is fitted with a two-hole rubber stopper and secured with string. The ot,her end is tied off. Inserted into the rubber stopper (flush with the other end) is a 6-mm glass tube about 1 m in length and a short piece of glass tubing through which to fill the cellophane bag. The bag is filledwith solution and the shortglass tube is closed off with rubber tubing and a pinch clamp. The bag is then immersed to the stopper in a 4-1 beaker full of water at pH 5.5-7.0. Two clamps are used for support of the tubing (see figure). The solution injected into the bag is a 10yo aqueous solution of sucrose a t pH 4.5. To this solut,ion is added methyl red indicator solution until the sucrose solution is bright,ly red. Changes in this system after the set-up can be observed in a few minutes. The first observation will be solution rising in the tube due to the osmotic pressure. After another time lapse the solution in the dialysis bag begins to turn yellow at the very top. This yellow coloration will slowly progress down the bag. The water outside the bag in the bottom of the beaker will turn yellow. If one observes closely, sucrose and indicator solution can be seen flowing out of the bottom of t,he dialysis bag. The t,ime for the best observation is when the yellow coloration in the dialysis bag has progressed one-half t,he way down the bag. 362
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lournol o f Chemicol Education
Apparatus demonrtroting osmotic ond hydrortatic presrurer in blood copillorie..
When one explains the observations of t,he color changes it is apparent that solvent, because of osmotir pressure, flows int,o the bag at the top. This increases the pH which changes the color of the indicator from red to yellow. The range of the indicator is from pH 4.2-6.3. This region of the dialysis bag then represents the venous end of a capillary where substances flow into the capillary. In the bottom of the dialysis bag, solvent plus sucrose and indirator flow out of the bag into the bott,om of the beaker causing the indicator to change color due t,o the higher pH. This flow outaward is not primarily due to hydrostatic pressure, but rather to the higher density of the sucrose solution and the ability of the sucrose and indicator to permeate the membrane. However, it does serve to demonstrate the hydrostatic pressure. This region represents the arterial end of the capillary where the hydrostatic pressure is higher and substances are forced from t,he capillary into the surrounding tissues. Since the sucrose molecules are permeable to the membrane, after a long period of time the osmotic pressure will go to zero and the entire system will be colored yellow.
