420 PROTOPLASM 



One of the most common examples of a disturbance in water 

 control in animals is the condition known as edema, or swelling of 

 tissues. Knowledge of the forces involved in the movement 

 of water through capillaries and membranes and the adsorption 

 of water on to surfaces has advanced sufficiently to make specific 

 statements possible on so complicated a medical problem as the 

 cause of edema. Ordinarily, edema will develop when the normal 

 permeability of the capillary wall is altered or when there is a 

 disturbance in equilibrium between the hydrostatic pressure and 

 the osmotic pressure of the blood. A common cause of altered 

 permeability is inflammation. Chemical or mechanical injury 

 of the capillary blood vessels may also cause edema. Normally, 

 the hydrostatic pressure of the blood balances the osmotic 

 effect; but when this balance of forces is disturbed in either 

 direction, edema develops. Thus, an increased hydrostatic 

 pressure gives rise to a loss of fluid from the circulating blood to 

 the tissue. This is known as cardiac edema. In so-called 

 nephrotic or hydremic edema, due to the increased permeability 

 of the capillaries of the kidneys, there is a leakage of blood 

 proteins. This persistent loss of plasma protein accounts for 

 the decrease in the osmotic effect. As long as the plasma protein 

 content is above 5 per cent, with a specific gravity of 1.023, there 

 is little likelihood of edema. Below this critical level, there is a 

 definite tendency toward edema. Deficit in plasma protein and 

 its associated edema may also be caused by a diet low in protein. 

 Edema is present in various diseases, whenever there is starva- 

 tion, malnutrition, or excessive catabolism, as in toxic conditions 

 with high temperatures. In postoperative or generalized edema, 

 the problem is more complicated and as yet not fully understood. 

 The primary causes of edema may be numerous and quite 

 different from one another. It is possible that the ultimate cause 

 in many cases is a change in acidity. 



Epilepsy is associated with an excessive water supply and 

 alkalinity, while diabetes is associated with dehydration of the 

 tissues and acidity. Surplus water in epilepsy collects in the 

 subarachnoid spaces. To bring the water balance down to 

 normal is the task of the physician. Epileptic convulsions 

 usually stop when a negative balance in body water has been 

 obtained, while a positive water balance will precipitate epileptic 

 seizures. Convulsions also cease after partial fasting or a 



