The Total Body Fluid (TBF) is the water that contains the dissolved electrolytes and non-electrolytes (the latter, electrically neutral organic molecules like glucose and cholesterol).
TBF has 2 compartments:
・an external one, the outside-of-cells, the extra-cellular fluid (ECF), that includes the blood plasma (the fluid of blood)
・an internal one, the inside-of-the-cells, or intracellular fluid (ICF).
How to Measure TBF?
TBF can be estimated by converting your body weight in lbs and dividing it by 4. So if you weigh 160 lbs (72.6kg), your TBF is 40 L. The ECF is 40% of the TBF. Thus, a 160 lb person with TBF 40 L has 16 L of ECF (outside cells) and 24L of ICF (inside cells).
Blood plasma is 8% of TBF, so a 160 lb man with 40 L TBF, can be estimated to have 3.2 L blood plasma. And since the plasma normally is 60% of Total Blood Volume (TBV), it calculates to TBV 5.3 Liters. (32/6=5.333…)
Since water passes easily and rapidly into and out of the cells, across the cell membranes between ICF and ECF, it maintains what is called osmotic equilibrium by going into or out of cells in response to unequal concentrations of the dissolved particles like excess Na+ inside a cell or like the glucose molecules outside the cells.
Each particle in fluid needs to be surrounded by a unit of water molecules. Since particles are constantly being added to the ECF by eating and drinking, and leaving the ECF mostly in the urine but to a lesser extent in feces, sweat and breathed air, and since some particles are disappearing from the ICF due to being used up by the cells’ metabolism, the water in cells is constantly shifting back and forth across cell membrane, causing a shrink and swell of the cell. This is most seriously affecting brain function because brain swelling cannot continue long and much without brain damage and death. So body control is always attempting to limit cell swell or cell shrink by using the kidneys to get rid of excess water or conserve it and adjust electrolyte concentrations across cell membranes.
From a practical standpoint the kidneys are the key organ to regulate water balance and they do it in an automatic way. Balancing against the kidneys are the brain’s sense of thirst that directs to drink more or less water and balance it with needed or not needed electrolytes.
Problems can develop in persons with an abnormal brain function that affects the sense of taste, like people with schizophrenia, who drink excessive amounts of water. Luckily, we humans have a gigantic capacity to drink excess water but it has limits especially if our kidneys or hearts are not functioning well. In heart or kidney failure, water and salt intake must be strictly controlled and usually limited.
In certain life situations, knowledge of water balance can be life-saving; for example, if your commercial jet crash lands in the Pacific Ocean and you’re on a lifeboat and your water runs out, catch fish and drink the fish’s squeezed fresh flesh fluid, but don’t eat the flesh because the raw fish flesh is too salty. By the way, in the Pacific, flying fish are frequent and easy to catch.
For the normal person in good health, water balance is automatic based on normal thirst and drinking and normal kidneys excretion. But if, for example, you catch cholera on a trip, then, a tremendous amount of body water is lost every hour in diarrhea and must be replaced by sports drinks rather than just plain water or you will go into shock and die, as most patients with cholera do.
Check out an article on Alkaline water on The Millennials™️!
To learn more about the ins and outs of body fluid water balance, check
- 2.13a The Water of Our Body – Internal Milieu
- 2.13b Blood Electrolytes & Body Water
- 2.13c Acidity and Alkalinity of the Body.
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