Biological role and pathways of water excretion
Water performs two fundamental functions in the human body. First, it provides an ideal medium for all metabolic processes and biochemical reactions. Second, it performs a transport function: it ensures the movement of ions, nutrients (e.g., glucose), oxygen, vitamins, as well as hormones and neurotransmitters. In addition, water transports intermediate and final products of tissue metabolism to sites of excretion or utilization.
The amount of fluid in the body is constantly decreasing. Every day, a person loses about 2.8 L of water through the following pathways:
- Urination (kidneys): 1.8 L.
- Respiration (lungs): 0.5 L.
- Sweating (skin): 0.4 L.
- Defecation (intestines): 0.1 L.
Along with the fluid, dissolved components are removed: ions, critically important metabolic products, and partially useful substances (amino acids, vitamins, hormones).
Sources of intake and body distribution
To compensate for losses, the body must intake precisely the same amount — 2.8 L of fluid per day. This volume consists of two fractions:
- Exogenous water (2.5 L): comes from outside. Of this volume, 1.5 L is drunk as fluids, and another 1.0 L is obtained as part of solid food.
- Endogenous, or metabolic water (0.3 L): is synthesized directly in tissues during the oxidation of nutrients.
The output of metabolic water depends on which substrate is oxidized. The leaders are fats, which yield 1.09 mL of water per 1 gram of substance. Carbohydrates generate 0.6 mL/g, and proteins only 0.44 mL/g. This is why diet affects thirst: a carbohydrate-fat diet reduces the need for drinking, while a high-protein diet increases it.
All water is distributed among internal reservoirs. The bulk is accounted for by tissue water (from 30 to 50 L), whereas about 5 L circulates in the bloodstream.
Internal exchange in the gastrointestinal tract
There is a continuous and intensive fluid exchange between the circulatory system and digestive glands. During the day, about 8.0 L of digestive juices are secreted into the lumen of the gastrointestinal tract.
The composition of this secretion is distributed as follows:
- Intestinal juice — 2.5 L.
- Gastric juice — 2.0 L.
- Saliva — 1.5 L.
- Pancreatic juice — 1.5 L.
- Bile — 0.5 L.
However, the body does not lose this volume. Total reabsorption occurs during digestion. The total volume of fluid reabsorbed from the GI tract back into the blood reaches 10.5 L/day. This figure includes 8 L of secreted juices and 2.5 L of exogenous water (from food and drink).
Transcapillary exchange (Starling mechanism)
Water continuously circulates between the blood and the interstitial space. The direction of its movement is determined by the interplay of two forces: hydrostatic pressure (HP), which "pushes" water out of the vessel, and oncotic pressure (OP) of plasma proteins, which "holds" water in.
The pressure dynamics along the capillary are as follows:
- Arterial end (filtration). Blood enters the capillary with a high HP of 40 mm Hg. Oncotic pressure is 30 mm Hg. Since HP is greater than OP (40 > 30), fluid rushes from the bloodstream into the tissues with a gradient of about 10 mm Hg. Water enters the tissue spaces.
- Venous end (reabsorption). As blood moves along the capillary, vascular resistance dampens the hydrostatic pressure, dropping it to 20 mm Hg. Meanwhile, oncotic pressure remains unchanged (30 mm Hg). Now the pulling force of proteins exceeds the hydrostatic pressure (30 > 20). Through osmosis, water returns from the tissue back into the blood.
Over 99% of the filtered water is successfully reabsorbed back into the capillary. The remaining negligible fraction (about 0.02%) is drained via lymphatic capillaries.
Briefly: answers to topical questions
Where are the osmoreceptors located that trigger the sensation of thirst when water balance changes?
Osmoreceptors that trigger the sensation of thirst when blood osmotic pressure increases are located in the anterior nuclei of the hypothalamus.
Mechanism:
- When there is a water deficit in the blood, osmotic pressure increases.
- The anterior nuclei of the hypothalamus (osmoreceptors) are stimulated.
- The signal is transmitted to the thirst center, generating the sensation of thirst, and water intake occurs.
- The signal is also transmitted to the supraoptic and paraventricular nuclei of the hypothalamus, which synthesize vasopressin.
For quick memorization of capillary pressure dynamics, use the "forty-thirty-twenty" rule. At the beginning of the vessel, hydrostatic pressure is 40, oncotic pressure is stable at 30, and at the end, hydrostatic drops to 20. First 40 beats 30 (filtration occurs), then 30 beats 20 (absorption occurs).
Class and exam questions
Why do people on a high-protein diet need to drink more water?
During the oxidation of one gram of proteins, a minimal amount of endogenous water is released — only 0.44 mL (unlike fats, which yield 1.09 mL). Due to the decrease in the proportion of metabolic fluid, the overall requirement for external water increases compensatorily.
What happens to the fluid that did not return to the venous end of the capillary?
The bulk of water (over 99%) is reabsorbed into the blood. The insignificant fraction of fluid that remains in the interstitium (about 0.02% according to the scheme) enters the lymphatic capillaries and ensures lymphatic drainage of tissues.
What volumes make up the 10.5 liters absorbed in the GI tract?
This figure is formed from two sources. 2.5 liters come from outside with food and drink, and 8 liters represent the fluid of digestive juices that was secreted into the GI lumen and then reabsorbed.
What else to study on this topic
- Water distribution: tissue reservoirs and circulating blood volume
- The relationship between fluid osmotic pressure and water loss processes
- Features of fluid secretion by digestive glands
- The effect of hydrostatic resistance on hemodynamics in the microvasculature
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Water balance and transcapillary water exchange" and 4,128 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the section "Regulation of Water-Salt Metabolism"
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.