Excretory Function of the Lungs
The respiratory system plays a crucial role in the removal of volatile compounds and water vapor. The mechanism of water elimination is based on a humidity gradient: inhaled air is generally not saturated with vapor, but upon reaching the alveoli, it becomes saturated due to evaporation from their vast surface area. As a result, exhaled air has 100% humidity. Through this pathway, the body loses approximately 0.5 L of water daily.
In addition to fluid, the pulmonary barrier excretes:
- Alcohol vapors;
- Agents used for inhalation anesthesia (chloroform, ether, nitrous oxide).
Excretion via Sweat Glands
The integumentary system participates in excretion primarily through the activity of sweat glands, which are distributed across the entire body surface. Their highest density is observed on the palms, soles, and in the axillary regions. The daily volume of eliminated water reaches 0.4 L.
In its qualitative composition, sweat resembles urine: it contains salts and end products of metabolism. The specific odor of the secretion is imparted by ammonia, and the acidic pH reaction of the medium is due to the presence of hydrogen ions. Urea and uric acid are also excreted with sweat.
The main difference between sweat and urine is a significantly lower concentration of substances. This is because sweat glands completely lack the concentration and reabsorption mechanisms typical of renal tubules.
Neurochemical Feature of Innervation
The activity of sweat glands is strictly controlled by the nervous system: sweating is invariably enhanced during sympathetic nervous system activation. However, a major physiological paradox lies here.
Traditionally, postganglionic fibers of the sympathetic nervous system release the neurotransmitter norepinephrine. The innervation of sweat glands is a unique exception to this rule—here, the terminals of postganglionic sympathetic fibers release acetylcholine. This fact is of critical importance in clinical pharmacology when selecting medications that affect sweating.
Role of the Gastrointestinal Tract
The excretory function of the GI tract is not limited solely to the evacuation of undigested food residues. Throughout the pathway of chyme transit and feces formation, the epithelial cells of the stomach, small intestine, and large intestine actively transport metabolic products from the bloodstream into the lumen of the digestive tube.
With feces, the body loses about 0.1 L of water per day and eliminates:
- Urea and uric acid;
- Various intermediate and end products of metabolism;
- Bilirubin—a degradation product of hemoglobin that is formed in the liver and enters the intestinal lumen as part of bile.