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 excretion is based on the humidity gradient: inhaled air is typically not saturated with vapor, but upon entering the alveoli, it becomes saturated with moisture due to evaporation from their vast surface area. As a result, exhaled air has a humidity of one hundred percent. Through this pathway, the body loses about 0.5 L of water daily.
In addition to fluid, the following are excreted through the pulmonary barrier:
- Alcohol vapors;
- Agents used for inhalational anesthesia (chloroform, ether, nitrous oxide).
Excretion via sweat glands
The integumentary system participates in excretion primarily through the action of sweat glands scattered across the entire body surface. Their highest density is observed on the palms, soles, and in the armpits. 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. Ammonia imparts a specific odor to the secretion, and the acidic reaction of the medium (pH) 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 the significantly lower concentration of substances. This is because the structures of the sweat glands completely lack the concentration and reabsorption mechanisms (backward absorption) characteristic of the renal tubules.
Neurochemical feature of innervation
The activity of sweat glands is strictly controlled by the nervous system: sweating is always enhanced upon excitation of the sympathetic division. However, this harbors a crucial physiological paradox.
Traditionally, postganglionic fibers of the sympathetic nervous system secrete the neurotransmitter norepinephrine. The innervation of sweat glands is a unique exception to this rule—here, acetylcholine is released from the terminals of postganglionic sympathetic fibers. This fact is of critical importance for clinical pharmacology when selecting drugs that affect sweating.
Role of the gastrointestinal tract
The excretory function of the gastrointestinal tract is not limited solely to the evacuation of undigested food residues. Along the entire path of chyme transit and fecal formation, the epithelial cells of the stomach, small, and large intestines 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 also gets rid of:
- Urea and uric acid;
- Various intermediate and end products of metabolism;
- Bilirubin—a product of hemoglobin degradation formed in the liver and entering the intestinal lumen as part of bile.
Briefly: answers to topical questions
What other organs and tissues, besides the lungs, skin, and gastrointestinal tract, take part in extrarenal excretion?
In addition to the indicated structures, the salivary glands and liver participate in extrarenal excretion.
- Salivary glands (gl. parotis, gl. submandibularis, gl. sublingualis) possess an excretory function and are capable of excreting substances from the blood into saliva (nitrogen metabolism products and inorganic ions).
- Liver—participates in the elimination of bilirubin, which is a hemoglobin breakdown product and is excreted along with the produced bile.
Class and exam questions
Why is sweat significantly less concentrated than urine?
Unlike renal nephrons, sweat glands lack physiological mechanisms for the reabsorption of water and concentration of the secretion.
Which neurotransmitter is released at the nerve endings stimulating sweating?
Acetylcholine is released in postganglionic sympathetic fibers heading to the sweat glands, which is an exception for the sympathetic nervous system.
Is the excretory function of the intestine reduced solely to the removal of undigested food?
No, the epithelial cells of the stomach and intestines also actively excrete metabolic products (e.g., urea and uric acid) from the blood into the lumen of the gastrointestinal tract.
What else to study on this topic
- Pharmacological significance of the cholinergic sympathetic innervation of sweat glands
- Mechanisms of participation of the lungs, gastrointestinal tract, and skin in the regulation of blood osmotic pressure
- The process of bilirubin formation in the liver and its excretion with bile
- Physics of moisture evaporation from the alveolar surface and thermoregulation
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