Excretory Organs and Homeostasis
The excretory system is a functional system whose primary task is to maintain the physiological constancy of the internal environment.
Major pathways for substance elimination from the body:
- Kidneys. The primary excretory pathway, responsible for urine formation and the removal of most soluble metabolites.
- Skin. Elimination of substances via sweat glands plays a major role in temperature and water-salt balance.
- Gastrointestinal tract (GIT). Participates in the excretion of water and salts.
- Lungs. Responsible for the evaporation of water during respiration and the elimination of gaseous waste products.
The coordinated action of these organs maintains stable osmotic and blood pressure, blood pH, partial pressure of respiratory gases, body temperature, and nutrient concentrations (e.g., glucose). Water and salts are continuously replenished by dietary intake, which is critical for regulating osmotic pressure.
Protein Metabolism and Non-Protein Nitrogen
The final products of fat and carbohydrate oxidation are water and carbon dioxide. However, protein catabolism yields nitrogenous compounds, high concentrations of which are toxic to cellular processes. Physiological mechanisms are aimed at keeping their levels minimal.
The amount of protein metabolism products in the blood determined after precipitation of plasma proteins is called non-protein nitrogen. Its normal range is 200–400 mg/L.
Non-protein nitrogen includes:
- Urea nitrogen (constituting the largest fraction—about 70%).
- Creatinine nitrogen.
- Creatine.
- Uric acid.
- Amino acids.
- Ammonia.
- Indican and certain other substances.
Functional Regulatory System
The regulation of metabolite levels operates as a closed-loop system. If the concentration of protein catabolism products in the blood increases (stimulus), this is detected by vascular chemoreceptors. They perform afferent feedback by transmitting signals to the central nervous system.
The central processing unit that coordinates the response is the hypothalamus, the highest autonomic center. Efferent mechanisms are then activated:
- Neural regulation (direct action on organs).
- Hormonal regulation (humoral action on target organs).
As a result, excretory processes are enhanced. The system balances between "input" (behavioral regulation, e.g., restricting dietary protein) and "output" (excretion via the kidneys, skin, and GI tract). The primary implementation of this response is urine formation.
Physiology of Defecation
The process of bowel evacuation is also governed by strict physiological reflexes. The gastrocolic reflex plays a vital role: when food enters the stomach, it reflexively causes contraction of the rectum and the urge to defecate. This mechanism is particularly prominent in infants and is used to establish a conditioned reflex for a regular time and place.
Normal defecation requires regularity (evacuation at the same time daily, optimally in the morning) and prompt execution upon the onset of the urge. Prolonged and frequent delays lead to autonomic dysfunction of the internal anal sphincter, chronic constipation, and profound impairment of subsequent rectal evacuation.