Excretory organs and homeostasis
The excretory system is a functional system whose task is to preserve the physiological constancy of the internal environment.
Main pathways of substance elimination from the body:
- Kidneys. This is the main excretion pathway, ensuring urine formation and the removal of most soluble metabolites.
- Skin. Excretion of substances through sweat glands is of paramount importance for temperature and water-salt balance.
- Gastrointestinal tract (GIT). Participates in the excretion of water and salts.
- Lungs. Ensure the evaporation of water during respiration and the removal of gaseous products.
The collaborative work of these organs maintains stable levels of osmotic and blood pressure, blood pH, partial pressure of respiratory gases, body temperature, and nutrient concentrations (e.g., glucose). At the same time, water and salts in the body are constantly replaced by new molecules coming from food and drink, which is critically important for regulating osmotic pressure.
Protein metabolism and residual nitrogen
The end products of fat and carbohydrate oxidation are water and carbon dioxide. However, protein metabolism leads to the formation of nitrogenous compounds, high concentrations of which are toxic to cellular processes. Physiological mechanisms are aimed at maintaining their minimal level.
The amount of protein metabolism products in the blood, determined after plasma protein precipitation, is called residual nitrogen. Its normal range is 200–400 mg/L.
Residual nitrogen includes:
- Urea nitrogen (makes up the largest share — 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. If the concentration of protein metabolism products increases in the blood (stimulus), this is detected by vascular chemoreceptors. They perform reverse afferentation, transmitting signals to the central nervous system.
The central link that processes information and coordinates the response is the hypothalamus — the highest autonomic center. Efferent mechanisms are activated as an effector response:
- Nervous regulation (direct action on organs).
- Hormonal regulation (humoral influence on target organs).
As a result, excretion processes are enhanced. The system balances between the "input" (behavioral regulation, e.g., restricting protein foods) and the "output" (excretion via kidneys, skin, and GIT). The primary pathway for implementing this response is urine formation.
Physiology of defecation
The process of bowel emptying is also subject to strict physiological reflexes. The gastrocolic reflex plays a crucial role: when food enters the stomach, it reflexively causes contraction of the rectum and the urge to defecate. This mechanism is particularly frequent in children and is used to form a systematic reflex for time and place.
Conditions for normal defecation require regularity (emptying at the same time, optimally in the morning) and execution of the act immediately upon the emergence of the urge. Prolonged and frequent delays lead to a distortion of the autonomic regulation of the internal anal sphincter, the development of chronic constipation, and profound disruption of subsequent rectal evacuation.
Briefly: answers to topical questions
Which specific hormones are involved in the humoral regulation of urine formation?
Hormones involved in the humoral regulation of urine formation include those that affect diuresis directly or through metabolic changes.
- Vasopressin (ADH) — enhances water reabsorption in the renal tubules, causing a decrease in diuresis.
- Aldosterone — increases sodium reabsorption, decreases diuresis.
- Natriuretic peptide (Na-uretic hormone) — enhances the excretion of sodium and water, causes an increase in diuresis.
- Angiotensin II, ACTH, adrenoglomerulotropin — stimulate aldosterone production.
- Melatonin — inhibits aldosterone secretion.
- Thyroid hormones, growth hormone (GH), sex hormones — increase diuresis due to the osmotic component (accumulation of protein metabolism products).
What are the mechanisms of tubular reabsorption in the nephron?
The mechanisms of tubular reabsorption vary depending on the nephron segment and include active and passive transport processes.
- Obligate (mandatory) reabsorption — occurs in the proximal convoluted tubules. Glucose, vitamins, and amino acids, as well as up to 70% of water and ions, are completely returned to the blood.
- Passive diffusion — in the descending limb of the loop of Henle, water moves passively into the interstitial fluid along an osmotic gradient.
- Active transport — in the ascending limb of the nephron loop, cells actively pump out sodium ions (the wall is impermeable to water).
- Facultative reabsorption — takes place in the distal convoluted tubules and is regulated by hormones depending on the body's needs.
Final urine concentration is completed in the collecting ducts.
Class and exam questions
What does residual nitrogen consist of?
It includes protein metabolism products: urea (about 70%), creatinine, creatine, uric acid, amino acids, ammonia, and indican. Normally, its level is 200–400 mg/L.
How does the nervous system detect the accumulation of metabolic waste products in the blood?
Changes in metabolite concentration in the bloodstream are detected by special vascular chemoreceptors. From them, the signal is transmitted to the central nervous system, specifically to the hypothalamus.
What is the normal daily volume of primary urine produced?
The kidneys filter about 180 liters of primary urine per day. However, 99% of this volume undergoes reabsorption, and only 1% is excreted as secondary urine.
Why are frequent delays in responding to the urge to defecate dangerous?
Ignoring urges disrupts the autonomic regulation of the internal anal sphincter. This inevitably leads to constipation and a disorder of rectal evacuation mechanisms.
What else to study on this topic
- Mechanisms of glomerular filtration in the kidneys
- Reabsorption processes in the renal tubules
- The role of antidiuretic hormone in water balance regulation
- Regulation of blood osmotic pressure
- Autonomic innervation of the digestive tract
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