Human Excretory System: Function, Organs, and Diseases
Excretion
Excretion is the expulsion of toxic substances from the blood. These substances are products of cellular metabolism (urea, uric acid, and carbon dioxide) and ingested salts. Its purpose is to remove harmful substances from the blood and maintain a constant concentration of dissolved salts.
Harmful substances:
- Urea: main substance derived from ammonia, formed in the metabolism of proteins. Ammonia is transformed into less toxic urea.
- Uric acid: A substance derived from ammonia.
- Dissolved minerals: Eliminated only when there is an excess in the blood.
- Foreign substances: Those not degraded by cells and accumulate in the blood (drugs and food additives).
The Urinary System
The urinary system removes water, urine, and toxic substances from the body. It comprises:
- Kidneys: Two organs near the back, surrounded by protective fat. They filter the blood, eliminating urea, uric acid, and excess minerals, producing urine.
- Ureters: Two tubes, one from each kidney, that carry urine to the bladder.
- Urinary bladder: A chamber where urine accumulates until its expulsion. The output has a muscle ring, a sphincter, which regulates the opening. It connects to the outside through the urethra.
- Urethra: Expels urine (urination). In males, it also conducts semen.
Kidney, Nephron, and Urine Formation
The kidney’s functional units are called nephrons. Kidneys are formed by thousands of nephrons.
Nephron
The glomerulus comprises capillaries where toxic substances and urea are collected by Bowman’s capsule. These substances circulate inside the nephron while water is reabsorbed. As the toxic substances move towards the collecting tube, urine is formed. All collecting tubes empty into the Malpighian pyramids in the renal pelvis, which lead the urine into the ureter. The ureters carry the urine to the bladder, which expels it through the urethra.
Diseases
Renal failure: Dysfunction of the nephrons (the kidney cannot remove urea from the blood, leading to its accumulation). The patient must undergo dialysis.
Nephritic colic: Very painful contraction of the ureter due to crystallized substances (stones). Drinking plenty of water is recommended to prevent this.
Cystitis: Inflammation of the urinary bladder caused by a bacterial infection. It produces a burning sensation when urinating and sometimes involuntary urine emission (urinary incontinence).
Prostatitis: Inflammation of the prostate (a gland only men have). In inflammation, the prostate exerts pressure on the urethra, causing difficulty in urination.
Other Excretory Organs
- Sudoriferous (sweat) glands: Produce sweat, which contains water and salts similar to dilute urine.
- Lungs: Expel carbon dioxide (CO2) from cellular respiration.
- Lacrimal glands: Expel water and mineral salts through tears.