Human Excretory System Functions and Urine Formation

The Human Excretory System

To carry out their functions, cells perform chemical reactions called cellular metabolism. During this process, waste substances are produced, the main ones being:

• Carbon Dioxide: It originates from the metabolic reactions that produce energy.
• Urea and Uric Acid: These proceed from the destruction of proteins and nucleic acids.
• Toxic Ingested Substances: Various harmful substances taken into the body.

Anatomy of the Kidney

The kidneys are two bean-shaped organs located in the back of the abdomen. Each kidney is composed of an outer part called the renal cortex and an internal cavity called the renal pelvis.

Each kidney is made up of more than one million nephrons, which are small tubes closed at one end and surrounded by blood capillaries. These are responsible for filtering the blood and producing urine. Each nephron ends in collecting ducts that drain into the renal pelvis. Additionally, the kidneys secrete substances that stimulate red blood cell production.

The Urinary Tract

The urinary tract consists of the ducts that connect the kidneys to the exterior:

• Ureters: These narrow tubes, about 28 cm long, emerge from each kidney to carry urinary flow to the bladder. When the bladder is too full, there is nerve stimulation (the voiding reflex) that provokes the release of urine through the urethra.

The Urethra

The urethra consists of ducts leading to the outside and has a sphincter that remains permanently closed while there is no nervous micturition reflex. In men, the urethra joins with the reproductive system, while in women, it is independent of it.

The Process of Urine Formation

The process of urine formation consists of the following stages: glomerular filtration and tubular reabsorption.

Glomerular Filtration

This is the passage of certain blood substances contained in the glomerulus into Bowman's capsule. This filtered liquid contains water and most of the molecules from the circulating blood plasma: excretory products (urea, uric acid, etc.) and substances useful to the body (glucose, vitamins, etc.). The filtrate in Bowman's capsule is similar to blood plasma but without proteins.

Tubular Reabsorption

During its journey through the nephron tubules, a large portion of the filtered substances is reabsorbed. Among these substances are nutrients (glucose, vitamins, amino acids), as well as water and mineral substances, which return to the blood to maintain constant concentrations.