Human Excretory System and Urinary Function

The Human Excretory System

All cells produce wastes and toxins; if these accumulate, they can be harmful. They must be eliminated through the function of excretion. The blood carries these substances to the organs responsible for their elimination. The organs involved in excretion include:

• The kidneys: They eliminate toxic substances through urine and form part of the urinary system.
• Lungs: They expel CO2.
• Liver: It eliminates wastes from medicines, drugs, and other substances by forming bile (which is also used in digestion).
• Sweat glands: They eliminate wastes through the production of sweat. Sweat is very similar to urine, but more diluted.

Anatomy of the Urinary System

The urinary system produces and expels urine. It consists of the following components:

The Kidneys

Two organs located in the upper part and back of the abdominal cavity. They produce urine by filtering wastes from the blood. They consist of three parts:

• The renal cortex: The outer part.
• The renal medulla: The inner part.
• The renal pelvis: A kind of funnel that collects the urine formed in the previous parts.

The Urinary Tract

• Ureters: These connect the kidneys to the bladder.
• The bladder: A reservoir where urine is collected.
• The urethra: This carries the urine outside of the body. In women, the urethra is shorter.

Nephrons: The Functional Units

The nephrons are millions of tiny structures that exist in every kidney. They are the functional units of the kidneys, meaning they are responsible for producing urine. While they have many parts, the primary components are:

• The glomerulus and Bowman’s capsule: A mass of tiny blood capillaries and a surrounding sac. These parts are located in the renal cortex.
• A tubule and a collecting duct: These are located in the renal medulla.

The Process of Urine Production

Urine is produced in every kidney by the nephrons through two main processes:

1. Filtration of the blood: Toxic substances, salt, and water move from the blood in the glomerulus to the Bowman’s capsule. This is the "first urine."
2. Reabsorption: Useful substances, such as water, mineral salts, and vitamins, are reabsorbed into the blood within the tubule. The urine then becomes more concentrated.

The urine then reaches the collecting duct, which brings it to the renal pelvis. From the renal pelvis, it travels to the bladder through the ureters.

Diseases of the Urinary System

• Cystitis: The inflammation of the bladder or urinary tract walls due to an infection. It causes pain and burning during urination, which becomes very frequent.
• Renal colic: Extremely severe pain caused by kidney stones.
• Renal failure: A condition where the kidneys stop filtering. This can lead to death. Sometimes hemodialysis is needed to cleanse the blood using a machine.