Biology » Human Biology » The Excretory System

Urinary System

How is it determined what’s waste and what’s not?

Shown above is a major process of maintaining homeostasis. Getting rid of waste and excess water. Such a basic process is actually very complex. It involves an intricate exchange of material through the kidney.

Urinary System

The kidneys are part of the urinary system, which is shown in the figure below. The main function of the urinary system is to filter waste products and excess water from the blood and excrete them from the body.

Kidney location in body

The kidneys are the chief organs of the urinary system.

Kidneys and Nephrons

The kidneys are a pair of bean-shaped organs just above the waist. A cross-section of a kidney is shown in the figure below. The function of the kidney is to filter blood and form urine. Urine is the liquid waste product of the body that is excreted by the urinary system. Nephrons are the structural and functional units of the kidneys. A single kidney may have more than a million nephrons!

kidney cutaway

Each kidney is supplied by a renal artery and renal vein.

As shown in the figure below, each nephron acts as a tiny filtering plant. It filters blood and forms urine in the following steps:

  1. Blood enters the kidney through the renal artery, which branches into capillaries. When blood passes through capillaries of the glomerulus of a nephron, blood pressure forces some of the water and dissolved substances in the blood to cross the capillary walls into Bowman’s capsule.
  2. The filtered substances pass to the renal tubule of the nephron. In the renal tubule, some of the filtered substances are reabsorbed and returned to the bloodstream. Other substances are secreted into the fluid.
  3. The fluid passes to a collecting duct, which reabsorbs some of the water and returns it to the bloodstream. The fluid that remains in the collecting duct is urine.
In-depth diagram of nephron

The parts of a nephron and their functions are shown in this diagram.

Excretion of Urine

From the collecting ducts of the kidneys, urine enters the ureters, two muscular tubes that move the urine by peristalsis to the bladder (see the figure above). The bladder is a hollow, sac-like organ that stores urine. When the bladder is about half full, it sends a nerve impulse to a sphincter to relax and let urine flow out of the bladder and into the urethra. The urethra is a muscular tube that carries urine out of the body. Urine leaves the body through another sphincter in the process of urination. This sphincter and the process of urination are normally under conscious control.

Summary

  • The kidneys filter blood and form urine. They are part of the urinary system, which also includes the ureters, bladder, and urethra.
  • Each kidney has more than a million nephrons, which are the structural and functional units of the kidney.
  • Each nephron is like a tiny filtering plant.
  • Urine volume and concentration is regulated through the same processes that regulate blood volume.
  • Antidiuretic hormone (ADH) is produced by the pituitary gland to control the amount of water that is reabsorbed through the collecting ducts.

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