The Excretory System - by Alexis England

The body is constantly trying to maintain homeostasis, or an internal balance. The excretory system helps to achieve this balance by maintaining proper concentrations of substances in the blood, whether by removing wastes or reabsorbing needed molecules like water.


The excretory system is responsible for three major functions:
  1. Osmoregulation, or the control of water concentration in the body.
  2. Removal of unnecessary material, particularly nitrogenous wastes.
  3. Regulation of blood concentration.
The excretory system is able to carry out these functions by:
  • Filtration
  • Re-absorption
  • Secretion and Excretion
A diagram of the excretory system, with major organs bold face.

Urinary Excretion


A diagram of the kidney with major parts.
A sample of tissue from the bladder.
The glomerulus (red) within the Bowman's capsule (blue).

A model of the nephron showing forms of re-absorption.

Kidneys : The kidneys are the major components of the excretory system. They are responsible for osmoregulation and filtration of the blood. Urea is formed in the liver, then travels down to the kidneys through the blood (urea is in sweat too!). Urea, along with excreted wastes, electrolytes, sugars, and other molecules, forms urine. The renal artery carries blood from the body into the kidneys, and the renal vein carries filtered blood from the kidneys into the vena cava.

  • Nephron - The nephron is the functional unit of a kidney. There are millions of these thin tubules that extend from the outer cortex of the kidney into the medulla. The blood enters the nephron and comes out as a filtrate, containing wastes, yes, but also vitamins and amino acids.

A Trip Through the Nephron

Blood enters the glomerulus, a ball of capillaries, for filtration. Ions, salts, sugars, and
some water selectively diffuses through the thin membrane of the capillaries into the surrounding Bowman's capsule and are re-absorbed into the body. The remaining filtrate continues to travel through tubules and the Loop of Henle, where more molecules are absorbed and the concentration of the blood is controlled. The final product, urine, contains only wastes, poisons, and excess solutes. It then travels through the collecting tubule to the ureter.

Ureters : A ureter runs down both sides, connecting the kidneys to the bladder. Muscular contractions push the urine through this tube and into the bladder.

Bladder : The bladder is a hollow sac that holds urine. It is able to expand significantly as more urine enters.

Urethra : The urethra is a small tube that runs from the bladder to outside the body, so that urine can be expelled.

Enzymes and Hormones

Anti-diuretic Hormone : A hormone responsible for regulating the concentration of water by determining how much H2O the body should reabsorb and how much should be expelled with urine.
Aldosterone : A hormone responsible for regulating the concentration of ions, like sodium, by determining how much to reabsorb from the filtrate in the nephron.
ATPases : Multiple enzymes that help create the concentration gradient necessary to move ions and wastes across the membrane for absorption and secretion.


  • The excretory system is regulated with assistance from the hormones produced by the endocrine system. ADH, for example, controls how much water diffuses through the membrane of the tubules in the nephron. Aldosterone controls the concentration of solutes, such as ions and amino acids, that can be re-absorbed by the blood.
  • The two sphincters at the bottom of the bladder are able to control urine release. The inner sphincter is involuntary and will open when the bladder is filling, but the external sphincter can be voluntarily controlled so you can 'hold' your pea.
  • The circulatory system has a huge effect on urinary production because good blood pressure is needed to properly filter the blood. A faster heart rate causes a faster trip through the excretory system.
  • Because other systems, including the digestive and respiratory systems, also secrete certain products like feces and carbon dioxide, anything that affects these systems, like exercise, also affects the excretory system.


The circulatory system actually plays a role because
  • Blood is the carrier of the various wastes and solutes that are filtered, absorbed, secreted, and excreted.
  • Renal arteries and veins carry blood to the kidney, but are considered part of the circulatory system.
  • Even within the neprhon, thanks to the glomerular capillaries, the circulatory and excretory systems are communicating in order to maintain proper balance in the blood.
The endocrine system also assists by
  • producing important hormones like ADH in the pituitary gland.
  • helping to regulate blood osmolarity via the JGA and ion pumps for ions such as sodium and potassium.

Other Forms of Excretion

  1. Via the Integumentary System: Urea is also found in sweat. During heat or exercise, sebaceous glands in the skin release a mixture of water and waste, known as sweat, that is excreted through the pores.

  2. Via the Respiratory System : After respiration, carbon dioxide is excreted from the lungs by breathing out.

  3. Via the Digestive System : At the end of digestion, defecation occurs. Feces is another mixture of excess solutes and waste that the body excretes through the rectum.