Excretory System

Excretion Overview

The excretory system is an organ system that removes metabolic waste from the body fluids of an organism, so as to help maintain internal chemical homeostasis and prevent damage to the body. Metabolic waste includes salts, CO2 and urea. The dual function of excretory systems is the elimination of the waste products of metabolism and to drain the body of used up and broken down components in a liquid and gaseous state. In humans, most of these substances leave the body as urine or fecal matter, although expulsion also does occur through exhalation and sweating. Excretion must not be confused with defecation, which is the removal of non-digested material from the digestive tract.

There are 4 excretory organs in human: the skin, the lungs, the liver, and the kidney.


The lungs in the respiratory system excrete some waste products, such as carbon dioxide and water. Cellular respiration occurs in every living cell in your body. It is the reaction that provides energy (in the form of ATP molecules) for cellular activities. If respiration stops, the cell no longer has energy for cellular activities, and the cell dies. As respiration occurs carbon dioxide is produced as a waste product. As the carbon dioxide accumulates in body cells, it eventually diffuses out of the cells and into the bloodstream, which eventually circulates to the lungs. In the alveoli of the lungs, carbon dioxide diffuses from the blood, into the lung tissue, and then leaves the body every time we exhale. Some water vapor also exits the body during exhalation.


The skin is another excretory organ that rids the body of wastes through the sweat glands. The skin excretes the sweat outside the body through numerous pores in the surface of this organ. Sweat is a mixture of three metabolic wastes: water, salts and urea. So as you sweat, your body accomplishes two things:

  1. Sweating has a cooling effect on the body
  2. Metabolic wastes are excreted


The liver (via the intestines) excretes bile pigments that result from the destruction of hemoglobin.  The liver as an excretory organ acts to breakdown some proteins and other nitrogenous compounds by a process called deamination. As a result of these reactions, a nitrogenous waste called urea is formed. The liver also helps in excreting toxic substances, drugs and their derivatives, bile pigments and cholesterol.


The major task of excretion, however, belongs to the kidneys, the major organ in the urinary system. If it fails the other organs cannot take over and compensate adequately. The kidneys are bean-shaped organs which are present on each side of the vertebral column in the abdominal cavity. Humans have two kidneys and each kidney is supplied with blood from the renal artery. The kidneys remove from the blood the nitrogenous wastes such as urea, as well as salts and excess water, and excrete them in the form of urine. This is done with the help of millions of nephrons present in the kidney. Nephrons (over a million per kidney) are microscopic units that filter blood of its metabolic waste products. Water and waste taken from blood into the nephron will go on to form urine. The glomerulus is a network or clump of capillaries located at the beginning of a nephron in the kidney. It serves as the first stage in the filtering process of the blood carried out by the nephron in its formation of urine. The filtrated blood is carried away from the kidneys by the renal vein (or kidney vein). The urine from the kidney is collected by the ureter, one from each kidney, and is passed to the urinary bladder. The urinary bladder collects and stores the urine until urination. The urine collected in the bladder is passed into the external environment from the body through an opening called the urethra.

Renin Angiotension Aldosterone System (RAAS)

If blood pressure is too low, the kidney releases renin, which triggers the formation of angiotensin II, which stimulates aldosterone release. The end result is to raise the blood pressure. The adrenal glands release aldosterone, which causes the distal tubules of the kidney to reabsorb more sodium ions which in turn causes more water reabsorption. ADH causes more water reabsorption in the kidney tubules, raising the blood pressure. High levels also causes vasoconstriction. Should the blood pressure be too high, all the above hormones stop releasing. Also, the heart can release atrial natriuretic peptide (ANP), which counteracts aldosterone and causes the kidney to excrete both more sodium ions and more water. ANP can also cause vasodilation. Blood plasma is mainly sodium ions and chloride ions while inside cells is mainly potassium and hydrogen phosphate ions. Blood osmolarity is determined predominantly by sodium ions and chloride ions. When blood osmolarity is too low, aldosterone is produced causing sodium ions and chloride ions to be reabsorbed. Potassium ions are regulated by aldosterone, whose production causes excretion of the potassium ions. Calcium and phosphate regulated by parathyroid hormone (PTH). PTH results in more calcium ion reabsorption in kidney tubules. It also causes bone to break down to release calcium and phosphates, and causes the small intestine to absorb more calcium.

Renin Aldosterone Angiotensin System
Source: Anatomy & Physiology, Connexions

Kidney Regulation of pH

Renal physiology has several powerful mechanisms to control pH by the excretion of excess acid or base. In response to acidosis, tubular cells reabsorb more bicarbonate from the tubular fluid, collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammonia genesis leads to increased formation of the NH3 buffer. In responses to alkalosis, the kidney may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells, and lowering rates of glutamine metabolism and ammonium excretion. Excreting bicarbonate ion makes blood more acidic and its reabsorption makes blood more basic. Bicarbonate bind H+, so it has the opposite effect on pH. Therefore, H+ secretion gets rid of acidity. Urine is concentrated urea in water, with some salt. Urea is a harmless form of toxic ammonia and it is a nitrogenous waste.

Amino acids → Ammonia → Urea → Excretion

Kidney Anatomy

By Davidson, A.J., Mouse kidney development (January 15, 2009), StemBook, ed. The Stem Cell Research Community, StemBook, doi/10.3824/stembook.1.34.1, http://www.stembook.org. [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

The cortex is the outer shell of the kidney and it contains the convoluted tubules. The medulla is the inner part of the kidney and contains the loop of Henle.

Haisook at English Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons 

The nephron is a functional unit of the kidney. It is made up of the glomerulus, Bowman’s capsule, proximal tubule, loop of Henle, distal tubule and a collecting duct. One collecting duct can be shared by many nephrons. The glomerulus is a balled network of capillaries located at the beginning of a nephron in the kidney. The glomerulus is surrounded by a cup-like sac known as Bowman’s capsule. The capillaries of the glomerulus filter the blood plasma into the capsule and the Bowman’s capsule empties the filtrate into the proximal tubule.

The proximal tubule is the convoluted tubule on the side of the Bowman’s capsule and it is the major site for reabsorption of nutrient, salts and water. It also does secretion of ions, except for potassium ion. The loop of Henle is a U shaped loop that dips into the renal medulla and the countercurrent multiplier mechanism occurs here. The descending limb is responsible for water reabsorption by osmosis and it is permeable to water but not to solute. Bottom of the loop is the most concentrated. The ascending limb is responsible for salt reabsorption and it is permeable to salt but not water. The collecting duct concentrates urine by water reabsorption by facilitated diffusion, when ADH is present. Water reabsorption in the collecting duct is possible because the loop of Henle has very high osmolarity (very concentrated) at the bottom. The distal tubule is the convoluted tubule on the side of the collecting duct. The collecting duct is hormone-controlled and it fine tunes the work done by the proximal tubule. It reabsorbs of salts and water based on the requirement of the body. The job of distal convoluted tubule, in response to aldosterone, is also to secrete potassium ions. Many nephrons collectively drain into one collecting duct. Both good and bad substances, as well as ions are filtered out, as long as it’s small enough.

The countercurrent multiplier creates an osmotic gradient down the loop of Henle, which is used by the collecting duct to concentrate urine. Sodium chloride pump on ascending limb drives the countercurrent multiplier.

Countercurrent means that on the descending limb of the loop of Henle, water flows out of the filtrate and on the ascending limb, salt flows out of the filtrate. The multiplier is the gradient-producing power of each individual sodium chloride pump multiplies down the length of the loop of Henle.

The longer the loop of Henle, greater the osmotic gradient and the more concentrated the urine that can be produced.

Urea at the bottom of the collecting duct can leak out into the interstitial fluid and back into the filtrate. It contributes to the high osmolarity at the bottom of the loop of Henle. The ureter, bladder and urethra aid in storage and elimination of urine. The collecting ducts drain into the ureter, which drains into the bladder. The bladder stores the urine untl excretion. The bladder has special epithelium (transitional epithelium) that can squish to accommodate storage of large amounts of urine. Urine gets excreted out of the bladder through the urethra.

Renal Anatomy
By Sunshineconnelly at English Wikibooks [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

Nephron Summary

Region Function
Proximal Convoluted Tubule  Reabsorption of ions, organic molecules, water, vitamins
 Descending Loop of Henle  Reabsorption of water
 Ascending Loop of Henle  Reabsorption of ions
 Distal Convoluted Tubule  Reabsorption of water and sodium ions. Secretion of potassium, acids, drugs, ammonia
 Collecting Duct  Reaborption of water


  • Kidney
  • Nephron
  • Renal Anatomy
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