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The Importance of Excreting Wastes

To maintain life processes, the body must eliminate waste products.
There are many organs in the body to help eliminate wastes, such as;
  • the lungs: eliminates carbon dioxide
  • the liver: eliminates lactic acid and bile pigments
  • the large intestine: eliminates solid wastes
  • the kidney: eliminates ammonia, urea, and uric acid


The Urinary System

The urinary system maintains homeostasis by regulating levels of ions such as sodium, regulating the water content of the blood, and also maintaining the pH of the blood. 

The urinary system filters waste products out of the blood and makes urine.

It consists of the following:




Anatomy of the Kidneys and Urine Formation

The kidneys are located near the middle of the back, one on either side of the spine. Above each kidney there is a adrenal gland, which produces hormones.

They are responsible for removing wastes, balancing blood pH, and maintaining water balance.




Inside the kidneys, the blood is filtered through a network of slender tubules called nephrons. The afferent arterioles, from the renal artery, supplies the nephrons with the blood.  The blood is transferred to the glomerulus, where it is filtered. The efferent arterioles carry the blood away from the glomerulus to a net of capillaries called peritubular capillaries that wrap around the kidney tubule. As the blood passes through the blood vessels of the nephron, all unwanted wastes are taken away before it leaves the kidney.

When the blood enters the kidneys, it consists of urea, glucose, proteins, toxic wastes, salts and water, and when it leaves the kidneys, the blood would have filtered out the urea and the toxic wastes.

The waste products of the blood and unneeded water are collected and urine is formed.

The fluids to be processed into urine enter the Bowman's capsule from the blood. Once the blood is filtered, the urine travels to the proximal tubule and is carried to the loop of Henle, which descends into the medulla of the kidney. The urine then moves through the distal tubule, the last segment of the nephron, and into the collecting ducts. The collecting ducts collect urine from many nephrons and it is gathered in the renal pelvis, which is located at the centre of each kidney.  From the renal pelvis the urine drains down into the bladder from the ureters.  The urethra then carries the urine from the bladder out of the body. 



Urine formation depends on three functions:
  1. Filtration is accomplished by the movement of fluids from the blood into the Bowman's capsule
  2. Reabsorption involves the transfer of essential solutes and water from the nephron back into the blood
  3. Secretion involves the movement of materials from the blood back into the nephron




Glomerulus and Bowman's capsule

     Water and dissolved solutes are filtered here as the blood is forced through the walls of the glomerulus, into Bowman's capsule by fluid pressure in the capillaries


Proximal Tubule

     Selective reabsorption of nutrients from filtrate back into blood by active and passive transport


Descending limb of loop of Henle

     Permeable to water, resulting in loss of water from filtrate by osmosis

     Salt becomes concentrated in filtrate as descending limb dips into the inner medulla of the kidney


Ascending limb of loop of Henle

     Permeable to salt, resulting in diffusion of salt out of ascending limb


Distal Tubule

     Selective reabsorption of nutrients from blood into nephron by active transport

     Helps regulate potassium and salt concentration of body fluids


Collecting Duct

     Urine formation; water, salt, urea, uric acid, and minerals




Antidiuretic Hormone (ADH)
Antidiuretic is a peptide hormone which promotes the reabsorption of water in the kidneys and regulates the concentration of urine.


The hypothalamus produces ADH and the posterior pituitary releases it into the blood.
When ADH is present, the collecting duct is very permeable to water, and the water is reabsorbed back into the kidney tissues and the blood. The resulting urine will be very concentrated.
When ADH is low in concentration or absent, the collecting duct remains impermeable to water, and a very dilute urine is formed.


Maintaining an Internal Balance | The Importance of Excreting Wastes | Chemical Signals Maintain Homeostasis | How Nerve Signals Maintain Homeostasis | The Maintenance of Balance by the Immune System | Key Terms | Quiz | Test

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