Excretory System Functions

Excretory System

• The excretory system is responsible for removing excess and unwanted substances from the body and maintaining a stable internal environment.
• The system comprises a number of organs and tissues, including the lungs, liver, kidneys, skin, and bladder.

Kidneys and Urinary Tract

• The kidneys are two bean-shaped organs located in the upper back on either side of the spinal column.
• The role of the kidneys is to filter blood, reabsorb the useful substances within the filtrate, and secrete the unwanted ones.
• The kidneys can hold up to 25% of a person's blood volume at one time.
• The urinary tract consists of: kidneys, ureters, bladder, and urethra.

Nephrons

• A kidney contains millions of nephrons, which are the functional unit of a kidney.
• Each nephron has the same structure, consisting of:
  • Glomerulus
  • Proximal convoluted tubule
  • Loop of Henle
  • Distal convoluted tubule
  • Collecting duct
• Multiple nephrons can feed into the same collecting duct.

Glomerulus and Bowman's Capsule

• The glomerulus is located inside the Bowman's capsule.
• The high pressure of blood in the glomerular blood vessels forces fluid through the walls of glomerular capillaries and into the Bowman's capsule.
• Only small molecules and water can pass through the glomerulus' membranes: blood cells and large proteins remain behind in the glomerular capillaries.

Proximal Convoluted Tubule

• Glucose, specific ions, and amino acids are reabsorbed into capillaries by active transport.
• About 65% of water is reabsorbed via osmosis.
• Some urea is incidentally reabsorbed by passive transport.
• Ammonia and some drugs are secreted into the nephron.

Loop of Henle

• The ascending limb actively pumps ions out of the tubule.
• Water exits the descending limb via osmosis and ions diffuse into the tubule down their concentration gradient.
• Water cannot exit the ascending limb as the membrane is impermeable to water.
• Some urea is secreted into filtrate.

Distal Convoluted Tubule and Collecting Duct

• Fine-tunes the composition of filtrate according to the body's requirements.
• May reabsorb more water and ions.
• May secrete more ions and toxins into filtrate.
• Further fine-tunes filtrate composition.
• May reabsorb more water.
• Secretes waste like ammonia into the duct.

Regulation of Water Balance and Blood Pressure

• The kidneys play an important role in the regulation of water balance and blood pressure.
• If you drink lots of water, the distal convoluted tubule and collecting duct do not reabsorb much water.
• Conversely, if you lose lots of water or don't drink enough, the distal convoluted tubule and collecting duct reabsorb lots of water into capillaries.
• This increases the concentration of urea in the urine.

Other Excretory Organs

• The lungs are responsible for removing carbon dioxide and water via exhalation.
• The skin is responsible for removing small amounts of nitrogenous waste and ions via sweating.
• The liver breaks down toxins and other waste materials in the blood that are then carried through to the urinary tract and excreted.
• The digestive tract can play a role in excretion, as waste material is released into the digestive tract and comes out in faeces.### Excretory System
• The excretory system is responsible for removing excess and unwanted substances from the body and maintaining the ideal concentration of water and solutes.
• The major component of the system is the kidneys and the urinary tract.

Kidneys

• The kidneys are two bean-shaped organs located in the upper back on either side of the spinal column.
• The kidneys filter blood, reabsorb useful substances, and secrete unwanted ones.
• Blood is delivered to the kidneys by a pair of blood vessels called the renal arteries.
• The kidneys can hold up to 25% of a person's blood volume at one time.

Nephrons

• Each kidney contains millions of nephrons, which are the functional units of a kidney.
• A nephron has a glomerulus, Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.
• Multiple nephrons can feed into the same collecting duct.

Glomerulus and Bowman's Capsule

• The glomerulus is a bunch of capillaries inside the Bowman's capsule.
• The high pressure of blood in the glomerular blood vessels forces fluid through the walls of glomerular capillaries and into the Bowman's capsule.
• The fluid that crosses the glomerular walls is called the primary filtrate.

Proximal Convoluted Tubule

• Reabsorption: ions, amino acids, water, glucose
• Secretion: ammonia, toxins
• Glucose, specific ions, and amino acids are reabsorbed into capillaries by active transport.
• About 65% of water is reabsorbed via osmosis.

Loop of Henle

• Reabsorption: water
• Secretion: urea
• The ascending limb actively pumps ions out of the tubule.
• Water exits the descending limb via osmosis.

Distal Convoluted Tubule

• Optional reabsorption: ions, water
• Secretion: ions, toxins
• Fine-tunes the composition of filtrate according to the body's requirements.

Collecting Duct

• Optional reabsorption: water
• Secretion: urea, ions, ammonia, toxins
• Further fine-tunes filtrate composition.

Urinary Tract

• The urinary tract consists of the kidneys, ureters, bladder, and urethra.
• Urine is produced in the kidneys, travels through the ureters to the bladder, and is excreted out of the body via the urethra.

Excretion

• The excretory system is not limited to the urinary tract, but also includes other organs such as the lungs, skin, liver, and digestive tract.
• The lungs excrete carbon dioxide, and the skin excretes some nitrogenous waste via sweating.
• The liver breaks down toxins and other waste materials in the blood that are then carried through to the urinary tract and excreted.
• The digestive tract can also play a role in excretion by releasing waste material into faeces.