Excretory System

• Excretion

  • The removal of wastes of metabolism
  • Examples:
  • Carbon Dioxide excreted by lungs
  • Alimentary canal passes out bile pigments
  • Urea secreted by the kidneys
• Urinary System

  • A ureter leaves each kidney
  • Carries Urine to bladder for storage
  • Empties to the outside via the urethra
• Kidneys

  • Is enclosed by the renal capsule
  • Under this is the renal cortex, renal medulla and renal pelvis
  • The renal hilum is where the vessels enter and leave
  • The Medulla consists of a number of renal pyramids which are separated by renal columns
• Renal = Kidney
• Nephron

  • Functional unit of the kidney
  • About 1.2 million nephrons in each kidney surrounded by a complex network of blood capillaries.
  • Each nephron consists of a renal corpuscle and a renal tubule.
• Renal Tubule

  • About 5cm long
  • It brings with a highly convoluted section called the proximal convoluted tubule
  • A 'hair pin' like section follows this known as the loop of Henle which has a descending and ascending limb
  • The second coiled section is known as the distal convoluted tubule.
  • The distal convoluted tubules of many nephrons join to form a collecting duct
  • Collecting ducts funnel into the ureter.
• Renal corpuscle

  • Blood enters the kidney through the renal arteries. The renal arteries divide into small arteries and arterioles. Each renal corpuscle is supplied by one afferent arteriole.
  • The afferent arteriole forms a knot of capillaries called the glomerulus.
  • The glomerulus is located within the glomerular capsule.
  • The capillaries eventually unite to form another arteriole, the efferent arteriole, which pass out the renal corpuscle.
• Peritubular network

  • After leaving the renal corpuscle; the efferent arteriole breaks up into a second capillary network known as the peritubular capillaries.
  • These capillaries surround the proximal and distal convoluted tubules, the ascending and descending limbs and the collecting ducts.
  • Venous blood drains away from this network of capillaries and eventually leaves the kidney in the renal vein.
• Urine Production

  • The formation of urine by the nephrons of the kidneys involves 3 major processes: glomerular filtration, selective reabsorption and tubular secretion.
• Glomerular filtration


  Process where fluid is forced out of the blood and is collected by the glomerular capsule
• Renal corpuscle


  • Where glomerular filtration takes place
• Glomerular filtration


  1. Fluid is forced out of the blood

  2. Fluid is collected by the glomerular capsule
• Pressure changes within the afferent and efferent arterioles

  Cause glomerular filtration
• Afferent arteriole


  • Leading into the glomerulus has a wider diameter than efferent arteriole
• Efferent arteriole


  Leaving the glomerulus has a narrower diameter
• Narrowing of the efferent arteriole

  Increases resistance to the flow of blood and produces a higher pressure in the glomerulus
• Glomerular filtration


  1. Water and dissolved blood components are forced through the two layers of single cells (arteriole and capsule)

  2. Filtrate is collected
• Glomerular filtration

  • Takes place in the renal corpuscle
  • When fluid is forced out of the blood and is collected by the glomerular capsule.
  • This is due to pressure changes within the afferent and efferent arterioles.
• Glomerular filtration

  • The afferent arteriole leading into the glomerulus has a wider diameter than efferent arteriole leaving it.
  • This narrowing of the efferent arteriole increases resistance to the flow of blood and produces a higher pressure in the glomerulus.
  • Water and dissolved blood components are forced through the two layers of single cells (arteriole and capsule) and is collected as the filtrate
• Filtrate

  • Contains all of the materials present in blood except from red and white blood cells and plasma proteins. These are 2 large to pass through the walls of the arterioles and capsule.
  • Filtrate consists of: water, salts, amino acids, fatty acids, glucose, urea, uric acids, creatinine, hormones, toxins and various ions.
  • 125ml of filtrate is produced per minute = 180L per day.
• Reabsorption

  • Many of the components of filtrate need to be reabsorbed back into the body because they are useful to the body.
• Selective Reabsorption

  • Water, glucose, amino acids, ions and urea are reabsorbed through the walls of the cells that line the renal tubule into the peritubular capillaries.
  • In the proximal convoluted tubule and loop Henle:
  • Ions such as potassium, chloride and bicarbonate are passively reabsorbed.
  • Water is passively reabsorbed by osmosis
  • Glucose and sodium is actively reabsorbed.
  • In the distal Convoluted tubule:
  • Sodium is actively reabsorbed.
• Facultative Reabsorption

  • Under the control of hormones the amount of water reabsorbed can be con trolled depending upon the needs of the body.
  • Occurs in the distal convoluted tubule and collecting duct.
• Tubular Secretion

  • Material such as potassium, hydrogen, creatinine and drugs are added to filtrate from the blood
  • This can be an active or passive process and is essential for regulating blood and urine pH
  • Occurs in the distal convoluted tubule
• Collection and Urination

  • What remains in the renal tubule after these 3 processes is drained into the colleting ducts and into the renal pelvis to produce urine.
  • Urine drains into the ureters and is pushed by muscular contractions to the urinary bladder for storage
  • The urethra carries urine from the bladder to the exterior of the body in the process urination.
• The formation of urine is maximized by the structure of the nephron
• Glomerular capsule


  • Surrounds the glomerulus to collect fluid filtered out of the blood capillaries
• Substances need to only pass through 2 layers of cell to form filtrate
• A large volume of blood is continuously supplied to the kidney to maintain the concentration gradient
• Efferent arteriole


  • Has a smaller diameter than the afferent arteriole to increase blood and force substances out in glomerular filtration
• Tubule


  • Has a large surface area for reabsorption and secretion due to the convolutions and a long loop
• Each kidney has over a million nephrons to increase total surface area
• Nephron Structure and function

  • The formation of urine is maximized by the structure of the nephron
  • The glomerular capsule surrounds the glomerulus to collect fluid filtered out of the blood capillaries.
  • Substance need to only pass through 2 layers of cell to form filtrate.
• Nephron Structure and function

  • A large volume of blood is continuously supplied to the kidney to maintain the concentration gradient.
  • The efferent arteriole has a smaller diameter than the afferent arteriole to increase blood and force substances out in glomerular filtration.
  • Each tubule has a large surface area for reabsorption and secretion due to the convolutions and a long loop.
  • Each kidney has over a million nephrons to increase total surface area.
• Liver and Excretion

  • The body primarily uses carbohydrates and fats as energy sources
  • Proteins are primarily 'body builders' and make up structural materials of cells. As long as the body has sufficient amounts of carbs and facts, proteins is not used as a source of energy.
  • Excess protein cannot be stored in cells of the body so processes are required to remove it from the body. - Known as deamination (occurs in the liver)
• Deamination

  1. The amino group (NH2) must first be removed from the amino acids
  2. Amino acid + oxygen -> carbohydrates + ammonia (NH3)
  3. Energy + carbon dioxide + ammonia -> urea + water

  • Urea is then excreted in urine