THE EXCRETORY SYSTEM AND THE FORMATION OF URINE

• What does the excrotory system do?

  The excretory system wastes from blood and maintain water balance (water in=water out)
• What is the law of water balance?

  The law of water balance states that the amount of water entering a system must equal the amount of water leaving the system.
• What is water balance important for?

  • water volume and blood pressure 
  • Too little water – low bp
  • Too much water – high bp
  • pH 
  • ionic balance (osmosis)
• What is metabolic wastes?

  • Wastes from protein metabolism
  • Amino acids that are used for energy are deaminated

  (amino group removed)
• Homeostasis involves the regulation of the excretory system which is responsible for maintaining water balance.
• The excretory system consists of the kidneys, ureters, bladder, and urethra.
• The kidneys are responsible for filtering blood, reabsorbing water and nutrients, and producing urine.
• The ureters transport urine from the kidneys to the bladder.
• The bladder stores urine until it is ready to be excreted.
• The urethra is the passageway through which urine is excreted.
• Control of water balance involves ADH, or Anti Diuretic Hormone, which is made in the hypothalamus, released by the posterior pituitary gland, and acts on the kidneys to maintain water balance.
• Secretion involves some molecules moving from the peritubular capillaries and interstitial fluid into the distal tube of the nephron, including N-containing waste (ammonia), H+ ions (involved in pH balancing), and some pharmaceuticals.
• Diuretics increase urine production.
• ADH increases the permeability of the proximal convoluted tube, leading to more water being reabsorbed back into the blood.
• Reabsorption occurs in the distal tubule, with K+, H+, detoxed poisons, and drugs moving into the nephron, and ammonia moving out of the nephron.
• Alcohol inhibits ADH, lowering permeability and leading to more water in urine, resulting in more urine.
• Caffeine inhibits Na+ reabsorption, leading to less osmosis and more urine.
• Reabsorption moves molecules out of the nephron, into the interstitial fluid and back into the blood.
• Na+ is actively pumped (membrane protein) and Cl- moves by passively by electrochemical attraction.
• The location, substance transported, active/passive transport, and direction of reabsorption can be found in the table below.
• Too little water results in low blood pressure.
• The Nephron is the filtration system in the kidney where blood is filtered as it moves from the glomerulus to the Bowman’s capsule.
• Filtrate starts out as the same concentration as plasma and concentrates urine to minimize water loss.
• Most of the water reabsorption occurs in the proximal convoluted tube and more water is removed in the descending loop of Henle.
• pH and ionic balance (osmosis) are important for maintaining water volume and blood pressure.
• 1400L of blood are filtered every day.
• Water will move out of the nephron passively through membrane proteins called aquaporins.
• Too much water results in high blood pressure.
• The glomerulus is a high blood pressure capillary that pushes fluid into the Bowman’s capsule, now called filtrate.
• Dialysis machine is used to excrete nitrogenous wastes.
• Reabsorption requires hormone ADH-antidiuretic hormone.
• Water volume and blood pressure need to be maintained.
• Glucose and amino acids (active) set up an osmotic gradient and hypertonic solution is outside the nephron.
• Metabolic wastes are wastes from protein metabolism and include amino acids that are used for energy which are deaminated (amino group removed).
• Blood cells (red and white), platelets, and plasma proteins do not get into the nephron and remain in blood.
• Reabsorption involves active (requires ATP) and passive transport.
• Metabolic wastes are excreted as NH4+ and are converted to less toxic urea in the liver before being excreted.
• The urinary system is best known for removing metabolic wastes from the body in the form of urine, but its importance goes far beyond that function.
• The kidneys filter large amounts of blood plasma, positioning them to detect changes in blood volume and composition, and respond accordingly.
• The kidneys are critical in maintaining steady state, or homeostasis, of many blood parameters, including blood volume and pressure, osmolality, concentrations of various solutes, blood pH, and red blood cell count.