Excretion and Homeostasis

• Kidney cortex:

  • dark outer layer
  • filtering out of blood occurs here
  • a dense capillary network carrying blood from renal artery
• Kidney medulla:

  • lighter in colour (than cortex)
  • contains tubules of nephrons that form pyramids of the kidney and collecting ducts
• Kidney pelvis:

  • Central chamber where urine collects before passing down ureter
• Bowman's capsule:

  • contains glomerulus
  • more blood goes into glomerulus than leaves due to ultrafiltration
• Proximal convoluted tubule (PCT):

  • found in cortex
  • substances required by body are reabsorbed
• Loop of Henle:

  • descending loop runs from cortex to medulla, then comes back up
  • changes water potential for absorption
• Distal convoluted tubule (DCT):

  • permeability of walls affected by ADH
  • regulation of pH and ion balance occurs here
• Collecting duct:

  • urine passes from medulla into pelvis
  • walls sensitive to ADH
• Ultrafiltration:

  • blood enters glomerulus through afferent artery and leaves through efferent artery (narrower) creating high pressure
  • blood is forced through capillary walls
  • fluid must pass through basement membrane (network of collagen fibres)
  • podocytes in wall of BC act as an extra sieve to prevent cells and proteins passing into nephron
  • filtrate contains glucose, salts, urea
• Reabsorption:

  • PCT cells have microvilli and many mitochondria
  • glucose, amino acids, vitamins and hormones are reabsorbed
  • sodium ions are actively transported out of the PCT cells into tissue fluid
  • glucose/amino acids enter the cells with sodium ions by facilitated diffusion (cotransport)
  • glucose and amino acids diffuse into blood capillary
• Loop of Henle:

  • descending limb is permeable to water
  • water moves out through osmosis
  • filtrate becomes concentrated
  • ascending limb is permeable to ions, not water
  • ions are actively pumped out, and conc. in medulla increases greatly
  • water cannot leave and filtrate becomes dilute
  • water in the collecting duct moves into medulla by osmosis due to gradient created in loop of henle.
• DCT:

  • if the body needs ions, they are actively transported out of the DCT
  • water can also leave
  • pH of blood is balanced
• Collecting duct:

  • water moves out by osmosis
  • permeability controlled by ADH
• osmoregulation: regulation of water potential and salt levels in the body
• ADH:

  • produced in hypothalamus, secreted by pituitary gland
  • increases the permeability of PCT and collecting duct
  • more ADH, less urine
• osmoreceptors:

  • monitor water potential of blood
  • if blood has low water potential, water leaves the osmoreceptor cells by osmosis, causing them to shrink
  • this stimulates neurosecretory cells
• neurosecretory cells:

  • act like nerve cells but release hormones into the blood
  • ADH manufactured in the cell body and passed down the axon to be stored in terminal bud
• Water is gained from food, drink, respiration
• water is lost in urine, sweat, exhaled air, faeces
• ADH on collecting duct wall:

  • ADH detected by CS receptors
  • cyclic AMP is released within cell as a secondary messenger
  • enzyme-controlled reactions occur
  • vesicles containing aquaporins fuse to membrane
  • more water is reabsorbed
• water in short supply:

  • water potential of blood becomes more negative
  • osmoreceptors detect this and nerve impulses are sent to pituitary gland which releases ADH into the blood
• water in excess:

  • nerve impulses to pituitary gland are stopped
  • ADH release inhibited
• Urine samples:

  • urine contains water, urea, salts and broken down products from chemicals
  • diseases may show up in urine i.e. glucose in urine is a sign of diabetes, creatinine/protein can show muscle damage
• Pregnancy tests:

  • hCG is produced when an embryo implants into uterus
  • some hCG is found in the mothers urine
  • pregnancy tests test for hCG
• Monoclonal antibodies:

  • antibodies from a single clone of cells produced to target a particular cell/chemical in the body
  • mouse injected with hCG produces antibodies
  • B cells that make antibodies are removed from spleen and fused with myeloma
  • hybridoma reproduces rapidly
  • antibodies collected and purified
• Pregnancy test:

  • wick soaked in urine
  • test contains mobile monoclonal antibodies (MA) with coloured beads attached
  • hCG binds to antibodies forming a complex
  • urine travels up test until a window
  • in the window, immobilised antibodies bind to the complex causing a coloured line to form
  • urine continues to second window
  • immobilised antibodies in second window attach to mobile antibodies and act as a control
• Anabolic steriods:

  • excreted in urine
  • tested for by vaporising urine with a solvent
  • passed along a tube creating a chromatogram
  • can be used to read the presence of drugs
• Dialysis - use of a partially permeable membrane to filter blood
• dialysis membrane - partially permeable membrane separates dialysis fluid from blood
• dialysis fluid - complex solution matches composition of ideal body fluids
• Haemodialysis - blood is taken from vein and passed through dialysis machine
• Peritoneal dialysis - dialysis fluid pumped into body cavity so exchange occurs across peritoneal membrane
• Causes of kidney failure:

  • kidney infection
  • raised blood pressure
  • genetic coonditions
• High blood pressure can cause protein/blood in the urine if basement membrane is damaged
• Kidney failure causes:

  • loss of electrolyte balance
  • build up of urea
  • high blood pressure
  • weak bones
  • pain and stiffness in joints
  • anaemia
• Glomerular Filtration Rate:

  • GFR is used to indicate kidney disease
  • blood test measures levels of creatinine in blood
  • eGFR is made from measuring creatinine clearance rate
  • too much creatinine is a sign of kidney failure
  • eGFR does not drop below 70 in healthy people
• Haemodialysis:

  • blood leaves body and is passed into dialysis machine
  • blood flows between partially permeable dialysis membranes
  • dialysis fluid contains plasma levels of glucose, some mineral ions and no urea
  • materials move by diffusion/osmosis and move according to conc. gradients
  • blood and fluid move in opposite directions to maintain a countercurrent exchange system
• Peritoneal dialysis:

  • occurs inside body
  • makes use of peritoneum
  • dialysis fluid is added to abdomen using a catheter
  • urea and excess minerals pass out of capillaries into tissue fluid then across the membrane into dialysis fluid
  • fluid is drained and new fluid is put in
• Transplant:

  • long term dialysis has side effects such as low blood pressure, sepsis and muscle cramps
  • healthy kidney can be placed in body and connected to blood supply
• Transplant problems:

  • immune system recognises kidney as non-self
  • immunosuppressant drugs make patient vulnerable
  • transplanted organs only last around 10 years
  • surgery can pose a risk