Excretion

• Excretion - the removal of metabolic waste from the body
• Metabolic waste must be removed to prevent toxins accumulating and inhibiting enzyme activity by altering pH

  E.g:

  • Carbon dioxide
  • urea or nitrogenous waste
  • bile pigments
• excretory product - carbon dioxide

  • CO2 + H2O -> H2CO3 (carbonic acid)
  • carbon acid -> H+ + HCO3-

  Dissociated H+ affect the pH of the cytoplasm in red blood cells

  > excess H+ can reduce pH of blood plasma altering activity of proteins

  >if change in pH is small, extra H+ are detected by the respiratory system in medulla oblongata

  >causes an increased breathing rate in the brain to remove excess co2
• Excretory substrate - Nitrogenous compounds

  body cannot store excess amino acids

  > they are transported to liver to be deaminated where toxic group is removed forming a toxic more soluble compound, ammonia

  > this is converted to less toxic and soluble compound, urea

  > this is transported to kidneys for excretion and remaining keto acid is used in respiration to release energy or converted to a carbohydrate for fat storage
• The skin sweats which contains uric acid, salts, water and ammonia.
• Structure of a kidney

  > blood supplied by a renal artery and drained by a renal vein

  > pelvis region leads into the ureter where urine passes before entering the bladder

  • 3 regions:

  A) cortex

  B) medulla

  C) pelvis

  
• Summary of kidneys

  
  

  1. Ultrafiltration in glomerulus/ bowman's capsule

  2. Selective reabsorption of glucose/ water – PCT

  3. Loop of Henle (in the medulla)- creates a Na+ gradient to enable reabsorption of water

  4. Reabsorption of water - DCT and collecting duct
• Glomerulus - specialised bundle of capillaries situated in the bowman's capsule between the afferent and efferent arteriole.
• Products of ultrafiltration

  > Efferent arteriole contains: RBCs, WBCs, large plasma proteins as they are too big and cannot pass through filters

  > Selectively reabsorbed filtrate contains: glucose, water, urea, salt (Na+, Ca2+), amino acids, vitamins, hormones
• Selective reabsorption of glucose and water in PCT

  1. Na+ actively pumped out of cells lining PCT into blood in capillaries
  2. conc. of ions in cell cytoplasm decreases, creating a conc. gradient
  3. ions diffuse down gradient from lumen of PCT into cells lining PCT through facilitated diffusion via a co-transport protein carrying glucose
  4. glucose diffuses from PCT epithelial cell into blood stream
  5. water moves into cells by osmosis
• Specialisation of cells lining proximal convoluted tubule

  > cell surface membrane in contact with tubule fluid:

  • microvilli - increases SA for absorption
  • co-transporter proteins - transport glucose / amino acids with Na+ from tubule into cells

  > opposite membrane of cells close to tissue fluid and capillaries:

  • highly folded - increase SA
  • contains Na/ K+ pumps

  > cell cytoplasm has many mitochondria providing ATP for AT
• Reabsorption of water in Loop of Henle

  1. mitochondria in walls of cells actively transport mineral ions out descending limb
  2. accumulation of ions outside nephron lowers WP in medulla
  3. water moves out of descending limb by osmosis and enters capillaries down conc. gradient (water is reabsorbed by blood)
  4. mineral ions diffuse into lower ascending limb
  5. mineral ions actively transported out upper ascending limb
  6. Water cannot follow by osmosis as ascending limb walls are impermeable to water
  7. Tubule contains dilute solution of urea and excess mineral ions
• How and why is the Loop of Henle arranged?

  arranged in hairpin countercurrent multiplier system

  • increases the efficiency of transfer of mineral ions from ascending limb to descending limb to create low WP in tissue fluid of medulla so water can be reabsorbed by the blood
• Selective reabsorption Distal convoluted tubule

  body reabsorbs substances subjective to needs and pH of blood

  • Na+ by active transport
  • water by osmosis
  • Cl- by diffusion
• Selective reabsorption Collecting duct

  > tubule fluid has a high WP due to dilute filtrate

  > duct carries fluid down through medulla to pelvis

  > water moves by osmosis from tubule fluid into surrounding tissue fluid then enters the blood capillaries and is carried away

  • amount of water reabsorbed depends on permeability of collecting duct walls
  • urine is very concentrated with excess minerals and urea and has a negative water potential
• How is water allowed to be moved out of collecting duct and into tissue fluid by osmosis

  > more water will move from duct filtrate of higher water potential

  to

  > tissue fluid of lower water potential

  then

  > into blood down the water potential gradient by osmosis

  • Na+ in tissue fluid generates a steep water concentration gradient so water moves out to capillaries
• Nephron structures locations in the kidney

  > Cortex - glomerulus, bowman's capsule, proximal convoluted tubule and distal convoluted tubule

  > Medulla - Loop of Henle and collecting duct
• Concentration changes in tubule fluid within kidneys

  > Glucose decreases in concentration as it is selectively reabsorbed from proximal tubule

  > Na+ / Cl- pumped in lower ascending limb by diffusion (conc. rises)

  > Na+ / Cl- pumped out of upper ascending limb by AT( conc. falls )

  > Na+ is removed from distal tubule and conc. rises as water is removed in the ducts

  > K+ also increase in conc. as water is removed and are actively transported into distal tubule and duct to be removed in urine

  > urea conc. rises as water is taken from distal tubule
• Osmoregulation - control of water potential in the body
• Histology of nephrons

  A) distal

  B) proximal

  C) bowman's capsule

  
• Histology of nephrons

  A) glomerulus

  B) bowman's capsule

  C) distal

  D) proximal

  E) glomerulus

  F) capillary

  
• Concentration of ADH in blood

  • Osmoreceptors in the hypothalamus detect low water potential in blood and shrink by osmosis
  • this stimulates Neurosecretory cells which produce ADH in their cell body
  • ADH moves down axon to terminal bulb in posterior pituitary gland and stored in vesicles
  • when stimulated they carry APs which cause release of ADH by exocytosis
  • ADH is transported to collecting ducts where they become more permeable so more water can be absorbed into blood
  • once water potential of blood rises, less ADH is released
• The more permeable the collecting duct walls in the kidney, the more water can be absorbed into the blood and less urine is produced
• Aquaporins - vesicles containing water-permeable channels that make the walls of the collecting duct more permeable.
• ADH - Antidiuretic hormone
• Effect of high levels of ADH on walls of collecting duct

  1> ADH binds to complementary cell surface receptors on tubule wall

  2> this triggers formation of cAMP from ATP causing a chain of enzyme controlled reactions in cell

  3> vesicles containing Aquaporins fuse with cell surface membrane

  4> walls become more permeable to water
• Effect of low levels of ADH on walls of collecting duct

  1> cell surface membrane invaginates, creating new vesicles removing aquaporins from the membrane

  2> this makes the walls less permeable and less water is absorbed by osmosis
• Ultrafiltration - filtration of the blood at a molecular level under pressure
• Selective reabsorption -

  • when certain molecules filtered out of the capillaries in the glomerulus (with nitrogenous waste products and water)
  • and are reabsorbed from the filtrate as they pass through the nephron in the PCT
• Effects of kidney failure

  • unable to regulate levels of water and electrolytes in the body
  • unable to remove urea from the blood
• Kidney function is assessed by glomerular filtration rate and analysing urine for substances

  • proteins in urine is an indication that filtration mechanism is damaged

  GFR - rate at which fluid enters the nephrons

  • normal reading: 90-120 CM3min-1
  • chronic kidney disease: below 60 cm3min-1
  • kidney failure: below 15 cm3min-1
• Treatment of kidney failure

  > Kidney transplant

  > Renal dialysis: waste products, excess fluid, mineral ions are removed from blood by passing over partially permeable dialysis membrane

  • enables exchange of substances between the blood and dialysis fluid containing correct conc.

  two types:

  • haemodialysis
  • Peritoneal dialysis
• Haemodialysis

  >blood from artery or vein passed through machine with an artificial dialysis membrane shaped forming artificial capillaries increasing surface area for exchange

  • heparin is added to prevent clotting
  • artificial capillaries are surrounded by dialysis fluid flowing in opposite direction to blood in a counter current system, increasing efficiency of exchange
  • any bubbles are removed before blood is returned to body via a vein

  > performed 2-3 times a week for 7 hours per session
• Peritoneal dialysis

  peritoneum (abdominal membrane) is used as a dialysis membrane

  • permanent tube inserted into patient and dialysis solution is poured in, filling spaces between abdominal wall and organs
  • solution is drained after 7 hours
  • patient can walk around and carry out at home
• Evaluation of a kidney transplant

  (+)

  • less time consuming than renal dialysis
  • improved quality of life and ability to travel
  • no longer chronically ill

  (-)

  • need to take immunosuppressant drugs
  • side effects of drugs: high blood pressure, susceptibility to other infections
  • need for major surgery under anaesthetic
  • lack of donors/availability
  • can be expensive
  • risks of organ rejection
• Use of monoclonal antibodies in pregnancy testing

  human embryo is implanted in uterine lining which releases hCG

  Monoclonal antibodies in testing kits bind to it:

  1. urine poured onto test stick
  2. hCG binds to mobile antibodies attached to a blue bead
  3. mobile antibodies move down test stick
  4. if hCG is present, it binds to fixed antibodies, holding bead in place, forming a blue line (positive test result, 2 lines)
  5. mobile antibodies with no hCG attached bind to another fixed site to show test is working in control zone (line that always appears)
• anabolic steroids increase protein synthesis within cells resulting in build up of muscle tissue.

  • testing for steroids involves analysing urine samples using gas chromatography
• Ultrafiltration - glomerulus

  1. blood enters glomerulus via afferent arteriole
  2. fluid pushed into bowman's capsule through high filtration pressure caused by differences in hydrostatic pressure between afferent and efferent arteriole
  3. water / glucose / ions forced out capillaries
  4. large proteins / RBCs too big to fit through gaps in capillary endothelium so remain in blood and leave via efferent arteriole
• High filtrate pressure in glomerulus is caused by

  • differences in hydrostatic pressure of blood between afferent and efferent arteriole
  • splitting of afferent arteriole into smaller capillaries in glomerulus - increasing blood pressure
• Specialisations that enable ultrafiltration

  1. Gaps in endothelium of capillary: allow blood plasma/ substances to pass out capillary
  2. Basement membrane of glomerulus: made of collagen, stops large proteins/ RBCs in plasma leaving capillaries.
  3. Epithelial cells of Bowman’s capsule: contains Podocytes ensure only certain molecules are filtered

  A) afferent

  B) basement membrane

  C) podocyte cell

  D) efferent