The kidneys

The kidneys

1. Overview

   • Kidneys are organs of excretion
   • They excrete waste products and regulate water potential
   • 1) Blood enters the kidney through the renal artery and then passes through the capillaries in the cortex of the kidneys
   • 2) As blood passes through the capillaries, substances are filtered out the blood and into long tubules that surround the capillaries - called ultrafiltration
   • 3) Useful substances are reabsorbed back into the blood from the tubules in the medulla and cortex - selective reabsorbtion
   • 4) The remaining unwanted substances eg urea pass along the tubules, then along the ureter to the bladder where it is expelled as urine
   • 5) The filtered blood passes out of the kidneys through the renal vein

2. Nephron filtration

   • Nephrons are long tubules with bundles of capillaries where the blood is filtered
   • 1) Blood from renal artery enters smaller arterioles in the cortex
   • 2) Each arteriole splits into a glomerulus - a bundle of capillaries looped inside a hollow ball called the bowmans capsule - which is where ultrafiltration takes place
   • The arteriole that takes blood into the glomerulus is called the afferent arteriole and the arteriole that takes blood out is called the efferent arteriole
     • Efferent arteriole is much smaller in diameter than afferent, so blood in glomerulus is at high pressure
   • 3) High pressure in the efferent arteriole forces liquid and small molecules in the blood out of the capillary and into the bowmans capsule
   • 4) The small molecules pass through 3 layers to get to the bowmans capsule
     • The capillary wall
     • the basement membrane
     • the epithelium of the bowmans capsule
   • 5) The liquid and small molecules now called filtrate, pass along the rest of the nephron and useful substances are reabsorbed on the way
   • 6) Finally filtrate travels through collecting duct and passes out of the kidney along the ureter

3. Nephron reabsorbtion

   • Selective reabsorption occurs as the filtrate flows along the PCT (proximal convoluted tubule), through the loop of Henle and along the DCT (distal convoluted tubule)
   • Useful substances leave the tubules of the nephrons and enter the capillary network that wrapped around them by diffusion
   • The epithelium of the PCT has microvilli to increase SA for the reabsorption of useful chemicals from the filtrate into the blood
   • Most solutes are taken up by fascilitated diffusion and active transport along the PCT
   • Water enters the blood by osmosis due to a more negative water potential in the blood
   • The filtrate that remains is urine, which passes along the ureter to the bladder
   • Urine is made of
     • Water and dissolved salts
     • Urea
     • Other substances like hormones and excess vitamins
   • Urine doesnt normally contain
     • Proteins and blood cells
     • Glucose, amino acids and vitamins

4. Controlling water potential

   • The kidneys regulate the water potential in the blood so the body has just the right amount of water
   • Water is lost from pissing, breathing and sweating
   • Dehydrated
     • If the water potential of the blood is too low, more water is reabsorbed by osmosis into the blood from the tubules of the nephrons - which results in more concentrated urine, and less water lost during excretion
   • Overhydrated
     • If the water potential of the blood is too high, less water is reabsorbed by osmosis into the blood from the tubules of the nephrons - this results in more dilute urine, so more water is lost during excretion
   • Loop of henle
     • Counter current multiplier mechanism
       • 1) Near the top of the ascending limb, Na+ and Cl- are actively pumped out into the medulla. The ascending limb is impermeable to water, so it stays in the tubule which creates a low water potential in the medulla, due to the high concentration of ions
       • 2) Due to the lower water potential in the medulla, than in the descending limb, so water moves out by osmosis, which makes the filtrate more concentrated - the water is reabsorbed in the blood through the capillary network
       • 3) Near the bottom of the ascending limb Na+ and Cl- ions diffuse out into the medulla, further lowering the water potential in the medulla, (the ascending limb is impermeable to water, so it stays in the tubule)
       • 4) The first 3 stages increases the ion concentration in the medulla, which lowers the water potential - this causes water to move out of the collecting duct by osmosis and the water in the medulla is reabsorbed into the blood through the capillary network
       • The volume of water taken up can be adjusted by changing the permeability of the collecting duct
   • ADH
     • The water potential of the blood is monitored by cells called osmoreceptors in the hypothalamus
     • When the osmoreceptors are stimulated by low water potential in the blood, the hypothalamus sends an impulse to the pituitary gland to make it release ADH
     • ADH makes the walls of the DCT and collecting duct more permeable to water
     • So more ADH = more water absorbed due to higher permeability
     • Blood ADH rises when your dehydrated
       • 1) water content of blood drops, so does water potential
       • 2) This is detected my osmoreceptors in the hypothalamus
       • 3) The posterior pituitary gland is stimulated to release more ADH into the blood
       • 4) More ADH means that the DCT and collecting duct are more permeable, so more water is reabsorbed into the blood by osmosis
       • 5) A small amount of concentrated urine is produced so less water is lost
     • Blood ADH falls when youre hydrated
       • 1) The water content of the blood rises, so does water potential
       • 2) This is detected by osmoreceptors in the hypothalamus
       • 3) The posterior pituitary gland releases less ADH into the blood
       • 4) Less ADH means that the DCT and collecting duct are less permeable, so less water is reabsorbed into the blood by osmosis
       • 5) A large amount of dilute urine is produced and more water is lost

5. Kidney failure

   • Kidney failure is when the kidneys stop working properly
   • Kidney failure is detected by measuring GFR (glomerular filtration rate)
   • Can be caused by
     • Kidney infections
     • High blood pressure
   • Problems it causes:
     • Waste products that the kidneys would normally remove build up in the blood
     • Fluid starts to accumulate in the tissues because kidneys cant remove excess water
     • The balance of electrolytes (ions) becomes unbalanced
     • Anaemia - lack of hameoglobin in the blood
   • Treatments
     • Renal dialysis
       • A machine is used to filter a patients blood
       • Patients need multiple long sessions every week and inbetween waste builds up and the patient feels worse
       • Mainly used to keep someone alive until a transplant is available
       • Very expensive and inconvenient for the patient
     • Kidney transplants
       • A new kidney transplanted into your body to replace a damaged one
       • The kidney has to be compatible - eg same blood type, same tissue type
       • Advantages
         • Cheaper
         • More convenient for patient
         • Patients dont feel unwell between dialysis sessions
       • Disadvantages
         • Major risky operation
         • The immune system may reject the transplant, so patient has to take drugs to suppress it

6. Urine tests

   • Urine can be used to test for drug use and pregnancy
   • Pregnancy
     • They detect the hormone hCG thats only found in urine of pregnant baby
     • A test strip has antibodies for hCG bound to a coloured bead
     • When urine is applied any hCG will bind to the antibodies on the beads
     • Urine moves up the test strip, carrying the beads with it where more antibodies are immobilised
     • If hCG is present the strip turns blue because the immobilised antibody binds to any hCG attach to the blue beads, which concentrates the blue beads making it blue
     • If no hCG is present the beads will pass through the area without binding to anything
   • Steroids
     • Steroids are found by detecting steroids or the compounds they are broken down into
     • They are detected by using gas chromotography and mass spectroscopy
     • Chromotography separates out compounds in the urine
     • Mass spectroscopy is used to analyse what compounds are present by converting them into ions
     • The results are then compared to results for known substances
   • Recreational drugs
     • they use test strips to test for most drugs with antibodies thatll bind to the drug or the compounds it is broken into
     • If a certain amount of the drug is present a colour change will occur

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