Module 5: Section 2 - Excretion as an example of homeostatic control



Excretion - the removal of metabolic waste from the body

Prevents the buildup of potentially fatal metabolic products - may alter pH or act as enzyme inhibitors

The excretory organs

  • Lungs - remove carbon dioxide
  • Liver - many metabolic roles - some substances are passed into the liver for excretion with faeces eg. billirubin, also involved in conversion of excess amino acids to urea
  • Kidney - removal of urea from blood to become part of urine
  • Skin - excretion is not the primary function but sweat contains excretory products eg. urea and salts
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The structure of the liver

Made up of liver lobules - hepatacytes arranged in rows

Hepatic artery - supplies the liver with oxygenated blood

Hepatic vein - takes away deoxygenated blood

Hepatic portal vein - brings blood from the small intestine

Bile duct - takes bile to the gall bladder for storage

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The functions of the liver

Formation of urea

  • Deamination = amino acid + oxygen → keto acid (from R and carboxyl groups) + ammonia (from amino group)
  • Orthnithine cycle = ammonia + carbon dioxide  urea + water, requires ATP, urea is less soluble or toxic 

Detoxification of alcohol

  • Alcohol  ethanal (uses ethanol dehydrogenase) → ethanoate (uses ethanal dehydrogenase)
  • Ethanoate combined with coenzyme A to form acetyl coenzyme A - enters respiration
  • Hydrogen atoms released from alcohol are combined with NAD to produce reduced NAD
  • If the liver has to detoxify too much alcohol, it uses up its stores of NAD and has insufficient left to deal with fatty acids from respiration - converted back to lipids and are stored as fat


  • Converts excess glucose to glycogen and stores the glycogen until needed
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The structure of the kidney

Bowman’s Capsule

  • Contains glomerulus
  • Three layes: endothelium (pores), basement membrane (messh), epithelial cells (podocytes)


  • Site of ultrafiltration
  • Tight, knot-like, high pressure capillary bed

Proximal Convoluted Tubule (PCT)

  • Involved in selective reabsorption
  • Re-absorbs valuable substances, such as glucose

Loop of Henle

  • Creates low water potential in the medulla
  • Allows water to be reabsorbed
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The structure of the kidney

Distal Convoluted Tubule (DCT)

  • Involved in osmoregulation
  • Varies the amount of water reabsorbed into the blood

Afferent arteriole

  • Brings blood from the renal artery

Efferent arteriole

  • Narrow vessel that restricts blood flow
  • Raises blood pressure
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Kidneys: Ultrafiltration

1. Blood from the renal artery enters smaller arterioles in the cortex

2. Each arteriole splits into a glomerulus - this is where ultrafiltration takes place

3. The arteriole that takes blood into each glomerulus is called the afferent arteriole, and the arteriole that takes the filtered blood away from the glomerulus is called the efferent arteriole

4. The efferent arteriole is smaller in diameter than the afferent arteriole, so the blood in the glomerulus is under high pressure

5. The high pressure forces liquid and small molecules in the blood out of the capilary and into the Bowman's capsule

6. Smaller molecules pass through the Bowman's capsule wall and enter the blood - larger moelcules eg. hormones can't pass through and stay in the blood

7. The filtrate passes along the rest of the nephron and selective reabsorption takes place

8. The filtrate flows through the collecting duct and passes out of the kidney through the ureter

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Kidneys: Selective reabsorption

1. Takes places as the filtrate flows along the PCT, through the loop of Henle and along the DCT

2. Useful substances leave the tubules of the nephron and enter the capilary network

3. The epithelium of the PCT has microvilli - large surface area

4. Useful solutes eg. glucose and amino acids are reabsorbed along the PCT by active transport and facilitated diffusion

5. Some urea is also reabsorbed by diffusion

6. Water enters the blood by osmosis because the water potential of the blood is lower than that of the filtrate - water is reabsorbed from the loop of Henle, DCT and the collecting duct

7. The filtrate that remaines is urine, which passes along the ureter to the bladder

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Kidneys: Regulating water potential

1. Near the top of the ascending limb of the loop of Henle, sodium and chloride ions are actively pumped out into the medulla. The ascending limb is impermeable to water, so water stays in - creates a low water potential in the medulla

2. Water moves out of the descending limb into the medulla by osmosis (down a water potential gradient). This makes the fitrate more concentrated. The water in the medulla is rebsorbed into the blood through the capilary network

3. Near the bottom of the ascending limb, sodium and chloride ions diffuse out of the medulla, further lowering the water potential in the medulla

4. The first three stages massively increase the ion concentration in the medulla, so water moves out of the collecting duct by osmosis and is reabsorbed into the capillary network

The longer an animal's loop of Henle, the more water they can reabsorb - more ions can be actively pumped out, creating a really low water potential

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Controlling water reabsorption

The volume of water reabsorbed into the capilaries is controlled by changing permeability, using ADH:

When you're dehydrated:

  • Osmoreceptors in the hypothalamus detect the drop in water potential of the blood (due to decreased water content)
  • The posterior pituitary gland is stimulated to release more ADH into the blood
  • More ADH means that the DCT and the collecting are more permeable, so more water is reabsorbed into the bloody by osmosis - less water is lost and urine is more concentrated

When you're hydrated:

  • Osmoreceptors in the hypothalamus detect the increase in water potential (due to increased water content) 
  • The posterior pituitary gland is stimulated to release less ADH into the blood
  • Less ADH means that the DCT and the collecting are less permeable, so less water is reabsorbed into the bloody by osmosis - more water is lost and urine is dilute
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Kidney failure

Can be detected by the GFR - low GFR indicates that the kidneys aren't working properly


  • Kidney infections - inflammation interderes with filtration or reabsorption
  • High blood pressure - damages the glomeruli so larger molecules can pass into the urine


  • Waste products build up and fluid accumulates in the tissues
  • The balance of electrolytes becomes unbalanced eg. blood may become too acidic
  • Long-term = anaemia
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Treating kidney failure

Haemodialysis - uses a machine to filter the patient's blood - blood flows on one side of a partially permeable membrane, dialysis fluid flows on the other

(+) 4 dialysis - free days a week, less risky than surgery

(-) Have to plan your life around sessions (3x a week for several hours), diet needs to be controlled

Peritoneal dialysis - the patient's own abdominal membrane is used as the filter

(+) Can be carried out at home, equipment is portable, fewer dietary restrictions

(-) Has to be carried out every day, increased risk of infection

Kidney transplant - new kidney is implanted into the patient's body

(+) Cheaper than being on dialysis for a long time, more convenient for the patient, patients don't have the problem of feeling unwell between haemodialysis sessions

(-) Major operation - risky, patient has to take immunosupressants

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Urine testing: Pregnancy

1. A stick is used with an application area that contains monoclonal antibodies for hCG bound to a coloured bead. Monoclonal antibodies are all identical to eachother

2. When urine is applied to the application area any hCG will bind to the antibody on the beads

3. The urine moves up the test stick, carrying the beads with it

4. The test stick has immobilised antibodies to hCG at the top of the stick

5. If there is hCG present a plie line will appear because the immobilised antibody binds to any hCG attached to the blue bead, concentrating the blue beads in that area

6. If there is no hCG present, the beads will pass through the area without binding to anything, so there will be no line 

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Urine testing: Steriods and drugs

Testing for steriods:

  • In gas chromatography the urine sample is vapourised and passed through a colum containing a polymer
  • Different substances move through the column at different speeds, so substances in the urine sample separate out
  • Ones the substances have separated out, a mass spectrometer converts them into ions, then separates them out accoring to their mass and charge
  • The results are analysed by a computer and by comparing them with the results of known substances it's possible to tell which substances were in the urine sample 

Testing for recreational drugs:

  • Usually starts with test stickss, which contain antibodies that the drug being tested for will bind to
  • A sample of urine is applied to the test stick and if a certain amount of the drug is present a colour change will occur, indicating a positive results
  • Positive results are usually confirmed using gas chromotography
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