1. Removal of materials from blood
Chapter 23

2. Role of lungs CO2 produced by cells is transported to the lungs
Pulmonary capillaries lie close to alveoli
Plasma has CO2 dissolved in it as bicarbonate (HCO3-)
In pulmonary circulation HCO3- enter red blood cells to form carbonic acid
Carbonic acid converted to CO2 & H2O by an enzyme
CO2 diffuses from blood to alveoli by diffusion
CO2 then excreted

3. Role of liver Conserves useful substances: Regulates glucose levels
Synthesises plasma proteins
Detoxifies toxic materials:
Chemical alteration e.g drugs
- Excreted in bile or by kidneys
Chemical breakdown
- H2O2 into Water & Oxygen
- by Catalase
- Alcohol into acetyl CoA (Excess = cirrhosis)
Conjugation (Chemical Attachment)
- Unwanted substances attached to glycine
- recognised as waste by the kidneys
Uptake by macrophages
- Removal of foreign particles

4. Bilirubin Red blood cells broken down by macrophage
- in liver, spleen, & bone marrow
Breakdown releases haemoglobin
- converted into bilirubin
- gives plasma it’s yellow colour
Liver removes it and conjugates it to another substance
Then joins onto bile (conjugation) = bile pigment
In gut, bacteria breakdown bilirubin into a brown pigment – into faeces
Jaundice caused by bilirubin build up:
- Liver disease preventing bilirubin absorption
- Bile duct blockage preventing release into intestine
- Very high red blood cell death

5. Production of urea
Excess amino acids undergo deamination in liver cells
Ammonia and organic acid formed
Organic acid enters Kreb’s cycle
Ammonia (v toxic) enters ornithine cycle:
Controlled by enzymes in liver cells
Ammonia + CO2 Urea + Water
Urea leaves via hepatic vein
- removed from blood by kidneys

6. Kidney Structure

7. Role of Kidneys - Ultrafiltration
Glomerulus wall has large pores
Wall attached to basement membrane
– permeable but with small pores
Small molecules allowed through
- not plasma proteins
Bowman’s capsule has cytoplasmic projections
– allow rapid passage of filtrate
High pressure maintained by:
- Existing pressure from renal artery supply
- ‘Bottle-neck’ effect – blood squeezed through glomeruli
- Total volume of 1500 litres of blood/day
- 180 litres of glomerular filtrate produced
- However only 1-2 litres of urine

8. Role of kidneys - Reabsorption
99% of water is reabsorbed
Concs of other substances vary little from arterial blood to glomerular filtate
But vary greatly between filtrate and urine
Proximal Convoluted Tubule (PCT):
Glucose (& amino acids) are reabsorbed by PCT epithelial cells
Epithelial cells are suitable because:
- Cells have microvilli
- Cells have mitochondria (active transport)
- Cells have carrier molecules
Virtually all glucose reabsorbed (exception – diabetes)
Salt (sodium & chloride ions) are actively pumped back into blood
Water is absorbed by osmosis (85%)

9. Role of kidneys – Reabsorption 2
Loop of Henle:
Water reabsorbed by osmosis in descending limb
Sodium & Chloride ions pumped back into tissue fluid in ascending limb
Water not reabsorbed in ascending limb (wall impermeable)
Therefore low water conc in surrounding medulla
Distal Convoluted Tubule (DCT):
Sodium Chloride reabsorbed
Water reabsorbed
Collecting Duct:
Large level of water reabsorption by osmosis if required
Reabsorption controlled by levels of ADH
Any remaining filtrate – released in urine (nitrogenous excretion)

11. ADH