Week 3 - Filtration by the glomerulus Flashcards
How does the renal artery divide?
Renal artery → segmental arteries → interlobar arteries → arcuate arteries → interlobular arteries → afferent arterioles
What is the role of afferent arterioles?
- The afferent arterioles each deliver blood to a single nephron
- The diameter of each afferent arteriole is slightly greater than the diameter of the associated efferent arteriole
Describe glomerular filtration
The diameter difference between the afferent and efferent arteriole increases the pressure of the blood inside the glomerulus
- This increased hydrostatic pressure helps to force the below components out of the blood in the glomerular capillaries
- – Most of the water
- – Most/all of the salts
- – Most/all of the glucose
- – Most/all of the urea
- Only 20% of the delivered blood is actually filtered, 80% exits via the efferent arteriole
- These are all filtered as they are relatively small particles
- RBCs and plasma proteins are not filtered because they are too large
- The water and solutes that have been forced out of the glomerular capillaries pass into the Bowman’s space and are called the glomerular filtrate or the ultrafiltrate
Why does the basement membrane and podocytes not allow protein movement?
They have negatively charged glycoproteins, which repel protein movement
- This charge can be lost in many diseases, causing proteinurea
Describe the renal filtration barrier
There are 3 layers to pass through
- Capillary endothelium
- – Water, salts, glucose
- – Filtrate moves between cells
- Basement membrane
- – Acellular gelatinous layer of collagen/glycoproteins
- – Permeable to small proteins
- – Glycoproteins repel protein movement
- Podocyte layer
- – Pseudopdia interdigitate to form filtration slits
Which forces are involved in plasma filtration?
- Hydrostatic pressure in the capillary (favours filtration)
- Hydrostatic pressure in the Bowman’s capsule (opposes filtration)
- Osmotic pressure difference between the capillary and tubular lumen (opposes filtration)
Net filtration pressure = 10mmHg
What is tubular reabsorption?
- Only about 1% of glomerular filtrate actually leaves the body
- The rest is reabsorbed into the blood while it passes through the renal tubules
- This recovery process is called tubular reabsorption
- There are 3 mechanisms
- – Osmosis
- – Diffusion
- – Active transport
- The majority of the glomerular filtrate is reabsorbed in the PCT
- – Includes some water and all of the glucose
- An energy demanding process
- – Mostly for the reabsorption of Na+ ions
- – Reabsorption of other things in the filtrate is coupled to the active reabsorption of Na+ ions
- – Na+ is pumped out of tubular cells across the basolateral membrane by 3Na-2K-ATPase
- – Na+ moves across the apical membrane down its concentration gradient
- – Water moves down the osmotic gradient created by the reabsorption of Na+
Describe renal secretion
- Provides a 2nd route, other than glomerular filtration, for solutes to enter the tubular fluid
- Useful as only 20% of plasma is filtered each time the blood passes through the kidney
- Helps to maintain blood pH
- Substances secreted into the tubular fluid:
- – Protons
- – Potassium
- – Organic anions and cations
Describe a model for organic cation secretion in the PCT
- Entry by passive carrier:
- – Mediated diffusion across the basolateral membrane down favourable concentration and electrical gradients, created by the 3Na-2K-ATPase pump
- Secretion into the lumen:
- – H+-OC+ exchanger that is driven by the H+ gradient created by the Na+-H+ antiporter
Describe reabsorption in the PCT
Reabsorption in the PCT is isosmotic
- Driven by sodium uptake
- Other ions accompany sodium to maintain electro-neutrality (e.g. chloride and bicarbonate)
- Solutes move from tubular lumen → intersticium → capillaries
- Reabsorption can be transcellular or paracellular
What is the role of active transport and co-transport in tubular reabsorption and secretion?
Different segments of the tubule have different types of Na+ transporters and channels in the apical membranes
- Allows Na+ to be the driving force for reabsorption, using the concentration gradient set up by 3Na-2K-ATPase
- – Proximal tubule = Na-H antiporter, Na-Glucose symporter (SGLUT)
- – Loop of Henle = Na-K 2Cl symporter
- – Early distal tubule = Na-Cl symporter
- – Later distal tubule and collecting duct = ENaC
How is glucose reabsorbed?
Reabsorbed in the PCT using the Na-glucose symporter (SGLUT)
- Moves against its concentration gradient
- Moves into the tubule cells
- Glucose then moves out of the tubule cell on the basolateral side by facilitated diffusion
- 100% of glucose is normally reabsorbed
- System has a maximum capacity (transport maximum, Tm)
- – If the plasma concentration exceeds Tm, the rest spills over into the urine
- – If this happens, water follows into the urine, causing frequent urination (polyuria)
What is clearance?
Clearance = the volume of plasma from which a substance can be completely cleared to the urine per unit time
- Clearance = (amount in urine x urine flow rate) / arterial plasma concentration
- Significance: Used to measure glomerular filtration rate
What is the glomerular filtration rate?
- The volume of plasma from which any substance is completely removed by the kidney in a given amount of time
- A measure of the filtration process of all the nephrons
- A measure of kidney function
- Normal GFR for males = 115-125 ml/min
- Normal GFR for females = 90-100 ml/min
How can you measure GFR?
- A substance must be freely filtered across the glomerulus
- This substance must not be reabsorbed, secreted or metabolised by the cells of the nephron
- Must pass directly into the urine
