Lecture 32 - Renal physiology continued

Question 1

Nephron processes - filtration

Answer 1

Is a specific form of ultrafiltration

Creates a plasma like filtrate of the blood

At the glomeruli is not particularly selective

Takes place at a rate of 125ml/min (180L/day)

Produces only 1.5L of urine per day (178.5L is going back into the system via reabsorption)

Question 2

What defines renal filtration?

Answer 2

Renal blood flow
Filtration barrier
Driving forces

Question 3

Renal blood flow

Answer 3

The renal corpuscle glomerulus has afferent and efferent arterioles supplying the glomerulus with blood for filtration

Renal blood supply
25% of cardiac output
1-1.2L/min (~400mL/100g/min) - very high because its job is to filter blood, only for filtration and there is not a significant amount of metabolism (high flow for filtration rather than metabolism)

Question 4

Filtration barrier

Answer 4

Filtration takes place in the glomerulus
The filtration membrane is composed of fenestrated endothelium, fused basement membrane and podocytes with food processes and filtration slits

Small substances (low molecular mass) are freely filtered whereas large substances (high molecular mass) are NOT filtered.

Question 5

Driving forces

Answer 5

Determined by…
Forces/pressure gradients between glomerular capillary/capsular space
Permeability of glomerular capillary
Surface area of glomerular capillary

There are 4 forces (pressure) determining net filtration pressure (10mmHg)
Glomerular hydrostatic pressure (GHP) = blood pressure (+50mmHg) - this is the main driving force that favours filtration)
Blood colloid osmotic pressure (BCOP) = albumin (-25mmHg) - BCOP is made up by plasma protein such as albumin which are very large so does not pass through the filtration barrier and stays in the capillary and this makes a force that opposes filtration and ultimately brings it to a hold once a lot of water has been filtered
Capsular hydrostatic pressure (CsHP) = Corpuscle embedded in tissue (-15mmHg) - Corpuscle is embedded in tissue which means that it cannot expand anywhere which makes up an opposing pressure
Capsular colloid osmotic pressure (CsCOP) = no protein in the capsular space (+0mmHg) - another favouring force which would be the osmotic pressure that sits in the capsular space but it is pretty much zero because there is no protein filtered so this favouring force is zero

Net filtration pressure = outwards pressures - inwards pressures (want this to be positive)
NFP= Glomerular hydrostatic pressure - capsular hydrostatic pressure - blood colloid osmotic pressure
NFP = 50mmHg-15mmHg-25mmHg = 10mmHg (positive therefore an outwards pressure)

Question 6

Filtration in kidney results in

Answer 6

An isotonic primary filtrate - this is because it is based on NaCl, osmolarity of blood and urine is primarily based on NaCl

Question 7

Renal clearance

Answer 7

It is a way of defining renal filtration 
Clearance = Us x V/Ps (in mL/min)
Us= conc of S in urine (mg/L) 
V = volume of urine per time 
Ps = conc of S in plasma (mg/L)  

The equation describes the rate at which substance S is cleared by the kidneys per unit time
The equation can be used for all substances that can be detected in plasma and urine

Question 8

Estimating GFR

Answer 8

GFR (glomerular filtration rate) is the amount of fluid filtered per unit time 
Normally - 180L/day or 125mL/min 
Tightly regulated 
Variation from person to person 
Slowly declines from age of 30 

We can estimate GFRR by calculating renal clearance of a substance that is not reabsorbed, secreted or metabolised by the kidneys
The best option is creatinine - a waste product from muscle occurring naturally in the body (it is cleared entirely by the kidneys)
Plasma creatinine is an indicator for kidney function - plasma creatinine will be low if both kidneys are working (125ml/min)
Plasma creatinine remains fairly normal, even if only one kidney is still working (GFR about 60ml/min) - this is due to redundancy

Can also use inulin which is a polysaccharide that is not metabolised in the body. It is not found in the body, must be injected

Question 9

Relationship between GFR and creatine

Answer 9

Plasma creatinine is an indicator for kidney function - plasma creatinine will be low if both kidneys are working (125ml/min)
Plasma creatinine remains fairly normal, even if only one kidney is still working (GFR about 60ml/min) - this is due to redundancy

Question 10

Filtration fraction

Answer 10

GFR/RPF

RBF = renal blood flow
25% cardiac output per min = 1.25L blood/min
Blood is about half plasma and half cells therefore RPF = 625mL plasma/min
But only 125ml/min filtered (GFR) - the rest flows through into the efferent arteriole i.e. only 20% (125/625) of plasma is filtered = filtration fraction (GFR/RPF)

Question 11

Filtered load

Answer 11

Filtered load = amount of a particular substance (solute) filtered per minute = GFR x solute plasma conc.

Question 12

The glomerular filtration rate (GFR) is defined as …

Answer 12

The volume of plasma filtered per time

Question 13

Amount of substance filtered per time

Answer 13

Filtered load

In g/min or mol/min

Question 14

Volume of urine produced per day

Answer 14

1.5L per day

Question 15

Amount of sodium filtered per time

Answer 15

Filtered load of sodium