Flow and filtration

Question 1

What percentage of the cardiac output goes to the kidney? Why? What else contributes to this?

Answer 1

Receives 20-25% - in order to generate the high pressure needed for ultrafiltration. Achieved also with direct aortic branching.

Question 2

What is glomerular filtration?

Answer 2

Formation of an ULTRAFILTRATE in glomerulus.

Question 3

What proportion of plasma is generally filtered in glomerular filtration?

Answer 3

20%. While 80% is retained in the plasma.

Question 4

How is the glomerular filtrate produced?

Answer 4

Passive process – as fluid is driven through semi-permeable walls of glomerular capillaries into Bowman’s capsule by (very high) cardiac hydrostatic pressure. Capillary walls (which are fenestrated) = highly permeable to fluids and small solutes, but not cells; proteins and drugs are carried by proteins. Filtration barrier = fenestrated CAPILLARY ENDOTHELIUM, modified BASAL MEMBRANE and PODOCYTES found outside the capillaries with feet that interdigitate.

Question 5

How does the afferent and efferent glomerulus arterioles differ?

Answer 5

Afferent is much larger in diameter. This helps with maintaining pressure in the glomerulus, so keeping gradient high enough for ultrafiltration. NOTE, the afferent arteriole emerges from above, and the efferent leaves below. [PHOTO 2].

Question 6

What is primary urine?

Answer 6

Clear ultrafiltrate fluid free from blood and proteins.

Question 7

What is the renal corpuscle?

Answer 7

Made up of the Bowman’s capsule, glomerulus and podocytes.

Question 8

How is the body’s level of excretion through the kidneys calculated?

Answer 8

Amount FILTERED + amount SECRETED – amount ABSORBED.

Question 9

What is the glomerular filtration rate?

Answer 9

Amount of fluid filtered from the glomeruli into the Bowman’s capsule per unit time (ml/min) – sum of rate of all functioning nephrons.

Question 10

What is Pgc, Pt and πgc?

Answer 10

Pgc = hydrostatic pressure in the glomerular capillaries. Pt = hydrostatic pressure in tubules. Other one = osmotic pressure in the glomerular capillaries.

Question 11

What is the normal value for GFR?

Answer 11

120 ml min-1 per 1.73m^2. 1.73m^2 is a typical value for the body surface area of an adult.

Question 12

What happens to GFR with age?

Answer 12

Decreases about 10 ml min-1 per decade after 40.

Question 13

What is the filtration fraction?

Answer 13

Ratio of renal plasma flow to amount of filtrate filtered by the glomeruli.

Question 14

What is the normal value for filtration fraction?

Answer 14

0.2.

Question 15

What is meant by ‘freely filtered’?

Answer 15

Concentration in the glomerular filtrate is the same as in the plasma.

Question 16

What factors affect GFR? (x4)

Answer 16

Pgc, πgc, Pt, Kf (number of functioning nephrons). Can be hormonally or neutrally controlled.

Question 17

What is myogenic autoregulation? Importance?

Answer 17

Vascular smooth muscle cells stretched when arterial pressures rise in vessels of the kidneys, Resistance therefore increases too. IMPORTANCE: this reduces blood flow to keep GFR constant.

Question 18

What events can affect GFR? (x4) How – for each?

Answer 18

1. Severe haemorrhage: decreases Pgc so will decrease GFR. 2. Nephron tubule obstruction: increases Pt/decreases Kf, so decreases GFR. 3. Reduced plasma protein concentration: decreases πgc, so decreases GFR. 4. Small blood pressure increase: no effect due to myogenic autoregulation.

Question 19

Why is GFR clinically important? (x2)

Answer 19

• A drop in the rate is a cardinal feature of renal disease, and excretory proteins will build up in plasma: Most renal diseases result in destruction of nephrons, and GFR is equal to sum of functioning nephrons. • Useful in determining correct drug dosage as some drugs are excreted from the body primarily from glomerular filtration.

Question 20

Why is creatinine concentration in GF clinically important?

Answer 20

Raised or lowered plasma creatinine concentration is diagnostic of renal disease.

Question 21

What are the three pressures involved in glomerular filtration?

Answer 21

Hydrostatic pressure in glomerular capillaries, which is opposed by hydrostatic pressure of the tubule and osmotic pressure of the glomerular capillaries.

Question 22

What is the calculation for ultrafiltration PRESSURE?

Answer 22

Puf = Pgc – Pt – πgc.

Question 23

What is the normal value for ultrafiltration pressure?

Answer 23

Net pressure of 10-20mmHg.

Question 24

What does ureteral obstruction do to ultrafiltration pressure?

Answer 24

Will increase Pt, hence decreasing Puf and GFR.

Question 25

What is renal clearance?

Answer 25

It is the extent to which substances passing through the kidneys are removed from the blood and out of the body. The renal clearance of something is therefore equal to the volume of plasma which supplies that amount of the substance excreted by the kidneys per unit of time. An important point to remember is that clearance will always be expressed as a volume of plasma per unit of time. CALULATIONS ARE IN CLINICAL NEPHROLOGY.

Question 26

Where does creatinine originate?

Answer 26

Endogenous waste produce from creatinine in MUSCLE metabolism, and amount released relatively constant. If renal function stable, then amount in urine stable.

Question 27

How is creatinine handled in the kidneys?

Answer 27

Freely filtered, and not reabsorbed or metabolised in the kidneys. Some is secreted into the urine in the PCT.

Question 28

What proportion of the ultrafiltrate is reabsorbed back into the blood?

Answer 28

99%.

Question 29

Why is it important to maintain osmolarity?

Answer 29

If osmolarity fluctuates too much, cell volumes will also fluctuate because solute concentration and water potential are interdependent e.g. if osmolarity is high and there are too many solutes in a cell, water will move in to balance the osmolarity, and volume will increase.

Question 30

What is the proportion of water found across the body?

Answer 30

65% = intracellularly. 35% = extracellularly (mostly interstitial fluid, then plasma, lymph and transcellular (cerebrospinal, synovial…).

Question 31

What is the total volume of water in the body?

Answer 31

40L.

Question 32

What is the circulating volume of water in the blood?

Answer 32

~2.5 litres.

Question 33

What is the volume of blood?

Answer 33

5 litres.

Question 34

How does composition of body fluids vary intracellularly and extracellularly: Na+, Cl-, HCO3-, K+, HPO42-, Ca2+, Mg2+ and Protein?

Answer 34

Look at photo.

Question 35

How is fluid removed from the body? (x4)

Answer 35

Sweat, faeces, respiration (all uncontrollable); urine (controllable).

Question 36

What proportion of water is RETAINED in the tubules at each region of the nephron? Where can we control this retention?

Answer 36

Look at photo – most is removed in the PCT (~70%). THE GREEN REGION IS WHERE WE EXERT SOME CONTROL.

Question 37

How important is the loop of Henle in control of water between mammals?

Answer 37

The ratio of the volume of the medulla to the cortex determines how concentrated the urine can be made in the loop of Henle. For mammals that have the highest medulla:cortex volume, they can concentrate their urine further?

Question 38

If inulin concentration was 1 mmol/L, what would be the plasma inulin concentration in the efferent arteriole?

Answer 38

STILL 1 mmol/L because water and everything else has moved out too in the same proportions.

Question 39

If inulin concentration was 1 mmol/L, what would be the plasma inulin concentration in the renal vein?

Answer 39

> 99% water, glucose, amino acids move back into blood. We can assume this is 100%. And, since no inulin is reabsorbed, concentration falls to 0.8mmol.L in the renal vein because 20% was filtered.