Renal Physiology: The Glomerulus

Question 1

The three key roles of the kidney

Answer 1

1. ) Maintain balance of salt/water/pH
2. ) Excrete waste products
3. ) Endocrine function: secreting hormones

Question 2

Renal Blood flow

Answer 2

Cardiac output is 5L/min. Renal blood flow is about a fifth of this, so is 1L/min. This results in a urine flow of 1ml/min

Question 3

How does the renal blood supply work?

Answer 3

Descending aorta-> renal artery-> interlobar artery-> arcuate artery-> interlobar artery-> afferent arteriole-> nephron: the glomerular capillary and the peritubular capillary

Question 4

Where are glomeruli found

Answer 4

In the cortex. The medulla only has tubules

Question 5

What are the two main capillary beds in a nephron and what takes place in each one?

Answer 5

1. ) Glomerular capillary: exchange of small solutes

2. ) Peritubular capillary: exchange of ions to modify your urine

Question 6

Ultrastructure of the glomerulus: what two features are present through looking at a microscopic image?

Answer 6

1. ) Podocytes

2. ) Foot processes

Question 7

Glomerular filtration: what forms the filtrate?

Answer 7

• Passage of fluid from the blood into Bowman’s space will form the filtrate
• Surface area for this = 1m2

Question 8

What makes up the filtration barrier

Answer 8

Urinary space
1.) Podocyte
2.) Foot processes
3.) Basement membrane
4.) Capillary endothelium
 Capillary

Question 9

What is the function of the foot processes?

Answer 9

They are negatively charged-> limits albulmin getting through as it is negatively charged as well

Question 10

What determines what crosses the filtration barrier

Answer 10

1. ) Pressure
2. ) Size of molecule
3. ) Charge of molecule
4. ) Rate of blood flow
5. ) Binding to plasma proteins e.g. calcium

Question 11

Therefore: examples of what can and can’t cross the filtration barrier

Answer 11

Can: small molecules/ions up to 10kDa can pass freely e.g. glucose and uric acid
Can’t: larger molecules become increasingly restricted + negatively charged anions (as fixed negative charge in glomerular B.M repels them)

Question 12

What is rhe only protein present in filtered fluid?

Answer 12

Tamm Horsfall protein. It is produced by tubule

Question 13

Determining the filtration barrier: pressure

Answer 13

• Two types of pressure:
  1. ) Hydrostatic pressure:
• glomerular capillaries: 45 mmHg
• bowman’s capsule: 10mmHg
  2. ) Osmotic pressure
• G.C: 25mmHg
• B.S: zero

Question 14

Glomerular filtration rate definition

Answer 14

Is the filtration volume per unit time (mins)

Question 15

How do we calculate GFR

Answer 15

(hydrostatic pressure in G.C - hydrostatic pressure in B.S) - (osmotic pressure in G.C - osmotic pressure in B.S)

Question 16

What does GFR also give

Answer 16

KF: this is the filtration coefficient

Question 17

Autoregulation 1 : how does it work?

Answer 17

• Pressure within afferent arteriole rises -> stretches vessel wall -> triggers contraction of smooth muscle -> arteriolar constriction
• Prevents an increase in systemic arterial pressure from reaching the capillaries
• reverse happens when systemic arterial pressure falls

Question 18

Autoregulation: tubuloglomerular feedback. How does this work?

Answer 18

• GFR of individual nephron regulated by the rate at which filtered fluid reaches the distal tubule
• cells of the macula densa (distal tubule) detect NaCl arrival
• Macula densa cells release prostaglandins in response to reduced NaCl delivery
• Acts of granular cells. Triggers renin release + activates the renin-angiotensin system

Question 19

How do we measure GFR

Answer 19

• calculated by measuring the excretion of a marker substancw (M):
• freely filtered/not secreted or absorbed in tubules/not metabolised

Question 20

Measurement of M in fluid

Answer 20

Concentration in fluid x volume of fluid
Pm = concentration of M in plasma
Um = concentration of M in urine
GFR = Um x urine flow rate/Pm

Question 21

Measurement of GFR in clinical practice

Answer 21

• Normal GFR = 125 ml/min

- Clinically: creatine in used to estimate GFR

Question 22

Why is creatine used to measure GFR

Answer 22

• It is a muscle metabolite: constantly produced

- Serum creatinine concentration will vary with muscle mass

Question 23

Filtration fraction calculation

Answer 23

FF = GFR/renal plasma flow
GFR = 120ml/min
Renal blood flow: 1000ml/min so renal plasma flow = 600ml/min
- FF = 120/600 = 0.2 or 20%

Question 24

Definition of renal clearance

Answer 24

Volume of plasma from which a substance is completely removed by the kidney per unit time (usually a min)

Question 25

Calculate clearance

Answer 25

Urine concentration x urine volume/plasma concentration