Week 1 Lecture 2

Question 1

what are the vascular elements of the basic renal processes?

Answer 1

blood enters through the renal artery > blood enters afferent arteriole (supplies blood to the glomerulus) > glomerulus (ball of capillaries where filtration takes place) > efferent arteriole (carries blood away from the glomerulus) > peritubular capillaries (capillaries hat surround the tubule where reabsorption and secretion take place) > renal vein

Question 2

what are the tubular elements of the basic renal processes?

Answer 2

the filtrate is collected by the bowman’s capsule which surrounds the glomerulus > fluid then flows into the proximal tubule (where the majority of reabsorption takes place) > then into the loop of Henle where further reabsorption takes place. This region is important for producing urine of varying concentrations > then into the Distale tubule/collecting duct (controlled reabsorption (esp Na+ and H2O) and secretion (K+ and H+) to determine final filtrate composition > renal pelvis collects urine and funnels it to the ureter/bladder

Question 3

What is the advantage of free filtration? Why not just filter out only the substances to be
excreted? isn’t it inefficient?

Answer 3

If we reabsorb you don’t require a specific mechanism to get rid of foreign substances. We need to have a channel that recognises a substance and transport it back into the body. Xenobiotics or foreign substances if they are smaller or water-soluble they can stay in the filtrate and be excreted from the kidney.

Question 4

what are the forces that drive filtration?

Answer 4

These are pathological circumstances.

the net driving force is the balance of:

• Glomerular capillary hydrostatic pressure. Determined by the blood pressure in the capillary bed.
• Plasma osmotic pressure (oncotic pressure). Driven by the presence of proteins in the plasma, but not in the filtrate.
• Bowman’s capsule hydrostatic pressure. Determined by the presence of fluid in the capsule.
• Net filtration pressure is the sum of these three forces.

Question 5

How would each of the following affect GFR?

1) Low plasma albumin (eg reduced albumin production)
2) Urinary tract obstruction (eg kidney stone, enlarged prostate)
3) Severe dehydrating diarrhea (loss of water in the stool)

Answer 5

1. will cause liver damage

Question 6

What are the basic renal processes?

Answer 6

Filtration, reabsorption and secretion are the three main renal processes.
• Filtration happens exclusively in the glomerulus and involves the free movement of fluids and small solutes from the glomerular capillaries into Bowman’s capsule.
• Reabsorption is the selective movement of substances from the tubular lumen back into the peritubular capillaries.
• Tubular secretion is the selective transfer of substances from the peritubular capillaries to the tubular fluid.

Question 7

~ 20% of plasma flowing through the glomerular capillaries is filtered; the remaining 80% of plasma flows via the efferent arteriole into the peritubular capillaries.

What would happen if this was reversed, and instead 80% of the plasma was filtered?

Answer 7

Due to the filtration of water and small solutes etc, these are all retained in the plasma that is filtered. If 80% was filtered we would be removing so much liquid matrix we would end up with a really concentrated sludgy fluid that passes through the efferent arteriole

Question 8

What are the barriers to filtration?

Answer 8

Substances moving from the glomerular capillaries into Bowman’s capsule must pass through 3 barriers:
– Glomerular capillary endothelium
– Basal lamina
– Bowman’s capsule
epithelium (podocytes)

Question 9

how can we detect glomerular damage?

Answer 9

• The presence of protein in the urine (proteinuria) can indicate damage to the filtration barrier in the kidney.
• Damage to the filtration barrier can be a sign of chronic kidney disease (CKD), which can progress to end-stage renal disease that may require dialysis or renal transplantation.
• Proteinuria is also a risk factor for cardiovascular disease mortality.
• Diabetes mellitus and hypertension are two common causes of glomerular damage.
• ~ 30% of Australians have one (or more) risk factors for CKD; early detection of proteinuria/albuminuria can help slow progression to end-stage renal disease.

Question 10

what are the two important differences between filtration in glomerular capillaries when compared to other capillary beds?

Answer 10

1. Glomerular capillaries are highly permeable

2. Filtration occurs through the entire length of the capillary bed.

Question 11

What is GFR (Glomerular Filtration Rate)?

Answer 11

GFR is defined as the volume of fluid that is filtered at the glomerulus per unit time.

Average GFR is ~ 125 mL/min (= 180 L/day).

GFR progressively declines with age; both reduced GFR and proteinuria can predict mortality risk. A marked reduction in GFR can be a sign of kidney damage and loss of normal renal function.

Question 12

Renal filtrate is different to blood because it lacks

Answer 12

proteins and cells. Only fluid and small solutes (ions like Na+, K-, Ca2+, Mg or carbohydrates like glucose etc) are filtered in the glomerulus

Question 13

Most of the filtered fluid is absorbed in

Answer 13

the proximal tubule

Question 14

Which structure is the most important for producing urine of different concentrations?

Answer 14

Loop of Henle

Question 15

Approximately how much of the renal plasma flow is filtered in the bowman’s capsule?

Answer 15

20%. the remaining 80% goes into the peritubular capillaries where 99% is reabsorbed

Question 16

Podocytes help form the

Answer 16

filtration slits. they are specialised epithelial cells that wrap around the glomerular capillaries and they have long projections that interdigitate together to form filtration.

Question 17

Which of these forces promotes filtration?

Answer 17

glomerular capillary pressure

Question 18

what factors influence GLOMERULAR FILTRATION RATE?

Answer 18

GFR = net filtration pressure x filtration coefficient

The filtration coefficient is determined by the surface area for filtration and the permeability (the more permeable, the more filtration) of the barrier.

Question 19

How would each of the following affect GFR?

1) Low plasma albumin (eg reduced albumin production)
2) Urinary tract obstruction (eg kidney stone, enlarged prostate)
3) Severe dehydrating diarrhoea (loss of water in the stool)

Answer 19

1) low plasma albumin can cause liver damage (cirrhosis) where they can not synthesise their normal plasma protein and thus a reduced albumin production occurs - this affects the GFR by decreasing plasma osmotic pressure. If you’ve got less plasma to produce you have less pressure to generate and therefore the next issue is the decrease in opposing force. This will result in an increase in GFR.
2) formation of solid mass (kidney stone) or an enlarged prostate gland that provides external pressure to compress that urinary tract. This will increase downstream resistance. This will cause a pressure increase in hydrostatic pressure in Bowman’s capsule which will increase the opposing force and thus the GFR will be decreased.
3) excessive loss of water through diarrhoea in the faeces will cause loss of water through the stool. This will increase plasma osmotic pressure which will increase the opposing force resulting in a decrease in GFR.