2 - Renal

Question 1

What is the fundamental unit of the kidney?

Answer 1

The nephron and it’s vascular structure.

Question 2

What is the initial step in urine formation? What is the speed of this?

Answer 2

Bulk filtration in the glomerulus.

This process filters ~100 ml/min of plasma in the normal human.

Question 3

What happens to the fluid once it is filtered by the glomerulus?

Answer 3

It undergoes net reabsorption in the tubule.

This returns ~99% of the filtered fluid to the ECF.

Question 4

What happens once the ~99% of fluid is returned to the ECF?

Answer 4

Some substances are transported from the ECF into the tubular lumen in a process called secretion.

Question 5

What are the common parameters of renal blood flow?

Answer 5

Blood flow: Renal art flow (RBF) = 20-25% of CO ~1200 ml/min

Renal Art plasma flow (RPF = RBF(1-Hct) ~700 ml/min

Question 6

What are the common renal parameters for filtration?

Answer 6

Glomerular filtration rate (GFR) ~ 125 ml/min

Filtration fraction (GFR/RPF) = 125/700 = 18% : Range: 15-25%

Filtered Load (GFR x plasma concentration = mass / time)

Question 7

What is the common renal parameter for reabsorption?

Answer 7

99% of filtered solute and water ~124 ml/min

Question 8

What is the common renal parameter for excretion?

Answer 8

Urine flow ~ 1ml/min

Question 9

What is the common renal parameter for venous blood flow?

Answer 9

~1199 ml/min

Question 10

What is secretion?

Answer 10

Transport of solutes from blood into tubular lumen?

Question 11

Where are the kidneys located? How much of the body weight are the kidneys? How much CO do they recieve?

Answer 11

On the posterior wall of the abdomen, outside of the peritoneal cavity.

Approx 0.5% body weight, yet they receive 20-25% of CO.

Question 12

How is the kidney divided?

Answer 12

Into 8-18 pyramids consisting of cortex, medulla, and inner medulla.

Question 13

What is the nephron composed of?

Answer 13

Bowman’s capsule - which combines with the glomerular capillaries to form the glomerulus, the proximal tubule, the loop of henle (thin descending, thin ascending limb, and thick ascending limb), distal tubule (early distal tubule), connecting tubule (late distal tubule), and the collecting duct system (cortical, outer medullary, and inner medullary segments)

Question 14

Describe the arterial blood flow to the kidneys. What is the purpose of these vessels?

Answer 14

Abdominal aorta > large segmental arteries > interlobar arteries > arcuate arteries > interlobular arteries.

Serve primarily as conduit to deliver blood to the high resistance afferent arterioles.

Question 15

How does pressure change in the afferent arterioles?

Answer 15

They provide a large resistance to blood flow and the pressure drops about 60 mmHg.

Question 16

What percentage of the nephrons are superficial cortical nephrons? Describe the blood flow and the hydrostatic pressure.

Answer 16

90% of the nephrons.

Blood flows through a second major vessel, the efferent arteriole, and leads into the peritubular capillaries that surround tubular structures in the renal cortex. Then to venous system.

Hydrostatic pressure is relatively low (20mmHg).

Question 17

The other ten percent of nephrons are what type? Describe the blood flow. What is the hydrostatic pressure?

Answer 17

Juxtamedullary (deep) nephrons.

Blood flows through a second major resistance vessel, the efferent arteriole, and leads into the vasa recta capillaries that surround the renal medulla.

Hydrostatic pressure is relatively low (20mmHg).

Question 18

Describe the structure of juxtamedullary (deep) nephrons.

Answer 18

Have long loops of henle that extend into the inner medulla and also have a thin ascending and descending limb.

Question 19

What is the function of the postglomerular peritubular capillaries (cortical nephrons) and vasa recta capillaries (juxtamedullary nephrons)?

Answer 19

To reabsorb fluid to the ECF.

Question 20

The renal circulation has two capillary beds in _________?

Answer 20

Series

Question 21

What is the difference in pressure in the glomerular capillaries compared to the postglomerular capillaries (vasa recta and peritubular)?

Answer 21

Glumerular have higher hydrostatic pressure that favors glumerular ultrafiltration (net filtration from capillaries)

Postglomerular capillaries have lower hydrostatic pressure which favors reabsorption.

Question 22

Where are resistance vessels located? What happens in these vessels?

Answer 22

Because and after the high pressure glomerular capillaries: afferent arteriole and efferent arteriole.

There is the largest drop in pressure in them.

Question 23

What is the initial step in urine formation? What is the net filtration pressure?

Answer 23

Glomerular filtration, which is a bulk process.

Approximately 100-125 ml/min

Net filtration pressure is 10mmHg (hydrostatic -glomerular oncotic - bowman’s capsule pressure).

Question 24

What is the second step of urine formation?

Answer 24

Reabsorption, which is approx 99% of the filtered load (amount of solute and volume filtered).

Question 25

What is the third step of urine formation?

Answer 25

Secretion of some solutes by the tubules in the tubular lumen.

Question 26

What is the fourth step of urine formation? How would you calculate this?

Answer 26

Excretion! Excretion = Filtration + secretion - reabsorption

Question 27

What is glomerular filtrate? What is its composition?

Answer 27

An ultrafiltrate of plasma formed by the net effect of starling forces to move fluid out of the capillaries into bowman's space. Same concentration of most salts and organic substances found in plasma. Large proteins and cellular elements are excluded from filtrate.

Question 28

What is another way that GFR can be expressed?

Answer 28

Flux (Jv) equation Jv = Ultrafiltration coefficient (Kf) X sum of all forces opposing and favoring filtration

Question 29

In the 70 kg man, what is the normal renal plasma flow (going into the afferent arteriole)? How much of this is filtered and how much of this goes through the efferent arteriole?

Answer 29

Renal plasma flow (RPF) in through the afferent arteriole is 700 ml/min. The GFR is 125 ml/min so what's not filtered, 575 ml/min is the plasma flow through the efferent arteriole (EffPF).

Question 30

What are the three layers of the filtration barrier in the glomerulus?

Answer 30

1. Capillary wall: 700 A fenestrations - permeable to small molecules, neg charged glycoproteins on surface 2. Basement membrane: porous matrix of ECF proteins including type 4 collagen, laminin, fibronectin and other - charged proteins 3. Podocytes: finger-like processes with negatively charged proteins and slit pores ~40 x 140 A

Question 31

What does the filtration barrier select by?

Answer 31

Size: more permeable to small molecules Charge-selective: more permeable to positively charged molecules

Question 32

What is the structure of the proteins covering slit pores?

Answer 32

A molecular lattice of proteins including nephrin, P-cadherin, and other cytoskeletal proteins form a membrane.

Question 33

What happens when nephrin is absent in the slit pore?

Answer 33

The molecular lattice bridging the split pore is altered, making it larger (10 nm as opposed to the normal 5nm). Excess albumin and proteins are filtered, leading to albuminuria and proteinuria (protein in the urine).

Question 34

How does filterability change with size? What about charge?

Answer 34

Filterability decreases as molecular size increases. Compared to neutral compounds, positively charged molecules have a high filterability and negatively charged molecules have a low filterability.

Question 35

What are factors that could alter the GFR?

Answer 35

Anything that alters ultrafiltration coefficient: change in hydraulic permeability or surface area of glomerular capillary membrane. Or anything that changes the driving force: Glom capillary hydrostatic pressure, bowman's space hydrostatic pressure, or glomerular capillary oncotic pressure

Question 36

How is GFR primarily regulated physiologically?

Answer 36

By alterations in vascular resistance that leads to changes in glomerular capillary hydrostatic pressure and renal blood flow.

Question 37

How does Ohm's law apply to renal blood flow?

Answer 37

Flow = Driving pressure / Vascular resistance Perfusion is kept constant. Vascular resistance is both upstream in the afferent arteriole and downstream in the efferent arteriole of the glomerular capillaries.

Question 38

How does glomerular colloid pressure pressure and Hct change based on distance along the glomerular capillary?

Answer 38

Near the afferent end, there is a lower colloid pressure and Hct. Near the efferent end, there is a higher colloid pressure and a higher Hct.

Question 39

What are some pathophysiological causes of decreased GFR?

Answer 39

Renal disease, diabetes, hypertension. Urinary tract obstruction. Increased plasma protein. Decreased MAP (minimal effects due to auto-regulation. Preglomerular constriction, and postglomerular dilation.

Question 40

How does protein concentration change in the glomerular capillaries? What effect does this have on colloid osmotic pressure?

Answer 40

Since large, neg charged molecules are not filtered across the glomerular membranes, the concentration of protein in the plasma in the glomerular capillaries must increase. There will be a greater colloid osmotic pressure in the plasma in the efferent end than the afferent end.

Question 41

What effect would changing the filtration fraction have on the change in glomerular colloid osmotic pressure?

Answer 41

An increase in the filtration fraction results in a bigger increase in colloid osmotic pressure. This makes sense because more fluid being filtered means that there more proteins in the plasma that exert the colloid pressure.

Question 42

What is the filtration fraction?

Answer 42

Glomerular Filtration Rate / Renal Plasma Flow