Tubular Function

Question 1

What happens to filtrate?

Answer 1

• completely reabsorbed: glucose (162g in filtrate, none in urine), amino acids, bicarbonate ion
• regulated and thus is partially reabsorbed: water (180L in filtrate, 1-2L in urine), sodium (570g in filtrate, 4g in urine), potassium, chloride
• excreted as waste: urea, creatinine (1.6g in filtrate, 1.6g in urine), drugs and drug metabolites, uric acid (8.5g in filtrate, 0.8g in urine)

Question 2

What is a difference between the ascending and descending limbs of the loop of Henle?

Answer 2

• descending limb is water permeable
• ascending limb is not water permeable (can move ions out, but can’t move water out)

Question 3

What is the collecting duct’s permeability to water?

Answer 3

-variably permeable to water

Question 4

What are the arcuate arteries? What other vessels branch from them?

Answer 4

• arteries run along the border of the medulla and cortex
• put off straight vessels called interlobular arteries
• interlobular arteries have afferent arterioles to each nephron

Question 5

Describe capillaries beds in renal tubules

Answer 5

• branches into a capillary bed through the tubules to collect things back into blood stream
• spans across the proximal/distal convoluted tubule and loop of Henle
• then goes to venous drainage
• vasa recta runs parallel to loop of Henle
• blood collects into arcuate vein

Question 6

How can reabsorption occur?

Answer 6

Active: requires energy

Passive: chemicals following their electrochemical gradients

Question 7

How does water move?

Answer 7

• osmosis
• passive mechanism by which water follows its concentration gradient through a semipermeable membrane
• osmosis is driven by the movement of the solutes from the tubules to the interstitial fluid which often requires energy
• about 90% of water reabsorption is obligatory (water is dragged along by the solutes being moved from tubules)
• much of the obligatory reabsorption of water will occur in the proximal convoluted tubule and descending loop of Henle because these areas have tubules which are permeable to water
• the last 10% is facultative which means it can increase or decrease depending on the amount required by the body
• facultative reabsorption of water is under control by ADH which makes the cells in the collecting duct permeable to water and thus water can leave the ducts

Question 8

Describe osmosis in the proximal convoluted tubule

Answer 8

• movement of water will be by osmosis
• this osmosis is linked to passive reabsorption (diffusion) of a number of ions as well as the waste product urea
• sodium transport is active
• most active area for reabsorption is the proximal convoluted tubule
• by the end of the PCT, almost 100% of organic solutes have been reabsorbed
• about 65% of water has been reabsorbed

Question 9

How does passive transport occur?

Answer 9

• the movement of solutes often does not involve additional energy being added
• some solutes can slip between the tight junctions of the cells (paracellular) or into or out of the cells of the tubules (transcellular) strictly by following their electrochemical gradients
• diffusion may be facilitated by transport proteins as in the movement of glucose from inside of the tubular cells to the interstitial fluid
• leakage channels also exist for some ions to facilitate their walk down the concentration gradient

Question 10

Describe passive sodium reabsorption

Answer 10

• fluid in tubule lumen
• apical membrane (part of brush border) to increase surface area for absorption
• tubule cells lining tubule with tight junctions
• basolateral membrane
• to move Na passively, concentration must be higher in the lumen than interstitial fluid
• can go paracellularly into the interstitial fluid and capillary

Question 11

What are the types of active transport?

Answer 11

• primary active transport functions strictly with the use of ATP
• secondary active transport uses the energy of the movement of ions down their concentration gradients to transport other solutes like ions and larger uncharged molecules like glucose or amino acids (symporter if secondary active transport protein moves the ion and solute molecule in same direction, antiporter if they move in opposite direction)
• both active transport mechanisms require a carrier protein to assist the movement of the ions and other solutes

Question 12

What happens when transport mechanism of transport proteins is saturated?

Answer 12

• transporters are proteins that can only bind and then move solutes at a fixed rate so they have a maximum rate at which they can function
• the saturation of the transport mechanism limits reabsorption of substances like glucose
• conditions like glucosuria occur strictly because the Tm of the active transporter has been reached
• the sign of glucosuria indicates that the pathology diabetes is present and the Tm has been reached for the Na-glucose symporter

Question 13

Are active and passive transport mechanisms happening at the same time?

Answer 13

• yes
• active process consumes about 6% of total ATP at rest

Question 14

Describe active sodium transport

Answer 14

• sodium moves down its concentration gradient into the cell
• can’t leave Na in the cell to maintain the membrane potential (and it would also cause water to come in)
• sodium potassium pump takes ATP and moves sodium out into interstitial fluid and potassium in

Question 15

Describe glucose movement in the proximal convoluted tubule

Answer 15

• sodium glucose symporter at apical membrane
• 2 sodiums going down concentration gradient is enough to pull 1 glucose into tubule cell
• sodium potassium pump moves Na into interstitial fluid
• glucose facilitated diffusion transporter on basolateral membrane allows glucose to go down its concentration gradient to go into interstitial fluid

Question 16

Describe the sodium/hydrogen antiporter in the proximal convoluted tubule

Answer 16

• CO2 being made and combines with H2O to form carbonic acid which rapidly becomes H+ and bicarbonate
• sodium hydrogen antiporter on basolateral membrane uses one Na to move a H+ into the lumen
• get rid of H+ from inside the body so you don’t become acidic
• sodium potassium ATP pump moves Na out of cell
• bicarb is saved and moves through bicarb facilitated diffusion transporter down its concentration gradient and be taken back into blood stream
• in kidney, you can kick out an H+ while also saving the bicarb (whereas through breathing you lose the bicarb)

Question 17

Describe the symporter system at the thick limb of the LOH

Answer 17

• Na+ K+ 2CL- symporter
• pulls in Na, K, and 2Cl (Na is the one moving down its concentration gradient, other ones come along with it)
• sodium and potassium pump moves sodium out
• Cl leaves through leak channel (if there is too much chloride there are lots of leak channels into filtrate, too little chloride there are leak channels back into interstitial fluid)
• potassium leak channels that let potassium back into filtrate usually (can go the other way also)
• set up a charge gradient by moving negative ions out so that it pulls positive ions from the lumen into interstitial fluid paracellulary

Question 18

Describe sodium reabsorption in collecting duct

Answer 18

• collecting duct is last chance to balance filtrate
• Na leakage channels (facilitates transcellular Na+ reabsorption) on apical membrane
• Na moves out through sodium potassium pump into interstitial fluid
• if we need potassium, there will be more leak channels on interstitial side
• if we don’t need potassium, it will leak into filtrate

Question 19

What occurs if kidneys fail?

Answer 19

• salts and waste products like urea build up and the pH of the blood goes down
• if you keep that salt, you also keep the fluid resulting in edema
• acidemia results from inability to excrete H+
• when K+ levels get too high, cardiac arrest occurs (potassium dysregulates heart rhythm)

Question 20

What are treatments for kidney failure?

Answer 20

• kidney dialysis: take blood out of veins, dialyze out the substances, and put blood back in
• peritoneal dialysis: port into peritoneal cavity and flush in fluid, sodium and ions will move into that fluid, flush fluid out
• kidney transplant

Question 21

Patient comes into ER. Generalized weakness, nausea, and overall sense of illness. Vital signs are normal but there is pitting edema. K+ levels are 7.5mmol/L (normal is 3.5-5)

Answer 21

-kidney failure

Question 22

What is the trigone?

Answer 22

• part of the bladder that doesn’t contract
• if whole bladder contracted it would close off tubes

Question 23

What does the deep transverse perineus and levator ani muscles do?

Answer 23

DTP:

• fibres that run up urethra
• striated

Levator ani:

-move up (affects urethra, vagina, and anus)

Question 24

Answer 24

Question 25

Describe the micturition reflex

Answer 25

- when the bladder fills with 200-400mL of urine the stretch receptors in the bladder wall are stimulated - stretch receptors send signal to sacral portion of the spinal cord - sensory input to this level triggers an autonomic reflex which sends parasympathetic motor signals to the detrusor muscle (smooth) to contract and the internal urethral sphincter (in males) to relax - internal sphincter is smooth muscle and thus is controlled by the ANS and is not under conscious control - external urethral sphincter (in males and females) is striated muscle and thus can be consciously controlled - other muscles like the levator ani and deep muscles of the perineum can help too - somatic nerves holding this sphincter closed are inhibited by micturition reflex - pressure buildup is usually not enough in adults, there has to be a conscious effort to relax the external sphincter before urine can pass (this can't last forever) - the bladder will eventually give up contracting but then more urine gets produced so eventually it will contract again harder and it will be more painful - detrusor muscle can overcome external sphincter