from blood plasma to urine 2

Question 1

Filtration facts

Answer 1

• 20% of plasma entering kidneys is filtered
• 125ml of filtrate/min
• 180L/day
• Average plasma volume = 2.75 L
• Plasma is filtered 65 times per day!
• Without reabsorption total plasma volume lost in 30 min!!!!

Question 2

Reabsorption

Answer 2

Filtrate contains substances we want to keep (eg. Water, electrolytes & nutrients)
• Reabsorption = selective movement of substances from tubules back into blood
• Reabsorbed substances travel back into the venous system via peritubular capillaries
• 180 L/day plasma filtered 178.5 L/day reabsorbed!

Question 3

Locations for reabsorption

Answer 3

• 60-70% of filtrate is reabsorbed in the proximal tubule
– Approx. all organic nutrients (glucose & amino acids) – 60-70% Na+ & Cl- + other ions
– 60-70% water
• LoopofHenle
– 25% water and Na+ reabsorption
* producing urine of multiple concentrations
• Distal tubule & collecting duct
– Hormonally controlled reabsorption of water and Na+

Question 4

How does reabsorption occur?

Answer 4

Molecules must move from the tubule lumen to the interstitial fluid & capillaries
– Transepithelial transport (1) – Paracellularpathway(2)
• Movement can be
– Passive – no steps require ATP
– Active – at least one step requires ATP

Question 5

What are the 5 barriers that must be crossed during transepithelial transport?

Answer 5

1. luminal cell membrane
2. cytosol
3. basolateral cell membrane
4. interstitial fluid
5. capillary wall

Question 6

Reabsorption overview

Answer 6

Filtrate entering the tubular system has the same solute concentration as the extra-cellular fluid.
A gradient must be produced to allow reabsorption…
(1) Na+ is reabsorbed by active transport to produce concentration & electrical gradients
(2) Movement of Na+ produces an
electrical gradient that favours the movement of anions into the ECF (eg. Cl-)
- creates osmolarity

Question 7

Reabsorption overview 2

Answer 7

(3) Movement of Na+ and anions increases the solute concentration in the ECF and water follows via osmosis
(4) Movement of water into the ECF concentrates the remaining solutes in the lumen allowing passive diffusion of solutes into the ECF (eg. Urea)

Question 8

Na+ reabsorption

Answer 8

• Nearly all Na+ is reabsorbed from the filtrate (back into blood)
• It is reabsorbed along the length of the tubule and plays different roles at each site

Question 9

Na+ reabsorption 2

Answer 9

Site> Percentage reabsorbed > Role

Proximal tubule
67%
Aids reabsorption of glucose, amino acids, water, Cl- and urea

Loop of Henle
25%
Allows production of urine of varying concentrations

Distal tubule & collecting duct
8%
Regulates ECF volume

Question 10

Na+ reabsorption steps

Answer 10

1) Na+ transport across the basolateral membrane is active (Na+ - K+ pump) and produces a Na+ concentration gradient (low in cell, high in the interstitial fluid)
2) Transport across the luminal membrane is passive and down the concentration gradient

Question 11

Na+ concentration gradient

Answer 11

Na+ concentration gradient aids the reabsorption of other essential molecules: The example of glucose

Question 12

Na+ concentration gradient

2

Answer 12

1. Na+ moving down EC gradient using SGLT protein pulls glucose into the cell against its concentration gradient
2. glucose diffuses out the basolateral side of the cell using the GLUT protein
3. Na+ is pumped out by Na+-K+ ATPase

Question 13

Actively reabsorbed molecules have a transport maximum

Answer 13

• Membrane carriers have specificity (only carry designated molecules)
• There are a limited number of carriers in the tubular cell membranes
• If the number of molecules in the filtrate exceeds the available carriers, some molecules will not be reabsorbed….

Question 14

Transport maximum (Tm) =

Answer 14

transport rate when carriers are saturated

Question 15

renal threshold

Answer 15

renal threshold = plasma concentration at which Tm is reached and molecule (eg. Glucose) first appears in the urine

Question 16

Secretion

Answer 16

• Some substances are secreted from the plasma into the filtrate.
– Reduce their concentration in the blood
* substances that have too high concentration in blood
• Commonly secreted substances – K+
– Organic ions
– H+ (hydrogen ion) or HCO3- (bicarbonate)

Question 17

K+ is secreted to maintain ECF concentration

Answer 17

• K+ concentration is low in the ECF
– membrane excitability
• K+ secretion from distal tubule
and collecting duct
– Hormonally controlled – Aldosterone

Question 18

Secretion of organic ions

Answer 18

• Bodily derived ions
– Prostaglandinsandepinephrine
• Xenobiotics = molecules foreign to the body
– Eg.Drugs
• Secretion occurs in the proximal tubule – Organicaniontransporters
– Organiccationtransporters
• Transporters are non-selective
– Transport many substances based on chemical structure
– Different drugs competitively bind same transporter slower rate of excretion from body