Urinary physiology 2: How do nephrons work?

What substances are reabsorbed in the proximal tubule of the nephron?

Na, Cl, glucose, amino acids, peptides, metabolites, urea, water

Name 1

tubular lumen

Name 2

tubular cell

Name 3

interstitial fluid

Name 4

capillary lumen

Name 5

Paracellular

Name 6

transcellular

Name 7

reabsorption

Name 8

secretion

Name 9

apical

Name 10

basal

Name 11

basolateral space

Where is secretion (into the proximal tubule) from

capillary to tubular fluid

Name the 2 types of transport mechanisms

active, passive

Name the 2 types of active transport

primary and secondary AT

Active transport

metabolic energy required

passive transport

no metabolic energy required

primary active transport

coupled to ATP hydrolysis

secondary active transport

relies on Na gradient set up by Na+K+ATPase

How is the proximal tubule adapted for reabsorption?

• located immediately after glomerulus

• convoluted

• peritubular blood has high oncotic pressure

• tubule cells

How are tubule cells (in the proximal convoluted tubule) adapted for reabsorption?

• brush border on apical surface

• infoldings of basal membrane

• many mitochondria

• carriers/transporters for different solutes

• tight junctions to reduce paracellular reabsorption

Which portion of the nephron does most transport occur in?

proximal tubule

Brush border (proximal tubule)

lots of infoldings on membrane

What does the brush border increase?

surface area

Antiport

counter transport

symport

co-transport

What transport mechanism is most reabsorption in the proximal tubule by?

secondary active transport

What drives movement of ions across the apical membrane?

sodium gradient

What enzyme is reabsorption in proximal tubule in basal and basolateral membranes driven by?

ATPase

Does reabsorption of glucose occur with or against the concentration gradient of glucose?

against conc grad

What is the reabsorption of glucose dependent on?

Na+ gradient

Where is almost all of filtered glucose resbsorbed?

proximal tubule

How much of filtered water is reabsorbed in the proximal tubule?

60-70%

Is reabsorption of water active or passive?

passive (osmosis)

Does the osmolarity of tubular fluid change in the proximal tubule?

NO

Where does variable reabsorption of water occur?

from the collecting duct

What is the osmotic effect in the proximal tubule aided by?

build up of solutes in clefts (basolateral spaces) between tubule cells

What drug affects cAMP?

aspirin

What drug affects bile salts?

penicillin

What drug affects prostaglandins?

several diuretics

What drugs affect adrenaline?

morphine

What drug affects noradrenaline?

isoprenaline

What drugs affect dopamine?

amiloride and atropine

What is the permeability of water like in the distal tubule?

low and constant

What gets reabsorbed in the distal tubule?

Na+ and Cl-

What is Na+ and Cl- reabsorbed via in the distal tubule?

NaCl symporter in apical membrane

What is the NaCl symporter in the apical membrane driven by? (distal tubule)

Na+K+ATPase in basal membrane

What is the normal osmolarity of the plasma of a dog?

290 mOsmol/L

What is the urine of a dog that had one bowl of water (500ml) in a day like?

less, more concentrated

What is the urine of a dog that has drunk lots of water (1.5L) in a day like?

more, dilute

What are the mechanisms that allow urine to be concentrated to an osmolarity higher than that of plasma?

• generate osmotic grad in medulla

• anatomical arrangement of LOH and CD

• max urine conc directly proportional to length of LOH

What is the maximum urine concentration directly proportional to?

length of LOH

What are the features of the descending limb? (what it’s permeable to)

permeable to water

What are the features of the thin ascending limb? (what is is permeable/impermeable to)

• impermeable to water

• permeable to Na+ & urea

What are the features of the thick ascending limb? (what is is permeable/impermeable to)

• impermeable to water & urea

• Na+ and Cl- actively removed

What is high osmolarity in the medulla created by?

countercurrent multiplier

What is the name of the vessels that supply blood to the medulla?

vasa recta

What does counter current blood flow in vasa recta prevent?

‘wash out’ of salts in the medulla

Where does the collecting limb run in relation to the ascending limb?

parallel to AL of LOH with flow in opposite direction

What causes increased permeability of water in the collecting duct?

ADH (vasopressin)

How does water move out of the collecting duct?

by osmosis due to hyperosmotic medulla

What are the cortical and medullary sections of the collecting duct impermeable to?

water, urea and NaCl

When does permeability of water in the collecting duct increase?

in presence of ADH

When is permeability of the collecting duct to urea increased?

in medullary section by ADH

What percentage of urea is reabsorbed in the proximal tubule?

50%

In what part of the nephron does urea conc increase?

Loop of Henle

Why does tubular urea concentration increase in the cortical collecting duct?

removal of water

What receptors detect change in plasma osmolarity?

osmoreceptors in hypothalamus

What does excess fluid do to blood fluid osmolarity?

lowers it

What lowers blood fluid osmolarity?

excess fluid

What does fluid deprivation do to body fluid osmolarity?

increases it

What increases blood fluid osmolarity?

fluid deprivation

What does increase in osmolarity do? (ADH)

increases circulating levels of ADH

What does increasing circulating levels of ADH do?

increase water reabsorption

What does ADH act on?

collecting duct

What does ADH do to the collecting duct?

inserts aquaporin channels

Where does ADH insert aquaporin channels?

apical membrane