Lecture 3 : Role of the Tubules in Urine Formation

Question 1

Glomerular filtration

Answer 1

dumps cell-free and protein-free blood filtrate into the container (glomerular capsule or Bowman’s capsule)

Question 2

The kidneys will process ____ of filtrate a day

Answer 2

180 L (47 gallons)

Question 3

Tubular reabsorption

Answer 3

reclaims what the body needs to keep (from tubule to blood) – which is ALMOST everything

Question 4

Tubular secretion

Answer 4

selectively adds to the waste container (from blood to tubule)

Question 5

Anything remaining in kidney tubule at the end is EXCRETED as ____

Answer 5

urine

Question 6

The ____ and the _____ are low pressure capillary beds adjacent to the renal tubule

Answer 6

peritubular capillaries

vasa recta

Question 7

Peritubular Capillaries:

Answer 7

Efferent arteriole gives rise to peritubular capillary bed

Surround PCT and DCT in cortex - will reclaim (reabsorb) most of the solutes and water from the filtrate

Question 8

Vasa recta:

Answer 8

Efferent arteriole gives rise to vasa recta – long straight capillaries

Follow nephron loop deep into medulla – maintain osmotic gradient that makes it possible to concentrate urine

Question 9

Processes we need to understand in the renal tubule:

Answer 9

Filtrate to blood
Blood to filtrate

Countercurrent mechanisms

Role of blood flow in vasa recta of medulla

Question 10

In PCT:

Answer 10

most reabsorption occurs here and is unregulated

maintains osmotic equilibrium

Question 11

Nephron loop:

Answer 11

only water leaves in descending limb

only salt leaves in ascending limb

(Exceptions to what you think about osmosis)

Question 12

In DCT and collecting duct:

Answer 12

hormones control reabsorption

Question 13

Aldosterone:

Answer 13

Na+ reabsorption

Question 14

ADH:

Answer 14

water reabsorption

Question 15

The renal tubule has a single layer of ___
but each region has a unique histology

Answer 15

epithelium

Question 16

The filtrate is modified through the processes of ___ and ____ which occur by transport mechanisms in the epithelial cells of the renal tubule.

Answer 16

reabsorption
secretion

Question 17

Proximal tubule histology:

Answer 17

simple cuboidal epithelium with microvilli

Question 18

Descending limb of nephron loop histology:

Answer 18

simple squamous epithelium

Question 19

Distal tubule histology:

Answer 19

simple cuboidal epithelium with few microvilli

Question 20

Thick ascending limb of nephron loop histology:

Answer 20

simple cuboidal epithelium with no microvilli

Question 21

Proximal Tubule Cells are Specialized for ___

Answer 21

Transport

Question 22

Microvilli

Answer 22

Increase surface area for absorption

Question 23

Mitochondrion

Answer 23

Provide the energy needed for the active transport processes

Question 24

The proximal convoluted tubule is the primary site for tubular reabsorption:

Answer 24

nearly ALL glucose, amino acids, and most electrolytes are reabsorbed in PCT, and about 65-70% of Na+ and water.

Question 25

Mechanisms of Tubular Reabsorption:

Answer 25

Transcellular Route and Paracellular Route

Question 26

Transcellular Route:

Answer 26

1) transport across the apical membrane 2) diffusion through the cytosol 3) transport across the basolateral membrane 4) movement through the interstitial fluid and into the capillary

Question 27

Paracellular Route:

Answer 27

1) movement through leaky tight junctions, particularly in the proximal convoluted tubule 2) movement through the interstitial fluid and into the capillary

Question 28

Transcellular transport mechanisms : review of passive transport processes

Answer 28

1) simple diffusion = fat-soluble molecules directly through phospholipid bilayer 2) osmosis = solvent like water through a specific channel protein or lipid bilayer 3) carrier-mediated facilitated diffusion = via a protein carrier (they are specific for a chemical)

Question 29

Transcellular transport mechanisms : Primary and secondary active transport

Answer 29

primary active transport = ATP-driven Na+K- pump stores energy by creating a steep concentration gradient for a Na+ entry into the cell secondary active transport = as Na+ diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell

Question 30

The Na-K ATPase pump moves _____ out of the cell and ____ into the cell for each ATP molecule that is hydrolyzed; this establishes a concentration gradient for Na+.

Answer 30

3 Na+ ions 2 K+ ions

Question 31

Other substances are co-transported with Na+ in secondary active transport :

Answer 31

glucose, amino acids, some ions, vitamins

Question 32

Sodium Potassium Pumps are on the ____

Answer 32

basal surface of the cuboidal cells of the PCT

Question 33

Secondary Active Transport Proteins are on the ____

Answer 33

apical surface of the cuboidal cells of the PCT

Question 34

The sodium gradient provides a driving force to move other substances

Answer 34

Sodium and glucose cotransport Sodium and amino acid cotransport Sodium and hydrogen ion antiport

Question 35

The majority of tubular reabsorption occurs in the _______

Answer 35

Proximal Convoluted Tubule (PCT) sodium, glucose, amino acids, ions, urea

Question 36

“Water follows solutes” =

Answer 36

“obligatory” water reabsorption by osmosis because aquaporins are always present in PCT cells

Question 37

_______ are specific and limited

Answer 37

Transport proteins

Question 38

Transport maximum

Answer 38

is # of transport proteins available in tubule cell membrane

Question 39

If the amount of a substance in the filtrate exceeds the number of transport proteins that are available, the transporters are _____

Answer 39

saturated

Question 40

When all the transport proteins are saturated for a given substance (ex. glucose), some of the substance _____

Answer 40

stays in the urine.

Question 41

Diabetes mellitus = ______ if transport maximum exceeded, glucose is excreted in the urine; water stays with glucose so increased urine volume = _____

Answer 41

high blood glucose diuresis

Question 42

Proximal Convoluted Tubule is always in osmotic equilibrium (isosmotic) with surrounding interstitial fluid and blood in ____

Answer 42

peritubular capillaries

Question 43

PCT CANNOT SEPARATELY ADJUST ___ AND WATER!

Answer 43

SALT

Question 44

Osmolarity at the end of the Proximal Convoluted Tubule

Answer 44

Osmolarity of blood entering kidney = 300 Filtrate is isosmotic with plasma at end of PCT : solutes and water have moved together Lost water on the way down so filtrate becomes more concentrated

Question 45

Osmolarity of the filtrate changes as it moves through the ____

Answer 45

nephron loop

Question 46

The kidneys have an osmotic gradient extending from the cortex through the depths of the _____

Answer 46

medulla

Question 47

The Renal cortex is ____ with arterial blood (~300 mOsm)

Answer 47

isosmotic

Question 48

An increasing osmotic gradient extends through the whole renal medulla creating the ______

Answer 48

medullary osmotic gradient

Question 49

The nephron loop dips into a ___ environment

Answer 49

salty the deeper it extends into the medulla, the saltier the interstitial space around the tubule

Question 50

The thick and thin limbs of the ______ function differently

Answer 50

nephron loop

Question 51

Simple squamous cells of thin descending limb:

Answer 51

no significant active transport

Question 52

Simple cuboidal cells of thick ascending limb:

Answer 52

active transport

Question 53

The Nephron Loop separates salt movement from _____

Answer 53

water movement

Question 54

Cells in the thin _____ have aquaporins that allow water to leave tubule by osmosis

Answer 54

descending limb

Question 55

Cells in the thick ______ have no aquaporins and are impermeable to water; they actively transport salts out of tubule into interstitial space

Answer 55

ascending limb

Question 56

In PCT-

Answer 56

most reabsorption occurs here and is unregulated

Question 57

Nephron loop -

Answer 57

only water leaves in descending limb only salt leaves in ascending limb

Question 58

______ creates an osmotic gradient through interactions of the two limbs of the nephron loop

Answer 58

COUNTERCURRENT MULTIPLIER

Question 59

The nephron loop acts as a _____

Answer 59

countercurrent multiplier

Question 60

Filtrate flows in the opposite direction in the two limbs of the nephron loop, = ______

Answer 60

countercurrent

Question 61

Tubule cells that line the descending limb of the nephron loop are (passively) permeable to water, but NOT to ____

Answer 61

salts

Question 62

Tubule cells that line the ascending limb of the nephron loop actively pump salts out of the tubule lumen into the interstitium, but:

Answer 62

WATER CANNOT LEAVE the ascending limb because these cells have no aquaporins

Question 63

The countercurrent multiplier depends on three properties of the nephron loop to establish the osmotic gradient:

Answer 63

1) filtrate flows in the opposite direction (countercurrent) through parallel sections of nephron loop 2) the descending limb is permeable to water, but not salt 3) the ascending limb is impermeable to water, but pumps out salts

Question 64

The countercurrent exchanger ensure the medullary gradient doesn’t get ______

Answer 64

washed away

Question 65

Vasa recta are very permeable to water and solutes:

Answer 65

So, blood within vasa recta remains nearly isosmotic with surrounding fluid, absorbing water and solutes without undoing osmotic gradient.

Question 66

Osmolarity at the start of the distal convoluted tubule :

Answer 66

At start of DCT, ~20% of filtered water ~10% of filtered Na+ remains in filtrate If we do no further adjustments to filtrate, we will make a finished urine that is more dilute than blood, and that carries wastes and some ions away

Question 67

The remainder of the tubule is where we ___ absorption and secretion to “fine tune” filtrate to the needs of the body

Answer 67

regulate

Question 68

At the DCT and Collecting Duct:

Answer 68

Make urine more dilute or more concentrated than blood Change ion concentrations further Adjust pH