Endocrine Control of Body Fluid Volume and Composition

Question 1

What property does the tubular fluid leaving the loop of Henle have in relation to the plasma?

Answer 1

It is hypo-osmotic to the plasma = 100 msomol/l

Question 2

What is the osmolarity of the interstitial fluid surrounding the renal cortex?

Answer 2

300 mosmol/l

Question 3

Where does the distal tubule empty into?

Answer 3

The collecting ducts

Question 4

What are the collecting ducts bathed in as they descend though the medulla?

Answer 4

Progressively increasing concentrations of surrounding interstitial fluid (300-1200 mosmol/l)

Question 5

What are the major sites for the regulation of ion and water balance?

Answer 5

The distal tubule and collecting duct

Question 6

Where do all the tubules of the kidney empty into?

Answer 6

The cortical collecting ducts

Question 7

What happens to filtered ion loads before they reach the distal tubule?

Answer 7

> 95% are reabsorbed before the filtrate reaches the distal tubule = residual load is very important for salt balance

Question 8

What mainly affects the fluid and NaCl regulation?

Answer 8

Hormones

Question 9

What are the effects of some hormones on the kidneys?

Answer 9

ADH = increases water reabsorption
Aldosterone = increases Na+ reabsorption and H+/K+ secretion
ANP = decreases Na+ reabsorption
PTH = increases Ca2+ reabsorption and decreases PO4- reabsorption

Question 10

What does the distal tubule have low permeability to?

Answer 10

Urea and water = urea is concentrated in the tubular fluid

Question 11

What does the concentration of urea in the distal tubule help to establish?

Answer 11

Osmotic gradient within the medulla

Question 12

What are the two segments of the distal tubule?

Answer 12

Early = Na+-K+-2Cl- transporter (NaCl reabsorption)

Late

Question 13

What are the functions of the late segment of the distal tubule?

Answer 13

Ca2+, Na+ and K+ reabsorption, and H+ secretion in the basal state
K+ secretion when K+ secretory cells are activated by aldosterone

Question 14

What are the two segments of the collecting duct?

Answer 14

Early = similar function to late distal tubule
Late = low ion permeability, permeability to water and urea influenced by ADH

Question 15

What is the first step in ADH secretion?

Answer 15

Octapeptide synthesised by the supraoptic and paraventricular nuclei in the hypothalamus

Question 16

Where is ADH transported from once it leaves the hypothalamus?

Answer 16

Transported down nerves to terminals where it is stored in the granules in the posterior pituitary

Question 17

How is ADH released?

Answer 17

Released into blood when action potentials down the nerves lead to Ca2+ dependent exocytosis

Question 18

What is the plasma half life of ADH?

Answer 18

10-15 minutes

Question 19

How does ADH increase permeability of the luminal membrane of the collecting duct to water?

Answer 19

By inserting new aquaporins

Question 20

What happens in the presence of maximal plasma ADH concentration?

Answer 20

Water moves from the collecting duct lumen along the osmotic gradient into the medullary interstitial fluid = enables hypertonic urine formation

Question 21

What happens if there is high ADH concentration?

Answer 21

High water permeability in the collecting duct = hypertonic urine (up to 1400 mosmol/l)

Question 22

What happens if there is low ADH concentration?

Answer 22

Low water permeability in the collecting duct = hypotonic urine (<50 mosmol/l)

Question 23

What happens to urine production in the presence of maximal ADH plasma concentration?

Answer 23

Small volumes of concentrated urine = tubular fluid equilibrates with interstitium via aquaporins

Question 24

What happens to urine production in the presence of minimal ADH plasma concentration?

Answer 24

Large volumes of dilute urine = collecting duct is impermeant to water so no water reabsorption

Question 25

What is the most important stimulus for ADH release?

Answer 25

Hypothalamic osmoreceptors

Question 26

What is the accessory pathway for ADH release?

Answer 26

Activation of left atrial stretch receptors

Question 27

What does decreased atrial pressure cause?

Answer 27

Increased ADH release = need large changes in plasma volume

Question 28

What effect does nicotine have on ADH?

Answer 28

Stimulates its release

Question 29

What effects does alcohol have on ADH?

Answer 29

Inhibits its release

Question 30

What effect does stimulation of stretch receptors in the upper GI tract have on ADH?

Answer 30

Exerts a feed-forward inhibition

Question 31

How is diabetes insipidus classed?

Answer 31

Central or nephrogenic (usually hereditary)

Question 32

What are the symptoms of diabetes insipidus?

Answer 32

Large volumes of dilute urine (up to 20l per day) | Constant thirst

Question 33

How is diabetes insipidus treated?

Answer 33

ADH replacement

Question 34

What is aldosterone?

Answer 34

Steroid hormone secreted by the adrenal cortex

Question 35

What is aldosterone secreted in response to?

Answer 35

Rising K+ concentration or falling Na+ concentration in the blood, or activation of the renin-angiotensin system

Question 36

What does aldosterone stimulate?

Answer 36

Na+ reabsorption and K+ secretion = causes Na+ retention which increases blood volume and pressure

Question 37

What normally happens to K+ in the nephron?

Answer 37

Normally 90% is reabsorbed in the early regions of the nephron (mainly the proximal tubule)

Question 38

What happens to the K+ not reabsorbed in the nephron when aldosterone is absent?

Answer 38

Reabsorbed in the distal tubule = no K+ is excreted in the urine

Question 39

What effect does an increase in plasma K+ concentration have?

Answer 39

Directly stimulates the adrenal cortex = aldosterone stimulates the secretion of K+

Question 40

What effect does a decrease in plasma Na+ concentration have?

Answer 40

Promotes indirect stimulation of aldosterone by means of the juxtaglomerular apparatus = aldosterone increases Na+ reabsorption in the distal and collecting tubules

Question 41

Where is renin released from?

Answer 41

Granular cells in the juxtaglomerular apparatus

Question 42

What does reduced pressure in the afferent arteriole cause?

Answer 42

More renin is released = more Na+ reabsorbed so blood volume increased and blood pressure restored

Question 43

What cells sense with amount of NaCl in the distal tubule?

Answer 43

Macula densa cells = if NaCl reduced more renin is released and more Na+ is reabsorbed

Question 44

What effect does increased sympathetic activity as a result of reduced arterial BP have?

Answer 44

Granular cells are directly innervated by the sympathetic nervous system so release more renin

Question 45

What are some conditions linked with abnormal RAA system?

Answer 45

Hypertension = abnormal increase in RAA system | Congestive heart failure = fluid retention due to low BP caused by reduced cardiac output

Question 46

How is congestive heart failure treated?

Answer 46

Low salt diet and loop diuretics

Question 47

What are the actions of ACE inhibitors?

Answer 47

Stop fluid and salt retention | Stop arteriolar constriction

Question 48

Where is atrial natriuretic peptide (ANP) produced?

Answer 48

Produced by heart and stored in atrial muscle cells

Question 49

When is ANP released?

Answer 49

When atrial muscle cells are stretched due to an increase in the circulating plasma volume

Question 50

What does ANP promote?

Answer 50

Excretion of Na+ and diuresis = reduces plasma volume | Also exerts effect on CV system to lower blood pressure

Question 51

Where is urine temporary stored?

Answer 51

In the bladder = emptied by micturition

Question 52

What happens to urine once it has been formed by the kidneys?

Answer 52

It is propelled by peristaltic contractions through the ureters to the bladder for temporary storage

Question 53

What controls micturition?

Answer 53

Micturition reflex and voluntary control

Question 54

How much urine can the bladder hold before stretch receptors in its walls starts the micturition reflex?

Answer 54

250-400ml

Question 55

What occurs in the micturition reflex?

Answer 55

Involuntary emptying of the bladder by simultaneous bladder contraction and opening of the internal and external urethral sphincters

Question 56

How can micturition be voluntarily prevented?

Answer 56

By deliberate tightening of the external sphincter and surrounding pelvic diaphragm

Question 57

What is water excess/deficit a response to?

Answer 57

Changes in ECF osmolarity

Question 58

How is ECF osmolarity monitored?

Answer 58

Hypothalamic osmoreceptors detect and initiate the priority mechanism for regulation of ECF osmolarity = signal to ADH and thirst

Question 59

How able are renal mechanisms to cope with water excess/deficit?

Answer 59

Renal mechanisms suffice during water excess, but during deficit it is necessary to increase intake

Question 60

What is salt excess/deficit a response to?

Answer 60

Changes in ECF volume (e.g salt deficit due to haemorrhage)

Question 61

What occurs in water diuresis?

Answer 61

Increased urine flow but not an increased solute excretion

Question 62

What occurs in osmotic diuresis?

Answer 62

Increased urine flow is as a result of a primary increase in salt excretion

Question 63

What are the rules around diuresis?

Answer 63

Any loss of solute in the urine must be accompanied by water loss (osmotic diuresis), but the reverse isn't true (water diuresis isn't necessarily accompanied by equivalent solute loss)