Endocrine control of body fluid volume and composition

Question 1

How does the osmolarity of the tubular fluid leaving the loop of henle compare to that of the surrounding interstitial fluid?

Answer 1

Hypo-osmotic

Question 2

Where does the distal tubule drain to?

Answer 2

Collecting duct (cortical)

Question 3

How does the interstitial fluid change as the collecting duct progresses?

Answer 3

Increasing osmolarity (collecting duct drills down into medulla)

Question 4

Where in the nephron is ion and water balance mainly regulated?

Answer 4

Distal tubule and collecting duct

Question 5

Where does most reabsorption of ions occur?

Answer 5

Proximal tubule BUT the remaining ions are essential for salt balance

Question 6

What is the main mechanism of salt and water regulation?

Answer 6

Hormonal

Question 7

Where do the hormones acting on the kidney act?

Answer 7

Distal tubule and collecting duct

do NOT act on proximal tubule or loop of Henle

Question 8

How permeable is the distal tubule to water and urea? What effect does this have?

Answer 8

Not very (unless ADH levels are high)
Urea is concentrated in the tubular fluid which helps maintain the corticomedullary gradient

Question 9

Describe what happens in the early and late parts of the distal tubule with regard to ion absorption

Answer 9

Early
 - Sodium, chlorine and potassium triple co-transporter (i.e salt reabsorption)
Late
 - Calcium reabsorption
 - Hydrogen secretion
 - Sodium reabsorption
 - Potassium reabsorption

Question 10

Is it the early or late distal tubule that is stimulated by hormones?

Answer 10

Late

Question 11

Describe what happens in the early and late collecting duct with regard to ion absorption

Answer 11

Early
 - Sodium reabsorption
 - Potassium reabsorption
 - Hydrogen secretion
 - Calcium reabsorption 
Late 
 - Low ion permeability 
 - Permeability to water varies with respect to ADH

Question 12

Describe vasopressin secretion

Answer 12

Peptide hormone synthesised (supraoptic and paraventricular nuclei) in the hypothalamus >
Transported down nerves >
Stored in the posterior pituitary >
Released into blood in response to calcium dependent exocytosis (induced by action potentials)

Question 13

Peptide hormones have a long half life. T/F

Answer 13

False - short half life

Question 14

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

Answer 14

ADH binds to type 2 vasopressin receptors on the basolateral membranes of tubular cells >
Increase in intracellular cyclic AMP >
Increased expression of aquaporins (water channels) at the apical membrane >
Increased permeability to water

Question 15

How is hypertonic urine formed?

Answer 15

In the presence of high ADH water moves from the collecting duct to the interstitial fluid along the osmotic gradient

Question 16

How is hypotonic urine formed?

Answer 16

In the presence of low ADH water cannot leave the collecting duct and so is retained in the urine

Question 17

Tubular fluid equilibrates with interstitial fluid via aquaporins under which condition?

Answer 17

High ADH

Question 18

The collecting duct is impermeable to water so none is reabsorbed under which condition?

Answer 18

Low ADH

Question 19

ADH affects salt and water reabsorption. T/F

Answer 19

False - ADH only has a direct effect on water reabsorption

Question 20

Describe the ADH hormone axis

Answer 20

Increase in plasma osmolarity >
Hypothalamic osmoreceptors >
Increase in thirst and ADH secretion >
ADH causes vasoconstriction + increased distal tubular and collecting duct permeability to water >
Decreased urine output >
Increased plasma volume and decreased plasma osmolarity

Question 21

When are left atrial volume receptors stimulated to produce ADH?

Answer 21

When there is a massive drop in ECF thus blood pressure and eventually decreased atrial stretch

Question 22

How can diabetes insipidus be classified?

Answer 22

Central

Nephrogenic

Question 23

Is diabetes insipidus hereditary?

Answer 23

Usually

Question 24

What are the symptoms of diabetes insipidus?

Answer 24

Large volumes of dilute urine

Constant thirst

Question 25

How is diabetes insipidus managed?

Answer 25

ADH replacement

Question 26

What medication (long term) induces diabetes insipidus?

Answer 26

Lithium as is used in bipolar disorder

Question 27

What controls feedforward ADH secretion?

Answer 27

Stretch receptors in the GI tract

Question 28

What effect do nicotine (i.e smoking) and alcohol have on ADH secretion?

Answer 28

Nicotine - increase ADH Alcohol - decrease ADH Nb - ecstasy also decreases ADH and thus poses a massive dehydration risk

Question 29

How does the tubular osmolarity change across the length of the nephron

Answer 29

Proximal tubule - low Loop of Henle - high then low Distal tubule - low (may rise if increased ADH) Collecting duct - low (may rise if increased ADH)

Question 30

What type of hormone is aldosterone and where is it secreted form?

Answer 30

Steroid | Adrenal cortex

Question 31

What mnemonics can be used to remember the hormones secreted from the adrenal gland?

Answer 31

Get my = glomerulosa - mineralocorticoids (aldosterone) Freaking gun = fasiculata - glucocorticoids (cortisol) Right away = reticularis - androgens (sex hormones) Magic = medulla - adrenaline, etc (magic rush)

Question 32

When is aldosterone secreted?

Answer 32

Directly in response to rising potassium Indirectly in response to falling sodium Indirectly as part of the renin-angiotensin-aldosterone system

Question 33

What effect does aldosterone have?

Answer 33

Increases sodium reabsorption Increases potassium secretion Nb - remember water follows salt so inc sodium helps to increased plasma volume

Question 34

What happens if a patient is not producing aldosterone?

Answer 34

Eventually leads to death due to low fluid volumes

Question 35

Where is potassium reabsorbed? How much potassium is excreted in the urine?

Answer 35

90% within the proximal tubule and 10% within the distal tubule. None

Question 36

How is potassium reabsorption affected by aldosterone?

Answer 36

Increase in potassium is detected by the adrenal cortex > Aldosterone release > Increased secretion of potassium

Question 37

Decreasing sodium indirectly stimulates aldosterone release how?

Answer 37

Lowered sodium levels detected within the juxtaglomerular apparatus stimulates the renin-angiotensin-aldosterone system

Question 38

Explain RAAS

Answer 38

Lowered ECF, sodium and BP > Kidneys detect and secrete renin > Renin acts on angiotensiogen (secreted by the liver) to covert it to angiotensin I > Angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE) in the lungs > Angiotensin II acts on the adrenal cortex to stimulate aldosterone > Aldosterone acts on the liver to decrease potassium and increase sodium reabsorption > Increased salt causing increased water retention and thus fluid repletion

Question 39

What does angiotensin II do?

Answer 39

Increases thirst Stimulates aldosterone release Increases ADH Causes arteriolar vasoconstriction

Question 40

How does the juxtaglomerular apparatus effect the RAAS?

Answer 40

Granular cells in the juxtaglomerular apparatus release renin in response to - decreased afferent blood pressure - decreased salt concentration - direct sympathetic stimulation

Question 41

How does aldosterone increase sodium reabsorption at the distal tubule and collecting duct?

Answer 41

Promotes increased expression of apical sodium channels | Increases number and activity of basolateral sodium potassium ATPases

Question 42

Abnormal activity of the RAAS causes what?

Answer 42

Hypertension

Question 43

What is the cause of fluid retention in congestive heart failure?

Answer 43

RAAS

Question 44

Where is atrial natriuretic peptide/hormone produced and stored?

Answer 44

Left atrium produces and atrial muscle cells store

Question 45

What is atrial natriuretic peptide released?

Answer 45

In response to mechanical stretch of the left atrium due to increased circulating plasma volume

Question 46

What does atrial natriuretic peptide do?

Answer 46

Increases excretion of sodium hence diuresis and thus reduction in circulating plasma volume Decreases activity of RAAS Arteriolar vasodilation Decreases sympathetic stimulation

Question 47

Which two mechanisms control micturation?

Answer 47

Micturation reflex | Voluntary control

Question 48

What initiates the micturation reflex?

Answer 48

Stretch receptors in the bladder wall

Question 49

What is the micturation reflex?

Answer 49

Involuntary contraction of the detrusor and relaxation of the internal urethral sphincter

Question 50

How can micturation be stopped by voluntary control?

Answer 50

Central pathways cause the relaxation of the detrusor and contraction of internal Voluntary contraction of external sphincter and levator ani

Question 51

What is the difference between water diuresis and osmotic diuresis?

Answer 51

Water - increased urine flow but no increased salt excretion Osmotic - increase in urine flow is a result of primary increase in salt excretion