Water and Electrolytes

Question 1

How is water “gained”?

Answer 1

Ingested (food and drink)

Water formed in metabolism

Question 2

How is water “lost”?

Answer 2

Excretion: urine, faeces
Evaporation: sweat, in expired air

Question 3

How much water is gained and lost in a day?

Answer 3

2,400ml

Question 4

What is a nephron?

Answer 4

The basic unit of the kidney

Question 5

What does the glomerulus do?

Answer 5

Filtration of plasma

Question 6

What is the renal blood flow per minute?

Answer 6

1200ml/min

Question 7

What is the “normal” glomerular filtration rate (GFR)?

Answer 7

120ml/min

Question 8

What creates the driving force for filtration?

Answer 8

Pressure

Question 9

What does the proximal convoluted tubule (PCT) do?

Answer 9

Reabsorption of 60-70% of the glomerular filtrate

Question 10

What is reabsorbed by the proximal convoluted tubule?

Answer 10

Ions

Small organic molecules

Question 11

What does the Loop of Henle do?

Answer 11

Important for concentration of urine - countercurrent exchange mechanism

Question 12

What does the distal convoluted tubule (DCT) do?

Answer 12

Reabsorption of water, sodium, chloride and calcium

Secretion of hydrogen and potassium

Question 13

Which hormones is the activity of the distal convoluted tubule controlled by?

Answer 13

Aldosterone, Atrial Natriuretic Hormone (ANH), Antidiuretic Hormone (ADH), Parathyroid hormone

Question 14

What do the collecting ducts do?

Answer 14

Water reabsorption, under the influence of ADH

ADH inserts aquaporins for water reabsorption

Question 15

What is ADH also known as?

Answer 15

Vasopressin

Question 16

Where is ADH produced?

Answer 16

The hypothalamus

Question 17

Where is ADH released from?

Answer 17

The posterior pituitary gland

Question 18

What does ADH act on?

Answer 18

The distal collecting tubules and collecting ducts to increase water permeability by insertion of aquaporins

Question 19

When is ADH secretion increased?

Answer 19

Decreased ECF volume (low pressure receptors in atria and great veins)
Increased ECF osmolarity (osmoreceptors in the hypothalamus)

Question 20

What effect does ADH have on blood vessels?

Answer 20

ADH causes vasoconstriction

Question 21

What happens if there is increased protein content in the blood?

Answer 21

Increased plasma osmolarity -> ADH is released. Water will be reabsorbed.

Question 22

How does RAAS regulate Na+ reabsorption?

Answer 22

Promotes reabsorption of Na+ in the DCT

Na+ is exchanged for K+ and H+

Question 23

What effect does ANH (ANP) have on Na+/

Answer 23

ANH increases the secretion of Na+

ANH has opposing effects to aldosterone

Question 24

What does renin do?

Answer 24

Converts angiotensinogen to angiotensin I

Question 25

What does angiotensin-converting enzyme (ACE) do?

Answer 25

Converts angiotensin I to angiotensin II

Question 26

What happens when angiotensin II acts on the adrenal cortex?

Answer 26

Adrenal cortex releases aldosterone

Question 27

What effect does aldosterone have on Na+?

Answer 27

Causes Na+ reabsorption in the cortical collecting ducts

Question 28

Disorders of ADH secretion can lead to what?

Answer 28

Hyposecretion of ADH: Diabetes insipidus
Large volumes of insipid urine produced

Hypersecretion of ADH: Syndrome of inappropriate ADH (SIADH)
Excess water retention

Question 29

What are the effects of increasing ECF K+ concentration on nerve function?

Answer 29

Increasing ECF [K+] -> depolarisation
Depolarisation causes axons to fire action potentials
But the membrane potential does not return to ‘resting’ levels
The result is that the axon remains in an extended refractory period

Question 30

What is the purpose of K+ salts in toothpaste?

Answer 30

Axons remains in extended refractory period - cannot fire more action potentials
Therefore, K+ used to desensitise ‘hypersensitive’ dentine