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Module 104 - Nutrition, metabolism & excretion > T3 L4 Formation of urine > Flashcards

T3 L4 Formation of urine Flashcards

1
Q

What molecules are reabsorbed back into the blood in the proximal tubule?

A 
Water
Sodium
Potassium
Chloride
Hydrogen carbonate
Glucose
Proteins & amino acids
Urea
Calcium
Magnesium
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2
Q

What molecules are secreted into the urine at the proximal tubule?

A 
Organic anions & bases
Hydrogen
Ammonium
Urea
Creatinine
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3
Q

What happens at the Loop of Henle?

A

Tubular fluid is further modified here

Aim is to recover fluid & solutes from glomerular filtrate

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4
Q

What are the 2 stages of the process occurring at the loop of Henle?

A

1) Extraction of water in the thin descending limb

2) Extraction of Na+ & Cl- in the thick ascending limb

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5
Q

What type of nephrons have a longer loop of Henle?

A

The juxtamedullary nephrons

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6
Q

Describe the thin descending limb

A

Cells are flat
No active transport of salts
Freely permeable to water via aquaporin-1 channels
Some passive movement of water via tight junctions

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7
Q

Describe the thick ascending limb

A

Tubular wall is impermeable to water as there are no channels
Specialised Na+/K+/2Cl- cotransporters (NKCC2)
Na+, K+ & Cl- are reabsorbed

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8
Q

Describe the process in the loop of Henle

A

Fluid entering from the proximal tubule is isotonic
Water is reabsorbed out of the descending loop of henle
By the tip of the loop of Henle, the filtrate is hypertonic
Solutes are pumped out of the ascending limb
By the end of the Loop of Henle the filtrate that enters the distal tubule is hypotonic

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9
Q

What does the counter-current multiplication do?

A

Creates a large osmotic gradient within the medulla
Facilitated by Na+/K+/2Cl- transport in the ascending limb
Permits the passive reabsorption of water from the tubular fluid in the descending Loop of Henle

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10
Q

Describe the process of the counter-current mechanism

A

1) Fluid enters tubule
2) Active transport of Na+, K+, Cl- into medullary interstitial fluid
3) Water moves out of descending limb by osmosis
4) Isosmotic state in descending limb creates an osmotic difference between the descending & ascending limb
5) More fluid enters tubule by pushing higher osmolarity fluid through by bulk flow

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11
Q

What happens to urea in the Loop of Henle?

A

Urea is freely filtered at the glomerulus
Some reabsorption in the PT but LOH & DT are relatively impermeable
Urea can diffuse out of collecting duct into medulla down its concentration gradient
Adds to osmolarity of medullary interstitium

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12
Q

Where does the remainder of mOsm come from?

A

Active transport of NaCl contributes 600-1000mOsm

The rest comes from urea

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13
Q

What does the distal tubule do?

A

Performs further adjustment of urine

Active absorption & secretion of solutes occurs here

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14
Q

What happens to sodium & chloride ions in the distal tubule?

A

They are actively reabsorbed from the tubular fluid & exchanged for potassium or hydrogen ions which are secreted into tubular fluid

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15
Q

How are principal cells involved in the exchange of Na+ for K+?

A

Cells are sensitive to aldosterone
When aldosterone is released the cells reabsorb more Na+
More water gets reabsorbed
BP increases
Exchange forms post of renin-angiotensin-aldosterone system

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16
Q

What cells are involved in the exchange of Na+ for H+ in the distal tubule & collecting duct?

A

Intercalated cells

17
Q

What are the subtypes for intercalated cells?

A

Alpha intercalated cells

Beta intercalated cells

18
Q

Describe the pathway from angiotensinogen to increased sodium reabsorption by the distal tubule in exchange for K+

A

Angiotensinogen –> angiotensin I –> angiotensin II –> increased secretion of aldosterone –> increased sodium reabsorption by distal tubule in exchange for K+

19
Q

What is the main function of the collecting duct?

A

Final modification of urine

20
Q

What is ADH (antidiuretic hormone)?

A

Most important hormone that regulates water balance
Also known as vasopressin or 8-arginine vasopressin
Released from posterior pituitary subsequent to hypothalamic inputs
Plasma half life is 10-15 minutes

21
Q

What does ADH do?

A

Acts on vasopressin V2 receptors on basal membrane of principal cells in distal tubule & collecting duct –> activation of intracellular water channels (AQP2)

22
Q

What happens if there is the maximal amount of circulating ADH?

A

Occurs if severely dehydrated
Collecting duct becomes permeable to water due to maximal AQP2 insertion so water reabsorption occurs
Reabsorbs up to 66% of water entering collecting duct
Delivery of fluid to collecting duct is low
Urine concentration can be reduced to 300ml/day
Very concentrated urine

23
Q

What happens if there is no circulating ADH?

A

Reabsorption of water occurs at various sites in nephron
Collecting duct becomes impermeable to water as there is no AQP2 so a large volume of water is excreted
Lack of ADH - diabetes insipidus
Dilute urine

24
Q

How is diabetes insipidus treated?

A

Give synthetic ADH

25
Q

What are the 2 main types of diabetes insipidus?

A

Nephrogenic

Neurogenic

26
Q

What is nephrogenic diabetes insipidus?

A

Due to inability of kidney to respond normally to ADH

Treatment is chlortalidone (diuretic) & indomethacin (anti-inflammatory)

27
Q

What is neurogenic diabetes insipidus?

A

Due to a lack of ADH production by the brain

Treatment is desmopressin (ADH analogue), vasopressin, carbamazepine (anti-convulsive)

28
Q

What are some other types of diabetes insipidus?

A

Dipsogenic

Gestational

29
Q

What is SIADH?

A

Syndrome of inappropriate ADH
Excessive release of ADH - due to head injury, drug effects (ecstasy) etc
Causes hyponatraemia & possible fluid overload

30
Q

What is the treatment for SIADH?

A

V2 receptor blocker such as demeclocycline or tolvaptan

31
Q

Where is ADH synthesised?

A

Hypothalamus

32
Q

Where is ADH stored & released?

A

Posterior pituitary

33
Q

What agents increase ADH release

A 
Nicotine
Ether
Morphine
Barbiturates
Anti-diuretic action
34
Q

What agents inhibit ADH release?

A

Alcohol

Diuretic action

35
Q

What happens to all the water & solutes reabsorbed from the kidney?

A

Taken back into peritubular vessels & vasa recta surrounding tubule

Module 104 - Nutrition, metabolism & excretion (31 decks)

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