Reabsorption (salt, water and glucose) and Osmoregulation (7.6)

Question 1

Outline the reabsorption of Na⁺, Cl^-, water, glucose and urea in the PCT

Answer 1

Question 2

Define tubular reabsorption and secretion and give examples of substances transported by these means

Answer 2

Ultrafiltration: Nondiscriminatory, passive filtration of blood plasma

Tubular reabsorption: The movement of solutes into the capillaries from the nephron tubule

Passive: Chloride, urea, bicarbonate and water

Active: Glucose, amino acids and proteins (pinocytosis)

Tubular secretion: The movement of solutes into the nephron tubule from the capillaries, for secretion in the urine

Organic acids (PAH); H⁺; toxins; K⁺

Question 3

Describe the relative tubular contribution with respect to these processes

Answer 3

PCT 65 - 70 %

LoH 25 %

DCT 5 %

CD 1 - 2 % (hormally regulated)

Question 4

State features of reabsorption occuring at the PCT

Answer 4

Question 5

Explain, with reference to the terms tubular transport maximum, the rationale for glucosuria in uncontrolled diabetes mellitus

Answer 5

Increased plasma glucose levels seen in uncontrolled diabetes mellitus.

Tm for the glucose transporter (SGLT2) is exceded.

Excess glucose is therefore excreted, leading to high urine glucose content.

Question 6

Describe the major H₂O compartments and their relative volumes (in L) for an average healthy adult male.

Answer 6

Question 7

Define osmolarity and osmolality

Answer 7

Osmolarity: Number of solute particles in 1 L of solution (mmol/L)

Osmolality: Number of solute particles in 1 kg of solute (mOsmol/kg)

Question 8

Give examples of sources of water gain and loss for water balance and explain why osmoregulation is important for physiological function

Answer 8

Water gain: Fluid intake, food, metabolism

Water loss: Sweat, urine, via skin and lungs, faeces

Importance of osmoregulation for physiological function: As cellular membranes are freely permeable to water movements changes in osmotic potential cause cells to swell or shrink

Question 9

Describe the renal and hormonal mechanisms involved in H₂O homeostasis and ECF osmolality

Answer 9

CORRECTION: UT-A1 is present in the CD (NOT LoH). Allows for reabsorption of urea from the collecting/papillary duct

ADDITION: ANP and BNP act to increase excretion of H₂O. ANP is released in reponse to atrial stretch

Question 10

Describe the effect of thirst and other influential factors on body water regulation

Answer 10

CORRECTION: UT-A1 inserted into the apical membrane of CD

Question 11

Outline the pathological mechanisms:

• Diabetes insipidus
• SIADH

Answer 11

Question 12

Outline the effect of thirst

Diagram

Answer 12