Introduction to Renal Physiology (pH and Electrolytes)

Question 1

Describe the distribution of body fluids and the composition of intra-cellular and extra-cellular fluids

Answer 1

The body is about 60% water by weight, distributed in two major compartments:

A. Intracellular Fluid (ICF):

~2/3 of total body water.
Found inside cells.
Major ions:
Cations: K⁺, Mg²⁺
Anions: Proteins, PO₄³⁻

B. Extracellular Fluid (ECF):

~1/3 of total body water.
Divided into:
Interstitial fluid (~75% of ECF)
Plasma (~25% of ECF)
Also includes transcellular fluids (CSF, pleural, synovial fluid).
Major ions:
Cations: Na⁺
Anions: Cl⁻, HCO₃⁻

Question 2

Describe fluid balance and the role of the kidney in regulating blood osmolality and volume i.e. water and electrolyte (sodium and potassium) balance

Answer 2

Fluid Balance:

Maintained by balancing intake (thirst) and output (urine, sweat, feces).
Key regulators: hypothalamus, ADH, aldosterone, ANP.

Kidney’s Role:

A. Osmolality Regulation:
Osmolality = concentration of solutes (mainly Na⁺) in plasma.
Controlled by:
ADH (vasopressin):
Secreted by posterior pituitary in response to ↑ osmolality or ↓ volume.
Increases water reabsorption in collecting ducts via aquaporins.
Concentrates urine, dilutes plasma.

B. Volume & Electrolyte Balance:
Sodium (Na⁺):
Aldosterone (from adrenal cortex) increases Na⁺ reabsorption in distal tubule and collecting duct.
Retains water → increases blood volume and BP.

Potassium (K⁺):
Also regulated by aldosterone.
Aldosterone promotes K⁺ excretion in urine.

NICE CG174 (Acute Kidney Injury) emphasizes monitoring fluid status and electrolyte balance in hospitalized patients to prevent or manage AKI.

Question 3

Describe glomerular filtration rate, creatinine, and their significance in recognising kidney injury

Answer 3

Glomerular Filtration Rate (GFR):

Volume of fluid filtered from glomerular capillaries into Bowman’s capsule per minute.
Normal GFR ≈ 90–120 mL/min/1.73 m²
Declines with age, disease, and dehydration.

Creatinine:

Waste product of muscle metabolism.
Excreted by the kidneys; used as a marker of GFR.

eGFR:

Estimated GFR based on serum creatinine, age, sex, and ethnicity.

Recognising Kidney Injury:

NICE AKI Definition (CG174):

AKI is diagnosed by any of:
1. Serum creatinine rise ≥26 μmol/L within 48 hours.
2. Serum creatinine rise ≥1.5× baseline in 7 days.
3. Urine output <0.5 mL/kg/hour for >6 hours.

NICE CKD Guidelines (NG203):

1. eGFR <60 mL/min/1.73 m² for >3 months = chronic kidney disease.
2. ACR (albumin:creatinine ratio) also used to assess kidney damage.

Question 4

Describe the role of bone and kidneys in Vitamin D and Calcium metabolism.

Answer 4

A. Vitamin D Metabolism:

1. Cholecalciferol (D3) from skin or diet.
2. Hydroxylated in the liver → 25(OH)D (calcidiol).
3. Further hydroxylated in the kidney → 1,25(OH)₂D (calcitriol, the active form).

B. Calcium Homeostasis:

Regulated by:
Parathyroid Hormone (PTH)
Vitamin D
Calcitonin (minor role)

Bone:

Acts as a calcium reservoir.
PTH stimulates bone resorption to raise plasma Ca²⁺.

Kidneys:

1. Reabsorb calcium in renal tubules.
2. Activate vitamin D (essential for intestinal Ca²⁺ absorption).
3. Excrete phosphate (which complexes with calcium).

NICE NG34: In CKD patients, monitor and manage calcium and phosphate metabolism to reduce cardiovascular risk and bone mineral disease.