Renal Physiology

Overview – Renal Physiology

Renal physiology encompasses the complex and vital processes by which the kidneys regulate fluid balance, electrolytes, blood pressure, and acid-base homeostasis. The kidneys also play critical roles in erythropoiesis and vitamin D activation. An understanding of renal physiology is fundamental to interpreting renal function tests, understanding disease mechanisms, and managing common clinical conditions such as hypertension and acidosis.

Physiological Functions of the Kidney

• Fluid conservation
• Electrolyte balance (Na⁺, K⁺, PO₄³⁻, HCO₃⁻)
• Excretion of metabolic waste (urea, creatinine, bilirubin)
• Acid-base homeostasis (H⁺/HCO₃⁻ balance)
• Blood pressure regulation (via fluid volume & renin-angiotensin system)
• Erythropoiesis (via erythropoietin secretion)
• Vitamin D activation

Hormonal Regulation in Renal Physiology

• Renin
  • Secreted by juxtaglomerular cells in response to renal hypoperfusion
  • Stimulates conversion of angiotensinogen to angiotensin I
  • Also causes afferent arteriole vasodilation to improve kidney perfusion
• Angiotensin II
  • Produced in the lungs from angiotensin I
  • Causes systemic vasoconstriction → ↑BP
  • Constricts efferent arterioles → maintains GFR
  • Stimulates aldosterone secretion
• Aldosterone
  • Secreted by adrenal cortex in response to angiotensin II, hyperkalaemia, or hyponatraemia
  • Promotes Na⁺ reabsorption, K⁺ excretion, and water reabsorption
• Anti-Diuretic Hormone (ADH)
  • Released from posterior pituitary in response to increased plasma osmolality
  • Increases water reabsorption in the collecting ducts → ↑plasma volume, ↓urine output

Urine Production and Excretion

Step 1 – Glomerular Filtration

• Passive, non-selective filtration of blood plasma into Bowman’s capsule
• Occurs across 3 layers:
  • Capillary endothelium
  • Glomerular basement membrane
  • Podocytes (visceral layer of capsule)

• Filtrate resembles plasma, but is protein-free (if membrane intact)
• Filtration rate (GFR):
  • Depends on net hydrostatic pressure and colloid osmotic pressure
  • Also influenced by membrane permeability and filtration surface area
  • Kidneys filter ~180L/day from ~1L/min blood flow (≈25% of cardiac output)
  • Most of this filtrate is reabsorbed

• GFR Regulation:
  • ↓GFR via sympathetic NS, endothelin, adrenaline
  • ↑GFR via prostaglandins, nitric oxide, bradykinin
  • Angiotensin II maintains GFR by efferent arteriole constriction

• Autoregulation:
  • Myogenic: smooth muscle contracts when stretched
  • Metabolic: low oxygen or nutrients → vasodilation
  • Maintains stable GFR across varying arterial pressures

Step 2 – Tubular Reabsorption

• Selective reabsorption of substances into peritubular capillaries
• ~99% of filtrate is reabsorbed
• Routes:
  • Transcellular (through cells)
  • Paracellular (between cells)
• Passive processes (e.g. osmosis for water)
• Active transport:
  • Eg: Na⁺ via Na⁺/K⁺-ATPase
  • All transporters have maximum capacities (e.g. glucose spills into urine in diabetes)
• Example: Na⁺ reabsorption in ascending limb
  • Stepwise: Passive diffusion into tubule cell → Active transport to interstitium → Uptake by capillaries

Step 3 – Tubular Secretion

• Active transfer of substances from capillaries → tubules
• Important for:
  • Eliminating drugs (e.g. penicillin)
  • Removing poorly filtered substances
  • K⁺ and H⁺ secretion (acid-base regulation)
• Sites of secretion:
  • Proximal tubule: organic acids/bases
  • Distal tubule & collecting duct: K⁺, H⁺, toxins

Micturition Reflex (Urination)

• Reflex pathway: spinal-cord based
• Voluntary control develops by 2–3 years (via cortical inhibition)
• Reflex initiation:
  • Bladder fills → stretch receptors fire → brain interprets urge
  • Conscious relaxation of external sphincter → detrusor contracts → urine released
• Phases:
  • Storage phase: bladder relaxed, sphincters contracted
  • Voiding phase: bladder contracts, sphincters relax
• Clinical relevance:
  • Neurological damage → incontinence
  • Infants rely on involuntary pathway

Summary – Renal Physiology

Renal physiology includes the filtration of blood, reabsorption and secretion of solutes, and tightly regulated hormonal control of fluid and electrolyte homeostasis. The kidneys maintain acid-base balance, regulate blood pressure, and contribute to haematopoiesis and vitamin D activation. A clear understanding of renal physiology is critical to managing electrolyte disturbances and renal pathology. For a broader context, see our Renal Overview page.