M3: Urinary/Renal System

Urinary System: Function

• The urinary system maintains internal body equilibrium and eliminates metabolic waste products.

• Waste Elimination: Filters and removes nitrogenous wastes (e.g. urea, creatinine) generated by cellular metabolism to prevent toxic build-up.

• Fluid & Electrolyte Balance: Regulates body water volume and essential electrolytes (Na+, K+, Ca2+) vital for nerve signal conduction and muscle contraction.

Renal Hormones

• Erythropoietin: Stimulates red blood cell production within the bone marrow.

• Renin: Enzyme secreted by the kidneys that plays a central role in controlling blood pressure.

• Active Vitamin D (Calcitriol): Activated by the kidneys to enable intestinal calcium absorption, supporting bone health and cellular processes.

Urinary System Anatomy

• Composed of four main structures: paired kidneys, paired ureters, a single urinary bladder, and a single urethra.

• Kidney Location: Paired bean-shaped organs positioned retroperitoneally (behind the abdominal peritoneal lining) on either side of the spine below the diaphragm.

• Asymmetry: The right kidney sits slightly lower than the left to accommodate the space occupied by the liver.

• Protective Layers: Enclosed by three distinct tissue layers: innermost renal capsule, middle adipose capsule, and outermost renal fascia.

Kidney Structure + Urine Flow

• Outer Renal Cortex: Structural zone extending from the outer renal capsule to the bases of the renal pyramids.

• Inner Renal Medulla: Contains cone-shaped renal pyramids separated by intervening renal columns.

• Urine Transport Pathway: Collecting ducts in renal pyramids -> Minor calyx -> Major calyx -> Renal pelvis -> Ureter -> Urinary bladder -> Urethra.

Ureters, Bladder & Urethra

• Ureters: Muscular tubes connecting each kidney to the bladder; move urine via peristaltic contractions to prevent backflow into the kidneys.

• Urinary Bladder: Hollow pelvic organ storing urine; stretch activation triggers the micturition reflex to initiate urination.

• Urethra: Terminal duct for urine excretion; shorter in females (solely urinary function) and longer in males (serves both urinary and reproductive functions).

Glomerular Filtration

• Filtering blood plasma from glomerular capillaries into the glomerular capsule

Tubular Reabsorption:

Moving essential water and solutes from tubular filtrate back into blood.

Tubular Secretion:

Transferring waste products and excess ions from blood into tubular filtrate for excretion.

Nephron: Structure

• Nephron: Microscopic functional unit of the kidney responsible for filtering blood and forming urine.

Histology of Nephron Components

• Renal Corpuscle: Consists of the glomerulus (capillary network) surrounded by Bowman's capsule (double-sided cup) to filter blood plasma into glomerular filtrate.

• Proximal Convoluted Tubule (PCT): Lined with simple cuboidal epithelium featuring a prominent apical microvilli brush border.

• Loop of Henle:

• Juxtaglomerular Apparatus (JGA): Specialized region where the final portion of the thick ascending limb contacts the afferent arteriole.

• Distal Convoluted Tubule (DCT):

Renin-Angiotensin-Aldosterone System (RAAS)

• Renin Enzyme: Synthesised and secreted by specialised juxtaglomerular (JG) cells within the juxtaglomerular apparatus of the kidneys.

• Secretion Triggers: Released into the bloodstream when the body senses a drop in blood pressure or reduced renal blood flow.

• RAAS Cascade: Renin acts on liver-derived angiotensinogen -> converts it into Angiotensin I -> Angiotensin-Converting Enzyme (ACE) converts Angiotensin I into Angiotensin II.

RAAS: Renal Actions of Angiotensin II

• Vasoconstriction: Acts as a potent systemic vasoconstrictor -> causes blood vessels to constrict -> increases peripheral resistance -> raises blood pressure.

• GFR Reduction: Constricts afferent arterioles -> reduces Glomerular Filtration Rate (GFR) -> conserves water and salts.

• Aldosterone Secretion: Triggers the release of aldosterone from the adrenal cortex.

RAAS: Aldosterone

Secretory Triggers & Cellular Action

• Triggers: Secreted by the adrenal cortex in response to Angiotensin II or elevated plasma potassium (K+) concentrations.

• Mechanism: Stimulates insertion of sodium (Na+) channels and Na+/K+ pumps into cells of the distal convoluted tubules (DCT) and collecting ducts.

Physiological Actions

• Sodium & Water Retention: Increases active reabsorption of Na+ -> drives passive reabsorption of water -> expands blood volume and supports blood pressure.

• Potassium Excretion: Promotes secretion of excess K+ into collecting ducts for urinary excretion.

RAAS: Erythropoietin (EPO) + RBC Production

Synthesis & Triggers

• Production Site: Synthesised by peritubular interstitial cells located in the kidneys.

• Secretion Trigger: Released into the bloodstream in response to decreased blood oxygen levels (e.g. high altitude or anaemia).

Actions on Erythropoiesis

• Target Site: Stimulates red blood cell (erythrocyte) production within the bone marrow.

• Physiological Effect: Increases the blood's oxygen-carrying capacity -> ensures adequate oxygen delivery to sustain cellular function.

RAAS: Vit D (Calcitriol)

Activation Pathway

• The kidneys activate vitamin D by converting inactive calcidiol into its active form, calcitriol.

Physiological Roles

• Intestinal Reabsorption: Enhances intestinal calcium reabsorption into the blood by promoting calcium-binding protein synthesis.

• Mineral Regulation & Bone Health: Regulates plasma calcium and phosphate levels for bone tissue mineralization, working alongside parathyroid hormone (PTH).

Urethra: Male vs. Female

• Urethra: Terminal duct connecting the urinary bladder to the exterior of the body for urine discharge.

• Female Urethra:

  • Shorter (~4 cm long), positioned posterior to the pubic symphysis.

  • Serves an exclusively urinary function.

  • Mucosal lining transitions from transitional epithelium (near bladder) -> non-keratinised stratified squamous epithelium (near external orifice).

• Male Urethra:

  • Longer (~20 cm long), serving dual urinary and reproductive functions (discharges urine and semen).

  • Divided into 3 anatomical regions: prostatic urethra (passes through prostate), membranous urethra (passes through deep perineal muscles), and spongy urethra (extends through penis).