The Urinary System and Fluid, Electrolyte, and Acid-Base Homeostasis

The Urinary System

Organs of the Urinary System

• Kidneys: Located retroperitoneally on either side of the vertebral column, between T12 and L3 vertebrae. They filter blood and produce urine.

• Ureters: Muscular tubes that transport urine from the kidneys to the urinary bladder.

• Urinary Bladder: A hollow, muscular organ in the pelvic cavity that stores urine until micturition.

• Urethra: A tube that conveys urine from the bladder to the exterior of the body.

Functions of the Kidneys

• Excretion of metabolic wastes (e.g., urea, creatinine, uric acid)

• Regulation of blood volume and pressure

• Regulation of electrolyte balance

• Regulation of acid-base balance

• Regulation of erythropoiesis (via erythropoietin secretion)

• Detoxification of blood

External Structure of the Kidney

• Location: Posterior abdominal wall, right kidney slightly lower due to liver.

• Support Structures: Renal fascia, perirenal fat capsule, and fibrous capsule.

• Coverings: Three layers protect and anchor the kidney.

The Nephron: The Basic Unit of Filtration

• Definition: The nephron is the microscopic functional unit of the kidney responsible for filtering blood and forming urine.

• Components: Renal corpuscle (glomerulus + Bowman's capsule) and renal tubule (proximal convoluted tubule, nephron loop, distal convoluted tubule).

• Significance: Each kidney contains about 1 million nephrons.

Major Structures and Subdivisions of the Kidney

• Macroscopic: Cortex, medulla (with renal pyramids), renal pelvis.

• Microscopic: Nephrons, collecting ducts, blood vessels.

Blood Flow Through the Kidneys

• Arterial Supply (largest to smallest): Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arterioles.

• Capillary Beds: Glomerulus (filtration), peritubular capillaries (reabsorption/secretion); juxtamedullary nephrons also have vasa recta.

• Venous Drainage (smallest to largest): Cortical radiate veins → arcuate veins → interlobar veins → renal vein.

Path of Filtrate and Urine Formation

• Filtrate Pathway: Glomerular capsule → proximal convoluted tubule → nephron loop (descending and ascending limbs) → distal convoluted tubule → collecting duct → papillary duct.

• Urine Pathway: Papillary duct → minor calyx → major calyx → renal pelvis → ureter → urinary bladder → urethra.

Types of Nephrons

• Cortical Nephrons: Located mostly in the cortex; short nephron loops; majority of nephrons.

• Juxtamedullary Nephrons: Long nephron loops extend deep into the medulla; important for concentrating urine.

Filtrate, Tubular Fluid, and Urine

• Filtrate: Fluid filtered from blood in the glomerulus.

• Tubular Fluid: Filtrate as it passes through the renal tubule, undergoing reabsorption and secretion.

• Urine: Final product after all modifications, ready for excretion.

Processes in Urine Formation

• Glomerular Filtration: Movement of plasma from glomerulus into Bowman's capsule.

• Tubular Reabsorption: Movement of substances from tubular fluid back into blood.

• Tubular Secretion: Movement of substances from blood into tubular fluid.

Filtration Membrane Structure

• Composed of fenestrated endothelium, basement membrane, and podocyte filtration slits.

Glomerular Filtration Rate (GFR)

• Definition: Volume of filtrate formed per minute by both kidneys.

• Average Value: About 125 mL/min in adults.

Net Filtration Pressure (NFP)

• Determined by hydrostatic and colloid osmotic pressures:

$NFP = (GHP) - (CHP + GCOP)$

• Where GHP = glomerular hydrostatic pressure, CHP = capsular hydrostatic pressure, GCOP = glomerular colloid osmotic pressure.

Regulation of GFR

• Myogenic Mechanism: Smooth muscle response to changes in pressure.

• Tubuloglomerular Feedback: Macula densa senses NaCl; adjusts afferent arteriole diameter.

• Renin-Angiotensin-Aldosterone System (RAAS): Increases GFR via vasoconstriction and aldosterone release.

• Atrial Natriuretic Peptide (ANP): Increases GFR by dilating afferent arteriole.

• Sympathetic Nervous System: Decreases GFR during stress.

Tubular Reabsorption and Secretion

• Reabsorption: Water, glucose, amino acids, and ions reabsorbed in the proximal tubule (active and passive transport).

• Secretion: Occurs mainly in the proximal and distal convoluted tubules; removes drugs, H+, K+ from blood.

Hormonal Regulation of Reabsorption and Secretion

• Renin-Angiotensin-Aldosterone System: Increases Na+ and water reabsorption.

• Antidiuretic Hormone (ADH): Increases water reabsorption in collecting ducts.

• Atrial Natriuretic Peptide (ANP): Inhibits Na+ reabsorption, increasing urine output.

Regulation of Urine Concentration and Volume

• Countercurrent Mechanism: Involves nephron loop and vasa recta; creates osmotic gradient for water reabsorption.

• Concentrated Urine: Produced when ADH is present; water reabsorbed from collecting ducts.

• Dilute Urine: Produced when ADH is absent; less water reabsorbed.

Physical and Chemical Properties of Urine

• Normal urine: Clear, pale yellow, slightly acidic (pH ~6), specific gravity 1.003–1.030.

• Composition: Water, urea, creatinine, ions, and other wastes.

Renal Clearance and GFR Measurement

• Renal clearance rate of a substance can be used to estimate GFR.

$C = \frac{U \times V}{P}$

• Where C = clearance, U = concentration in urine, V = urine flow rate, P = concentration in plasma.

Urine Transport, Storage, and Elimination

• Ureters: Muscular tubes with peristaltic contractions move urine to bladder.

• Urinary Bladder: Transitional epithelium allows stretching; detrusor muscle contracts during micturition.

• Urethra: Shorter in females; longer and divided into regions in males.

• Micturition Reflex: Stretch receptors trigger bladder contraction and relaxation of internal urethral sphincter.

• Voluntary Control: External urethral sphincter under conscious control.

Fluid, Electrolyte, and Acid-Base Homeostasis

Fluid Homeostasis

• Fluid Compartments: Intracellular fluid (ICF, ~2/3 of body water) and extracellular fluid (ECF, ~1/3; includes plasma and interstitial fluid).

• Major Electrolyte Concentrations: Na+ and Cl- high in ECF; K+ and phosphate high in ICF.

• Osmotic Pressure: Generated by solute concentration differences; drives water movement between compartments.

• Hydrostatic Pressure: Pushes water out of compartments; opposed by osmotic pressure.

• Water Gain: Ingestion, metabolic water production.

• Water Loss: Urine, sweat, feces, respiration.

• Regulation: Thirst mechanism, ADH, aldosterone, and ANP control intake and output.

• Dehydration: Water loss exceeds intake; cells shrink.

• Overhydration: Excess water intake; cells swell.

Electrolyte Homeostasis

• Major Electrolytes: Sodium (Na+), potassium (K+), calcium (Ca2+), chloride (Cl-), phosphate (PO43-).

• Hormonal Regulation: Aldosterone (increases Na+ reabsorption), ANP (decreases Na+ reabsorption), parathyroid hormone (PTH) and calcitonin (regulate Ca2+ and phosphate).

• Calcium and Phosphate: Inversely regulated; PTH increases Ca2+ and decreases phosphate reabsorption.

Acid-Base Homeostasis

• Blood pH: Normal range 7.35–7.45; maintained by buffer systems, respiratory, and renal mechanisms.

• Buffer Systems: Bicarbonate, phosphate, and protein buffers stabilize pH.

• Relationship of PCO2 and HCO3- to pH:

$CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$

• Increased CO2 lowers pH (acidosis); decreased CO2 raises pH (alkalosis).

• Respiratory Regulation: Hypoventilation increases CO2 (acidosis); hyperventilation decreases CO2 (alkalosis).

• Renal Regulation: Kidneys secrete H+ and reabsorb HCO3- to adjust pH.

• Compensation: Respiratory or renal adjustments correct acid-base imbalances (e.g., metabolic acidosis compensated by hyperventilation).

Integration of Homeostatic Mechanisms

• Cardiovascular System: Adjusts blood pressure and volume to maintain fluid balance.

• Endocrine System: Hormones (ADH, aldosterone, ANP, PTH) regulate water, electrolyte, and acid-base balance.

• Urinary System: Final effector for excretion and reabsorption to maintain homeostasis.

Summary Table: Major Hormones in Fluid and Electrolyte Balance

Hormone	Source	Main Effect	Target
Antidiuretic Hormone (ADH)	Posterior pituitary	Increases water reabsorption	Collecting ducts
Aldosterone	Adrenal cortex	Increases Na+ reabsorption, K+ secretion	Distal tubule, collecting duct
Atrial Natriuretic Peptide (ANP)	Atria of heart	Decreases Na+ reabsorption	Collecting duct
Parathyroid Hormone (PTH)	Parathyroid glands	Increases Ca2+ reabsorption, decreases phosphate reabsorption	Distal tubule

Additional info: These notes expand on the learning objectives by providing definitions, mechanisms, and examples relevant to the urinary system and fluid, electrolyte, and acid-base homeostasis, as covered in a typical college-level Anatomy and Physiology course.