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

The Urinary System

Overview and Functions

The urinary system is essential for maintaining homeostasis by regulating the composition and volume of blood, eliminating metabolic wastes, and controlling fluid and electrolyte balance. It consists of the kidneys, ureters, urinary bladder, and urethra.

• Eliminates excess water, salts, and physiological wastes.

• Kidneys receive about 25% of cardiac output and produce urine.

• Ureters transport urine from kidneys to the bladder.

• Urinary bladder stores urine until elimination.

• Urethra expels urine from the body.

Key functions:

• Adjust blood volume and pressure

• Regulate plasma concentrations of sodium, potassium, chloride, and other ions

• Stabilize blood pH

• Conserve valuable nutrients

• Remove drugs, toxins, and metabolic wastes

Kidney Structure

• Location: Retroperitoneal, between posterior body wall muscles and parietal peritoneum.

• Renal fascia: Anchors kidneys to surrounding structures.

• Hilum: Medial indentation; entry/exit for renal artery, vein, nerves, and ureter.

Internal Structure

• Fibrous capsule: Covers kidney surface and lines renal sinus.

• Renal cortex: Superficial region.

• Renal medulla: Inner, darker region.

• Renal pyramids: Conical structures in medulla; tip is the renal papilla.

• Renal columns: Separate adjacent pyramids.

• Kidney lobe: Contains a pyramid, overlying cortex, and adjacent columns (6–18 per kidney).

Pathway of Urine

• Urine produced in kidney lobes → minor calyx → major calyx (fusion of 4–5 minor calyces) → renal pelvis (funnel-shaped, continuous with ureter).

Nephron Structure and Function

Nephron Overview

The nephron is the functional unit of the kidney, responsible for filtering blood and forming urine.

• Each kidney contains about 1.25 million nephrons.

• Consists of the renal corpuscle (site of filtration) and renal tubule (modifies filtrate).

Nephron Segments

1. Renal corpuscle: Glomerular capsule + glomerulus (capillary network). Filtration occurs here.

2. Proximal convoluted tubule (PCT): Reabsorbs nutrients from filtrate.

3. Nephron loop (Loop of Henle): Establishes osmotic gradient in medulla.

4. Distal convoluted tubule (DCT): Adjusts composition by secretion and absorption.

Collecting System

5. Collecting duct: Carries fluid through medulla.

6. Papillary duct: Collects from multiple collecting ducts, delivers to minor calyx.

Types of Nephrons

• Cortical nephrons: 85%, located in cortex, short loops.

• Juxtamedullary nephrons: 15%, long loops extending deep into medulla, essential for water conservation and urine concentration.

Renal Circulation

Blood Flow Through the Kidneys

• Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arterioles → glomerulus

• Blood leaves glomerulus via efferent arteriole → peritubular capillaries (cortical nephrons) or vasa recta (juxtamedullary nephrons) → cortical radiate veins → arcuate veins → interlobar veins → renal vein

Capillaries of the Nephron

• Peritubular capillaries: Surround renal tubule, involved in reabsorption and secretion.

• Vasa recta: Long, straight capillaries parallel to nephron loop in juxtamedullary nephrons; crucial for urine concentration.

Nephron Innervation

• Innervated by sympathetic postganglionic fibers from celiac plexus and inferior splanchnic nerves.

• Regulate blood flow, pressure, and renin release.

Renal Physiology and Urine Formation

Overview

The kidneys maintain homeostasis by regulating blood volume and composition, concentrating urine, and excreting metabolic wastes such as urea, creatinine, and uric acid.

Three Processes in Urine Formation

1. Filtration: Blood pressure forces water and solutes from glomerular capillaries into the capsular space.

2. Reabsorption: Removal of water and solutes from tubular fluid into peritubular fluid.

3. Secretion: Transport of solutes from peritubular fluid into tubular fluid.

Urine Formation by Nephron Region

• Renal corpuscle: Filtration produces ~180 L/day of filtrate (similar to plasma, minus proteins).

• PCT: Reabsorbs 60–70% water, 99–100% organic substrates, 60–70% sodium and chloride ions.

• Nephron loop: Reabsorbs 25% water, 20–25% sodium and chloride; creates medullary concentration gradient.

• DCT: Reabsorbs variable water (usually 5%, influenced by ADH) and sodium (influenced by aldosterone).

• Collecting system: Reabsorbs variable water (usually 9.3%, influenced by ADH) and sodium (aldosterone).

Renal Corpuscle and Filtration

• Afferent arteriole: Delivers blood to glomerulus.

• Glomerular capsule: Inner visceral layer (podocytes) and outer parietal layer; capsular space in between.

• Podocytes: Cells with foot-like processes forming filtration slits; restrict passage of large solutes.

• Efferent arteriole: Smaller diameter, elevates glomerular pressure, aids filtration.

• Juxtaglomerular complex: Releases renin when glomerular pressure falls.

Water Reabsorption

• Obligatory reabsorption: Occurs in PCT and descending limb; recovers ~85% filtrate volume.

• Facultative reabsorption: Occurs in DCT and collecting system; regulated by ADH.

Effects of ADH on Urine Volume

• Without ADH: No water reabsorbed in DCT/collecting duct; high urine volume, low osmotic concentration.

• With ADH: Aquaporins form, increasing water reabsorption; urine volume decreases, osmotic concentration increases.

Normal and Abnormal Urine Volumes

• Normal: ~1200 mL/day, 1000 mOsm/L.

• Polyuria: Excessive urine output (hormonal/metabolic causes).

• Oliguria: 50–500 mL/day.

• Anuria: 0–50 mL/day.

Renal Failure and Treatment

Renal Failure

• Inability to maintain homeostasis; affects all body systems.

• Symptoms: reduced urine, fluid/pH imbalance, hypertension, anemia, CNS issues.

Acute Renal Failure

• Rapid onset (days); may recover if cause is removed.

• Causes: toxins, ischemia, obstruction, trauma, allergic reactions.

Chronic Renal Failure

• Gradual, irreversible; managed by restricting water, salt, protein.

• Acidosis common; treated with bicarbonate.

Dialysis

• Passive diffusion across a selectively permeable membrane.

• Hemodialysis uses artificial membrane to filter blood.

• Renal transplant is the only cure for severe failure.

Urine Storage and Elimination

Urinary Tract Anatomy

• Ureters: Muscular tubes (30 cm) from kidneys to bladder.

• Urinary bladder: Hollow, muscular organ; stores up to 1 L urine.

• Urethra: Extends from bladder neck to exterior; longer in males (also transports semen).

Urinary Bladder Structure

• Wall layers: Mucosa, submucosa, muscularis (detrusor muscle).

• Rugae: Folds that disappear as bladder fills.

• Trigone: Triangular area between ureteral openings and urethra.

• Neck: Surrounds urethral opening.

Urethral Sphincters

• Internal sphincter: Involuntary smooth muscle at bladder neck.

• External sphincter: Voluntary skeletal muscle in urogenital diaphragm.

Micturition Reflex

1. Bladder stretch receptors activated as urine volume increases.

2. Afferent signals to sacral spinal cord.

3. Parasympathetic efferents stimulate detrusor muscle contraction.

4. Voluntary relaxation of external sphincter allows urination.

• Urge to urinate at ~200 mL; after voiding, <10 mL remains.

• Voluntary control develops by age 2.

Urinary Disorders

Common Disorders and Clinical Signs

• Pyelonephritis: Kidney infection.

• Renal calculi: Kidney stones.

• Dysuria: Painful/difficult urination (cystitis, urethritis).

• Edema: Proteinuria leads to tissue swelling.

• Fever: Indicates infection (higher in kidney infections).

• Incontinence: Inability to control urination (stress, urge, overflow types).

• Urinary retention: Inability to void despite normal kidney function (often due to enlarged prostate in males).

Urinalysis

• Examines urine for pH, glucose, ketones, bilirubin, proteins, hemoglobin.

• Pregnancy test detects hCG hormone.

Fluid, Electrolyte, and Acid-Base Balance

Fluid Compartments

• Intracellular fluid (ICF): Cytosol; varies by sex and body composition.

• Extracellular fluid (ECF): Includes plasma and interstitial fluid; varies by sex and tissue type.

Solid Components

• 40–50% of body mass; includes proteins, lipids, minerals, carbohydrates.

• Minerals: Inorganic, dissociate to form electrolytes.

Fluid and Mineral Balance

• Fluid balance: Water intake equals water loss (via digestive tract, metabolism, feces, urine, evaporation).

• Fluid shifts: Rapid water movement between ECF and ICF in response to osmotic gradients.

• Mineral balance: Ion absorption (intestine/colon) equals ion excretion (kidneys, sweat glands).

Sodium and Blood Volume Balance

• Sodium balance: Gains equal losses; ECF volume changes to maintain Na+ concentration.

• Blood volume: ECF volume changes affect blood volume; homeostatic mechanisms respond to disturbances.

Acid-Base Balance

• pH: Measure of hydrogen ion concentration; normal plasma pH is 7.35–7.45.

• Acidosis: pH below 7.35 (acidemia); dangerous, affects CNS, heart, blood pressure.

• Alkalosis: pH above 7.45 (alkalemia); less common but also dangerous.

• CO2 and pH: CO2 + H2O → H2CO3 (carbonic acid); increased CO2 lowers pH, decreased CO2 raises pH.

Buffer Systems

• Buffer: Combination of weak acid and its anion; temporarily resists pH changes.

• Major buffer systems:

  1. Phosphate buffer system

  2. Protein buffer systems (e.g., hemoglobin, amino acids)

  3. Carbonic acid–bicarbonate buffer system

• Hemoglobin buffer: RBCs absorb CO2, convert to carbonic acid, buffer H+ with hemoglobin.

• Protein buffers: Amino/carboxyl groups accept/release H+.

• Carbonic acid–bicarbonate: HCO3– combines with H+ to form H2CO3, which dissociates to CO2 and H2O.

Compensation Mechanisms

• Renal compensation: Kidneys secrete/generate H+ or HCO3–.

• Respiratory compensation: Adjusts CO2 elimination via breathing rate.

Acid-Base Disorders

• Metabolic acidosis: Excess acid production or loss of HCO3–; compensated by increased respiration and renal H+ secretion.

• Respiratory acidosis: CO2 retention (hypoventilation); compensated by increased respiration and renal H+ secretion.

• Metabolic alkalosis: Excess H+ loss or HCO3– gain; compensated by decreased respiration and renal HCO3– excretion.

• Respiratory alkalosis: Excess CO2 loss (hyperventilation); compensated by decreased respiration and renal HCO3– excretion.

Key Equations and Concepts

• CO2 and pH relationship:

$\mathrm{CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-}$

• pH calculation:

$\mathrm{pH = -\log_{10}[H^+]}$

Summary Table: Major Buffer Systems

Buffer System	Location	Main Components	Function
Phosphate	ICF, renal tubules	H2PO4–, HPO42–	Buffers pH in cytosol and urine
Protein	ICF, plasma	Amino acids, hemoglobin	Accept/release H+ to buffer pH
Carbonic acid–bicarbonate	ECF	H2CO3, HCO3–	Buffers acids from metabolism

Example: Urine Pathway

• Urine is transported by the ureters, stored within the urinary bladder, and eliminated through the urethra.

Example: Acid-Base Compensation

• If kidneys conserve HCO3– and eliminate H+, they are compensating for acidosis.

Additional info: Some explanations and context have been expanded for clarity and completeness, including the summary table and equations.