Chapter 25: The Urinary System

Main Functions of the Urinary System

The urinary system is essential for maintaining homeostasis by regulating the composition and volume of blood, removing metabolic wastes, and controlling water and electrolyte balance.

• Excretion of wastes: Removes nitrogenous wastes (e.g., urea, creatinine) from the body.

• Regulation of blood volume and pressure: Adjusts water and sodium levels.

• Electrolyte balance: Maintains proper concentrations of ions such as sodium, potassium, and calcium.

• Acid-base balance: Regulates blood pH by excreting hydrogen ions and reabsorbing bicarbonate.

• Hormone production: Produces erythropoietin (stimulates red blood cell production) and renin (regulates blood pressure).

Structure of the Kidney

The kidneys are bean-shaped organs located retroperitoneally in the upper posterior abdomen. Each kidney is covered by a fibrous capsule and surrounded by adipose tissue for protection.

• Location: On either side of the vertebral column, between T12 and L3.

• Coverings: Renal capsule, adipose capsule, and renal fascia.

• Internal structure: Cortex (outer layer), medulla (inner region with pyramids), renal pelvis (collects urine).

Urinary Tract Anatomy

The urinary tract includes the kidneys, ureters, bladder, and urethra, each with distinct roles in urine formation and excretion.

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

• Bladder: Hollow organ that stores urine until excretion.

• Urethra: Tube that carries urine from the bladder to the outside of the body.

Male vs. Female Urinary Systems

While the basic structure is similar, there are key differences between male and female urinary systems.

• Urethra length: Longer in males (also passes through the prostate and penis); shorter in females.

• Location: Female urethra opens anterior to the vaginal opening; male urethra opens at the tip of the penis.

Renal Tissue Divisions and Blood Vessels

The kidney is divided into cortex and medulla, with major blood vessels supplying each region.

• Cortex: Contains renal corpuscles and convoluted tubules.

• Medulla: Contains renal pyramids and collecting ducts.

• Major blood vessels: Renal artery, interlobar arteries, arcuate arteries, interlobular arteries.

Pathway of Blood Through the Kidney

Blood enters the kidney via the renal artery, passes through smaller arteries and capillaries, and is filtered in the glomerulus.

• Renal artery → segmental arteries → interlobar arteries → arcuate arteries → interlobular arteries → afferent arterioles → glomerulus → efferent arterioles → peritubular capillaries/vasa recta → renal vein.

Renal Corpuscles, Tubules, and Nephron Structure

The nephron is the functional unit of the kidney, consisting of a renal corpuscle and renal tubule.

• Renal corpuscle: Includes the glomerulus and Bowman's capsule.

• Renal tubule: Proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.

• Types of nephrons: Cortical (short loops) and juxtamedullary (long loops, important for urine concentration).

Filtration, Reabsorption, and Secretion

Urine formation involves three main processes: filtration, reabsorption, and secretion.

• Filtration: Occurs in the glomerulus; water and solutes move from blood to Bowman's capsule.

• Reabsorption: Useful substances are reclaimed from the filtrate back into the blood (mainly in PCT).

• Secretion: Additional wastes are secreted from blood into the tubule.

Equation for Glomerular Filtration Rate (GFR):

Where is the filtration coefficient, is glomerular capillary hydrostatic pressure, is Bowman's space hydrostatic pressure, and is glomerular capillary oncotic pressure.

Filtration Membrane

The filtration membrane consists of three layers: capillary endothelium, basement membrane, and podocyte filtration slits. It allows water and small solutes to pass but blocks proteins and cells.

Glomerular and Peritubular Capillaries

Glomerular capillaries are specialized for filtration, while peritubular capillaries are involved in reabsorption and secretion.

Juxtaglomerular Complex

The juxtaglomerular apparatus regulates blood pressure and GFR by releasing renin and sensing sodium concentration.

• Macula densa: Senses sodium in the distal tubule.

• Juxtaglomerular cells: Release renin.

Regulation of GFR

GFR is regulated by blood pressure, blood volume, sodium content, and hormones such as angiotensin II, aldosterone, and ADH.

• Increase GFR: High blood pressure, increased blood volume.

• Decrease GFR: Low blood pressure, dehydration, constriction of afferent arteriole.

Countercurrent Mechanism

The countercurrent mechanism in the nephron loop and vasa recta helps concentrate urine by creating a gradient for water reabsorption.

Urine Formation and Elimination

Urine is formed in the nephrons, collected in the renal pelvis, transported by ureters, stored in the bladder, and eliminated via the urethra.

Bladder Anatomy and Function

The bladder is lined by transitional epithelium and has a muscular wall (detrusor muscle) for contraction during urination.

Chapter 26: Fluid, Electrolyte, and Acid-Base Balance

Fluid Compartments

Body fluids are distributed between intracellular fluid (ICF) and extracellular fluid (ECF) compartments.

• ICF: Fluid within cells (~2/3 of total body water).

• ECF: Fluid outside cells, including plasma and interstitial fluid (~1/3 of total body water).

Electrolyte Balance

Electrolytes such as sodium, potassium, chloride, and calcium are vital for nerve function, muscle contraction, and fluid balance.

• Sodium (Na+): Major ECF cation, regulates water balance.

• Potassium (K+): Major ICF cation, important for cardiac and muscle function.

• Chloride (Cl-): Major ECF anion, helps maintain osmotic pressure.

• Calcium (Ca2+): Needed for bone health, muscle contraction, and nerve signaling.

Osmotic Pressure and Non-Electrolytes

Osmotic pressure is determined by solute concentration; non-electrolytes (e.g., glucose, urea) also contribute to osmolarity.

Acid-Base Balance

The body maintains pH within a narrow range (7.35–7.45) using buffers, respiratory regulation, and renal mechanisms.

• Buffers: Bicarbonate, phosphate, and protein systems resist changes in pH.

• Respiratory system: Regulates CO2 (acid) levels via ventilation.

• Renal system: Excretes H+ and reabsorbs HCO3- to control pH.

Normal ranges: PCO2: 35–45 mmHg; HCO3-: 22–26 mEq/L.

Compensation for Acid-Base Imbalances

The body compensates for respiratory or metabolic acidosis/alkalosis via changes in breathing or kidney function.

• Respiratory compensation: Adjusts CO2 excretion.

• Renal compensation: Adjusts H+ and HCO3- excretion/reabsorption.

Homeostasis and Disruption

Homeostasis is maintained by coordinated actions of the cardiovascular, endocrine, and urinary systems. Disruption can lead to acidosis or alkalosis.

• Examples of disruption: Kidney failure, respiratory disease, severe vomiting or diarrhea.

Summary Table: Key Features of Urinary and Fluid Balance Systems

Additional info: Some explanations and examples were expanded for clarity and completeness, including equations and normal ranges for acid-base balance.