The Urinary System

Overview and Functions

The urinary system maintains homeostasis by regulating the composition, volume, and pressure of blood, and by excreting metabolic wastes. It also has important endocrine functions.

• Main functions: Excretion of wastes, regulation of blood volume and pressure, regulation of electrolytes, acid-base balance, and hormone production (e.g., erythropoietin, renin).

• Components: Two kidneys, two ureters, one urinary bladder, and one urethra.

Kidney Anatomy

• External coverings: Renal capsule, adipose capsule, and renal fascia protect and anchor the kidneys.

• Internal anatomy:

  • Cortex: Outer region containing renal corpuscles and convoluted tubules.

  • Medulla: Inner region with renal pyramids and columns.

  • Renal pelvis: Funnel-shaped structure collecting urine from calyces.

  • Calyces: Minor and major calyces collect urine from papillae.

Blood Supply and Nerve Innervation

• Blood flow sequence: Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arterioles → glomerular capillaries → efferent arterioles → peritubular capillaries/vasa recta → cortical radiate veins → arcuate veins → interlobar veins → renal vein → inferior vena cava.

• Nerve supply:

  • Sympathetic: Regulates blood flow and renin release.

  • Parasympathetic: Less significant, may modulate renal function.

Nephron Structure and Function

The nephron is the histological and functional unit of the kidney, responsible for urine formation.

• Renal corpuscle: Consists of the glomerulus (fenestrated capillaries) and Bowman's capsule (with podocytes and filtration slits).

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

• Types of nephrons:

  • Cortical nephrons: Short loops, mostly in cortex.

  • Juxtamedullary nephrons: Long loops, important for urine concentration.

Vascular Components

• Glomerulus: Site of filtration; high-pressure capillary bed.

• Peritubular capillaries/vasa recta: Surround tubules; involved in reabsorption and secretion.

Juxtaglomerular Apparatus (JGA)

• Components: Juxtaglomerular (granular) cells and macula densa.

• Functions: Regulates blood pressure and filtration rate via renin release and tubuloglomerular feedback.

Urine Formation

• Three steps: Glomerular filtration, tubular reabsorption, and tubular secretion.

• Glomerular filtration: Driven by net filtration pressure (NFP):

• Glomerular filtration rate (GFR): ~180 L/day in adults.

• Regulation: Intrinsic (renal autoregulation) and extrinsic (neural and hormonal) controls.

• Tubular reabsorption: Active and passive processes; includes co-transport, transport maximum (TM), and water reabsorption (obligatory and facultative).

• Tubular secretion: Adds substances (e.g., K+, H3O+, ammonium, creatinine) to filtrate.

Regulation of Urine Concentration

• Mechanisms: Countercurrent multiplier (loop of Henle), countercurrent exchanger (vasa recta), and antidiuretic hormone (ADH) action.

• Body hydration status: Kidneys adjust urine concentration in response to dehydration or overhydration.

Renal Clearance

• Definition: Volume of plasma cleared of a substance per unit time; used to assess kidney function.

Urine Characteristics

• Physical: Color, clarity, pH (4.5–8.0), specific gravity (1.001–1.035).

• Chemical: 95% water, 5% solutes (urea, creatinine, uric acid, ions).

Other Urinary Organs

• Ureters: Transport urine from kidneys to bladder.

• Urinary bladder: Stores urine; features trigone, detrusor muscle, and rugae.

• Urethra: Conducts urine out; has internal and external sphincters; structural differences between sexes.

Micturition Reflex

• Bladder fills and stretches, activating mechanoreceptors.

• Afferent signals to sacral spinal cord and brain; efferent parasympathetic signals contract detrusor muscle and relax internal sphincter.

• Voluntary control via external sphincter.

• Urinary incontinence: Inability to control urination.

• Urinary retention: Inability to void urine.

Fluid, Electrolyte, and Acid-Base Balance

Body Fluid Compartments

• Total body water: ~40 L in adults.

• Intracellular fluid (ICF): ~25 L (within cells).

• Extracellular fluid (ECF): ~15 L (includes interstitial fluid ~12 L and plasma ~3 L).

Fluid Movement and Edema

• Capillary dynamics: Bulk fluid movement governed by hydrostatic and osmotic pressures:

• Edema: Excess fluid in interstitial space; caused by increased capillary pressure, decreased plasma proteins, lymphatic obstruction, or increased capillary permeability.

Water Balance

• Intake: Ingested liquids, food, metabolic water.

• Output: Urine, sweat, feces, insensible loss (lungs, skin).

• Regulation: Thirst mechanism (stimulated by increased osmolality or decreased plasma volume); ADH controls water reabsorption.

• Disorders:

  • Dehydration: Negative water balance; symptoms include thirst, dry mucosa, low urine output.

  • Hypotonic hydration: Water intoxication; dilutional hyponatremia; symptoms include cerebral edema, confusion.

  • Edema: Tissue swelling due to excess interstitial fluid.

Electrolyte Balance

• Electrolytes: Salts, acids, bases; dissociate into ions in water.

• Functions: Essential minerals, control osmotic movement, maintain acid-base balance.

• Key ions: Sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+).

Acid-Base Balance

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

• Acids and bases: Acids release H+; bases accept H+. Strong acids/bases dissociate completely; weak ones do not.

• Regulation of H+: By chemical buffers, respiratory system, and renal mechanisms.

Chemical Buffer Systems

• Bicarbonate buffer:

• Phosphate buffer: Important in ICF and urine.

• Protein buffer: Proteins act as amphoteric molecules (can accept or donate H+).

Respiratory Regulation

• CO2 + H2O H2CO3 $\leftrightarrow$ H+ + HCO3-

• High CO2: Respiratory acidosis; low CO2 (hyperventilation): Respiratory alkalosis.

Renal Mechanisms

• Kidneys excrete or reabsorb H+ and HCO3- to maintain pH.

Acid-Base Imbalances

Male Reproductive System

Anatomy

• Scrotum: Contains testes; dartos and cremaster muscles regulate temperature (~3°C below body temp).

• Testes: Seminiferous tubules (sperm production), interstitial (Leydig) cells (testosterone production).

• Penis: External (glans, shaft, prepuce) and internal (corpora cavernosa [2], corpus spongiosum [1] with penile urethra) structures.

• Duct system: Epididymis (sperm maturation), ductus deferens, urethra (prostatic, membranous, spongy).

• Accessory glands: Seminal vesicles, prostate gland, bulbourethral (Cowper's) glands; produce seminal fluid.

Physiology

• Spermatogenesis:

  • Mitosis: Spermatogonia produce primary spermatocytes.

  • Meiosis I: Primary spermatocyte → two secondary spermatocytes.

  • Meiosis II: Secondary spermatocytes → four spermatids.

  • Spermiogenesis: Spermatids mature into spermatozoa (head with DNA and acrosome, midpiece with mitochondria, tail for motility).

  • Sertoli (sustentacular) cells: Support, nourish, and protect developing sperm.

• Male sexual response: Erection (parasympathetic), emission/ejaculation (sympathetic).

• Hormonal regulation: Hypothalamic-pituitary-gonadal (HPG) axis: GnRH → FSH & LH → testosterone, ABP, inhibin.

Female Reproductive System

Anatomy

• Ovaries: Produce ova and hormones; held by ligaments; contain follicles (primordial, primary, secondary, Graafian), corpus luteum, and corpus albicans.

• Duct system: Uterine (fallopian) tubes (isthmus, ampulla, infundibulum with fimbriae); uterus (fundus, body, isthmus, cervix); uterine wall (perimetrium, myometrium, endometrium with stratum functionalis and basale); vagina (fornix, introitus, hymen, acidic pH 3.5–4).

• External genitalia (vulva/pudendum): Mons pubis, labia majora (homologous to male scrotum), labia minora (homologous to penile urethra), vestibule, clitoris (homologous to penis).

• Mammary glands: Modified sweat glands for milk production.

Physiology

• Oogenesis:

  • Oogonia (stem cells) → primary oocytes (arrested in prophase I until puberty).

  • Meiosis I (at ovulation): Primary oocyte → secondary oocyte + first polar body.

  • Meiosis II (if fertilized): Secondary oocyte → ovum + second polar body.

• Ovarian cycle:

  • Follicular phase: Follicle growth.

  • Ovulatory phase: Ovulation (release of secondary oocyte).

  • Luteal phase: Corpus luteum activity (progesterone production).

• Uterine cycle:

  • Menstrual phase: Shedding of stratum functionalis.

  • Proliferative phase: Regeneration of endometrium.

  • Secretory phase: Preparation for implantation.

• Hormonal regulation: Cyclic changes in FSH, LH, estrogen, and progesterone control ovarian and uterine cycles.

• Puberty and menarche: Onset of reproductive capability and first menstruation.

• Menopause: Cessation of ovarian function and menstruation.

Lactation

• Milk production: Stimulated by prolactin; occurs in mammary glands.

• Milk ejection (letdown): Stimulated by oxytocin in response to suckling.

• Control mechanisms: Neuroendocrine reflexes involving hypothalamus and pituitary hormones.

Additional Topics

Metabolic Syndrome and Microbiota

• Metabolic syndrome: Cluster of conditions (obesity, hypertension, dyslipidemia, insulin resistance) increasing risk for cardiovascular disease and diabetes.

• Microbiota: Community of microorganisms in the body, especially the gut; important for digestion, immunity, and metabolic health.

• Prevention/management: Healthy diet, regular exercise, weight control, and medical management as needed.