Urinary System: Anatomy, Histology, and Physiology Study Guide

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Kidneys: Filter blood, remove waste, regulate fluid and electrolyte balance, and produce urine.
Ureters: Transport urine from kidneys to the urinary bladder.
Urinary Bladder: Stores urine until elimination.
Urethra: Conducts urine from the bladder to the exterior of the body.

Location: Retroperitoneal, on either side of the vertebral column, between T12 and L3.
Shape: Bean-shaped organs.
Covering Layers:
- Renal Capsule: Fibrous outer covering.
- Adipose Capsule: Fatty layer for protection.
- Renal Fascia: Connective tissue anchoring the kidney.
Hilum: Medial indentation where the renal artery enters, and the renal vein and ureter exit.

Renal Sinus: Central cavity containing the renal pelvis, calyces, blood vessels, nerves, and fat.
Renal Pelvis: Funnel-shaped structure collecting urine from major calyces.
Major Calyces: Collect urine from minor calyces.
Minor Calyces: Collect urine from renal papillae.
Renal Cortex: Outer region containing nephrons and medullary rays.
Renal Medulla: Inner region containing renal pyramids.
Renal Pyramids: Cone-shaped tissues containing nephron loops and collecting ducts.
Renal Papilla: Tip of each pyramid, releases urine into minor calyx.
Renal Columns: Extensions of cortex between pyramids.
Renal Lobe: Consists of a pyramid, overlying cortex, and adjacent columns.
Renal Lobule: Subdivision of a lobe, centered around a collecting duct.

Renal artery → Segmental arteries → Interlobar arteries → Arcuate arteries → Interlobular (cortical radiate) arteries → Afferent arterioles → Glomerulus → Efferent arterioles → Peritubular capillaries/vasa recta → Interlobular veins → Arcuate veins → Interlobar veins → Renal vein

Nephron: Functional unit of the kidney; consists of the renal corpuscle and renal tubule.
Collecting Ducts: Receive urine from multiple nephrons and transport it to the renal pelvis.

Glomerulus: Capillary network for filtration.
Bowman's Capsule: Double-walled structure surrounding the glomerulus.
- Parietal Layer: Simple squamous epithelium.
- Visceral Layer (Podocytes): Specialized cells with primary and secondary processes (pedicels) forming filtration slits.
- Capsular Space: Space between parietal and visceral layers where filtrate collects.

Proximal Convoluted Tubule (PCT): Reabsorbs water, ions, and nutrients from filtrate.
Loop of Henle (Nephron Loop): Descending and ascending limbs; establishes osmotic gradient in medulla.
Distal Convoluted Tubule (DCT): Further reabsorption and secretion; regulated by hormones.

Glomerular Filtration: Movement of water and solutes from blood into Bowman's capsule.
Tubular Reabsorption and Secretion: Reabsorption of useful substances back into blood; secretion of additional wastes into tubule.
Water Conservation: Regulation of water reabsorption to concentrate urine.

Filtration Membrane Components:
- Fenestrated endothelium of glomerular capillaries: Allows passage of plasma but not blood cells.
- Glomerular basement membrane: Blocks large proteins.
- Filtration slit diaphragm (between podocyte pedicels): Restricts passage of medium-sized proteins.
Filtered: Water, glucose, amino acids, ions, urea.
Retained: Blood cells, large proteins.
Filtration Pressures:
- Glomerular Blood Hydrostatic Pressure (BHP): Drives filtration out of glomerulus.
- Capsular Hydrostatic Pressure (CsHP): Opposes filtration.
- Blood Colloid Osmotic Pressure (COP): Opposes filtration by drawing water back into capillaries.
Net Filtration Pressure (NFP): The overall pressure driving filtration.
- Calculated as:
Afferent vs. Efferent Arterioles: Afferent arteriole is wider, creating higher pressure in glomerulus.
Glomerular Filtration Rate (GFR): Volume of filtrate formed per minute by both kidneys.
GFR Control Mechanisms:
- Autoregulation (local adjustment of arteriole diameter).
- Autonomic regulation (sympathetic nervous system).
- Hormonal regulation (renin-angiotensin-aldosterone system).
Renin–Angiotensin–Aldosterone System (RAAS): Increases blood pressure and GFR; Angiotensin II constricts efferent arteriole, stimulates aldosterone and ADH release.

Tubular Reabsorption: Movement of substances from filtrate back into blood (mainly in PCT).
Tubular Secretion: Movement of substances from blood into filtrate (mainly in DCT and collecting duct).
Transport Maximum (Tm): Maximum rate of reabsorption for a substance; exceeded Tm leads to excretion in urine (e.g., glycosuria in diabetes).
Glycosuria: Glucose in urine; Aminoaciduria: Amino acids in urine.
Loop of Henle: Establishes osmotic gradient for water reabsorption (countercurrent multiplier system).
Distal Convoluted Tubule (DCT): Site of hormone-regulated reabsorption and secretion (e.g., aldosterone, parathyroid hormone).

Collecting System: Collects urine from nephrons; final site for water reabsorption.
Hormones Affecting Collecting Duct:
- Antidiuretic Hormone (ADH): Increases water reabsorption.
- Aldosterone: Increases sodium (and water) reabsorption.
- Atrial Natriuretic Peptide (ANP): Decreases sodium reabsorption.
Diuretics: Substances that increase urine output by inhibiting water or sodium reabsorption.

Filtration at glomerulus → Reabsorption and secretion along tubules → Water conservation in collecting ducts → Urine formation.

Collecting duct → Minor calyx → Major calyx → Renal pelvis → Ureter → Urinary bladder → Urethra → Exterior

Tube conducting urine from bladder to outside.
Male Urethra: Longer, passes through prostate and penis; carries urine and semen.
Female Urethra: Shorter, opens anterior to vaginal opening; higher risk of urinary tract infections.

Reflex contraction of the bladder (detrusor muscle) and relaxation of urethral sphincters to allow urination.
Detrusor Muscle: Smooth muscle in bladder wall responsible for contraction during urination.