Anatomy of the Renal System and Urine Formation

Gross anatomy/organs of the renal System
- 2 kidneys: responsible for urine formation
- 2 ureters: transport urine from kidneys to bladder
- Urinary bladder: stores urine
- Urethra: carries urine outside the body

General Location
- posterior to the parietal peritoneum on the posterior abdominal wall.
- Lateral to the spine; the right kidney is slightly lower (inferior) than the left due to the liver's position.
- Either side of vertebral column; protected by any trauma from bony vertebrae
- Average size: ~11 cm long, ~5 cm wide, ~130 grams (around the size of a small fist)

Anatomical Features
#image below shows kidney located OUTSIDE of peritoneal cavity
- Renal Capsule: fibrous connective tissue that surrounds each kidney.
- Adipose Tissue: Provides cushioning outside the capsule for protection.
- Renal Fascia: Thin connective tissue layer that anchors kidneys to the abdominal wall.

Kidney external anatomy
- Renal artery: delivers oxygenated blood from heart to kidneys
- Renal vein: delivers deoxygenated away from kidneys and to the heart

Hilum: Medial concave surface where the renal artery and nerves enter, and the renal vein, ureter, and lymphatics exit.
- opens into renus sinus (filled with fat and loose CT)
Regions of the Kidney
- Outer Cortex: Contains renal columns (extensions of corticol tissue into medulla) and blood vessels.
- Inner Medulla: Contains renal pyramids, which project into the cortex.
  - Renal pyramids
    - forms a boundary between the cortex and medulla
    - the apex medially points to the renal papilla
    - pappilae extends into minor calyces (funnel shaped chamber) 8-20 per kidney
    - minor calyces funnel into major calyces
    - renal papilla → minor calyces → major calyces → renal pelvis → ureter
    - renal pelvis: single large funnel-shaped chamber which narrows to form the ureter at the hilum

Functional Unit of the Kidney
- Approximately 1.3 million nephrons per kidney, each ~50-55 mm in length.
- Blood enters nephron for filtration (filtrate/urine is produced)
- Urine flow: nephron → papillary ducts → minor calyces → major calyces → renal pelvis → ureter
Types of Nephrons
- Juxtamedullary Nephrons: 15%, deep renal corpuscle, long loop of Henle.
- Cortical Nephrons: 85%, located near the cortex, shorter loop of Henle.

Nephron Structure
- Located in the cortex
  - Renal Corpuscle: Glomerulus and Bowman capsule; site of blood filtration.
    - glomerulus: network of capillaries
    - bowman capsule: first part of the nephron where blood enters through afferent ateriole, filters in the glomerulus where filtrate sits in bowman capsule
    - eventually enters PCT and filtered blood exits through efferent arteriole
      - PARIETAL LAYER - outer, simple squamous epithelium (cuboidal in PCT)
      - VISCERAL LAYER - podocytes (specialised cells) assists w/ filtration
  - Proximal Convoluted Tubule (PCT): Major site for reabsorption.
  - Distal Convoluted Tubule (DCT): Fine-tuning of urine content.
- Extended into the medulla
  - Loop of Henle: Essential for creating a concentration gradient.
  - Collecting Duct: Drains multiple nephrons into the renal pelvis.

Proximal convoluted tubule
- simple cuboidal epithelium w/ many microvilli and mitochondria
- lots of surface area (convoluted, not flat)
- active reabsorption: Na+, K+ & Cl-
Loop of Henle
- thick areas - simple cuboidal epithelium
- thin areas - simple squamous epithelium
- osmosis/diffusion
Distal convoluted tubule
- simple cuboidal epithelium w/ less microvilli and mitochondria
- active reabsorption
Collecting duct
- simple cuboidal epithelium

Glomerular has specific characteristics that enable blood to be filtered
- Fenestrae: Large pores in the glomerular capillaries increase permeability.
- Basement Membrane: Provides additional filtration barrier.
- Filtration Slits: Between podocyte cell processes to facilitate filtration.

Glomerular Filtration: Blood enters the nephron, and liquid is filtered through the glomerulus into Bowman capsule.
Tubular Reabsorption: Essential nutrients, water, and ions are reabsorbed in PCT and loop of Henle.
Tubular Secretion: Waste products are secreted into the DCT, adding to the filtrate.

Pressure Mechanism: Smooth muscle contractions move urine through the ureters via peristalsis from kidneys to bladder.
Ureteral Structure: Lined with transitional epithelium to accommodate urine flow.

Bladder Anatomy: Hollow muscular organ located in the pelvic cavity, includes the trigone area where the ureters enter.
Urethra Structure: - Different in males (longer, extends to penis) and females (shorter, opens anterior to vagina).
- top of urethra - transitional epithelium
- rest of urethra - stratified columnar
Sphincters:
- Internal (smooth muscle) - prevents urine leakage
- External (skeletal muscle) - control of urine flow (starts&stops)

Common waste products excreted include urea, creatinine, and metabolic waste from protein breakdown.