The Urinary System Overview

The urinary system is essential for filtering blood, regulating ionic composition, pH, osmolarity, and volume, and excreting waste.

Renal Structure
- Intra/retro/sub-peritoneal positioning of organs
- Internal and external features of the kidney
- Features of the nephron and renal blood supply
- Features of the bladder
Renal Function
- Features of glomerular filtration in the nephron and regulation
- Resorption and secretion processes in nephron tubules
- Renin-angiotensin-aldosterone system and hormonal regulation
- Histology of nephron related to function

Located retroperitoneally
Surrounded by peritoneum:
- Intraperitoneal: organs surrounded by serosa (e.g. ileum, jejunum)
- Retroperitoneal: organs behind the peritoneum (e.g. kidneys)
- Subperitoneal: organs below the peritoneum (e.g. urinary bladder)
Kidney Structure:
- Outer cortex and inner medulla organized in pyramids
- Renal papilla at pyramid apex
- Minor calyx -> major calyx -> renal pelvis -> ureter for urine drainage

Monitoring blood composition:
- Regulation of ionic composition (e.g. Na+, K+, Ca2+, Cl-, phosphate)
- Regulation of blood pH, osmolarity, glucose, volume, and blood pressure
- Secretion of hormones (e.g. renin, erythropoietin, calcitriol)

Renal blood flow is crucial; features include:
- Renal artery, interlobar arteries, and glomerular capillaries
- Nephron: responsible for blood filtration and urine formation
- Afferent arterioles -> glomerulus for filtration
- Efferent arterioles -> peritubular capillaries for reabsorption

Three Processes:
1. Glomerular Filtration: Plasma filtered into the nephron
2. Tubular Reabsorption: Na+, Cl-, and water reabsorbed into the blood
3. Tubular Secretion: Additional waste products secreted from blood into nephron
Filtration Fraction: 20% of plasma is filtered, 99% of fluid is reabsorbed

Enhancing Factors:
- Thin filtration membrane
- Large surface area of glomerular capillaries
- High blood pressure within glomerular capillaries
Filtration Membrane Composition:
- Prevents passage of formed elements and large proteins
- Allows water and small solutes through

Important Pressures:
- Promoted by glomerular hydrostatic pressure (Pgc)
- Opposed by tubular hydrostatic pressure (Pt) and blood colloid osmotic pressure (πgc)
- Net filtration pressure (NFP) ~10 mmHg
GFR Calculation:
GFR = Kf imes P{uf}
where Kf = filtration constant

1. Renal Autoregulation:
- Myogenic mechanism: smooth muscle contraction in response to stretch
- Tubuloglomerular feedback: by macula densa cells
2. Neural Regulation:
- Sympathetic nervous system activation causes vasoconstriction
3. Hormonal Regulation:
- Atrial natriuretic peptide (ANP): vasodilation and increased GFR
- Renin-angiotensin-aldosterone system: decreases GFR

Countercurrent Multiplier Mechanism: Long loop of Henle creates osmotic gradient in medulla
Hormonal Regulation:
- ADH (Vasopressin): Increases water reabsorption in collecting ducts
- Aldosterone: Enhances Na+ reabsorption
- ANP: Decreased reabsorption and increased urine output

Proximal Convoluted Tubule: simple cuboidal with brush border
Loop of Henle: thin segment (simple squamous), thick segment (cuboidal)
Distal Convoluted Tubule and Collecting Duct: receptors for ADH and aldosterone

Substances filtered or not filtered into Bowman’s Capsule:
- Erythrocytes: Not filtered
- Water, glucose, Na+, urea, amino acids: Filtered