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Urinary System: Structure, Function, and Regulation (Anatomy & Physiology Study Notes)

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Urinary System

Overview

The urinary system is essential for maintaining homeostasis by filtering blood, removing metabolic wastes, and regulating fluid and electrolyte balance. It consists of the kidneys, ureters, urinary bladder, and urethra.

  • Key Functions: Excretion of waste, regulation of ion levels, maintenance of blood pH, conservation of nutrients, regulation of fluid volumes, erythrocyte production, and urine storage/elimination.

  • Main Organs: Kidneys, ureters, urinary bladder, urethra.

Functions of the Urinary System

Major Roles

  • Filters cellular waste products from the blood, especially urea.

  • Regulates ion levels (e.g., Na+, K+, Ca2+).

  • Regulates blood pH by excreting H+ and reabsorbing HCO3-.

  • Conserves nutrients such as glucose and amino acids.

  • Regulates fluid volumes to maintain blood pressure and hydration.

  • Regulates erythrocyte production via erythropoietin secretion.

  • Urine storage and excretion through the bladder and urethra.

Kidneys: Location & Gross Anatomy

Position and Structure

  • Location: Posterior abdominal wall, adjacent to lower vertebrae (T11–L3).

  • Covered by: Fibrous capsule and packed in adipose tissue for protection.

  • Major anatomical features: Renal artery and vein, adrenal gland, ureter, hilum (entry/exit site for vessels), cortex, medulla.

Kidney: Internal Anatomy

Regions and Structures

  • Cortex: Superficial, lighter region.

  • Medulla: Deeper, darker region containing 5–10 renal pyramids.

  • Hilum: Site where vessels enter/exit the kidney.

  • Sinus: Internal cavity that drains urine.

Medulla Details

  • Renal pyramids: Triangular-shaped structures.

  • Renal papilla: Apex of pyramid, where urine drains into minor calyces.

  • Renal columns: Extensions of cortical tissue between pyramids.

  • Lobe: Pyramid plus surrounding cortical tissue.

Urine Collection Structures

  • Minor calyces: Collect urine from pyramids.

  • Major calyces: Collect urine from minor calyces.

  • Renal pelvis: Collects urine from major calyces.

  • Ureter: Collects urine from renal pelvis and transports to bladder.

Blood Flow in Kidneys

Pathway and Importance

  • Kidneys receive 20–25% of cardiac output.

  • Blood flow sequence:

    1. Renal artery

    2. Interlobar artery

    3. Arcuate artery

    4. Cortical radiate artery

    5. Afferent arteriole

    6. Glomerular capillaries

    7. Efferent arteriole

    8. Peritubular capillaries

    9. Cortical radiate vein

    10. Arcuate vein

    11. Interlobar vein

    12. Renal vein (exits kidney)

Pathway of Urine out of the Body

Sequential Flow

  • Glomerular capsule

  • Proximal convoluted tubule

  • Loop of Henle

  • Distal convoluted tubule

  • Collecting duct

  • Papillary duct

  • Renal papilla

  • Minor calyx

  • Major calyx

  • Renal pelvis

  • Ureter

  • Bladder

  • Urethra

  • Out of body

Nephrons

Structure and Types

  • Basic functional unit of kidneys

  • ~1 million nephrons per kidney

Types of Nephrons

  • Cortical nephrons: Located near the outer cortex, have short loops of Henle.

  • Juxtamedullary nephrons: Located near the border of cortex and medulla, have long loops of Henle (important for concentrating urine).

Renal Corpuscle

Components

  • Glomerulus: Cluster of capillaries where filtration occurs.

  • Glomerular (Bowman's) capsule: Sac surrounding the glomerulus, collects filtrate.

Urine Formation

Three Main Processes

  • Glomerular filtration: Movement of water and solutes from blood into the glomerular capsule.

  • Tubular reabsorption: Selective movement of substances from filtrate back into blood.

  • Tubular secretion: Active transport of substances from blood into the filtrate for excretion.

Glomerular Filtration

Mechanism and Location

  • Occurs in the renal corpuscle (glomerulus + capsule).

  • Driven by pressure differences across the filtration membrane.

Glomerular Filtration Pressures

Types of Pressures

  • Glomerular hydrostatic pressure (GHP): Outward force pushing water/solutes out of blood.

  • Colloid osmotic pressure (COP): Inward force due to plasma proteins pulling water back into capillaries.

  • Capsular hydrostatic pressure (CHP): Inward force from fluid in the capsule opposing filtration.

  • Net Filtration Pressure (NFP): Overall pressure driving filtration.

Net Filtration Pressure Equation:

Typical values:

Pressure Type

Value (mm Hg)

Glomerular hydrostatic pressure

+60

Plasma colloid osmotic pressure

-32

Capsular hydrostatic pressure

-18

Net filtration pressure

+10

Glomerular Filtration Rate (GFR)

Definition and Regulation

  • GFR: Rate at which filtrate is formed in all nephrons per minute.

  • Normal GFR: ~120 mL/min/1.73 m2

  • Regulated by:

    • Renal autoregulation (self-adjustment by kidney)

    • Neural regulation (sympathetic nervous system)

    • Hormonal regulation (renin-angiotensin-aldosterone system)

Renin-Angiotensin-Aldosterone System (RAAS)

Hormonal Control of GFR

  • Renin: Released by kidneys in response to low blood pressure/volume.

  • Angiotensin II: Causes vasoconstriction, increases blood pressure, stimulates aldosterone release.

  • Aldosterone: Promotes Na+ and water reabsorption, increasing blood volume and pressure.

RAAS Pathway:

Step

Effect

Renin release

Converts angiotensinogen to angiotensin I

ACE (from lungs)

Converts angiotensin I to angiotensin II

Angiotensin II

Vasoconstriction, stimulates aldosterone

Aldosterone

Increases Na+ and water reabsorption

Consequences of Abnormal GFR

Hyperfiltration

  • GFR too high: Substances not reabsorbed, risk of dehydration and loss of nutrients.

  • Associated diseases: Diabetes, glomerular disease.

Hypofiltration

  • GFR too low: Nearly all substances reabsorbed, waste products not adequately excreted.

  • Associated diseases: Chronic kidney disease, stroke.

Summary Table: Key Features of Urinary System

Feature

Description

Filtration

Removal of water/solutes from blood into nephron

Reabsorption

Return of useful substances to blood

Secretion

Active transport of waste into nephron

GFR

Rate of filtration, regulated by NFP and RAAS

Hyperfiltration

High GFR, risk of nutrient loss

Hypofiltration

Low GFR, risk of waste retention

Concept Checks

  • Functions NOT performed by urinary system: Maceration, auscultation.

  • Urinary system removes: Urea (not bile).

  • Kidney regions: Outer cortex, inner medulla.

  • Renal corpuscle: Glomerulus + glomerular capsule.

  • Site for vessel entry/exit: Hilum.

  • Pressures affecting GFR: Decreased albumin lowers plasma colloid osmotic pressure, increasing GFR.

  • Abnormal GFR: Hyperfiltration (not enough reabsorption), hypofiltration (not enough excretion).

Additional info: These notes expand on the original slides by providing definitions, context, and tables for clarity. All equations and values are standard for undergraduate anatomy & physiology courses.

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