Urinary System Flashcards

The Urinary System

Introduction to the Urinary System

The urinary system consists of:

• Kidneys (x2)
  • Produce urine (H2O, ions, soluble substances).
• Ureters (x2)
  • Receive urine from the kidneys.
  • Carry urine to the bladder via gravity and peristalsis.
• Bladder
  • Receives and stores urine.
  • Contraction in muscular wall leads to urination.
• Urethra
  • Carries urine from the bladder to the external environment.

Functions of the Urinary System

The urinary system:

• Filters blood plasma.
• Regulates concentrations of Na^+/K^+/Cl^- ions (and others).
• Removes drugs, toxins, and waste products from the bloodstream.
• Conserves valuable nutrients, preventing nutrient loss via urine.
• Regulates blood volume and pressure by removing fluid (H_2O) from blood, decreasing blood volume and pressure.
• Regulates blood pH and glucose levels by removing H^+ ions and glucose from the blood to maintain homeostasis.
• Releases hormones:
  • Erythropoietin: stimulates RBC formation.
  • Calcitriol: stimulates Ca^{2+} absorption in the GIT.

Anatomy of the Kidney

• Paired organs that sit either side of the vertebral column.
• The left kidney is slightly superior to the right due to the liver.
• Retroperitoneal: sit behind the peritoneum (abdominal sac around the intestines).
• Protected by:
  • 11th and 12th ribs.
  • Visceral organs (anteriorly).
  • Fat.

Connective Tissue Layers of the Kidney

Each kidney is protected and supported by three connective tissue layers:

• Fibrous capsule:
  • The entire organ is covered by a capsule of collagen fibers.
• Perinephric/perirenal fat:
  • A thick, cushioning layer of adipose tissue.
• Renal fascia:
  • A dense, fibrous outer layer that anchors the kidney to surrounding structures.

Superficial Anatomy of the Kidney

• Hilum
  • Entry/exit point for the renal artery, renal nerves, renal vein, and ureter.
• Renal artery
  • Proportionally very large in diameter.
  • Supplies the kidney with ~20% of resting cardiac output (to be filtered).
• Renal vein
  • Takes blood out of the kidney.
• Ureter
  • Takes urine to the bladder.

Internal Anatomy of the Kidney

• Renal cortex
  • Outermost ~1cm of the kidney.
  • Where filtration and reabsorption occur.
  • Waste products and H_2O are removed from the blood.
  • Useful products (glucose, proteins, AA’s) are reabsorbed from filtrate back into the blood.
• Renal medulla
  • ~2-3cm region below the cortex.
  • Regulates the concentration of urine.
• Renal sinus
  • A central cavity that contains structures such as:
    • The renal pelvis
    • The renal calyces
    • Blood vessels
    • Fat

Structures Within the Renal Medulla

Within the renal medulla, there are:

• Renal pyramids
  • Conical structures that extend from the cortex to the renal sinus.
  • 8-18 per kidney (average).
  • Transport urine from the cortex to the sinus.
  • The apex (facing the sinus) = the renal papilla.
• Renal columns
  • Bands of tissue that separate adjacent renal pyramids.
• Kidney lobes (functional units)
  • Consist of:
    • A renal pyramid
    • An overlying renal cortex
    • Adjacent tissues of the renal columns
  • Where urine is produced.

Filtrate Drainage into the Renal Sinus

Filtrate (urine) produced in each kidney lobe is transported to the ureter via a series of structures:

(1) Renal papilla

• Filtrate passes from the renal pyramid to the renal sinus.

(2) Minor calyces

• Collects urine produced by a single kidney lobe.

(3) Major calyces

• Fusion of 4-5 minor calyces.
• Collects urine from minor calyces.

(4) Renal pelvis

• Large funnel-shaped chamber
• Continuous with the ureter

(5) Ureter

• Drains the urine from the kidney to the bladder.
• Every ~30 seconds, a peristaltic wave sweeps along the ureter.

The Ureters, Bladder & Urethra

The Ureters

• A pair of muscular tubes that extend from the kidneys to the urinary bladder (posterior wall).
• ~25-30 cm long and retroperitoneal.
• Firmly attached to the posterior abdominal wall.
• Mucosa:
  • Consists of transitional epithelium (cuboidal squamous).
  • Allows for expansion of ureter diameter.
• Muscularis:
  • Facilitates peristalsis (movement of urine).
  • Upper 2/3: Two layers of smooth muscle (inner longitudinal, outer circular).
  • Lower 1/3: Three layers of smooth muscle (inner longitudinal, middle circular, outer longitudinal).

The Bladder

• Hollow organ that temporarily stores urine.
• Mucosa:
  • Consists of transitional epithelium (cuboidal squamous).
  • Rugae: facilitates expansion.
• Muscularis:
  • Facilitates expulsion of urine.
  • Three layers of smooth muscle (inner longitudinal, middle circular, outer longitudinal).
• Sphincters:
  • Bands of muscle that control urine flow.
  • Internal urethral sphincter (involuntary).
  • External urethral sphincter (voluntary).

The Urethra

• Transports urine from the bladder to the exterior of the body.
• ~20-25 cm in males and ~4cm in females.
  • In females: Transports urine only.
  • In males: Transports urine and semen.
• Mucosa:
  • Epithelium varies along the length of the urethra.
    • Proximal end: Transitional.
    • Distal end: Stratified squamous.
    • Males have a middle zone of pseudostratified columnar epithelium.
• Muscularis:
  • Facilitates expulsion of urine.
  • Two layers of smooth muscle (inner longitudinal, outer circular).

Micturition (Urination)

• Urine reaches the bladder by peristaltic contractions of the ureters.
• When the bladder is full, stretch receptors in the bladder wall trigger the micturition reflex.
• The urge to urinate generally appears when the bladder contains ~200mL.
  • If ignored, urine will continue to accumulate in the bladder.
  • At ~500 mL, muscle contractions force the internal urethral sphincter open.

The Micturition Reflex

1. Detrusor muscle (surrounding bladder) contracts in response to a full bladder (involuntary).
2. Internal urethral sphincter relaxes (involuntary).
3. Brain sends instruction.
4. External urethral sphincter relaxes (voluntary).
5. Urine flows through the urethra and out of the body.

Voluntary Control

• If the neurological pathway for control of the external urethral sphincter has not yet been established, urination will occur involuntarily (e.g., infants).
• At 2-3 years, the brain is mature enough to execute voluntary control of the external urethral sphincter.
• If the external urethral sphincter is not powerful enough to prevent it, urination will occur involuntarily (e.g., elderly).

Structures of the Nephron

The Nephron

• Nephrons are the functional units of the kidney.
  • ~1 million nephrons per kidney.
• Cortical nephrons (~ 85%)
  • Located almost entirely within the renal cortex.
  • Important for excreting waste products in urine.
• Juxtamedullary nephrons (~15%)
  • Have long nephron loops that extend deep into the renal medulla.
  • Essential for producing concentrated urine.

Components of the Nephron

The nephron consists of three major components:

• Renal corpuscle:
  • Where water and dissolved solutes are pushed out of the blood and into the renal tubule.
  • Site of blood filtration.
• Renal tubule:
  • Where the components of the filtrate are altered (via secretion and absorption).
  • Site of filtrate modification.
• Collecting system:
  • Tubular fluid (urine) from each nephron empties into the collecting system (→ minor calyces).

The Nephron: Renal Corpuscle

• The ‘start’ of the nephron.
• Spherical filtration structure consisting of:
  • Glomerulus (capillary network).
    • Afferent arterioles – going in.
    • Efferent arterioles – going out.
    • Efferent are smaller diameter = ↑ glomerular pressure.
  • Glomerular capsule (Bowman’s capsule).
• Blood pressure forces water/solutes out of the glomerular capillaries into the capsule space and then into the renal tubule.
• Filtration of blood from the glomerulus into the glomerular capsule via the filtration membrane.

The Filtration Membrane

1. Fenestrated endothelium stops cells and platelets.
2. Basement membrane stops large proteins.
3. Filtration slits between pedicels (foot processes of podocytes) stop medium-sized proteins.

• Blood enters the glomerulus (via afferent arteriole) → filterable blood components move into the capsular space (= filtrate) → nonfilterable components exit the glomerulus (via efferent arteriole)
  • (1) Glomerular capillary endothelium is fenestrated (blocks cells)
  • (2) Underlying basement membrane (blocks large/charged proteins)
  • (3) Filtration slits - gaps between adjacent pedicels – (blocks medium-sized proteins)
• ~20% of blood pumped by the heart each minute will undergo filtration.

The Nephron: Renal Tubule

• Proximal Convoluted Tubule (PCT)
  • Beginning of the renal tubule.
  • Reabsorption of essential substances from the filtrate back into the blood.
  • Cells have microvilli to aid reabsorption.
• Nephron Loop (Loop of Henle)
  • Middle segment of the renal tubule.
  • Descending and ascending portions.
  • Descending = reabsorption of water.
  • Ascending = reabsorption of Na+ and Cl- from the filtrate.
• Distal Convoluted Tubule (DCT)
  • Last segment of the renal tubule.
  • Only 15-20% of the initial filtrate volume reaches the DCT.
  • Adjusts filtrate composition via reabsorption and secretion.

The Nephron: Collecting Duct & Papillary Duct

• The Collecting Duct
  • Filtrate is carried through the osmotic gradient in the renal medulla.
  • Water may/may not be reabsorbed depending on hydration status.
• The Papillary Duct
  • Collects filtrate from multiple collecting ducts and delivers it to a minor calyx.

Blood Flow Through the Kidneys

• About 20% of the blood pumped by the heart each minute is sent to the kidneys for filtration. The remaining 80% goes to the rest of the body for gas exchange.

1. O_2-rich blood from the heart arrives via the renal artery.
2. The renal artery divides into segmental arteries in the renal sinus.
3. The segmental arteries branch into interlobar arteries, which run within the renal columns.
4. Interlobar arteries continue to branch into smaller and smaller vessels, culminating in the afferent arterioles that supply each nephron (i.e., the blood vessel entering the glomerulus).
5. The efferent arteriole carries blood from the glomerulus to the peritubular capillaries.
6. The peritubular capillaries surround the entire renal tubule.
   • Collect water and solutes absorbed by the nephron.
   • Deliver other solutes to the nephron for secretion.
7. The peritubular capillaries drain into cortical veins, which carry filtered blood back to the inferior vena cava.

Renal Physiology

• The kidneys maintain homeostasis by regulating the volume and composition of blood.
• Involves three distinct physiological processes:

1. Filtration
   • Only occurs in the renal corpuscle of the nephron.
   • Solutes within the blood pass through the filtration membrane and into the nephron.
2. Reabsorption
   • Transport of water and solutes from the tubular fluid (i.e., filtrate) into the peritubular fluid (→ bloodstream).
3. Secretion
   • Transport of solutes from the peritubular fluid into the tubular fluid (i.e., filtrate).

Glomerular Filtration at the Renal Corpuscle

• Driven by blood pressure.
• ~180 L/day filtrate enters the glomerular capsules.
• ~178-179 L/day is reabsorbed back into the blood.
• ~ 900mL – 2L of urine is produced.
• Filtration is enhanced by:
  • Thinness of the filtration membrane.
  • Large surface area of glomerular capillaries.
  • High glomerular BP (due to ↓ size efferent arteriole).
• Glomerular Filtration Rate (GFR)
  • Formula based on creatinine levels (waste product), age, and gender.
  • Roughly equates to kidney function.
  • E.g., GFR 60 = kidneys working at ~60%.

Reabsorption at the Proximal Convoluted Tubule

• Reabsorption of essential substances from the filtrate back into the blood via channels, pumps, and co-transporters:
  • Water
  • Glucose
  • Ions
  • Small proteins
  • Organic nutrients
• >99% of glucose, amino acids, and other organic nutrients are reabsorbed – water follows along with it.
• Cells have microvilli to aid reabsorption.

Reabsorption at the Nephron Loop (Loop of Henle)

Descending Loop of Henle

• Further reabsorption of water from the filtrate.
• Filtrate becomes more concentrated (saltier).
• Osmolarity: 400 → 1200 mOsm/L

Ascending Loop of Henle

• Reabsorption of Na^+ and Cl^- from the filtrate.
• Impermeable to water.
• Filtrate becomes more dilute (less salty).
• Osmolarity: 1200 → 100 mOsm/L

Secretion and Reabsorption at the Distal Convoluted Tubule

• Only 15-20% of the initial filtrate volume reaches the DCT.
• Adjusts filtrate composition via reabsorption and secretion.
• Secretion of substances into the filtrate via exchange pumps and carrier proteins
  • Ions (Na^+ reabsorbed in exchange for K^+)
  • Acids (H^+ secreted in exchange for Na^+)
  • Drugs and toxins – via carrier proteins
• Variable reabsorption of:
  • Water (hormonally controlled)
  • Na^+ & Ca^{++}

Reabsorption at the Collecting Ducts

• Where filtrate from each nephron empties into the collecting system (carried through the renal medulla → minor calyces).
  • If you are hydrated
    • Collecting ducts do not reabsorb any more water (filtrate remains 100 mOsm/L)
  • If you are dehydrated
    • Collecting ducts reabsorb additional water (filtrate conc. increases 100 → 1200 mOsm/L) OR

Summary of the Functions of each Nephron Segment

Metabolic Waste Products

• The kidneys allow the removal of excess water, salts, and metabolic wastes through the production of urine
• Three important metabolic waste products:
  • Urea
    • Most abundant waste product
    • A by-product of amino acid breakdown in the liver
  • Creatinine
    • Generated in skeletal muscle tissue through the breakdown of creatine phosphate (involved in muscle contraction)
  • Uric acid
    • A by-product of recycling the nitrogenous bases of RNA molecules

Body Fluid

• Represents 55-60% of body mass.
• Declines with age (as fat increases).
• Locatons:
  • Intracellular fluid (ICF)
  • Extracellular fluid (ECF)
• Exchange occurs across:
  • Cell membranes: ICF ⇄ ECF
  • Capillary walls: ECF ⇄ blood plasma

Body Fluid - Location

• ICF: 2/3
• ECF: 1/3
  • Interstitial fluid: 80%
  • Plasma: 20%

Water Gain

• Metabolic water
  • Generated in reactions
  • NOT regulated
• Ingested foods
• Ingested liquids
  • Regulated in the hypothalamus
  • Triggered by a drop in blood pressure/blood volume

Water Loss

• Through the kidneys
  • The body’s fluid volume is determined by the extent of salt loss through urine
  • Water ‘follows’ salt out (osmosis)
  • Depends on dietary salt levels
• Evaporation from the skin
• Exhaled from lungs
• Component of faeces

Regulation of Water Loss

1. Eating a salty meal: Increases Na^+ & Cl^- ions in blood
2. Water moves from: ICF → ECF → blood plasma (via osmosis)
3. This increases: blood volume & urine output

Electrolytes

• Electrolytes = charged ions in solution
  • E.g. sodium (Na^+), potassium (K^+), calcium (Ca^{++}), chloride (Cl^-), phosphate (PO_4^{3-})
• Electrolytes are involved in multiple crucial functions:
  • Control osmosis
  • Maintain acid-base balance
  • Carry electrical currents

Sodium (Na^+)

• Most abundant in ECF
• More consumed than required (excreted)
• Action potentials

Potassium (K^+)

• Most abundant in ICF
• Action potentials

Calcium (Ca^{2+})

• Most abundant mineral in the body
• Blood clotting, neurotransmitter release

Phosphate (PO_4^{3-})

• Calcium phosphate in bone, teeth
• Buffers acidic body fluids & urine