Lecture Module B-7: Urinary System Review Questions

Objective 1 - Introduction

Major functions of the urinary system:
- Excretion
- Elimination
- Homeostatic functions
Definitions:
- Excretion: Removal of organic wastes from body fluids.
- Elimination: Discharge of waste products (urination).
Homeostatic functions:
- Regulating blood volume and pressure.
- Regulating plasma ion concentration.
- Stabilizing blood pH.
- Conserving nutrients.
- Assisting in detoxification.

Glomerulus: A ball of capillaries part of the renal corpuscle.
Supporting cells: Mesangial cells (smooth muscle cells).
Outer glomerular capsule lining: Simple squamous epithelium.
Podocytes: Large inner visceral epithelial "foot" cells covering glomerular capillaries.
Pedicels: "Feet" of podocytes.
Filtration membrane layers:
1. Glomerular capillary endothelium
2. Lamina densa
3. Filtration slits of podocytes
Filtration: Passive process in renal corpuscle.
Presence of plasma proteins in filtrate: No.

Functions of the renal tubule:
- Reabsorption of nutrients
- Reabsorption of water
- Secretion of wastes
Parts of the renal tubule:
- Proximal convoluted tubule
- Nephron loop
- Distal convoluted tubule
Proximal convoluted tubule function: Reabsorption.
Nephron loop limbs: Descending and ascending limbs.
Function of nephron loop: Concentrates tubular fluid via water movement.
Functions of distal convoluted tubule:
- Secretion of ions, acids, drugs, toxins
- Reabsorption of water
- Reabsorption of sodium/calcium
Juxtaglomerular complex (JGA): Releases erythropoietin and renin, increasing RBC count and blood pressure.

Nephrons drain into: Collecting duct.
Collecting ducts converge: Into a papillary duct.
Microscopic collecting ducts: Determine final osmotic concentration and volume of urine.

Urine production: Typically concentrated; failure to concentrate leads to dehydration.
Processes of urine formation:
- Filtration
- Reabsorption
- Secretion
Filtration location: Renal corpuscle.
Reabsorption and secretion location: Renal tubule.
Filtration process: Passive; small solutes pass through.
Presence of proteins in urine: No, due to restriction at the filtration membrane.
Transport in reabsorption and secretion: Active and passive (diffusion, osmosis, carrier-mediated transport).
Reabsorption direction: Out of the renal tubule into the bloodstream.
Secretion direction: Into the renal tubule from the bloodstream.
Transport Maximum/Renal Threshold:
- Carrier proteins can be saturated.
- Solute concentration increases until saturation.
- Renal threshold is the plasma concentration at which a compound appears in urine.
- Glycosuria is glucose in urine.
- Amino acids may appear in urine post protein-rich meal.
Reabsorption location: Primarily in proximal convoluted tubule.
Secretion location: Proximal and distal convoluted tubules.
Nephron types: Cortical (85%) and juxtamedullary (water conservation).
Osmolarity: Total number of solute particles per liter.

Filtration pressure: Average pressure forcing out water and dissolved materials.
Glomerular hydrostatic pressure: Higher than normal capillaries (needed for blood into efferent arterioles).
Glomerular filtration rate (GFR): Amount of filtrate produced per minute.
Creatinine clearance test: Estimates GFR.
GFR control:
- Autoregulation
- Hormonal regulation
- Autonomic regulation
Responses to blood flow/pressure changes:
- Reduced flow triggers afferent dilation.
- Increased flow triggers afferent constriction.
Hormonal regulation: Via renin-angiotensin-aldosterone system.
Renin release trigger: Decline in blood pressure.
Renin effects: Activates angiotensin II, constricts efferent arterioles, stimulates secretion of aldosterone and antidiuretic hormone, stimulates sympathetic nervous system.

Aldosterone: Stimulates sodium channels synthesis along distal convoluted tubule and collecting duct.
Natriuretic peptides (ANP & BNP): Oppose aldosterone.
ADH: Increases osmotic water movement into the bloodstream.
Diuresis: Elimination of urine.
Renal function summary:
- Glomerulus: Produces filtrate.
- PCT: Reabsorption of water, organic substances, ions.
- Nephron loop: Filtrate concentration.
- DCT: Final filtrate adjustments; reabsorption or secretion.
- Collecting ducts: Final filtrate adjustments; ADH effect.
- Vasa recta: Absorbs solutes, maintains concentration gradient of medulla.

Urination: Also called micturition.
Micturition reflex: Detrusor muscle contraction, voluntary relaxation of the external urethral sphincter, causes relaxation of the internal urethral sphincter.
Voluntary control in infants: No.
Incontinence: Inability to control urination voluntarily.
Age-related changes:
- Decline in functional nephrons.
- Reduction of GFR.
- Reduced sensitivity to ADH.

Fluid compartments: Intracellular fluid (ICF), extracellular fluid (ECF).
ICF volume: 2/3 of body fluid volume.
ECF components: 80% interstitial fluid.
Body weight: 60% water.
Plasma: Sodium (chief cation), chloride (chief anion).
ICF: Potassium (chief cation), phosphate (chief anion).
Solutes in plasma and ICF: Proteins are major solutes.
Fluid balance: Maintain proper volume and composition of ICF/ECF.
Factors for composition balance: Fluid, electrolyte, acid-base.
Water gain/loss systems: Digestive for gain, urinary for loss.
Electrolyte systems: Digestive for absorption, urinary for excretion.
Acid-base balancing: Urinary (secreting hydrogen ions) and respiratory (eliminating CO2).