Chapter 24: Urinary System

What are the main functions of the kidneys?

• Maintain chemical balance in the blood

• Filter blood and remove wastes

• Eliminate excess water, toxins, and waste as urine

What are the three main nitrogenous waste products?

• Urea – from protein breakdown

• Uric acid – from nucleic acid breakdown

• Creatinine – from muscle metabolism (creatine phosphate)

What organs are part of the urinary system?

• Kidneys

• Ureters

• Urinary bladder

• Urethra

Where are the kidneys located?

• Retroperitoneal (behind the peritoneum)

• Lateral to vertebrae T12–L3

• Protected partly by the 12th rib

What is the renal hilum?

• The indented area on the kidney's surface

• Entry/exit point for blood vessels, nerves, and ureter

What is the fibrous (renal) capsule?

A tough outer layer that surrounds and protects the kidney

What are the internal regions of the kidney?

• Renal cortex – outer layer

• Renal medulla – contains renal pyramids

• Renal pelvis – collects urine and leads to ureter

What is the renal pyramid?

• Cone-shaped tissue in the medulla

• Ends at the papilla, where urine drains into minor calyces

What is the renal column?

• Tissue between the renal pyramids

• Part of the cortex that extends inward

What is the function of the renal pelvis?

Funnels urine from the kidney into the ureter

How much blood do the kidneys receive?

About 25% of the heart’s total output goes to the kidneys through the renal arteries

What is the blood flow path through the kidney (arteries)?

1. Aorta

2. Renal artery

3. Segmental artery

4. Interlobar artery

5. Arcuate artery

6. Cortical radiate artery

7. Afferent arteriole

8. Glomerulus (capillaries)

What happens after blood is filtered in the glomerulus (veins)?

1. Efferent arteriole

2. Peritubular capillaries or vasa recta

3. Cortical radiate vein

4. Arcuate vein

5. Interlobar vein

6. Renal vein

7. Inferior vena cava

What is the nephron?

• The basic functional unit of the kidney

• Filters blood and forms urine

What are the two main parts of a nephron?

1. Renal corpuscle – where filtration begins

2. Renal tubule – where filtration is processed into urine

What makes up the renal corpuscle?

• Glomerulus – capillary network

• Glomerular (Bowman’s) capsule – surrounds the glomerulus

What are the segments of the renal tubule?

1. Proximal convoluted tubule (PCT)

2. Nephron loop (Loop of Henle)

   • Descending limb

   • Ascending limb

3. Distal convoluted tubule (DCT)

4. Collecting duct

What is the function of the proximal convoluted tubule?

• Reabsorbs most nutrients, water, and ions

• Lined with cells that have microvilli to increase surface area

What is the function of the nephron loop (loop of Henle)?

• Concentrates urine by creating a salt gradient in the medulla

• Descending limb: reabsorbs water

• Ascending limb: reabsorbs salts (Na⁺, Cl⁻)

What is the function of the distal convoluted tubule?

• adjusts salt and pH balance

• Reabsorbs calcium, sodium, and other ions

What is the collecting duct?

• Collects urine from many nephrons

• Helps conserve water (under hormone control like ADH)

What are the two types of nephrons?

• Cortical nephrons – 85%, short loops, mostly in cortex

• Juxtamedullary nephrons – 15%, long loops, help concentrate urine

What are peritubular capillaries?

• Tiny capillaries that surround the renal tubules

• Reabsorb water and solutes from the filtrate

What is the vasa recta?

• Long, looping capillaries around the nephron loop

• Found in juxtamedullary nephrons

• Key in urine concentration

What are the three basic processes of urine formation?

1. Filtration – Blood is filtered at the glomerulus

2. Reabsorption – Useful substances are taken back into the blood

3. Secretion – Extra wastes are added into the filtrate

Where does filtration occur?

• In the renal corpuscle

• Blood plasma (minus proteins) leaves the capillaries and enters the glomerular capsule

What is the filtration membrane?

A three-layer filter between blood and the glomerular capsule made of:

1. Fenestrated endothelium (leaky capillaries)

2. Basement membrane

3. Podocyte filtration slits

What can pass through the filtration membrane?

✅ Water
✅ Ions (Na⁺, K⁺)
✅ Glucose
✅ Amino acids
✅ Urea
❌ Most proteins and blood cells are too big to pass through

What is reabsorption?

• Movement of valuable substances (like water, glucose, ions) from the filtrate back into the blood

• Mainly occurs in the proximal convoluted tubule

What is secretion?

• Active transport of unwanted substances from the blood into the tubule

• Helps remove drugs, toxins, excess ions, and wastes

What percent of filtrate is reabsorbed?

• About 99% of the fluid filtered at the glomerulus is reabsorbed

• Only about 1% becomes urine

What structures are involved in secretion and reabsorption?

• Peritubular capillaries (for cortical nephrons)

• Vasa recta (for juxtamedullary nephrons)

Which part of the nephron makes the final adjustments to the fluid (filtrate) before it officially becomes urine?

• Distal convoluted tubule and collecting duct

• Controlled by hormones like aldosterone and ADH

What is the role of the collecting duct?

• Receives filtrate from multiple nephrons

• Adjusts water reabsorption depending on body needs (hormone-controlled)

What is the Juxtaglomerular Apparatus (JGA)?

A specialized structure near the glomerulus that helps regulate blood pressure and filtration rate

What are the main parts of the Juxtaglomerular Apparatus?

• Granular cells – in afferent arteriole, secrete renin

• Macula densa – in DCT, sense salt levels

• Mesangial cells – support and help in communication

What does renin do?

• Released by granular cells

• Starts the renin-angiotensin-aldosterone system (RAAS)

• Raises blood pressure and conserves sodium and water

What are the ureters?

• Tubes that carry urine from kidneys to the bladder

• Have smooth muscle to push urine (peristalsis)

• Enter bladder at an angle to prevent backflow

What are the layers of the ureter wall?

• Mucosa – transitional epithelium

• Muscularis – inner longitudinal + outer circular muscle

• Adventitia – connective tissue

What is the urinary bladder?

• A muscular sac that stores and expels urine

• When full: expands upward into the abdomen

• When empty: lies in the pelvis

What muscle makes up the bladder wall?

• Detrusor muscle – three layers of smooth muscle

What is the trigone of the bladder?

• Triangular area between the openings of the ureters and urethra

• Common site of UTIs

What is the urethra?

A tube that carries urine from the bladder to the outside of the body

How does the epithelium of the urethra change?

• Transitional epithelium (near bladder)

• Stratified/pseudostratified columnar (middle)

• Stratified squamous (near opening)

What are the urethral sphincters?

• Internal sphincter – smooth muscle, involuntary

• External sphincter – skeletal muscle, voluntary

How is the male urethra different from the female?

• Male: ~20 cm, 3 regions (prostatic, membranous, spongy)

• Female: ~3–4 cm, only carries urine

What is micturition?

• The act of urinating

• Controlled by brain + spinal cord reflex

• Involves relaxation of sphincters and contraction of detrusor muscle

What nervous systems control micturition?

• Parasympathetic: Stimulates urination

• Sympathetic: Inhibits urination

• Somatic: Voluntarily controls external sphincter

Urinary Tract Infections (UTIs)

• More common in females

• Burning during urination

Renal Calculi

• Kidney stones

• Crystallized calcium, salts, or uric acid

Bladder Cancer

• 3% of cancers

• More common in men

Kidney Cancer

Starts in tubule cells (epithelium)

Aging Effects on Urinary System

• Fewer & smaller nephrons

• Less efficient filtration

• Bladder muscle weakens

• Delayed urge to pee