Urinary System

What are the six primary functions of the urinary system?

1. Filters blood and excretes wastes.

2. Regulates blood's chemical composition (pH, osmolarity).

3. Regulates blood pressure (via blood volume and renin).

4. Produces erythropoietin (stimulates RBC production).

5. Produces calcitriol (active Vitamin D for calcium absorption).

6. Performs gluconeogenesis (glucose production during fasting).

Describe the location and protective layers of the kidneys.

• Location: Retroperitoneal, on either side of the vertebral column at T12-L1.

• Layers (from inner to outer):

  1. Fibrous capsule

  2. Adipose capsule (for cushioning)

  3. Renal fascia (for anchoring)

What are the structure and function of the ureters?

• Function: Transport urine from kidneys to bladder.

• Structure: Transitional epithelium, two smooth muscle layers, fibrous adventitia.

Describe the urinary bladder's location, function, and key structural feature.

• Location: Pelvic floor.

• Function: Stores urine.

• Key Feature: Has three layers of smooth muscle (detrusor muscle) for expulsion.

How does the male and female urethra differ, and what structure controls urine flow?

• Difference: Length (longer in males).

• Flow Control: Internal (involuntary) and external (voluntary) urethral sphincters.

Trace the path of filtrate/urine from the renal corpuscle to the urethra.

1. Renal Corpuscle → 2. Proximal Convoluted Tubule (PCT) → 3. Loop of Henle → 4. Distal Convoluted Tubule (DCT) → 5. Collecting Duct → 6. Papillary Duct → 7. Renal Papilla → 8. Minor Calyx → 9. Major Calyx → 10. Renal Pelvis → 11. Ureter → 12. Bladder → 13. Urethra

What is the function of the Renal Corpuscle (Glomerulus & Bowman's Capsule)?

It is the site of filtration, where fluids and solutes are forced from the blood into Bowman's capsule to form filtrate.

What processes occur in the Proximal Convoluted Tubule (PCT)?

Reabsorption of most water, nutrients, and ions. Secretion of wastes and drugs.

What is the primary function of the Loop of Henle?

To create a medullary osmotic gradient via the countercurrent mechanism, which is vital for concentrating urine.

What processes occur in the Distal Convoluted Tubule (DCT) and Collecting Duct?

Fine-tuning of reabsorption and secretion under hormonal control (e.g., Aldosterone, ADH). The Collecting Duct is the primary site for water reabsorption regulated by ADH.

What is the role of the Peritubular Capillaries?

They surround the renal tubules and are crucial for reabsorption (substances from tubules to blood) and secretion (substances from blood to tubules).

Trace the pathway of blood through the renal system from the left ventricle to the right atrium.

Left Ventricle → Aorta → Renal Artery → Segmental → Interlobar → Arcuate → Interlobular Arteries → Afferent Arteriole → Glomerulus → Efferent Arteriole → Peritubular Capillaries/Vasa Recta → Interlobular → Arcuate → Interlobar Veins → Renal Vein → Inferior Vena Cava → Right Atrium

What are the four pressures that determine Net Filtration Pressure (NFP)?

1. Glomerular Capillary Hydrostatic Pressure (P_GC): Promotes filtration (~60 mmHg).

2. Bowman's Capsule Oncotic Pressure (π_BC): Promotes filtration (0 mmHg).

3. Bowman's Capsule Hydrostatic Pressure (P_BC): Opposes filtration (~15 mmHg).

4. Glomerular Oncotic Pressure (π_GC): Opposes filtration (~29 mmHg).

What is the formula for Net Filtration Pressure (NFP) and the normal value?

• Formula: NFP = (P_GC + π_BC) - (P_BC + π_GC)

• Normal Value: 16 mmHg

What is the relationship between Net Filtration Pressure (NFP) and Glomerular Filtration Rate (GFR)?

GFR is directly proportional to NFP. If NFP increases, GFR increases. If NFP decreases, GFR decreases.

What are the two types of intrinsic autoregulation of GFR and how do they work?

1. Myogenic Response: Increased BP stretches afferent arteriole, causing it to constrict, which decreases GFR back to normal.

2. Tubuloglomerular Feedback: Macula densa cells detect high filtrate salt (from high GFR) → signal juxtaglomerular (JG) cells → afferent arteriole constricts → GFR decreases.

How does the Sympathetic Nervous System (extrinsic control) affect GFR?

During stress, it releases norepinephrine, causing vasoconstriction of afferent arterioles, which decreases GFR to redirect blood to vital organs.

What is the overall effect of the Renin-Angiotensin-Aldosterone System (RAAS) on GFR?

It decreases GFR. Angiotensin II constricts afferent arterioles and mesangial cells, reducing blood flow and filtration surface area.

What is the effect of Atrial Natriuretic Peptide (ANP) on GFR?

It increases GFR by relaxing mesangial cells (increasing filtration surface area) and suppressing Na+ and water reabsorption.

What is the purpose of Tubular Reabsorption?

To reclaim water and essential solutes (glucose, amino acids, ions) from the filtrate back into the blood.

What is the purpose of Tubular Secretion?

To remove waste products and regulate blood pH by moving substances (like H+ and K+) from the blood into the filtrate.

How is Sodium (Na+) reabsorption regulated?

Primarily by Aldosterone, which increases Na+ reabsorption in the DCT and collecting duct, leading to increased water reabsorption and higher blood volume/pressure.

How is Water reabsorption regulated?

Primarily by Antidiuretic Hormone (ADH), which makes the collecting duct permeable to water, allowing for water reabsorption and concentrated urine.

How is Potassium (K+) secretion regulated?

By Aldosterone, which promotes K+ secretion into the filtrate in the collecting duct to excrete excess potassium.

How is Hydrogen Ion (H+) secretion regulated?

By blood pH. When blood is too acidic, H+ secretion increases. When blood is too alkaline, H+ secretion decreases. This is managed by intercalated cells.

What is osmolarity?

The number of solute particles per liter of solution. It determines the direction of water movement via osmosis.

What is the role of the Countercurrent Mechanism in the Loop of Henle?

To create and maintain a high osmotic gradient in the renal medulla, which is essential for the kidneys to produce concentrated urine.

How is Dilute Urine formed?

When water needs to be excreted, ADH is low, making the collecting duct impermeable to water. Dilute filtrate is excreted. ANP also promotes dilute urine.

How is Concentrated Urine formed?

When water needs to be conserved, ADH is high, making the collecting duct permeable to water. Water is reabsorbed into the hyperosmotic medulla, concentrating the urine.

Describe the Micturition Reflex.

1. Bladder fills (~200 mL) and stretch receptors are activated.

2. Signals are sent to the sacral spinal cord, initiating the reflex.

3. Impulses are sent to the brain, creating the urge to urinate.

4. If it's time to void, the brain signals the pons to relax the internal and external urethral sphincters and contract the detrusor muscle.

5. If not, the external sphincter remains voluntarily closed.