Urinary System and Kidney Function

Primary Functions:
- Filters toxic waste from the blood (specifically nitrogenous waste).
- Regulates water volume, ion balance, pH levels, influences red blood cell production, and blood pressure.
Main Focus: Understanding urine production (pee).

Common misconception: Kidneys are merely filters that trap waste.
Actual function: Kidneys mostly remove everything from blood and selectively reabsorb necessary components before sending waste to the bladder.
Analogy: Cleaning out the fridge involves removing all items, not just the bad ones, to identify what to keep and what to discard.

Example scenario: Consuming a 32-ounce protein smoothie.
Protein is hydrolyzed into amino acids, which enter the bloodstream to build/repair cells, but produces waste.
Amino acids contain nitrogen in their amine groups; excess amino acids are converted into storable carbohydrates or fats.
Byproduct of this process: Ammonia (NH₃), which is toxic.
The liver converts ammonia into urea (less toxic), which kidneys filter out to produce urine.
Explanation for strong urine odor: Urea can degrade back to ammonia.

Kidneys:
- Structure: Bean-shaped, dark red organs located retroperitoneally (behind the abdominal cavity).
- Composed of three layers:
1. Cortex: Outermost layer.
2. Medulla: Contains cone-shaped tissues that secrete urine into tubules.
3. Renal Pelvis: Funnel-shaped structure that moves urine to ureter using peristalsis.
Blood Supply:
- Kidneys receive about 20% of total blood volume; filter around 120 to 140 liters daily.
- Hemodynamic significance: Renal arteries deliver nearly a quarter of blood pumped by the heart each minute.

Definition: About a million nephron units per kidney, responsible for urine production.
Three Main Functions:
1. Filtration: Removes waste from blood.
2. Reabsorption: Reclaims necessary substances.
3. Secretion: Eliminates additional waste.
Components:
- Renal Corpuscle: Round structure where filtration occurs, contains:
- Glomerular Capsule: Cup-shaped part housing the glomerulus (a cluster of capillaries).
- Porous Endothelium: Permits passage of fluids while blocking larger molecules.
- Filtrate: All filtered substances from blood into the glomerulus.
- Renal Tubule: Long and winding tube that processes the filtrate; consists of:
1. Proximal Convoluted Tubule (PCT)
2. Loop of Henle (nephron loop)
3. Distal Convoluted Tubule (DCT)

Proximal Convoluted Tubule (PCT):
- Structure: Walls of cuboidal epithelial cells; contains mitochondria for ATP production to power sodium pumps.
- Function: Actively reabsorbs sodium and other valuable substances (glucose, ions).
Loop of Henle:
- Consists of:
- Descending Limb: Primarily allows water reabsorption due to salt gradient in medulla.
- Ascending Limb: Actively pumps out salts, increasing concentration gradient.
Distal Convoluted Tubule (DCT) and Collecting Duct:
- Remaining filtrate transitions to urine; additional reabsorption may occur (esp. water).
- Both structures allow final adjustments in the concentration of urine.

Urea's role in kidney function: Used to enhance osmotic gradient for further water reabsorption.
Urea travels through the collecting duct, enhancing the osmotic gradient in the medulla.
Cycle of urea: Passively escapes urine, returns to the loop of Henle, enhancing its filtration ability.
This process ensures efficient reabsorption of water and minerals crucial for survival.

Definition: The last step that eliminates extra waste into the filtrate, mainly through active transport.
Components secreted: hydrogen, potassium, certain organic acids and bases.
Comparison: Similar to emptying pockets of additional waste before throwing trash away.

Urinary system is more complex than a simple filtration system; involves intricate processes of reabsorption and secretion.
Next focus: Regulation of absorption/excretion in the urinary system and impact of misregulation.