Urine Formation 1 - 3.17.25

Excretion: Separation and elimination of waste products
Key Nitrogenous Wastes:
- Urea: 50% (H2N)2CO
- Uric acid
- Creatinine
- Ammonia

Process Flow:
1. Glomerular filtration
2. Tubular reabsorption
3. Tubular secretion
Components:
- Afferent arteriole
- Glomerulus
- Efferent arteriole
- Glomerular capsule
- Renal tubule and collecting duct
- Peritubular capillary
- Resulting substance: Urine

Process: Water and solutes move from plasma into nephron via hydrostatic pressure.
Key Structures:
- Afferent arteriole
- Glomerulus
- Efferent arteriole
- Glomerular capsule

Capillary wall:
- Fenestrated capillaries hold back blood cells and large proteins
Basement membrane:
- Holds back most large molecules

Filtration slits:
- Spaces between cells of the visceral layer
- Allow passage of most small molecules (e.g., water, electrolytes, glucose)
- Molecules bound to plasma proteins (e.g., calcium, iron) are restricted.

Turned Back:
- Blood cells, plasma proteins, large anions, protein-bound minerals, hormones, most molecules > 8 nm in diameter
Passed Through Filter:
- Water, electrolytes, glucose, amino acids, fatty acids, vitamins, urea, uric acid, creatinine

High Glomerular Blood Pressure:
- Influenced by small diameter of efferent arteriole
Net Filtration Pressure (NFP):
- Outward pressures exceed inward pressures, promoting filtration
Sensitivity:
- Kidneys are sensitive to hypertension.

Definition: Amount of filtrate formed each minute by both kidneys.
Calculation: GFR = NFP x Kf
Typical Rates:
- Males: ~125 mL/min or 180 L/day
- Females: ~105 mL/min or 150 L/day
Urine Excretion: 1-2 L daily, with 99% of filtrate reabsorbed.

Importance: Ensures stability despite systemic fluctuations.
High GFR: Too much fluid lost as urine.
Low GFR: Wastes reabsorbed.
Controls:
- Intrinsic controls: renal autoregulation
- Extrinsic controls: nervous or hormonal regulation

Components:
- Macula densa: Chemoreceptors monitoring NaCl content
- Granular cells: Monitor blood pressure in afferent arteriole
- Mesangial cells: Transmit signals between granular cells and macula densa

Response to High GFR:
- Reduces GFR
- High NaCl sensed by macula densa
- Causes constriction of afferent arteriole
- Signals to granular cells to contract via mesangial cells

Function: Address large changes in blood pressure.
Neural control: Sympathetic stimulation constricts afferent arteriole, reducing GFR and urine output, redirecting blood away from kidneys.

Mechanism: Indirect renal regulation
Trigger: Decrease in systemic BP detected by baroreceptors.
Renin Release: From granular cells in response to sympathetic input and decreased stretch.
Sequence: Renin -> Angiotensinogen to Angiotensin II via ACE in lungs.

Functions:
- Vasoconstriction of efferent arteriole, raising glomerular blood pressure
- Lowers BP in peritubular capillaries, enhancing absorption
- Stimulates aldosterone and ADH secretion, promoting salt and water retention
- Stimulates thirst through the hypothalamus, increasing systemic blood pressure.

Location: Primarily in the proximal convoluted tubule (PCT).
Process:
- Necessary water and solutes returned to blood.
- Produces hypertonic tubular fluid by reabsorbing almost all organic materials, H2O, and some ions selectively.

Mechanisms:
- Active transport through tubule walls
- Symports: bind Na+ and another solute simultaneously
- Na+-H+ antiports and Na+-K+ pumps drive reabsorption.
- Alters osmotic and electrical gradients.

Driving Force: Osmotic gradient.
Aquaporins: Channels in tubule cells facilitating water reabsorption.
- Always present in PCT (obligatory water reabsorption).
- Present in collecting ducts only if ADH is present (facultative water reabsorption).
- Solvent drag: Water carries along dissolved solutes.

Substances:
- Glucose, amino acids, vitamins, other ions.
Transport Mechanisms:
- Some co-transported with Na+, while others are carried with water.
- Lipid-soluble solutes follow concentration gradient across tubule wall.
Nitrogenous Wastes:
- Almost all uric acid and ~50% urea secreted back later.

Key Factors for absorption:
Absorbs all materials that are passing out of nephron are returned to system through these capillaries
Specifically associated with cortical nephrons
- High interstitial hydrostatic pressure (HPIF) due to reabsorbed fluid.
- Low capillary hydrostatic pressure (HPC) from narrow efferent arterioles.
- High colloid osmotic pressure (COPC) due to retained plasma proteins during filtration.

Definition: Maximum amount of solute renal tubules can reabsorb (mg/min).
Significance: Sets the maximum reabsorption rate, each solute having its own Transport maximum; saturation of transporters impedes reabsorption.

Definition: Presence of glucose in urine.
Normal Filtration Rate: Glucose is typically reabsorbed at ~125 mg/min.
Glucose Tm: 320 mg/min; high blood glucose may exceed Tm, resulting in increased glucose in urine and reduced H2O reabsorption, linked with diabetes mellitus.
Expect glucose to be reabsorbed before turned into urine so no glucose in the urine unless you have high glucose levels in the blood, indicating a possible disorder of glucose metabolism.