25.5 Physiology of Urine Formation

Glomerular Filtration

Glomerular filtration is the kidney's first step in urine formation. Fluid and solutes pass selectively from blood in the glomerular capillaries into Bowman's capsule. This process is driven by Starling forces, and the rate at which it happens (the GFR) is one of the most important clinical measures of kidney health.

Forces in Glomerular Filtration

Four Starling forces determine the net filtration pressure (NFP) across the glomerular capillary wall. Two promote filtration, and two oppose it.

Hydrostatic pressures (physical pushing forces):

• Glomerular blood hydrostatic pressure ($P_{GC}$) promotes filtration by pushing fluid out of the capillaries and into Bowman's capsule. This is the main driving force behind filtration.
• Bowman's capsule hydrostatic pressure ($P_{BC}$) opposes filtration by pushing back against incoming fluid.

Osmotic (colloid oncotic) pressures:

• Glomerular capillary colloid osmotic pressure ($\pi_{GC}$) opposes filtration. Plasma proteins like albumin can't cross the filtration membrane, so they pull water back into the capillary by osmosis.
• Bowman's capsule osmotic pressure ($\pi_{BC}$) would promote filtration, but it's typically negligible because the filtrate is nearly protein-free.

The NFP equation puts these together:

$NFP = P_{GC} - P_{BC} - \pi_{GC} + \pi_{BC}$

A positive NFP means net filtration occurs (fluid moves into Bowman's capsule). A negative NFP would mean reabsorption back into the capillary, but this rarely happens under normal conditions.

Definition and Measurement of GFR

Glomerular filtration rate (GFR) is the volume of fluid filtered from the glomerular capillaries into Bowman's capsule per unit time. Normal GFR is approximately 125 mL/min, which works out to about 180 L/day. That sounds enormous, but the vast majority of that filtrate gets reabsorbed by the tubules.

GFR can be measured or estimated in a few ways:

• Inulin clearance is the gold standard. Inulin is an exogenous substance that is freely filtered at the glomerulus and is neither reabsorbed nor secreted by the tubules. That makes it a perfect marker. GFR is calculated as:

$GFR = \frac{U_{In} \times V}{P_{In}}$

where $U_{In}$ is urine inulin concentration, $V$ is urine flow rate, and $P_{In}$ is plasma inulin concentration.

• Creatinine clearance is more practical for clinical use. Creatinine is produced endogenously by muscle metabolism, is freely filtered, and is only minimally secreted by the tubules. Because a small amount is secreted, creatinine clearance slightly overestimates true GFR, but it's close enough for routine assessment. Clinicians often estimate GFR from serum creatinine levels using equations (CKD-EPI or MDRD) that account for age, sex, and body size.

Factors Affecting GFR

Renal blood flow directly influences GFR. More blood delivered to the glomerulus means more fluid available for filtration. Renal vasodilation increases GFR; renal vasoconstriction decreases it.

Afferent and efferent arteriolar resistance is a high-yield concept. Think of the glomerulus as sitting between two adjustable valves:

• Afferent arteriole constriction reduces blood flow into the glomerulus, lowering $P_{GC}$ and decreasing GFR.
• Efferent arteriole constriction restricts blood flow out of the glomerulus, which backs up pressure inside, raising $P_{GC}$ and increasing GFR. (However, severe efferent constriction can eventually reduce renal blood flow enough to decrease GFR.)

Glomerular capillary permeability affects how easily fluid crosses the filtration membrane. Damage to the membrane (as in glomerulonephritis) can increase permeability, allowing proteins and even blood cells into the filtrate. Scarring (glomerulosclerosis) decreases permeability and reduces GFR.

Systemic factors also play a role:

• Hypertension raises $P_{GC}$, which tends to increase GFR (though autoregulation normally compensates within a range of about 80–180 mmHg mean arterial pressure).
• Hypoalbuminemia (low plasma albumin) decreases $\pi_{GC}$, reducing the osmotic opposition to filtration and increasing GFR.

The juxtaglomerular apparatus (JGA) regulates GFR through tubuloglomerular feedback and the renin-angiotensin-aldosterone system (RAAS). When the macula densa cells of the JGA detect decreased $NaCl$ delivery (signaling low GFR), they trigger renin release, which ultimately produces angiotensin II. Angiotensin II preferentially constricts the efferent arteriole, helping restore GFR.

Renal Tubular Function and Urine Output

Process of Tubular Reabsorption

Once filtrate enters Bowman's capsule, the tubules selectively reclaim what the body needs and let waste products pass through to become urine. Tubular reabsorption is the movement of substances from the tubular lumen back into the peritubular capillaries. The proximal convoluted tubule (PCT) handles the bulk of this work, reabsorbing roughly 65% of filtered water, sodium, and most filtered glucose and amino acids. The loop of Henle further reabsorbs water and solutes to build the medullary osmotic gradient.

Reabsorption relies on several transport mechanisms:

1. Active transport

   • Primary active transport uses ATP directly to move substances against their concentration gradient. The $Na^+/K^+$ ATPase pump on the basolateral membrane of tubular cells is the key example; it keeps intracellular $Na^+$ low, which drives many other transport processes.
   • Secondary active transport harnesses the $Na^+$ gradient created by that pump to move another substance. For example, $Na^+$/glucose cotransporters (SGLT proteins) in the PCT use the favorable $Na^+$ gradient to pull glucose from the lumen into the cell against glucose's own concentration gradient.

2. Passive transport

   • Simple diffusion moves substances down their concentration gradient with no energy input. Urea and water (through aquaporins) follow this route.
   • Facilitated diffusion uses carrier proteins to move substances down their gradient. Glucose exits the basolateral side of PCT cells via GLUT transporters this way.

The net effect of reabsorption is to conserve glucose, amino acids, and other nutrients (so they don't appear in urine), maintain fluid and electrolyte balance by regulating $Na^+$, $Cl^-$, and water reabsorption, and help regulate acid-base balance through reabsorption of $HCO_3^-$ and secretion of $H^+$.

The countercurrent multiplication system in the loop of Henle deserves special attention. The descending limb is permeable to water but not solutes, so water leaves by osmosis as the tubule dips into the increasingly concentrated medulla. The ascending limb is impermeable to water but actively pumps out $Na^+$, $K^+$, and $Cl^-$. This creates and maintains the medullary osmotic gradient (ranging from about 300 mOsm/L at the cortex to 1200 mOsm/L at the inner medulla), which is essential for concentrating urine in the collecting duct under the influence of ADH.

Calculation of Urine Output

The basic relationship for daily urine output is:

$\text{Urine Output} = \text{GFR} - \text{Reabsorption} + \text{Secretion}$

Normal urine output is about 1–2 L/day, which means the tubules reabsorb the vast majority of the 180 L filtered daily.

The filtration fraction (FF) tells you what proportion of renal plasma flow actually gets filtered:

$FF = \frac{GFR}{RPF}$

Normal FF is approximately 20%, meaning one-fifth of the plasma flowing through the kidneys is filtered at the glomerulus.

Example calculation:

1. Given: $GFR = 125 \text{ mL/min}$, $RPF = 625 \text{ mL/min}$
2. $FF = \frac{125}{625} = 0.20 = 20\%$
3. Total daily filtrate: $125 \text{ mL/min} \times 1440 \text{ min/day} = 180 \text{ L/day}$
4. If reabsorption = 178.5 L/day and secretion is negligible:
5. $\text{Urine output} = 180 - 178.5 = 1.5 \text{ L/day}$ (within the normal range)

Signs of Kidney Dysfunction

Recognizing these clinical signs connects the physiology you've learned to real patient scenarios.

• Decreased GFR is the hallmark of impaired kidney function. As GFR drops, waste products accumulate in the blood. Elevated serum creatinine and blood urea nitrogen (BUN) are the classic lab findings. Oliguria (urine output < 400 mL/day) suggests a significant reduction in GFR.
• Proteinuria is the presence of protein (especially albumin) in the urine. Normally, the filtration membrane blocks large proteins. Proteinuria can result from increased glomerular permeability (damage to the membrane) or impaired tubular reabsorption of the small amount of protein that does get filtered.
• Hematuria is the presence of red blood cells in the urine. It may indicate glomerular damage or pathology elsewhere in the urinary tract. It can be microscopic (detected only by lab testing) or macroscopic (visibly discolored urine).
• Electrolyte imbalances arise from impaired tubular function:
  • Hyperkalemia (elevated blood $K^+$) can result from reduced potassium excretion and is dangerous because of its effects on cardiac rhythm.
  • Metabolic acidosis (decreased blood pH) occurs when the kidneys can't excrete enough $H^+$ or reabsorb enough $HCO_3^-$.
• Edema (fluid retention) develops when the kidneys can't adequately excrete salt and water. It can appear as peripheral edema (swelling in the extremities) or pulmonary edema (fluid in the lungs).
• Urinalysis is a straightforward diagnostic tool that can detect many of these abnormalities, including protein, blood, glucose, and changes in specific gravity or pH.

Urine Storage and Elimination

Micturition Reflex

Micturition is the process of emptying the urinary bladder. The micturition reflex is triggered when the bladder fills to approximately 200–300 mL, stretching the bladder wall enough to activate stretch receptors (mechanoreceptors) in the detrusor muscle.

The reflex involves coordination between three systems:

1. Parasympathetic signals (via the pelvic splanchnic nerves, S2–S4) stimulate contraction of the detrusor muscle and relaxation of the internal urethral sphincter (smooth muscle, involuntary).
2. Sympathetic input (via the hypogastric nerve, T11–L2) dominates during bladder filling, promoting detrusor relaxation and internal sphincter contraction to maintain continence.
3. Somatic (voluntary) control of the external urethral sphincter (skeletal muscle) comes from the pudendal nerve (S2–S4). This is the sphincter you consciously relax when you decide to urinate, and it's the reason you can override the reflex when the timing isn't appropriate.

In infants, micturition is purely reflexive. Voluntary control of the external sphincter develops as the nervous system matures, typically by age 2–3.