8.3 Urine Formation and Excretion

Urine Formation and Tubular Secretion

Glomerular Filtration

Urine formation starts at the glomerulus, where blood pressure forces fluid and small solutes out of the glomerular capillaries and into Bowman's capsule. The resulting fluid is called the ultrafiltrate, and it contains water, glucose, amino acids, urea, and ions.

Two things make this filtration work:

• High hydrostatic pressure in the glomerular capillaries (about 55 mmHg) provides the driving force. This pressure is unusually high compared to other capillary beds because the afferent arteriole is wider than the efferent arteriole.
• The filtration barrier (fenestrated endothelium, basement membrane, and podocyte foot processes) keeps large molecules like plasma proteins and blood cells out of the filtrate.

The glomerular filtration rate (GFR) is roughly 125 mL/min, which means the kidneys filter about 180 liters of fluid per day. Obviously, you don't excrete 180 liters of urine, so the next steps reclaim most of that fluid.

Tubular Reabsorption

As the ultrafiltrate moves through the nephron tubule, the vast majority (about 99%) gets reabsorbed back into the peritubular capillaries. Most reabsorption happens in the proximal convoluted tubule (PCT), which reclaims nearly all glucose, amino acids, and bicarbonate, along with about 65% of filtered sodium and water.

• Glucose and amino acids are reabsorbed via secondary active transport (sodium-dependent cotransporters)
• Water follows solutes by osmosis
• Waste products like urea are only partially reabsorbed, so they become more concentrated in the remaining tubular fluid

Tubular Secretion

Tubular secretion is the reverse of reabsorption: substances move from the peritubular capillaries into the nephron tubule. This is the body's way of removing substances that weren't adequately filtered at the glomerulus.

Key substances secreted include:

• Hydrogen ions (for acid-base balance)
• Potassium ions (regulated by aldosterone in the distal tubule and collecting duct)
• Ammonium ions and creatinine
• Certain drugs like penicillin and morphine

Both reabsorption and secretion are regulated by hormones (aldosterone, parathyroid hormone) and local factors (pH, concentration gradients). Together, these three processes (filtration, reabsorption, secretion) determine the final composition of urine.

Countercurrent Multiplication System

Loop of Henle Structure and Function

The loop of Henle is the U-shaped segment of the nephron that dips down into the renal medulla and back up again. Its job is to build a concentration gradient in the medullary interstitium, which the kidney later uses to concentrate urine.

The two limbs have very different properties:

• Descending limb: permeable to water, but not to solutes. Water leaves the tubule by osmosis as the filtrate moves deeper into the increasingly salty medulla.
• Ascending limb: impermeable to water, but actively transports solutes (sodium, potassium, chloride) out of the tubule and into the interstitium.

Because fluid flows in opposite directions through the two limbs (countercurrent flow), the system multiplies a small difference in osmolarity at each level into a large gradient across the entire medulla.

How the Concentration Gradient Forms

Here's the step-by-step process:

1. Filtrate enters the descending limb from the PCT. As it descends, water moves out by osmosis into the hyperosmotic interstitium, concentrating the filtrate.
2. The now-concentrated filtrate rounds the hairpin turn and enters the ascending limb.
3. In the thick ascending limb, the NKCC2 cotransporter (sodium-potassium-2 chloride cotransporter) actively pumps $Na^+$, $K^+$, and $2Cl^-$ out of the tubule into the interstitium. Because this limb is impermeable to water, the interstitium becomes saltier while the tubular fluid becomes more dilute.
4. The solutes deposited in the interstitium draw more water out of the descending limb, which in turn delivers more concentrated fluid to the ascending limb. This cycle repeats, multiplying the gradient.
5. Urea recycling from the inner medullary collecting duct adds to the interstitial osmolarity, contributing roughly 40-50% of the medullary gradient.

The result: interstitial osmolarity ranges from about 300 mOsm/L at the cortex to roughly 1200 mOsm/L at the tip of the medulla.

Urine Concentration

The medullary gradient does nothing on its own. It's the collecting duct that uses this gradient to determine final urine concentration.

• Antidiuretic hormone (ADH) controls how permeable the collecting duct is to water. When ADH is present, it inserts aquaporin-2 channels into the collecting duct membrane, allowing water to flow out by osmosis into the hyperosmotic medulla. This produces concentrated urine.
• Without ADH, the collecting duct stays relatively impermeable to water, and dilute urine is excreted.

Disrupting the countercurrent system impairs the kidney's ability to concentrate urine. Loop diuretics like furosemide block NKCC2 in the thick ascending limb, which washes out the medullary gradient and leads to large volumes of dilute urine. Medullary damage from disease has a similar effect.

Factors Influencing Urine Concentration

Hormonal Factors

ADH (vasopressin) is the primary hormone controlling urine concentration. Released by the posterior pituitary in response to dehydration or high blood osmolarity, it acts on collecting duct cells to insert aquaporin-2 water channels, increasing water reabsorption.

• Too little ADH causes diabetes insipidus: the kidneys can't concentrate urine, so patients produce massive volumes of dilute urine (sometimes 15-20 L/day).
• Too much ADH causes SIADH (syndrome of inappropriate ADH secretion): excessive water retention leads to dilutional hyponatremia.

Aldosterone, released by the adrenal cortex, acts on the distal tubules and collecting ducts to increase $Na^+$ reabsorption and $K^+$ secretion. Because water follows sodium, aldosterone indirectly promotes water retention and affects urine concentration. Excess aldosterone (primary hyperaldosteronism/Conn syndrome) causes hypertension and hypokalemia.

Osmotic and Volume Factors

Blood osmolarity is monitored by osmoreceptors in the hypothalamus. When osmolarity rises (dehydration, high salt intake), these receptors stimulate ADH release, and the kidneys produce concentrated urine. When osmolarity drops, ADH secretion is suppressed, and dilute urine is excreted.

Blood volume and pressure influence urine output through the renin-angiotensin-aldosterone system (RAAS):

• Decreased blood volume or pressure → RAAS activation → increased $Na^+$ and water reabsorption → reduced urine volume
• Increased blood volume or pressure → RAAS suppression → increased $Na^+$ and water excretion → increased urine volume

Solute load also matters. The amount of urea and other waste products in the filtrate affects the medullary osmotic gradient. High protein intake increases urea production and can enhance concentrating ability. Renal diseases like glomerulonephritis or renal failure impair urea handling and reduce the kidney's concentrating capacity.

Other Factors

Diuretics increase urine volume through different mechanisms depending on their class:

• Loop diuretics (furosemide) block NKCC2 in the thick ascending limb, disrupting the countercurrent gradient. These are the most potent diuretics.
• Thiazide diuretics (hydrochlorothiazide) block the NCC (sodium-chloride cotransporter) in the distal convoluted tubule, reducing sodium reabsorption at that segment.
• Caffeine and alcohol also act as mild diuretics. Alcohol inhibits ADH release, which is why drinking alcohol leads to increased urine output and dehydration.

Age-related changes reduce GFR and impair concentrating ability over time, making older adults more susceptible to dehydration and fluid imbalances.

Genetic factors can cause inherited forms of nephrogenic diabetes insipidus. Mutations in the aquaporin-2 gene or the ADH receptor gene (V2 receptor) make the collecting duct unresponsive to ADH, even when ADH levels are normal.

Micturition Reflex and Neural Control

Micturition Reflex Pathway

Micturition is the process of emptying the bladder. It's controlled by a spinal reflex that can be modulated by higher brain centers.

The reflex pathway works in these steps:

1. As the bladder fills, stretch receptors in the bladder wall detect increasing volume.
2. Afferent signals travel via the pelvic nerves to the sacral spinal cord (segments S2-S4).
3. The spinal cord sends efferent signals back through the pelvic nerves, causing the detrusor muscle (smooth muscle of the bladder wall) to contract and the internal urethral sphincter (smooth muscle, involuntary) to relax.
4. For urination to actually occur, the external urethral sphincter (skeletal muscle, voluntary) must also relax. This sphincter is innervated by the pudendal nerve (S2-S4) and is under conscious control.

This voluntary control over the external sphincter is what allows you to decide when and where to urinate, rather than emptying the bladder every time the reflex fires.

Supraspinal Control of Micturition

The spinal reflex doesn't act alone. Higher brain centers coordinate and regulate the process:

• The pontine micturition center (PMC) in the brainstem coordinates detrusor contraction with sphincter relaxation so that urination is smooth and complete. It receives input from the periaqueductal gray (PAG) and the hypothalamus about bladder fullness.
• The prefrontal cortex and other cortical areas exert voluntary control over the PMC and the external sphincter. These regions handle the decision of when and where urination is socially appropriate.
• Damage to the prefrontal cortex or its descending connections can lead to urinary incontinence or loss of social inhibition around urination.

Disorders of Micturition

Urinary incontinence is involuntary urine leakage. The three main types have distinct mechanisms:

• Stress incontinence: urine leaks during activities that increase abdominal pressure (coughing, sneezing, lifting). The underlying problem is weak pelvic floor muscles or urethral sphincter insufficiency.
• Urge incontinence: sudden, intense urge to urinate followed by involuntary leakage. Caused by involuntary detrusor contractions (overactive detrusor).
• Overflow incontinence: the bladder never fully empties, so it eventually overflows. Causes include impaired detrusor contractility or bladder outlet obstruction (e.g., benign prostatic hyperplasia, urethral stricture).

Urinary retention is the inability to completely empty the bladder, resulting in high residual urine volume. Common causes include bladder outlet obstruction, detrusor underactivity, and neurological disorders (spinal cord injury, multiple sclerosis).

Overactive bladder (OAB) syndrome presents with urinary urgency, frequency, and nocturia, with or without urge incontinence. It's associated with involuntary detrusor contractions and increased bladder sensitivity. Treatment options include behavioral modifications, pelvic floor muscle training (Kegel exercises), anticholinergic medications, and neuromodulation.