Unit 4: Renal Physiology in Mammals

2. Mechanism of Urine Formation

Urine formation occurs through three sequential physiological processes: Glomerular Filtration, Tubular Reabsorption, and Tubular Secretion.

Fundamental Relationship:
Excretion Rate = Filtration Rate - Reabsorption Rate + Secretion Rate

Step 1: Glomerular Filtration

Glomerular filtration is a non-selective bulk-flow filtration process driven by hydrostatic and osmotic pressures across the glomerular membrane. Blood entering the glomerulus is filtered, yielding a protein-free cell-free filtrate in Bowman's space.

Starling Forces Governing Glomerular Filtration

Net Filtration Pressure (NFP) is calculated using the following formula:

NFP = Glomerular Hydrostatic Pressure - (Capsular Hydrostatic Pressure + Glomerular Blood Colloid Osmotic Pressure)
NFP = HPg - (HPc + OPg)

• Glomerular Hydrostatic Pressure (HPg): Blood pressure inside glomerular capillaries (~55 mmHg). Promotes filtration. Elevated due to efferent arteriole diameter being smaller than afferent arteriole diameter.
• Capsular Hydrostatic Pressure (HPc): Fluid pressure exerted by filtrate inside Bowman's space (~15 mmHg). Opposes filtration.
• Glomerular Blood Colloid Osmotic Pressure (OPg): Osmotic pressure exerted by trapped plasma proteins in capillary blood (~30 mmHg). Opposes filtration.

Calculating Net Pressure:

NFP = 55 - (15 + 30) = 10 mmHg

Glomerular Filtration Rate (GFR)

GFR is the total volume of filtrate formed by all nephrons of both kidneys per minute. In a healthy adult, standard GFR is approximately 125 mL/min (or 180 L/day). Since average daily urine output is ~1.5 L, greater than 99% of filtered fluid is reabsorbed.

Autoregulation of GFR

Kidneys maintain a relatively constant GFR across systemic blood pressure variations (80-180 mmHg) via intrinsic autoregulatory mechanisms:

1. Myogenic Mechanism: Vascular smooth muscle cells in afferent arterioles contract automatically when stretched by increased systemic blood pressure, reducing renal blood flow. Conversely, when pressure drops, arterioles relax and dilate, maintaining flow and GFR.
2. Tubuloglomerular Feedback (TGF): When GFR rises, tubular fluid flow accelerates, reducing time for NaCl reabsorption in the loop of Henle. High NaCl reaches the macula densa, which releases paracrine signals (ATP, Adenosine) causing afferent arteriole constriction, bringing GFR back to normal.

Step 2: Tubular Reabsorption

Tubular reabsorption is the selective movement of filtered solutes and water from the tubular lumen across epithelial cells back into peritubular capillary blood.

Reabsorption by Segment

• Proximal Convoluted Tubule (PCT): The major site of reabsorption.
  • Reabsorbs 100% of filtered glucose and amino acids via sodium-dependent secondary active co-transporters (e.g., SGLT2 for glucose).
  • Reabsorbs ~65-70% of filtered Na+ and water.
  • Reabsorbs ~65% of K+, ~80-90% of HCO3-, and significant amounts of Cl-, phosphate, and urea.
  • Water reabsorption in PCT occurs via Obligatory Water Reabsorption: Water automatically follows active solute transport osmotically through Aquaporin-1 channels.
• Loop of Henle:
  • Descending Limb: Reabsorbs ~15% of filtered water; impermeable to solutes.
  • Thick Ascending Limb (TAL): Reabsorbs ~25% of filtered Na+, K+, and Cl- via the Na+-K+-2Cl- cotransporter (NKCC2). Highly impermeable to water.
• Distal Convoluted Tubule (DCT) and Collecting Duct: Site of fine-tuning solute and water balance under tight hormonal regulation (Facultative Reabsorption). Na+ reabsorption is regulated by Aldosterone; water reabsorption is regulated by ADH.

Step 3: Tubular Secretion

Tubular secretion is the transfer of substances from peritubular capillaries or tubule epithelial cells into the tubular lumen for excretion. It serves as an active elimination mechanism for substances not cleared by filtration.

• Hydrogen Ions (H+): Secreted in PCT, DCT, and collecting ducts to maintain arterial blood pH within narrow physiological limits (7.35-7.45).
• Potassium Ions (K+): Secreted in late DCT and collecting duct principal cells under the direct influence of Aldosterone.
• Organic Acids and Bases: Secretion of metabolic waste products (uric acid, creatinine) and xenobiotics/drugs (penicillin, aspirin, NSAIDs) primarily in PCT.
• Ammonia (NH3 / NH4+): Synthesized and secreted by proximal tubular cells during acidotic states.

3. Countercurrent Mechanism

The countercurrent mechanism is the process by which the kidney establishes and maintains a hyperosmotic medullary interstitial gradient (ranging from 300 mOsm/L at the corticomedullary border to 1200-1400 mOsm/L at the inner renal papilla). This gradient is essential for producing concentrated urine and conserving body water.

Countercurrent System Components:
1. Countercurrent Multiplier: Loop of Henle (Establishes the medullary gradient).
2. Countercurrent Exchanger: Vasa Recta (Maintains the medullary gradient).

1. Countercurrent Multiplier: Loop of Henle

The term 'countercurrent' refers to fluid flowing in opposite directions in adjacent parallel limbs of the hairpin loop (descending vs. ascending).

Mechanism of Gradient Creation

1. Active Transport in Thick Ascending Limb (TAL): Cells of the TAL actively pump Na+, K+, and Cl- out of the tubular lumen into the medullary interstitium via the NKCC2 symporter. Because TAL is strictly impermeable to water, solutes accumulate in the interstitial fluid without water following, making the medullary interstitium hypertonic and leaving tubular fluid dilute (~100 mOsm/L).
2. Single Effect: The active solute transport establishes a maximum horizontal concentration gradient of 200 mOsm/L between the tubular fluid inside TAL and the surrounding interstitial fluid at any given horizontal level.
3. Osmotic Equilibration in Descending Limb: The descending limb is highly permeable to water (contains Aquaporin-1) but impermeable to solutes. As fluid flows down, water moves osmotically out of the lumen into the hypertonic interstitium until tubular fluid osmolality balances interstitial osmolality.
4. Multiplication of Gradient: Continuous inflow of new tubular fluid pushes the concentrated fluid at the loop turn deeper toward the inner apex. Repeated continuous operation of active solute transport in TAL coupled with water withdrawal from the descending limb multiplies the horizontal 200 mOsm/L single effect into a dramatic vertical osmotic gradient along the length of the medulla (300 mOsm/L to 1200 mOsm/L).

2. Countercurrent Exchanger: Vasa Recta

The vasa recta are hairpin-loop blood vessels that run parallel to the juxtamedullary loops of Henle. They act as passive countercurrent exchangers, supplying metabolic oxygen and nutrients to medullary tissue without washing out the hyperosmotic medullary gradient established by the multiplier.

• Descending Vasa Recta: As blood flows down into the hypertonic deep medulla, water moves passively out of capillaries into the interstitium, while solutes (Na+, Cl-, urea) diffuse passively into blood. Blood osmolality increases, reaching ~1200 mOsm/L at the loop tip.
• Ascending Vasa Recta: As blood turns and ascends toward the cortex through decreasingly osmolal interstitium, the osmotic movement reverses: solutes diffuse out of capillaries back into the interstitium, while water is reabsorbed back into blood capillaries.
• Net Outcome: High medullary solute concentration is preserved in the renal medulla, while water reabsorbed from descending limbs and collecting ducts is carried away into systemic circulation.

Role of Urea Recycling in Medullary Hyperosmolality

Urea accounts for roughly 40-50% of the total hyperosmotic gradient in the inner renal medulla.

1. In cortical and outer medullary collecting ducts, water is reabsorbed in the presence of ADH, increasing urea concentration in the tubular lumen because these segments are impermeable to urea.
2. When high concentration urea reaches the Inner Medullary Collecting Duct (IMCD), ADH activates specific urea transporters (UT-A1 and UT-A3), causing urea to diffuse rapidly down its concentration gradient into the inner medullary interstitium.
3. This accumulated interstitial urea contributes significantly to inner medullary hypertonicity.
4. Interstital urea then diffuses into the thin limbs of Henle's loop, recirculating back through the nephron tubules (urea recycling).

4. Hormonal Regulation of Water Balance

Systemic water balance is strictly monitored to maintain extracellular fluid (ECF) osmolality around a baseline setpoint of ~285 to 295 mOsm/kg H2O. The primary hormone controlling water reabsorption is Antidiuretic Hormone (ADH), also known as Vasopressin.

ADH Synthesis, Secretion, and Triggers

• Synthesis: ADH is synthesized in cell bodies of magnocellular neurosecretory neurons located within the supraoptic and paraventricular nuclei of the hypothalamus.
• Storage and Release: Axonally transported down the hypothalamo-hypophyseal tract and stored in nerve terminals in the posterior pituitary gland (neurohypophysis), from where it is secreted into circulation.
• Primary Stimulus: Increased plasma osmolality (as small as a 1% rise above baseline) detected by hypothalamic osmoreceptors located in the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO).
• Secondary Stimulus: Decreased extracellular fluid volume or arterial blood pressure (5-10% decrease) sensed by arterial baroreceptors (carotid sinus and aortic arch) and low-pressure atrial stretch receptors.

Cellular Mechanism of Action on Collecting Ducts

1. Circulating ADH binds specifically to Vasopressin V2 Receptors located on the basolateral membrane of principal cells in the late DCT and collecting ducts.
2. V2 receptor binding activates a stimulatory G-protein (Gs), which stimulates adenylyl cyclase, catalyzing conversion of ATP to intracellular cyclic AMP (cAMP).
3. Elevated cAMP activates Protein Kinase A (PKA).
4. PKA triggers phosphorylation and exocytotic insertion of vesicles containing Aquaporin-2 (AQP2) water channels into the apical membrane facing the tubular lumen.
5. Water enters principal cells rapidly from lumen via AQP2 and exits basolateral membranes passively into interstitium via constitutively expressed Aquaporin-3 (AQP3) and Aquaporin-4 (AQP4) water channels.
6. Result: Facultative water reabsorption increases, producing a low volume of hypertonic, concentrated urine.
7. Absence of ADH: AQP2 channels are internalized via endocytosis back into intracellular vesicles. Apical membrane becomes impermeable to water, leading to excretion of large volumes of dilute urine (diuresis).

Osmoreceptors and the Thirst Mechanism

Water balance regulation operates via a dual efferent limb system combining ADH secretion (renal water conservation) and thirst drive stimulation (water intake behavior).

• When ECF osmolality rises, hypothalamic osmoreceptor cells shrink osmotically, sending action potentials to the cerebral cortex to induce sensation of thirst.
• Drinking fluid dilutes plasma ECF, restoring volume and osmolality, shutting off both thirst drive and ADH secretion via negative feedback.

Important Clinical Note: Diabetes Insipidus
A condition characterized by complete or partial inability to concentrate urine, leading to severe polyuria (excessive urine output) and polydipsia (excessive thirst).
- Central Diabetes Insipidus: Inadequate synthesis or release of ADH by hypothalamus/posterior pituitary.
- Nephrogenic Diabetes Insipidus: Inability of renal collecting ducts to respond to ADH (mutations in V2 receptors or Aquaporin-2 channels).

5. Regulation of Acid-Base Balance

Normal systemic arterial blood pH must be maintained within a tight physiological range of 7.35 to 7.45. Systemic blood pH below 7.35 represents acidosis; pH above 7.45 represents alkalosis.

While blood chemical buffers react instantaneously and lungs excrete volatile carbonic acid (CO2) within minutes, the kidneys provide the definitive long-term regulation of acid-base balance over hours to days by excreting non-volatile metabolic acids (fixed acids like sulfuric acid and phosphoric acid) and adjusting plasma bicarbonate ([HCO3-]) levels.

Henderson-Hasselbalch Equation for Blood pH:
pH = 6.1 + log ([HCO3-] / (0.03 * PCO2))

Three Primary Renal Acid-Base Mechanisms

1. Reabsorption of Filtered Bicarbonate (HCO3-).
2. Excretion of Hydrogen Ions (H+) and Generation of New Bicarbonate.
3. Excretion of Titratable Acids and Ammonium Ions (NH4+).

1. Reabsorption of Filtered Bicarbonate

Approximately 4300 mmol of HCO3- is filtered daily by glomeruli, and ~99.9% must be reabsorbed to prevent severe metabolic acidosis. The PCT reabsorbs ~80-90% of filtered bicarbonate.

Step-by-Step PCT Reabsorption Mechanism

1. PCT cells secret H+ into tubular lumen via the Na+/H+ Exchanger-3 (NHE3) on the apical membrane.
2. Secreted H+ combines with filtered lumen HCO3- to form carbonic acid (H2CO3).
3. Apical membrane-bound enzyme Carbonic Anhydrase IV (CA IV) rapidly converts luminal H2CO3 into carbon dioxide (CO2) and water (H2O).
4. CO2 is nonpolar and diffuses freely across apical membrane into PCT cytoplasm.
5. Inside PCT cytoplasm, intracellular Carbonic Anhydrase II (CA II) hydrates CO2 with H2O to regenerate H2CO3, which dissociates into H+ and HCO3-.
6. The generated intracellular HCO3- is transported across basolateral membrane into peritubular capillaries via the Na+/3HCO3- Cotransporter (NBCe1).
7. The intracellular H+ is recycled back into lumen via NHE3 to continue the process.

Note: Filtered bicarbonate ion itself cannot cross apical epithelial membranes directly; it is reabsorbed via conversion to CO2 gas.

2. Secretion of H+ and Generation of New Bicarbonate

To offset fixed daily metabolic acid production (~50-100 mEq/day), kidneys must excrete free H+ and generate new replacement HCO3- back into blood. This occurs mainly in the late DCT and collecting duct via Intercalated Cells.

• Type A Intercalated Cells (Active during Acidosis):
  • Secrete H+ into tubular lumen using apical membrane H+-ATPase (proton pump) and H+/K+-ATPase translocators.
  • Intracellular carbonic anhydrase generates new HCO3- from CO2 and H2O, which exits basolateral membrane into peritubular capillary blood via Anion Exchanger 1 (AE1 / Cl-/HCO3- exchanger).
  • Net effect: Increases blood pH and restores plasma bicarbonate pools.
• Type B Intercalated Cells (Active during Alkalosis):
  • Possess reverse polarity: apical Pendrin (Cl-/HCO3- exchanger) secretes excess HCO3- into tubular lumen, while basolateral H+-ATPase pumps H+ back into peritubular blood.
  • Net effect: Decreases blood pH and lowers plasma bicarbonate pools.

3. Urinary Buffering Systems

The minimum achievable urinary pH is approximately 4.5 (representing a 1000-fold free H+ concentration gradient compared to plasma). Free H+ ions alone can account for only ~0.1% of excreted metabolic acid. To excrete large amounts of H+, secreted hydrogen ions must combine with luminal buffers.

A. Phosphate Buffer System (Titratable Acid)

Filtered monohydrogen phosphate (HPO4(2-)) combines with secreted H+ in tubular fluid to form dihydrogen phosphate (H2PO4(-)), which cannot be reabsorbed and is excreted in urine.

H+ + HPO4(2-) -> H2PO4(-)

B. Ammonia Buffer System (Ammoniagenesis)

The primary mechanism for renal response to chronic acidosis.

1. PCT cells metabolize the amino acid Glutamine, generating two ammonium ions (NH4+) and two new bicarbonate ions (HCO3-).
2. New HCO3- diffuses into peritubular capillaries to raise blood pH.
3. NH4+ is secreted into tubular lumen (substituting for H+ on NHE3 translocators) and excreted in urine bound with chloride as ammonium chloride (NH4Cl).

6. Hormonal Regulation of the Volume of Urine

Urine volume regulation is tightly coupled to extracellular fluid (ECF) volume and blood pressure maintenance. Four major systemic hormonal systems coordinate control over final urine volume: RAAS (Renin-Angiotensin-Aldosterone System), ADH, Aldosterone, and ANP (Atrial Natriuretic Peptide).

1. Renin-Angiotensin-Aldosterone System (RAAS)

RAAS is a hormonal cascade triggered by decreased effective circulating arterial volume or renal hypoperfusion.

Triggers for Renin Secretion from JGA Granular Cells

• Decreased renal arterial blood pressure (detected directly by JGA intrarenal baroreceptors).
• Decreased NaCl concentration delivered to macula densa cells.
• Increased sympathetic stimulation of renal nerves via beta-1 adrenergic receptors.

The Cascade

1. Granular JGA cells secrete the enzyme Renin into systemic blood.
2. Renin acts on liver-synthesized circulating protein Angiotensinogen, cleaving it to yield the inactive decapeptide Angiotensin I (Ang I).
3. Angiotensin-Converting Enzyme (ACE) (bound primarily to pulmonary vascular endothelium) cleaves Ang I to produce the highly potent octapeptide Angiotensin II (Ang II).

Physiological Actions of Angiotensin II

• Direct Systemic Vasoconstriction: Constricts peripheral vascular arterioles, raising total peripheral resistance and systemic blood pressure.
• Efferent Arteriole Constriction: Preferentially constricts renal efferent arterioles, raising glomerular hydrostatic pressure to maintain normal GFR during periods of low renal perfusion.
• Stimulates PCT Transport: Increases apical NHE3 activity, directly promoting Na+, Cl-, and water reabsorption in PCT.
• Hypothalamic Activation: Stimulates thirst center and ADH release.
• Adrenal Cortex Activation: Stimulates the zona glomerulosa of the adrenal cortex to synthesize and release Aldosterone.

2. Aldosterone

Aldosterone is a steroid hormone that acts on the distal tubule and cortical collecting duct principal cells.

Mechanism of Action

1. Diffuses passively across basolateral membrane of principal cells and binds intracellular Mineralocorticoid Receptors (MR).
2. Hormone-receptor complex translocates into nucleus, inducing transcription of specific genes.
3. Increases synthesis and apical membrane insertion of Epithelial Sodium Channels (ENaC).
4. Increases synthesis and basolateral insertion of Na+/K+-ATPase pumps.
5. Increases mitochondrial ATP production to power active transport.

Physiological Outcome

Promotes active Na+ reabsorption and secondary passive water reabsorption (conserving volume) while increasing K+ and H+ secretion into lumen. Reduces total urinary Na+ excretion, maintaining ECF volume while reducing urine output volume.

3. Atrial Natriuretic Peptide (ANP)

ANP is a peptide hormone synthesized and stored in cardiac atrial myocytes, functioning as a powerful physiological antagonist to RAAS and ADH.

Triggers and Actions

• Trigger: Excessive stretch of cardiac atrial walls caused by hypervolemia, high venous return, or elevated ECF volume.
• Renal Actions:
  • Dilates afferent arterioles and selectively constricts efferent arterioles, increasing NFP and GFR.
  • Inhibits Renin secretion from JGA granular cells.
  • Inhibits Aldosterone synthesis from adrenal cortex.
  • Directly inhibits ENaC channels in medullary collecting ducts, blocking active Na+ reabsorption.
  • Inhibits ADH secretion and ADH action on collecting duct epithelium.
• Net Outcome: Causes Natriuresis (increased urinary excretion of sodium) and Diuresis (increased urine volume excretion), lowering ECF volume and restoring blood pressure back toward normal setpoints.