Concise Exam Notes - Mechanism for Concentrating & Diluting Urine

Mechanism for Concentrating & Diluting Urine:

  • Kidneys maintain body fluid osmolarity by altering urine composition.
  • Conserve water: excrete small volume of concentrated urine.
  • Rid excess water: excrete large volume of dilute urine.
  • Osmolarity of ECF controls water excretion.
    • Decreased ECF osmolarity: dilute fluid, large urine volume.
    • Increased ECF osmolarity: concentrated fluid, small urine volume.
  • Absence of ADH: dilute urine produced.
  • Presence of ADH: concentrated urine produced.
  • ADH produced by hypothalamus, stored in posterior pituitary.
  • Osmoreceptors in hypothalamus detect plasma osmolarity changes.
    • Increased osmolarity: stimulates ADH release, water reabsorption via aquaporin 2 in distal tubules and collecting ducts.
    • Decreased osmolarity: inhibits ADH release.
  • Dilute urine production involves negative feedback of ADH release.

Excretion of Concentrated Urine:

Requires:

  • High ADH concentration.
  • High osmolarity of renal interstitial fluid (ISF).
  • High renal ISF osmolarity produced by countercurrent system:
    • Counter current multiplier mechanism: LOH and collecting ducts.
    • Counter current exchange mechanism: vasa recta.

Role of Countercurrent Multiplier:

  • Creates high osmolarity in renal ISF for water reabsorption.
  • Active co-transport of Na^+ K^+ Cl^- in thick ascending limb of LOH.
  • Active transport of ions (especially Na^+) by P cells in distal tubules and collecting ducts, influenced by aldosterone.
  • Passive diffusion of urea from medullary collecting tubule into medullary interstitium (when ADH is present).
  • ADH increases collecting tubule permeability to water and urea, concentrating urea and increasing its diffusion into renal ISF.
  • Interstitial osmolarity at the tip of loop of Henle reaches 1200-1400 mOsmol/L with ADH.

Steps Involved in Causing Hyperosmotic Renal Medullary Interstitium:

  • Fluid entering LOH from PCT: 300 mOsm/L.
  • Active ion pump in thick ascending LOH establishes a 200 mOsmol/L gradient.
  • Fluid in descending LOH and ISF reach osmotic equilibrium.
  • Hyperosmotic fluid flows from descending to ascending limb.
  • Additional ions pumped into interstitium, raising ISF osmolarity, fluid in descending LOH equilibrizes with ISF.
  • Repetitive reabsorption of NaCl by thick ascending LOH creates a concentration gradient, raising interstitial osmolarity to 1200-1400 mOs/L.
  • Counter current multiplier: Repetitive NaCl reabsorption and inflow from PCT.

Role of Urea:

  • Contributes ~40% of renal medullary ISF osmolarity.
  • Urea is freely filtered.
  • Proximal tubule and medullary collecting duct are permeable to urea.
  • Urea concentration increases as water is reabsorbed in proximal tubules, leading to some diffusion into interstitial space.
  • Ascending LOH, distal tubule, and cortical collecting duct are relatively impermeable to urea.
  • High ADH concentration leads to water reabsorption in collecting tubule and increased urea concentration.
  • Urea diffuses into renal interstitium, contributing to hyperosmotic environment.
  • Medullary urea cycling: Urea diffuses into thin LOH, then to distal tubule and collecting duct, repeating the cycle to maintain hyperosmolarity.
  • Water depletion: Urea reinforces medullary hypertonicity.
  • Excess water: Urea helps lower medullary hypertonicity.

Role of Counter Current Exchange Mechanism (Vasa Recta):

  • Vasa recta are permeable to solute and water, reaching osmotic equilibrium with medullary interstitium.
  • Low blood flow minimizes solute loss from medullary interstitium.
  • Descending limb:
    • Water lost, solutes (Na+, Cl-, Urea) enter.
  • Ascending limb:
    • Water added, solutes lost into interstitium.
  • Solutes recirculate in LOH and vasa recta, maintaining hypertonicity.
  • Increased plasma protein concentration and osmotic pressure in descending vasa recta.
  • Oncotic pressure causes capillaries to take in fluid in ascending vasa recta.
  • Net result: Blood leaving medulla has slightly greater osmolarity than entering blood.
  • ADH presence: large H2O reabsorption from distal tubule and collecting duct because of high surrounding osmolarity of medullary ISF.

Diuresis:

  • Increased urine volume.

Types of Diuresis:

  • Water Diuresis:
    • Increased excretion of free water, hypo-osmolar urine.
    • Caused by excess water consumption or alcohol (inhibits ADH secretion).
    • Pathological: diabetes insipidus (ADH deficiency or insensitivity).
  • Osmotic Diuresis:
    • Non-absorbable/poorly absorbed substance causes diuresis.
    • Examples: Mannitol, excess salt, excess glucose (diabetes mellitus).
  • Pressure Diuresis:
    • Increased perfusion pressure washes out medullary osmotic gradient.
    • Role in long-term regulation of blood pressure.

Diuretics:

  • Substances that suppress solute reabsorption, increasing urine flow and decreasing ECF volume.
  • Used in management of edema and hypertension.

Sites of Action / Type of Diuretics:

  • Loop of Henle (loop diuretics eg frusemides).
    • Inhibit Na^+-K^+-2Cl^- cotransport in thick ascending LOH.
  • Distal convoluted tubules (thiazide diuretics).
    • Inhibit Ca^{2+} and Na^+ transport into tubular cells.
  • Collecting ducts (aldosterone antagonists eg amiloride).
    • Blocks sodium diffusion from filtrate through Na^+ channels into principal cells.
  • Carbonic anhydrase inhibitors eg. Acetazolamide (diamox).
    • Causes moderate diuresis.

Control of ECF Osmolarity:

  • Plasma [Na^+] regulated within 140-145mEq/L.
  • Linked to ECF osmolarity regulation.
  • Plasma osmolarity averages 300mOsm/L.

Mechanisms Involved:

  • Osmoreceptors - ADH mechanism:
    • Increased ECF osmolarity (increased plasma [Na^+]) causes osmoreceptor cells in anterior hypothalamus to shrink.
    • Signals sent to supraoptic and paraventricular nuclei, relayed to posterior pituitary.
    • ADH released, increasing water permeability in late distal tubule, cortical and inner medullary collecting ducts.
    • Increased water reabsorption, excretion of small volume of concentrated urine.
    • Water conserved, solutes excreted, diluting ECF.
    • Opposite events occur with decreased ECF osmolarity (less ADH released, increased water excretion).
  • Thirst mechanism:
    • Thirst: conscious desire for water.
    • Stimulation of thirst center (anterolateral preoptic nuclei of hypothalamus) increases drinking (polydipsia).

Stimuli for Thirst:

  • Increased ECF osmolarity: dehydration, stimulation of thirst center, water intake dilutes ECF.
    • Threshold for thirst: [Na^+] increases by 2mEq/L or ECF osmolarity increases by 4mOsm/L.
  • Dryness of mouth and mucous membrane of the esophagus.
    • Provides immediate relief from thirst before water absorption.
    • Prevents overhydration.
  • Decreased ECF volume and arterial blood pressure.
    • Stimulates thirst via cardiopulmonary and systemic baroreceptors.
    • Angiotensin II also stimulates thirst.
  • ADH and thirst mechanisms work together to regulate ECF osmolarity or [Na^+] of ECF.

Role of Aldosterone:

  • Adrenal cortical hormone.
  • Stimulates Na^+ reabsorption in distal convoluted tubule and cortical collecting ducts.
  • Low [Na^+] (low osmolarity) and low ECF volume stimulate aldosterone release.
  • Increases renal reabsorption of Na^+, increasing osmolarity and ECF volume.
  • High [Na^+] (high osmolarity) inhibits aldosterone secretion.
  • Increases renal excretion of Na^+, decreasing osmolarity.

Renin-Angiotensin System:

  • Major controller of aldosterone secretion.
  • Decreased ECF volume stimulates renin secretion via:
    • Renal sympathetic nerves.
    • Juxtaglomerular cells (intrarenal baroreceptors).
    • Macula densa (low sodium or chloride concentration).
  • Aldosterone increases renal reabsorption of Na^+.

Atrial Natriuretic Peptide (ANP):

  • Peptide hormone secreted by atria in response to atrial distention.
  • Inhibits Na^+ reabsorption, increasing Na^+ excretion.
  • Stimulus: expansion of plasma volume due to increased body Na^+, increased atrial distention.

ACID-BASE BALANCE:

  • pH of the body is carefully regulated.
  • Maintenance of normal [H^+] is an important aspect of homeostasis.
  • H^+ are produced as a result of body metabolism.

Sources of H+:

  • CO2 produced during oxidation of glucose and triglyceride. CO2 is an acid (H^+) generator and functions as the single most important weak acid (H2CO3) in the body fluid. Because CO2 is volatile, it is normally eliminated through the lungs.
  • Aerobic metabolism: fixed acids such as sulfuric acid, phosphoric acid and lactic acid (produced during exercise). These forms of acids (non volatile or fixed) are excreted mainly as titratable acid and as ammonium salts.

Only common source of alkali: consumption of fruits and vegetables.

  • Concentration of H^+ in body fluid is extremely small.
  • Plasma [H^+] about 0.00000004nEq/1 (4 \times 10^{-8})
  • pH unit developed.
  • Ph = log \frac{1}{[H^+]} = -log [H^+]
  • Normal pH of ECF is between 7.35 - 7.45 (Ave 7.4).
  • Acidosis: arterial pH below 7.40.
  • Alkalosis: arterial pH above 7.40.
  • Range of pH of ECF compatible with life: 6.8-7.7.
  • Urine pH can vary from 4.50-8.00.

Regulation of Normal [H+] by 3 Systems:

  • Chemical buffering and ionic exchanges.
  • The pulmonary ventilation (Reduction of H2CO3 by elimination of CO₂).
  • The renal system / mechanism (Reduction of non carbonic acid).

Chemical Buffering and Ionic Exchange:

  • First line of defense against pH changes.
  • Buffers take up excess H^+ or release H^+.
  • Buffer: substance that can bind or release H^+ to keep pH constant.
  • Acid: proton donor.
  • Base: proton acceptor.
  • Most effective buffers: weak acids with strong conjugate bases (e.g., CO2 + HCO3^-).
  • General buffer equation: HA \rightleftharpoons H^+ + A^-
  • If strong acid added, equilibrium shifts left, tying up H^+.
  • If base added, H^+ and OH^- form H_2O, driving dissociation of HA.

Henderson-Hasselbalch Equation:

pH = pK + log \frac{[A^-]}{[HA]}

  • Buffering capacity is maximum when [A^-] = [HA].
  • Efficient buffer: pK close to pH of body fluid.

Buffer Systems of the Body:

  • Bicarbonate/Carbonic Acid Buffer (HCO3^-/H2CO_3).
  • Disodium hydrogen phosphate/monosodium dihydrogen phosphate (HPO4^{2-}/H2PO_4^-).
  • Plasma protein (Protein/H^+ protein).
  • Erythrocyte haemoglobin buffer system (Hb^+/HHb).

These buffer systems obey the isohydric principle.

The Bicarbonate/ Carbonic Acid Buffer:

  • Major extracellular buffer.
  • Consists of weak acid H2CO3 and bicarbonate salt (e.g., NaHCO_3).
  • H2CO3 formed by reaction of CO_2 with water:

CO2 + H2O \rightleftharpoons H2CO3

  • Reaction catalyzed by carbonic anhydrase (CA).
  • Bicarbonate salt occurs as sodium bicarbonate:

NaHCO3 \rightleftharpoons Na^+ + HCO3^-

Adding strong acid (HCl):

H^+ + HCO3^- \rightleftharpoons H2CO3 \rightleftharpoons CO2 + H_2O

  • HCO3^- decreases, H2CO3 increases, increasing CO2 and H_2O production.
  • Lungs eliminate excess CO_2 via increased ventilation.

Adding strong base (NaOH):

NaOH + H2CO3 \rightleftharpoons NaHCO3 + H2O

  • Weaker base (NaHCO_3) replaces stronger base (NaOH).
  • Decreased CO2 inhibits respiration, kidneys excrete excess HCO3^- (alkaline urine).
  • Not a powerful buffer under physiological conditions (pK = 6.1) but most important because its components are easily regulated.
  • Lungs regulate [H2CO3] (i.e., [CO2]); kidneys regulate [HCO3^-].
  • Main buffer utilized by kidneys to buffer H^+ secreted by tubules.

Phosphate Buffer:

H2PO4^- \rightleftharpoons HPO_4^{2-} + H^+

  • Major intracellular buffer (with intracellular protein) and buffers renal tubular fluid.
  • PK is 6.8.
  • Becomes important in buffering urine in distal nephrons.
  • H^+ buffered by phosphates are excreted as titratable acid (amount of alkali needed to titrate urine back to normal pH of blood).

Protein Buffer System:

H Prot \rightleftharpoons H^+ + Prot

  • Most powerful and plentiful buffer in intracellular fluid and plasma.
  • Proteins made of amino acids with free carbonyl groups:

RCOOH \rightleftharpoons RCOO^- + H^+

And free amino group:

RNH3 \rightleftharpoons RNH2 + H^+

  • PK of many amino acids is close to 7.4.
  • Buffering takes time because cell membrane is relatively impermeable to H^+ but permeable to CO_2.

Erythrocytes Haemoglobin (Hb) Buffer System:

HHb \rightleftharpoons Hb + H

  • Primarily a noncarbonated buffer of the blood
  • Buffering action is due to imidazole groups of histidine residue (38 in number).
  • Reduced Hb (deoxygenated-Hb) is a weaker acid and better buffer than HbO_2.

Respiratory Regulation of Acid-Base Balance:

  • Second line of defense against acid-base disturbance.
  • Adjusts [H2CO3] downwards in metabolic acidosis, upwards in metabolic alkalosis.
  • Normal level of CO_2 in blood is 1.2 - 1.3 mmol/L.
  • Fall in pH (increased CO2) stimulates respiratory center, leading to hyperventilation, blowing off alveolar CO2 and raising pH.
  • Rise in pH (decreased CO2) inhibits respiration, causing hypoventilation, CO2 retention, and lowering pH.
  • Respiratory system acts as negative feedback controller of [H^+].
  • Has 1-2x buffering capacity of all chemical buffers.

Renal Regulation of Acid-Base Balance:

  • Third and final line of action in acid-base regulation.
  • Kidneys control acid-base balance by excreting acidic urine (minimum pH = 4.5) or basic urine (maximum pH = 8.0).

Reabsorption of HCO_3^- and Secretion H^+:

  • Occurs in all parts of the tubules except the descending and ascending thin limps of LOH.
  • 80-90% of the HCO_3^- (and H^+ secretion) occur in the proximal tubule.
  • Thick ascending LOH reabsorbs another 10% of the filtered HCO_3^- and the rest of the reabsorption takes place in the distal tubule and collecting duct.

Two different mechanisms:

  • Reabsorption of HCO_3^- and secretion H^+ in the proximal convoluted tubule by secondary active transport
    • Filtered HCO_3^- cannot be absorbed directly (tubular cells impermeable).
    • Converted to H2CO3, with H^+ secreted into lumen in exchange for filtered Na^+.
    • H2CO3 dissociates into CO2 and H2O. CO_2 diffuses into tubular cell.
    • Inside cell, CO2 combines with H2O to regenerate new H2CO3 molecule, catalyzed by carbonic anhydrase.
    • H2CO3 dissociates to form HCO_3^- and H^+.
    • Newly generated HCO_3^- diffuses into renal ISF and peritubular capillary blood.
    • Net result: For every H^+ secreted, a HCO_3^- enters the blood.
  • Reabsorption from distal and collecting tubules by primary active transport
    • Tubular cells of these sections secrete H^+ by primary active transport (intercalated cells).
    • Carbonic anhydrase catalyzes the combination of CO2 and H2O in the intercalated cells to form H2CO3.
    • H2CO3 then dissociates into HCO_3^- that is reabsorbed into the blood and H^+ secreted into the tubular lumen by means of H^+-ATPase pump in the luminal membrane.
    • Secretion is enhanced by aldosterone.
  • In alkalosis: Less reabsorption and more excretion of HCO_3^-.
  • In acidosis: All the filtered HCO_3^- is reabsorbed, excess H^+ secreted combines tubular buffers.

THE TUBULAR BUFFERS (ACIDIFICATION OF URINE): MECHANISM FOR EXCRETION OF H+

  • Phosphate buffers (mainly in distal tubule and Collecting duct)
    • Aid in buffering the secreted H^+. It is composed of HPO4^{2-} and H2PO4^-. There are excess HCO3^- in the tubular fluid, most of the secreted H^+ combine them (HCO_3^- ions).
    • However, once all the HCO3^- has been reabsorbed and is no longer available to combine with H^+, any excess H^+ then combines with HPO4^{2-} to form H2PO4^-, which is excreted as Na salt (NaH2PO4-acidic salt).
    • Filtered HCO_3^- is insufficient to buffer the H^+ secreted.
    • Secreted H^+ will lower filtrate pH, stopping secretion at pH 4.5.
    • Phosphate buffer aids in buffering these H^+ secreted.
    • Composed of HPO4^{2-} and H2PO_4^-, concentrated in tubular fluid due to water reabsorption.
    • Occurs in distal tubules and collecting ducts.
    • H that are secreted aid in buffering these H^+ secreted. With the addition of a new HCO3^- to the blood. This is one of the mechanism by which the kidneys are able to replenish the ECF stores of HCO3^-
    • Under normal conditions, some of the filtered phosphate is reabsorbed and the remainder that is available is not enough for buffering H^+ therefore much of the buffering of excess H^+ in the tubular fluid in acidosis occurs through ammonia buffer system.
  • Buffering by Ammonia
    • If all the buffers in the filtrate used up and maximum H^+ gradient reached (pH below 4.5), tubular cells synthesize ammonia (NH_3) from glutamine and other.
    • NH3 diffuses into lumen and reacts with H^+ to form ammonium ion (NH4^+. Which are lost in the urine along with Cl as ammonium chloride (NH_4CI).
    • For every H^+ secreted that forms NH4^+ (each NH4^+ excreted), one molecule of HCO_3^- is added to the plasma.
    • This makes this the dominant mechanism for acid eliminated.
    • The long-term renal compensatory mechanism.

STRUCTURE AND FUNCTION OF THE URETERS AND THE BLADDER

  • Ureters conduct urine from kidneys to bladder, entire urinary passages are adapted to withstand hypertonic and potentially toxic fluid and prevent ECF water from being drawn into the urine.
  • Ureters enter bladder muscular wall obliquely which tends to keep the ureters closed.

Bladder:

  • a hollow viscus organ which is made up of smooth muscle called detrussor.
  • Arranged in spiral, longitudinal and circular fashion. It is composed of 2 parts
    • The body: the major portion where the urine collects
    • The neck: a funnel shaped extension of the body.
  • A small triangular area called trigone lies Immediately above the neck where the bladder opens into the posterior urethra.
  • The 2 ureters enter the bladder at the upper most angles.
  • the urethra is the outlet for the bladder content.
  • At the neck of the bladder, the urethra is surrounded by internal urethral sphincter that is formed by smooth muscle (detrussor). This is tonically contracted to prevent bladder emptying until the pressure rises above a critical threshold.
  • External sphincter is formed by the skeletal muscle and is under voluntary control of nervous system.

Nerve Supply

  • Parasympathetic nerve: from sacral plexus (S2, 3, 4) convey both afferent (sensory) and efferent (motor ) impulses to and from ureters and bladder. These nerves contract the bladder and relax the internal sphincter during micturition reflex
  • Sympathetic nerve Supply : via hypo gastric nerves (1,2,3).The afferent convey information about fullness of the bladder and pain while the efferent constrict the internal sphincter in males during the sympathetic mediated seminal emission and ejaculation thereby preventing reflex of semen into the bladder.
  • Somatic nerve: The pudendal nerves (S2, 3, 4) Supply the external sphincter for voluntary control of micturition.

MICTURITION:

  • The process of emptying the urinary bladder, i.e. the process of urination. It involves 2 processes or steps.2. Progressive filling of the bladder until the pressure rises to a critical or (threshold) value.
  • A neuronal reflex called the micturition reflex which empties the bladder.

The Filling of the Bladder

  • As urine forms in the kidney it collects in the pelvis of the kidney. As the pressure in the pelvis increases, it initiates peristaltic contraction in the ureter which move downwards and carry urine into the bladder.
  • This peristaltic contraction occurs 1-5 times per minute and trickles urine into the bladder. This movement is aided by gravity
  • Parasympathetic stimulation enhances (frequency) while sympathetic stimulation inhibits peristalsis (↓frequency).

Cystometrogram:

  • As the bladder fills, the pressure at first shows little change, the tension produced initially is not maintained because of receptive relaxation so that when the bladder is extended by the entry of certain volume, the wall tension or intravesical pressure initially rises and falls again. This behavior is also partly explained by the law of Laplace which states that for a spherical viscus with elastic wall, the distending pressure (P) increases twice as the tension (T) in the wall but inversely as the radius (R).

P = \frac{2T}{R}

  • The curve shows an initial slight rise in pressure when the 1st increment in volume is produced (Ia). A long nearly flat segments with further increments (Ib) and a sudden sharp rise in pressure (II) as the micturition reflex is triggered.
  • The 1st urge to void is felt at a bladder volume of about 150ml. A short lived contraction of the bladder accompanied by a sense of fullness occurs and the fades away. This is known as micturition wave. Each new addition, provokes similar reactions (micturition wave) until the volume of about 300-400ml is reached when an intense urge to urinate is coupled with a vigorous micturition reflex. This leads to emptying of the bladder.

THE MICTURITION REFLEX:

  • The micturition reflex is a sacral parasympathetic reflex which may be inhibited or facilitated by voluntary control from higher centres in the CNS (very similar to defecation reflex).
  • Stimulation of stretch receptors in the wall of the bladder when the urine volume reaches 300 - 400ml causes impulses to flow from the parasympathetic afferents through polysynaptic connections in the sacral cord, and down the parasympathetic efferent which causes the bladder to contract and the internal urethral sphincter to relax with emptying of the bladder through the urethra.
  • In infants, micturition reflex always leads to the emptying of the bladder (automatic bladder), but by the second year, voluntary control of micturition begins, as centres of the brain and brain stem(i.e. CNS) inhibit the sacral centres for the reflex, and facilitate them when the circumstance is convenient for urination. By learned ability, the external sphincter is tonically contracted.
  • Micturition can be initiated voluntarily when a person contract (his or her) abdominal muscles, which increases the pressure in the bladder and allows extra urine to enter the bladder neck and posterior urethra under pressure, thus stretching their walls. This stimulates the stretch receptors which triggers micturition