Comprehensive Notes on Mechanisms for Concentrating and Diluting Urine and Maintaining Acid-Base Balance

Mechanism for Concentrating & Diluting Urine

Kidneys maintain body fluid osmolarity by altering urine composition.
To conserve water, kidneys excrete small volumes of concentrated urine (high solute concentration).
To rid excess water, kidneys excrete large volumes of dilute urine (low solute concentration).

Excretion of Dilute Urine

The body controls ECF osmolarity, not water content directly.
Decreased ECF osmolarity leads to excretion of large volumes of water in urine.
Increased ECF osmolarity leads to excretion of small volumes of water to reabsorb more water and dilute body fluids.
Absence of ADH results in dilute urine production.
Presence of ADH results in concentrated urine production.
ADH is produced in the supraoptic and paraventricular nuclei of the hypothalamus and stored in the posterior pituitary gland.
Osmoreceptor cells in the hypothalamus detect changes in plasma osmolarity.
Increased plasma osmolarity stimulates osmoreceptor cells, leading to ADH release.
ADH acts on distal convoluted tubules and collecting ducts, making walls permeable to water by inserting aquaporin 2 channels.
Decreased plasma osmolarity inhibits osmoreceptor cells and ADH release.
Dilute urine production involves the negative feedback mechanism of ADH release.

Excretion of Concentrated Urine

More complex process involving juxtamedullary nephrons with long loops of Henle (LOH).
Essential conditions for concentrated urine:
- High concentration of ADH.
- High osmolarity of the renal interstitial fluid (ISF) to reabsorb large volumes of water.
High renal ISF osmolarity is produced by the countercurrent system/mechanism.
Countercurrent system: Inflow runs parallel, counter to, and in close proximity to the outflow.
- Counter current multiplier mechanism - Loops of Henle and collecting ducts
- Counter current exchange mechanism - Vasa recta.

Role of Counter Current Multiplier

Creating a region of high osmolarity in the renal ISF is necessary for water reabsorption.
Gradient of increasing osmolarity is created by active solute pumps.
Major mechanisms involved:
- Active co-transport of $Na^+$ $K^+$ $Cl^-$ in the thick ascending limb of LOH.
- Active transport of ions, especially $Na^+$ , by P cells in distal tubules and collecting ducts into the medullary interstitium under aldosterone influence.
- Great passive diffusion of urea from medullary collecting tubule into the medullary interstitium when ADH is present.
- ADH makes collecting tubule permeable to water and urea. High osmolarity in the collecting tubule concentrates urea, increasing its diffusion into renal ISF.
Interstitial osmolarity at the tip of the loop of Henle in presence of ADH is about 1200-1400 mOsmol/L.

Steps Involved in Causing Hyperosmotic Renal Medullary Interstitium

Step 1: Fluid entering the LOH from the PCT has a solute concentration of 300mOsm/L.
Step 2: Active ion pump in the thick ascending LOH reduces concentration inside the tubule while raising intestinal fluid concentration, establishing a gradient of 200mOsmol/L.
Step 3: Tubular fluid in the descending LOH and ISF quickly reach osmotic equilibrium because of osmosis of water out of descending limb. Interstitial osmolarity is maintained at 400mOsm/L because of continuous transport of ions out of fluid into the LOH.
Step 4: Additional flow of fluid into the LOH from PCT causes the hyperosmotic fluid previously formed to flow into the ascending limb.
Step 5: Additional ions are pumped into the interstitium in the ascending limb until a 200mOs/L osmotic gradient is established, with ISF osmolarity rising to 500mOs/L. Fluid in the descending LOH reaches equilibrium with the hyperosmotic medullary ISF.
Step 6: As hyperosmotic tubular fluid from the descending LOH flows into the ascending LOH, still more solutes are continuously pumped out of the tubule and deposited into the medullary interstitium.
These steps are repeated, adding more solute to the medulla in excess of water, gradually trapping solutes and multiplying the concentration gradient, raising the interstitial osmolarity to 1200-1400mOs/L. Repetitive reabsorption of NaCl and continuous inflow of new NaCl from PCT into the LOH is called counter current multiplier.
Step 7: Urea, a metabolic product of protein, contributes about 40% of the osmolarity of renal medullary ISF. Only the proximal tubule and the medullary part of the collecting duct are permeable to urea. In the proximal tubules, as water is osmotically reabsorbed, urea concentration increases, so that some urea diffuses out of the tubules into the interstitial space.
As the filtrate flows up the ascending LOH and into the distal tubule and cortical collecting duct, little urea is reabsorbed because these segments are impermeable to urea. In the presence of high concentration of ADH, water is reabsorbed rapidly from the collecting tubule and the urea concentration increases. Then as the tubular fluid flows into the inner medullary collecting duct, more water reabsorption takes place causing an even higher concentration of urea in the fluid.
This causes the diffusion of urea into the renal interstitium because this segment is highly permeable to urea and ADH increases this permeability even more. This contributes to the establishment of hyperosmotic renal medullary interstitium. The high concentration of urea in the renal medullary interstitium causes some diffusion of urea into the thin LOH which is relatively permeable to urea from where it moves to distal tubule and colleting duct. This cycle is thus repeated and therefore called medullary urea cycling. This recycling adds to the high medullary osmolarity and indeed maintains the already created hyperosmolarity.
When the body is water depleted and maximum water reabsorption is required, urea reinforces the hypertonicity of the medulla. But when the body needs to get rid of excess water, urea helps to lower the medullary hypertonicity (by allowing increase urea excretion).

Role of Counter Current Exchange Mechanism (Vasa Recta)

Vasa recta are derived from the efferent arterioles of the Juxtamedullary nephrons and are in juxtaposition with the loop of Henle (LOH).
These vessels are permeable to solute and water and reach osmotic equilibrium with the medullary interstitium.
Blood flow in this region is small (1-2% of renal blood flow) and sluggish, supplying O2 and nutrients while removing CO2 and waste products, minimizing solute loss from the medullary interstitium.
Descending Limb: As blood flows down, encountering hypertonic interstitium, water is lost (by osmosis) and solutes (Na+, Cl-, and Urea) enter.
Ascending Limb: As blood leaves the medulla, water is added, while solutes are lost into the interstitium.
These solutes recirculate in both the LOH and vasa recta capillary loops, maintaining the hypertonicity of the medullary interstitium.
Counter current exchange traps solutes in the medullary interstitium.
As water is removed from the descending vasa recta, plasma protein concentration and osmotic pressure increase. In the ascending vasa recta, oncotic pressure causes capillaries to take in fluid (by osmosis).
Water reabsorbed by the nephron is returned to the general circulation without disturbing the solute concentration in the medullary interstitium.
Net result: blood leaving the medulla is only slightly greater in osmolarity than that entering.
When ADH is present, large amounts of H₂O will be reabsorbed by osmosis from the distal tubule and collecting duct due to the high surrounding osmolarity of medullary ISF, leading to production of concentrated urine.

Diuresis

Diuresis: An increase in urine volume.
Three types of diuresis:

Water Diuresis

Increased excretion of free water leading to hypo-osmolar urine.
Free water: The quantity of water that must be removed from the urine so that it attains the osmolarity of plasma.
Occurs as a normal physiological response to excess water or alcohol consumption, which inhibits ADH secretion.
Pathological causes: diabetes insipidus (ADH deficiency due to hypothalamus or posterior pituitary gland disease) or nephrogenic diabetes insipidus (insensitivity of collecting ducts to ADH).

Osmotic Diuresis

Caused by substances that are water soluble, freely filterable but non-absorbable or poorly absorbed by the kidney.
Examples: Mannitol, excess salt, excess glucose (as in diabetes mellitus).

Pressure Diuresis

Increased perfusion pressure within normal limits does not normally increase cortical blood flow (due to autoregulation of RBF).
Autoregulation is less efficient in the medulla; thus, increased medullary blood flow washes out the osmotic gradient, resulting in diuresis.
May play a role in long-term regulation of blood pressure since ECF volume is reduced.

Diuretics

Substances that act on the renal tubule to suppress the reabsorption of solutes (osmotic diuresis) leading to decrease in ECF volume.
Important in the therapeutic management of edema and hypertension.

Sites of Action / Type of Diuretics

Loop of Henle (Loop diuretics): e.g., frusemides. Inhibit the $Na^+-K^+-2Cl^-$ cotransport system in the thick ascending LOH; most powerful.
Distal convoluted tubules (Thiazide diuretics): Inhibit $Ca^{2+}$ and $Na^+$ transport into the tubular cells.
Collecting ducts (Aldosterone antagonists): e.g., amiloride, which acts on the principal cells to block sodium diffusion from the filtrate through $Na^+$ channels into the cells.
Carbonic anhydrase inhibitors: e.g., Acetazolamide (diamox), causes moderate diuresis.

Control of ECF Osmolarity

Plasma [Na] is normally regulated within the close range of 140-145mEq/L.
Regulation of ECF osmolarity and Na concentration are closely linked because Na (average-142mEq/L) is the most abundant ion in the extracellular compartment.
Plasma osmolarity averages 300mOsm/L, and this must be precisely controlled because they determine the distribution of fluid between the compartments.

Mechanisms involved in its control

Osmoreceptors - ADH mechanism

Water deficit causes increase in extracellular osmolarity († plasma [Na]).
Stimulation (shrinkage of osmoreceptors in anterior hypothalamus).
ADH release from posterior pituitary gland.
Increase in $H_2O$ permeability in late distal tubule & collecting ducts.
Increase $H_2O$ reabsorption (in proportion to $Na^+$ ).
Decrease $H_2O$ excretion.
*Increase in ECF osmolarity (↑ plasma [Na]) causes the osmoreceptor cells located in the anterior hypothalamus to shrink.
*The shrinkage of the osmoreceptors sends signals to supraoptic and paraventricular nuclei (in the hypothalamus).which relays to the posterior Pituitary gland.
*This causes the release of ADH which is stored in their secretary granules in their nerve ending.
*ADH enters the blood stream and is transported to the kidneys, where it increases the permeability of the late, distal tubule, cortical and inner medullary collecting ducts.
*The increase water permeability in these segments causes increase water reabsorption and excretion of small volume of concentrated urine.
*Thus water is conserved while Na and other solutes continue to be excreted in the urine. This water reabsorption causes the dilution of the solutes in ECF thereby correcting the initial excessively concentrated ECF.

Thirst mechanism

Thirst: A conscious desire for water.
Thirst sensation is produced by the stimulation of the thirst centre located anterolaterally in the preoptic nuclei of the hypothalamus.
- Stimulation of thirst center increase drinking (polydipsia).
- The cells of these thirst centers functions almost as the osmoreceptors to activate thirst mechanism in the same way as osmoreceptors stimulates ADH releases.
- Stimuli for thirst:
Increase ECF osmolarity: Causes dehydration and stimulation of thirst centre resulting to the production of thirst sensation that leads to intake of water. The water taken will then dilute the ECF volume and restore osmolarity to normal. The threshold for thirst (drinking) occurs when [Na+] increases by 2mEq/L or ECF osmolarity increases by 4mOsm/L.
Dryness of mouth and mucous membrane of the esophagus: Can elicit the sensation of thirst. As a result of this mechanism, a thirsty person may receive relief from thirst almost immediately after drinking water, even though the water has not been absorbed from GIT and has not yet had effect on ECF osmolarity. This is important to prevent overhydration as it takes about 30-60mins for water to be reabsorbed and distributed throughout the body after drinking.
Decrease in ECF volume and Arterial Blood Pressure: Stimulate thirst by a pathway that is independent of the one stimulated by increased plasma osmolarity eg decrease ECF volume due to haemarrhage stimulate thirst even without any increase in osmolarity. This is due to neutral input from the cardiopulmonary and systematic baroreceptors in the circulation. Angiotensin II also stimulates thirst and other actions of angiotensin II on the kidney help in restoration of blood volume and pressure.

Role of Aldosterone

Aldosterone: An adrenal cortical hormone that stimulates Na+ reabsorption in the distal convoluted tubule and cortical collecting ducts.
Low [Na+] (i.e. low osmolarity) and low ECF volume stimulate aldosterone release, which increases renal reabsorption of Na+, increasing osmolarity and ECF volume towards normal.
High [Na+] (high osmolarity) inhibits aldosterone secretion leading to renal excretion of Na+ and decrease in osmolarity.

Renin-Angiotensin System

A major controller of aldosterone secretion.
Decreased ECF volume stimulates renin secretion:
- Stimulation of renal sympathetic nerves to the juxtaglomerular cell by extra renal baroreceptors reflexes.
- Pressure decreases sensed by the juxtaglomerular cells themselves acting as intra renal baroreceptors.
- A signal generated by low sodium or chloride concentration in the filtrate passing through the distal tubule (macula densa).
Aldosterone increases renal reabsorption of Na+.

Atrial Natriuretic Peptide (ANP)

A peptide hormone synthesized and secreted by cells in the atria in response to atrial distention.
Acts on tubules to inhibit Na+ reabsorption, increasing Na+ excretion.
The actual stimulus for ANP is expansion of plasma volume that accompanies an increase in body Na+.
Specific stimulus is increased atrial distention.

Acid-Base Balance

Maintenance of normal [H+] is an important aspect of homeostasis.
H+ are produced as a result of body metabolism and a small change in pH greatly influence cellular enzymes.
The concentration of H+ in the body fluid is extremely small. The plasma (H+) is about 0.00000004nEq/1 (4 x $10^{-8}$ )
pH = -log [H+].
Therefore, a more convenient unit for expressing (H+) known as pH unit was developed.pH is inversely related to [H+].
Therefore a low pH corresponds to a high [H+] and a high pH corresponds to a low [H+]. The normal pH of ECF is between 7.35 - 7.45 (Ave 7.4). Acidosis is present whenever the arterial pH is below 7.40 and alkalosis is present whenever it is above 7.40. The range of pH of ECF compatible with life is 6.8-7.7. Urine pH can vary from 4.50-8.00 and helps to correct for change in [H+] in the body.

Sources of H+

Greatest source is CO₂ produced during oxidative metabolism.
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 of other substances: 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.
The only common source of alkali in the body is through the consumption of large amounts of fruits and vegetables

Regulation of Normal [H+] accomplished by:

Chemical buffering and ionic exchanges
The pulmonary ventilation (Reduction of $H2CO3$ by elimination of $CO_2$ )
The renal system / mechanism (Reduction of non carbonic acid).

Chemical Buffering and Ionic Exchange

Chemical buffering and ionic shifts serve as the 1st line of defense against changes in the pH of the body fluids.
Chemical buffers inside and outside the cells instantly take up excess H+ when acids are liberated or release H+ when alkali are liberated.
A buffer is a substance which has the ability to bind or release H+ in solution and therefore keep the pH of a solution constant despite addition of acid or alkali (base).
An acid is a substance that acts as proton (hydrogen atom without its orbital electron) donor while a base is a substance that accepts proton (i.e. H+) in solution.
In principle the most effective buffers are weak acids whose conjugate bases are strong eg
- $CO2 + HCO3$ (Carbonic acid/bicarbonate buffer):
 HA
 ot
 ightharpoonup H^+ + A^- Where $H1A$ is undissociated (weak) acid and $A^-$ represent any anion.
If an acid stronger than $HA$ is added to this system, the equilibrium is shifted to the left. $H^+$ are tide up in the formation of more undissociated $HA$ , so the increase in $[[H^+]]$ is much less than it would otherwise be. Whereas is a base is added to the solutions, $H^+$ and $OH^-$ react to form $H_2O$ . This distabilizes the equilibrium so that more $H1A$ dissociates to liberate more $H^+$ into the system thereby limiting the decrease in $H^+$ concentration, which otherwise would have occurred. [H+][A-]

Henderson-Hasselbalch equation for buffers

According to the law of mass action and from the buffer equation above the following relationship applies $K= [HA]$ Where K is the dissociation constant for the weak acid (HA)
H+=K [HA]
[A-]
Applying negative logarithms to convent the [H+] to pH, we obtain the Henderson - Hasselbalch equationpH=-pK+ log [A-]
[HA]
It will be seen from the above that if the quantities of A- and HA are equal, pH will be equal to pK. Therefore, the buffering capacity of a buffer system is maximum if it dissociates so that that the amount of free anion (A-) equals the undissociated weak acid (HA).It follows then that to be an efficient buffer, the pK should be close to the pH of the relevant body fluid.

Buffer Systems of the body

The major buffer systems in the body are:
- Bicarbonate/carbonic acid buffer ( $HCO3/H2CO_3$ )
- Disodium hydrogen phosphate/monosodium dihydrogen phosphate ( $HPO4/H2PO_4$ )
- Plasma protein (Protein/H+ protein)
- Erythrocyte haemoglobin buffer system ( $Hb^+/HHb$ )
These buffer systems obey the isohydric principle which states that when several buffers exist in common solution, all buffer pairs are in equilibrium with the same concentration of H+. Therefore a change in one buffer pair affects all buffer pairs equally

Bicarbonate/ Carbonic Acid Buffer

This is the major extracellular buffer. It consist of a solution of weak acid $H2CO3$ and a bicarbonate salt eg NaHCO3.
The body forms $H2CO3$ (1 component) by the reaction of Co2 with water.CO2 + H2O
ot
ightharpoonup H2CO3 This reaction is slow and very small amount of $H2CO3$ is formed in the absence of the enzyme carbonic anhydrase (CA) which catalyses the reaction. CA is abundant in the lung alveoli where CO2 is released and in the renal tubules where CO2 react with $H2O$ to form $H2CO_3$ .
CA CO2+H2O
ot
ightharpoonup H2CO3
ot
ightharpoonup H^+ + HCO3 + Na^+ When a strong acid such as HCI is added to the bicarbonate buffer solution, the increased H+ released from the acid (HCI-> H+ CI) are buffered by HCO-3: H^++ H^+ + HCL ot ightharpoonup H^+ + Cl HO3 ot ightharpoonup H2 CO3 ot ightharpoonup CO2 + H H_2O From these reactions, one can see that H+ from the strong acid (HCI) reacted with HCO-3 to form very weak acid.The excess CO2 greatly stimulates respiration which eliminates CO2 from ECF. Thus the lungs compensate by eliminating CO2 via increase ventilation.

Phosphate Buffer: H2PO4

ot
ightharpoonup^+ HPO4+ H Protein Buffer System H prot ot ightharpoonup^+H++ Prot $They are most powerful and plentiful buffer occurring in intracellular fluid as well as plasma:$ RCOOH ot ightharpoonup RCOO+H^+ RNH3
ot
ightharpoonup ^+RNH_2+H Erythrocytes Haemoglobin (Hb) Buffer SystemHHb ⇄ Hb + H The buffering action of Hb (protein) is due mainly to the imidazole groups of the histidine residue is a weaker acid than oxidized

Respiratory Regulation of Acid-Base Balance

Role: Adjust concentration of H2CO3 $(i.e.$ CO_2) downwards in metabolic acidosis and upwards in metabolic alkalosis to minimize pH change.
Normal level of CO_2 in blood is 1.2 - 1.3 mmol/L.
A fall in the pH (CO2) of ECF stimulates respiratory centre leading to hyperventilation which blows off much alveolar CO2 and lowers the [$H2CO3$] of blood and therefore raises the pH towards normal.
*Conversely, a rise in pH (CO2) above normal inhibits respiration (respiratory Centre) and the resulting hypoventilation causes CO2 retention (H2CO3) rises and the pH falls towards normal. The respiratory system acts as a typical negative feedback controller of (H+).
*↑ [H] Alveolar Ventilation ↓ PC02

Has 1-2x buffering capacity of all the chemical buffers put together..

Renal Regulation of Acid-Base Balance

Kidneys control acid-base balance by excreting either acidic urine (minimum pH = 4.5) to remove excess acid from ECF or basic urine (maximum pH = 8.0) to remove excess base.

Reabsorption of HCO_3 $and Secretion of$ H^+

Secretion and reabsorption occurs in all parts of the tubules except the descending and ascending thin limbs of LOH.
80-90% of the (and H secretion) occur in the proximal tubule, so that only small amount of flows into the distal tubules and collecting ducts. In the thick ascending LOH, another 10% of the filtered is reabsorbed and the rest of the reabsorption takes place in the distal tubule and collecting duct. The reabsorption of and secretion of are by 2 different mechanisms, both of which require the activities of the enzymes carbonic anhydrate.
Reabsorption of and secretion in the proximal convoluted tubule by secondary active transport:
*H secretion and HCO3 reabsorption occurs in all parts of the tubules except the descending and ascending thin limps of LOH. About 80-90% of the HCO3 (and H secretion) occur in the proximal tubule, so that only small amount of HCO3 flows into the distal tubules and collecting ducts. In the thick ascending LOH. another 10% of the filtered HCO3 is reabsorbed and the rest of the reabsorption takes place in the distal tubule and collecting duct. The reabsorption of HCO3 and secretion of H+ is by 2 different mechanisms, both of which require the activities of the enzymes carbonic anhydrate.."

Tubular Buffers (Acidification of Urine): Mechanism for Excretion of H^+

Phosphate buffers (mainly in distal tubule and Collecting duct)

Mechanisms for Excretion of H+

Phosphate buffer aid in buffering these H secreted. It is composed of HPO4 $and$ HPO4$$ The process of H secretion into the tubules is the same as described earlier. and excreted as tatrable acid..

Buffering by Ammonia

*the kidney are able to replenish the ECF stores of HCO3 (consumed during HCO3 buffering of H).

Structure and Function of the Ureters and the Bladder

The ureters conduct urine from the pelvis of each kidney into the urinary bladder, which stores the urine.
The entire urinary passages are adapted to withstand hypertonic and potentially toxic fluid and prevent ECF water from being drawn into the urine. The ureters enter the bladder muscular wall obliquely which tends to keep the ureters closed
The bladder is a hollow viscus organ which is made up of smooth muscle called detrussor that are arranged in spiral, longitudinal and circular fashion.

It is composed of 2 parts:

The body which is the major portion where the urine collects.
The neck which is a funnel shaped extension of the body.
Immediately above the neck, lies a small triangular area called trigone. At the lowest part of the trigone, the bladder opens into the posterior urethra while 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. Little below the internal sphincter is external sphincter. This is formed by the skeletal muscle and is under voluntary control of nervous system.

Nerve supply

*From sacral plexus (S2, 3, 4) convey both afferent (sensory) and efferent (motor) impulses to and from ureters and bladder via the pelvic splanchnic nerves. These nerves contract the bladder and relax the internal sphincter during micturition reflex.
*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.:*Supply the external sphincter for voluntary control of micturition.

MICTURITION

This is the process of emptying the urinary bladder, i.e. the process of urination. It involves 2 processes or steps.
- 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.
As the bladder fills, the pressure at first shows little change.A plot of the relationship between the intravesical fluid volume and pressure is known as Cystometrogram.*.
The 1st urge to void is felt at a bladder volume of about 150ml (micturition wave). A short lived contraction of the bladder accompanied by a sense of fullness occurs and the fades away. 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).

In infants, micturition reflex always leads to the emptying of the bladder (automatic bladder), but by the second year, voluntary control of micturition begins.