WATER BALANCE

18.15.1 Water Gain

Water gains occur by ingestion of water along with food and also from metabolic water. Metabolic water is derived from the chemical reactions of cellular metabolism in the mitochondria.

FIGURE 18.7 Countercurrent multiplier in loop of Henle.

18.15.2 Water Loss

Water loss from the body is classified as either an insensible loss or a sensible loss. Insensible losses are associated with vapor losses and occur constantly by evaporation from the skin and by loss of water vapor in exhaled air. Inhaled air becomes saturated with water vapor in the respiratory pas­sages and lungs, but there is no body mechanism to remove moisture from the respiratory gases before exhalation. Sensible losses are the visible losses in the form of urine, feces, and body secretions that leave the body. Sensible losses can become excessive in certain conditions, such as diarrhea, and threaten body stores of water.

18.15.3 Regulation of Acid-Base Balance

The regulation of acid-base balance actually means regula­tion of [H⁺] in the body fluids. The [H⁺] is usually expressed in terms of pH, which is the negative log of the hydrogen ion concentration. pH = log 1 /[H⁺ ] = -log[H⁺]. The pH of the extracellular fluid (ECF) is generally 7.4. Relatively small pH changes are associated with correspondingly large changes in H+ concentration.

1. 1. Chemical buffering - immediately combining with acid or base to prevent excessive changes in H⁺ concentration.

2. Respiratory adjustment of blood CO2 concentra­tion - regulates the removal of CO₂ (and therefore, H₂CO₃) from the extracellular fluid.

3. Excretion of H⁺ or HCO₃^- ions by the Kidneys - can excrete either acid or alkaline urine, thereby readjusting the extracellular fluid H⁺ concentration toward normal during acidosis or alkalosis.

A chemical buffer system consists of a mixture of a weak acid and its conjugate base, example is a solution of car­bonic acid and bicarbonate ion. The Henderson-Hasselbalch equation describes the relationship between pH and the mixture of a weak acid and its conjugate base as pH= pKa log [Base]∕[Acid]. Ka is the dissociation constant of the weak acid and Pka is the negative logarithm of Ka. pKa is defined as the pH at which the acid and base of the buffer system are in equal concentration. Thus, pH = pKa when base and acid are equal.

When an acid is added to the buffer system, a shift of pH is resisted, but the ratio of base to acid decreases and pH is somewhat depressed. If further acid will be added, buf­fers system will be exhausted and acidemia will result. The restoration requires the formation and secretion of H+ ions by the kidney tubules. If instead strong base were added to the body fluids, it would react with the acid of the buffer and thus form more buffer base.

18.15.4 Bicarbonate Buffer System

The bicarbonate buffer system is the principal buffer of the blood and the ECF. The weak acid component of the bicarbon­ate buffer system is H2CO3 and the conjugate base is HCO3^-. pH = pK + log [HCO₃^-]/ [H₂CO₃]. Bicarbonate is formed in the body by the reaction of carbon dioxide with water.

When a strong acid (Hcl) is added to the bicarbonate buf­fer solution, the increased H⁺ from the acid is buffered by HCO3-. As a result, more H2CO3 is formed, causing higher production of CO₂ & H₂O. The excess CO₂ greatly stim­ulate respiration which eliminate the CO₂ from the ECF. Opposite reaction takes place when strong base, NaOH is added to bicarbonate system. Here concentration of H₂CO₃ decreases, and net result is decreased concentration of blood CO₂ level. This inhibits respiration and decreases the rate of CO₂ expiration. For bicarbonate buffer system, pK is 6.1

18.15.5 Phosphate Buffer System

It plays a major role in buffering renal tubular fluid and intracellular fluids (ICF). The main component is H₂PO₄ and HPO4- so when strong acid (Hcl) is added.

Here strong acid, Hcl is replaced by weak acid NaH₂PO₄ and the decrease in pH is minimized. When a strong base, NaOH is added, the OH- is buffered by the H2PO4 to form additional amount of HPO4- & H2O. Phosphate buffer sys­tem has pK of 6.8 which is very near to the normal pH of 7.4. The phosphate buffer is especially important in tubular fluid of kidney for two reasons. Phosphate usually becomes highly concentrated in the tubules, hence increasing the buffer power of the phosphate system.

18.15.6 Protein Buffer System

Proteins are among the most plentiful buffers in the body because of their high concentration intracellularly. Approximately 60-70% of the total chemical buffering of the body fluid is inside the cells, and most of this results from the intracellular proteins. In addition to the high con­centration of proteins in the cells, another factor that con­tributes to their buffering power is the fact that the pK of many of the proteins system are very close to intracellular pH.

18.15.7 Excretion and Osmoregulation

The maintenance of fluid balance is critical for maintaining many bodily functions, including metabolic and biochemi­cal reactions, transport of nutrients and maintenance of tem­perature. The average adult animal has roughly 60% fluid volume. These body fluids may again be subcategorized into intracellular and extracellular fluid compartments.

Intracellular fluid is all the fluid contained within cells. The intracellular fluid makes up roughly two-thirds of the total body volume.

The remaining one-third of fluid volume in the body is constituted by extracellular fluid and may further divide into its interstitial, intravascular and transcellular compartments.

The maintenance of fluid homeostasis in each of these compartments is mainly dependent on the concentration of electrolytes and this electrolyte is responsible for genera­tion of osmotic pressure. This process regulation of osmotic pressure is known as osmoregulation.

To maintain homeostasis, it is essential that the excre­tion of water and electrolytes match an individual’s intake.

The glomerular filtration rate value in an average human being is 180L∕day. The advantage of this high filtration rate in terms of osmoregulation is that it allows the kidneys to rapidly and precisely regulate the volume and composition of body fluids.

At the hypothalamus level, osmoreceptor response to extracellular fluid hypertonicity, and facilitate ADH release from axons down to the posterior pituitary into the circulation. ADH serves a primary function to increase sol­ute-free water reabsorption in the nephrons to bring down body fluid hypertonicity.

18.15.8 Pathophysiology of Electrolyte Imbalance

Due to excessive release of ADH one important disease occurs named as Syndrome of inappropriate ADH secre­tion (SIADH). This excessive ADH release may results from hypothalamic hyperactivity, or other sources like small-cell carcinoma. Increased ADH promotes free-water reabsorption from the filtrate, leading to an inappropriately elevated urine osmolality compared to blood plasma and by consequence, hyponatremia may develop.

In both, chronic or acute kidney disease glomerular func­tion affected so heavily, thereby reducing the production of the filtrate. Typically, this leads to increased water reten­tion, increased potassium retention and dilution of plasma sodium concentrations due to reduced water excretion.

Oedema is one of the consequences of kidney dys­function. Oedema is the presence of excess fluid in either the intracellular or the extracellular fluid compartment. Extracellular oedema, may occur clinically due to acute or chronic kidney failure, mineralocorticoid excess, decreased plasma proteins or decreased hepatic synthesis of proteins.