L1 Water metabolism

Water Metabolism

• Water is the predominant chemical component of living organisms.
• Daily normal allowance for an adult (60kg) is approximately 1800-2500ml/day.
• Water is a neutral substance with a pH of 7.

Regulation of Water Balance

• Water balance is regulated by:
  • Hypothalamic mechanisms controlling thirst.
  • Antidiuretic hormone (ADH).
  • Retention or excretion of water by the kidneys.

Functions of Water

• Essential constituent of living cells. No living cell can resist drying.
• Solvent action forms a great number of crystalline and colloidal solutions.
• Serves as a universal medium for intracellular and extracellular chemical reactions.
• Plays a role in maintaining intracellular and extracellular pH.
• Important for absorption of food material from the intestine, reabsorption from kidney tubules, transport of food stuffs, manufacturing of secretions.
• Regulates body temperature.
• Acts as a lubricant preventing friction and drying.
• Mechanical buffer preventing injury to the nervous system.

pH

• pH is the negative logarithm of the hydrogen ion concentration (pH = -log [H+]).
• The square brackets around the H+ mean "concentration".
• pH symbol: p = power, H = Hydrogen.
• Water is a neutral solution because ionization gives equal amounts of H+ and OH- ions (10−7 = 7).
• Low pH values correspond to high concentrations of H+ ions.
• High pH values correspond to low concentrations of H+ ions.

pH Scale

• The pH scale ranges from 1 to 14.
• Values below 7 are acidic, values above 7 are alkaline.
• pH 7 is neutral.

Clinical Importance of pH

• Normal human blood pH is approximately 7.3-7.4.
• Blood pH below 7.3 is acidosis.
• Blood pH above 7.4 is alkalosis.
• Both acidosis and alkalosis are fatal conditions as changes affect enzyme activity (each enzyme has an optimal pH).

Acid-Base Balance

• Acid-base balance is vital for controlling hydrogen ion (H+) balance.
• Healthy kidneys and lungs control acid-base balance
• Normal ratio of Na bicarbonate (NaHCO3) to carbonic acid (H2CO3) is 20:1.
• Alterations in the concentration of one component are immediately compensated to maintain the ratio.
• Significant alterations result in acidosis or alkalosis.
• Body cells can only function within a narrow range of pH.

Clinical Abnormalities of Blood pH

• Acidosis (below 7.3):
  • Respiratory acidosis: Caused by CO2 buildup due to lung issues, respiratory depression, or breathing air with excess CO2.
  • Metabolic acidosis: Caused by reduced bicarbonate level (e.g., severe muscle exercise, ketoacidosis in uncontrolled diabetes, increase of proteins in diet, failure of kidney to excrete acids, vomiting with significant loss of alkaline intestinal and pancreatic juices).
• Alkalosis (above 7.4):
  • Respiratory alkalosis: Caused by hyperventilation (e.g., high altitude, high fever).
  • Metabolic alkalosis: Caused by excessive vomiting with HCl loss, and diets high in salt content.

Regulation of Blood pH

• Systems responsible for regulating blood pH include:
  • Buffers
  • Kidneys
  • Respiratory system

Buffers

• Normal metabolism produces CO2, metabolic acids (e.g., lactic acid, ketone bodies), and inorganic acids (e.g., sulfuric acid).
• CO2 is a major source of acid reacting with water to produce carbonic acid.
• Buffers are crucial to maintain a pH compatible with life.
• Buffers work by substituting strong acids/bases with weaker ones, reducing free H+ and minimizing pH changes.
• Solutions resist pH changes when acids or alkalis are added moderately.

Types of Buffers

• Mixture of weak acid and its salt with strong base (e.g., carbonic acid and its salt, sodium bicarbonate, and acetic acid and sodium acetate).
• Mixture of weak base and its salt with strong acid (e.g., ammonium hydroxide and its salt, ammonium chloride).

Examples of Buffer Systems

• Carbonic acid/Bicarbonate buffer system: Found mainly in extracellular fluid.
• Phosphate buffer system: Found in intracellular fluids and RBCs.
• Protein buffers: Include amino acid buffers, hemoglobin and oxyhemoglobin in RBCs, and plasma protein buffers.

Mechanism of Buffer Action

• Any buffer consists of basic and acidic parts.
• Strong acid reacts with the basic part of the buffer, minimizing pH change.
• Strong base reacts with the acidic part of the buffer, minimizing pH change.

Renal Regulation of pH

• Kidneys regulate pH and acid-base balance through excretion of excess acids or bases.
• Mechanisms include:
  • Excretion of hydrogen (H+) and potassium ions in tubules (acidifying the urine) responding to acidosis.
  • Reabsorption of bicarbonate (HCO3−) responding to acidosis, and excreting it in response to alkalosis.
  • Production and excretion of ammonia and ammonium ions (NH4+).

Excretion of Hydrogen

• Occurs in proximal, distal, and collecting tubules.
• CO2 combines with H2O to form H2CO3.
• H2CO3 dissociates into bicarbonate (HCO3−) and H+.
• H+ is secreted in exchange for Na+.
• Net production of HCO3− and net excretion of H+.

Na-H Exchange

• Exchange of sodium with hydrogen against concentration gradient via ATP hydrolysis.
• H+ ions extruded into tubular fluid in exchange for Na+ ions during acidosis.
• Process is inhibited during alkalosis.

Ammonia and Ammonium Excretion

• Ammonia is released by glutaminase enzyme action on glutamine in renal tubular cells.
• Ammonia diffuses across the cell membrane into tubular lumen.
• It is converted to ammonium ions which cannot cross the membrane, and is excreted with anions (e.g., phosphate, chloride, sulfate).

Bicarbonate Reabsorption

• Bicarbonate reabsorption primarily occurs in proximal convoluted tubules.
• Excreted hydrogen reacts with bicarbonate to form carbonic acid.
• Carbonic acid dissociates into CO2 and H2O.
• CO2 diffuses back and reforms carbonic acid, then bicarbonate.
• Approximately 90% of filtered bicarbonate is reclaimed, matching sodium reabsorption.

Important Buffer System

• Bicarbonate/carbonic acid is the main extracellular fluid buffer system.

Test your knowledge on the crucial roles and functions of water in living organisms. This quiz covers topics such as water balance regulation, daily water allowance, and the physiological importance of water. Perfect for students of biology and health sciences looking to refine their understanding of this essential substance.