Total Body Water (TBW) and Distribution

Total Body Water (TBW) and Distribution

• TBW makes up around 60% of body weight in young adult males, 51% in young adult females, and 45% in obese individuals.
• In a 70 kg young adult male, TBW is about 42 liters.
• Intracellular fluid (ICF) constitutes about 2/3 of TBW, equivalent to about 28 liters or 40% of body weight.
• Extracellular fluid (ECF) constitutes about 1/3 of TBW, equivalent to approximately 14 liters or 20% of body weight.
• Intravascular fluid (plasma) is about 1/4 of the ECF volume, around 3.5 liters or 5% of body weight.
• Extravascular fluid is about 3/4 of the ECF volume, around 10.5 liters or 15% of body weight.

Functions of Body Water

• Acts as a medium for chemical and enzymatic reactions.
• Functions as a medium for physical processes like diffusion and filtration.
• Works as an ionizing medium, regulating pH and body fluids osmolality.
• Regulates body temperature through heat absorption, distribution, and evaporation.
• Serves as a lubricant in joints and potential spaces, such as the pleura.
• Works as a refractive medium in the eye.
• Cerebrospinal fluid acts as a mechanical buffer to protect the brain.
• Facilitates the exchange of O2 and CO2 in the lungs and tissues.

Composition of ECF and ICF

• ECF composition is almost the same throughout, except for protein concentration.
• ECF has a protein concentration of about 17 mEq/liter in the plasma and about 5 mEq/liter in the interstitial fluid.
• The main cation in ECF is Na+ (about 142 mEq/liter), while the main anion is Cl- (about 103 mEq/liter).
• Other cations in ECF include K (about 4 mEq/liter), small amounts of Ca2+, and Mg2+.
• Other anions in ECF include HCO3 (about 28 mEq/liter), proteins, and small amounts of HPO4- and SO4-.
• ECF contains non-electrolytes like glucose, cholesterol, urea, uric acid, creatinine, bile pigments, and phospholipids.
• ECF has a pH of about 7.4 and an osmolality of about 300 mOsm/liter.
• Main cations in ICF include K+ and Mg2+ (about 140 mEq/liter and 58 mEq/liter, respectively), along with a small amount of Na+ and very little Ca2+.
• Main anions in ICF include HPO4- and protein (about 75 mEq/liter and 40-45 mEq/liter, respectively), along with small amounts of Cl-, HCO3-, and SO4-.
• ICF has a pH of about 7 due to its low HCO3- content, while its osmolality is equal to that of the ECF.

Fluid Exchange Between ECF and ICF

• Cell membranes are permeable to water, allowing continuous fluid exchange and resulting in equal osmolality between ECF and ICF.
• Hypertonic NaCl injection increases ECF osmolality.
• Water transfers from ICF to ECF until osmotic equilibrium, decreasing ICF volume, causing cell dehydration and shrinkage (crenation in red blood cells).
• Hypotonic NaCl solution injection reduces ECF osmolality.
• Water transfers from ECF to ICF until osmotic equilibrium, increasing ICF volume, causing cell swelling and potential rupture (hemolysis in red blood cells).
• Isotonic solutions injection increase ECF volume (edema) without significantly affecting body fluids' osmolality.

Measurement of TBW

• Uses the indicator dilution principle.
• A known amount of an indicator substance is injected intravenously.
• The indicator used should penetrate the cell membranes to be dispersed in both the ECF and ICF.
• Commonly used indicators are deuterium oxide (D₂O or heavy water), tritium oxide (³H₂O), and aminopyrine.
• Allow sufficient time for distribution in TBW, then measure its concentration in the plasma.
• Divide the injected amount of indicator by its concentration in the plasma to calculate the volume of distribution (TBW volume).

Measurement of ECF Volume

• Measured using the indicator dilution principle.
• Indicator should not penetrate cell membranes, remaining dispersed in the ECF only.
• Common indicators include inulin, sucrose, and mannitol.

Measurement of ICF Volume

• Cannot be measured directly.
• First measure TBW and ECF volumes.
• Subtract ECF volume from TBW volume to calculate ICF volume.
• Interstitial fluid volume also cannot be measured directly.
• Measure ECF and plasma volumes first.
• Subtract plasma volume from ECF volume to calculate interstitial fluid volume.

Water Balance

• Balance between daily water gain and water loss.
• Under normal conditions, both are equal (2300 ml/day each).
• Balance maintains normal water balance and keeps TBW constant.

Water Gain

• Normally about 2300 ml/day.
• Derived from exogenous water (oral route), the main source.
• Exogenous water averages 2000 ml/day.
• Includes water and other fluids that are drunk (about 1400 ml/day).
• Also includes water in eaten food like meat, vegetables, and bread (about 600 ml/day).
• Endogenous water is formed inside the body as a result of oxidation of H2 in foodstuffs.
• Endogenous water is normally about 300 ml/day.

Water Loss

• In a comfortable atmospheric temperature (about 20°C), young adult individuals lose about 2300 ml of water/day.
• 1400 ml lost in urine.
• 100 ml lost in feces.
• 350 ml lost by evaporation from the respiratory tract.
• 450 ml lost from the skin (insensible perspiration through diffusion and evaporation).
• Water loss through the skin and respiratory tract is commonly called insensible water loss.

Control of Water Balance: Water Gain

• Amount of water gain is controlled mainly by the thirst sensation.
• Thirst is the conscious desire for water that urges drinking.
• Thirst is produced by stimulation of a thirst center in the anterior hypothalamus (= central mechanism of thirst).
• Decrease in ECF or plasma volumes (= hypovolemia) stimulates the thirst center through angiotensin II and cardiovascular reflexes.
• Increased osmolality (or tonicity) of fluids directly stimulates the thirst center.
• Decreased salivary secretion in hypovolemia causes dryness in the buccal cavity and pharynx, triggering the peripheral mechanism of thirst.

Control of Water Loss

• The amount of water loss is controlled mainly by adjusting the urine volume.
• Antidiuretic hormone (ADH or vasopressin) is largely determined by water loss.
• ADH is released from the posterior pituitary gland.
• ADH increases water reabsorption from distal segments of the renal tubules in the kidneys.
• ADH release stimulated by hypovolemia and hypertonicity.
• ADH release inhibited by hypervolemia and hypotonicity.

Disorders of Water Balance: Dehydration

• Condition of negative water balance.
• Occurs when water loss exceeds water gain due to excessive sweating, vomiting, diarrhea or prolonged water deprivation.
• Manifestations include cold, pale, dry, and wrinkled skin.
• Sunken eyes and depressed fontanelles in infants.
• Loss of body weight and muscle weakness with rapid fatigue.
• Marked thirst and dry mouth.
• Rise in body temperature due to sluggish blood flow in the skin.
• Hypotension, weak pulse, and circulatory failure in severe cases.
• Increased plasma proteins concentration and elevation of serum Na+ level and hematocrit value.
• Oliguria or anuria, potentially leading to acidosis and uremia.
• Exhaustion of nerve cells leading to excitation, delirium, coma, and death if 20% of TBW is lost.
• Infants and children are more vulnerable than adults due to higher ECF/ICF ratio, kidneys' reduced ability to retain water, and greater insensible water loss.
• Hypovolemia increases ADH release and stimulates the thirst center, retaining water and increasing water ingestion.
• Treatment involves giving fluids either orally or intravenously.

Disorders of Water Balance: Hydration

• Positive water balance condition.
• Occurs when water gain exceeds water loss due to excessive ADH secretion, and when a dehydrated person drinks excess water without salt.
• Generalized edema, including pulmonary edema in severe cases.
• Excessive salivation with nausea and vomiting.
• Decreased plasma osmolality, hematocrit value, and Na+ concentration.
• Signs of water intoxication due to swelling of nerve cells.
• Includes asthenia, tremors, ataxia, convulsions, confusion, drowsiness, coma, and death.
• Hypervolemia and plasma hypotonicity decrease ADH release and inhibit the thirst center, which decreases water excretion and minimizes water ingestion.
• Treatment includes hypertonic saline solutions intravenously, and diuretic drugs.
• TBW also can be decreased by dialysis.

Blood Composition

• Blood consists of cells suspended in plasma, a clear yellowish fluid.
• Cells constitute 40-45% of blood volume.
• Includes red blood cells, white blood cells, and platelets.
• Red blood cells, or erythrocytes, number about 5 million per mm³.
• Decreased red blood cells cause anemia, and increased red blood cells result in polycythemia.
• White blood cells, or leukocytes, number 4000-11000 per mm³.
• Decreased white blood cells cause leukopenia, and increased white blood cells cause leukocytosis.
• Platelets, or thrombocytes, number about 300000 per mm³.
• Decreased platelets cause thrombocytopenia, and increased platelets cause thrombocytosis.

Blood Plasma

• Plasma constitutes 55-60% of total blood volume.
• Composed of 90% water and 10% dissolved solutes.
• Organic substances are plasma proteins (7.1%), lipids, hormones, enzymes, nutrients, and waste products (2%).
• Inorganic substances (0.9%) include electrolytes such as Na+, K+, Cl-, HCO3-, Ca2+, and PO43-.

Role of Bone Marrow in Hemopoiesis

• In early fetal life, hemopoiesis (blood cell formation) occurs outside the bone marrow, in the liver and spleen (=extramedullary hemopoiesis).
• After birth, all red blood cells and platelets and most white blood cells are formed in the bone marrow (=medullary hemopoiesis).
• In children, marrow in all bones is active and is called red marrow.
• Red marrow becomes confined to flat bones (sternum, ribs, vertebrae) and upper parts of long bones (humerus, femur) around age 20.
• The shafts of long bones contain inactive fatty or yellow marrow.
• Red cell count in blood is 500 times more than the white cell count.
• About 75% of marrow cells are leukocyte-producing cells, and only 25% are maturing red cells.
• Life span of white cells is much shorter than that of red cells.

Properties of Blood

• Blood color is red due to hemoglobin.
• pH of arterial blood is 7.4.
• pH of venous blood is 7.36.
• Blood is opaque due to its cellular elements.
• Blood specific gravity is about 1060 (cells are about 1090, plasma is 1025-1030).
• The blood viscosity is 5 times that of water due to cellular elements and plasma proteins.
• Plasma viscosity is 2 times that of water.
• Osmotic pressure of plasma is about 5500 mmHg, from crystalloids (electrolytes, glucose, urea, etc).
• Plasma proteins contribute about 25 mmHg, which is called plasma colloid osmotic pressure or oncotic pressure.
• Total plasma osmolality is 290-300 mOsm/liter.
• Plasma proteins contribute by only 0.5% (less than 2 mOsm/liter).
• Solutions with the same osmolality as plasma are isoosmotic or isotonic, e.g. 0.9% NaCl solution.
• Solutions with higher osmolalities than plasma are hyperosmotic or hypertonic.
• Solutions with lower osmolalities are hypoosmotic or hypotonic.
• A 5% glucose solution is initially isotonic, but after glucose metabolism, its effects are similar to hypotonic solutions.

General Functions of the Blood

• Links various body systems together and helps in producing homeostasis (maintaining a constant internal environment).
• Essential for nutrition by transporting food from the GIT to the tissues.
• Essential for respiration by transporting O2 from lungs to tissues and CO2 in the opposite direction.
• Necessary for excretion by transporting waste products from tissues to excretory organs (urea and creatinine to kidneys).
• Essential for the regulation of metabolism by transporting hormones from endocrine glands to tissues and regulating their secretion.
• Essential for regulation of body temperature by transporting heat to the skin.
• It plays an essential role in maintaining acid-base balance and keeping the pH of body fluids constant (by its buffer systems).
• The regulation of water balance requires excess fluids to be removed via the kidneys.