Body Fluids: ECF and ICF

• Body fluid averages close to 60% of body composition for males and a little over 50% for females.
• The average infant is 70-75% water, and the percentage drops as people age.
• Lean people have a greater percentage of water to body ratio because fat is mostly water-free.
• Body fluid is divided into extracellular fluid (ECF) and intracellular fluid (ICF).

Extracellular Fluid (ECF)

• Accounts for approximately 1/3 of the body fluids.
• Located outside of the body's cells, including interstitial fluids, blood plasma, cerebrospinal fluid (CSF), lymph, serous liquids, synovial fluid, G-I fluids, and fluids inside the eyeball.
• Major ions involved: chloride (CL), sodium (Na⁺), and bicarbonate (HCO3).

Intracellular Fluid (ICF)

• Accounts for approximately 2/3 of the body fluids.
• Located inside the body's cells.
• Major ions involved: phosphate (PO₄²⁻), magnesium (Mg²⁺), potassium (K⁺), and proteins (negatively charged ones).
• The cell membrane uses mechanisms like selective permeability, osmosis, and active transport to control the movement of ions between ICF and ECF.
• Changes in solutes affect water movement's distribution, and water movement occurs passively due to the concentration of solutes (osmotic gradients).

Water

• Water is the largest component of the human body.
• Newborns are usually over 70% water at birth.
• The percentage of water decreases if one has a great gain in fat (obesity).
• Daily water intake averages 2,500 ml. daily, with two-thirds from liquid ingestion and one-third from ingested food.
• The amount varies with metabolism, environment, and the health of the individual.
• Daily water loss averages approximately 2500 ml. daily, and the average lost should equal the average taken in.
• Water is lost through urine (1,200 - 1,800 ml daily), skin (450-500 ml daily), respiration (300-400 ml daily), feces (100-150 ml daily) , and mucus and tears (small amount).
• Factors like metabolism, environment, and health influence the amount of water lost.
• Insensible water loss accounts for 750-900 ml and occurs through evaporation from the respiratory tract and diffusion through the skin.
• Some internal secretions are mostly reabsorbed: saliva (1,500 ml/day), gastric secretions (1,500-2,500 ml/day), bile (500-1,000 ml/day), pancreas (700-1,000 ml/day), and intestines (2,000-3,000 ml/day).

Nonelectrolytes vs. Electrolytes

• Nonelectrolytes are compounds with covalent bonds that do not form ions when dissolved.
• Nonelectrolytes include most organic compounds like glucose.
• Electrolytes, also called ions, are compounds that have at least one ionic bond.
• Electrolytes dissociate into cations (ions with positive charges, like sodium) and anions (ions with negative charges, like chloride).
• Electrolytes include acids, bases, and salts, and can be inorganic or organic (citric acid and lactic acid).
• Electrolytes carry an electrical current, control water movement (osmosis), help maintain acid-base balance, and keep the body electrically and chemically balanced.

Edema

• Edema is excess fluid in the body's tissue.

Extracellular Edema

• Characterized by an excess extracellular fluid accumulation within the interstitial spaces.
• It is usually due to increased capillary pressure or permeability, decreased lymph return, decreased plasma protein levels, kidney problems that result in retention of water or salt, or cardiovascular problems (especially heart problems).

Intracellular Edema

• Characterized by an excess fluid accumulation within the cell.
• Can be caused by cells' failure to remove specific ions, metabolic problems resulting in imbalance between electrolytes and fluids, or circulatory problems resulting in imbalance between electrolytes and fluids.

Major Ions in Extracellular Fluid (ECF)

Chloride (CL)

• The major extracellular anion, balances osmotic pressure, is part of stomach acid (HCL), follows sodium passively and exits the body primarily via urine and sweat.
• Aldosterone indirectly regulates chloride levels.

Sodium (Na⁺)

• The major extracellular cation, necessary for action potential conduction, plays a major role in fluid and electrolyte balance and osmosis, and exits the body via urine, sweat, and feces.
• ADH and aldosterone regulate sodium levels.

Major Ions in Intracellular Fluid (ICF)

Phosphate

• Most common forms include H₂PO₄⁻, HPO₄²⁻, and PO₄³⁻.
• It is an anion, mostly present in the adult as calcium phosphate salts in bone, and is involved in bone and teeth structure and buffering.
• Parathormone and calcitonin hormone regulates phosphate levels.

Magnesium (Mg²⁺)

• It is a cation, mainly in bone and intracellular fluid.
• Magnesium is a cofactor for enzymes used in protein and carbohydrate metabolism, nervous system transmission, heart functioning, and is involved in the sodium pump.
• It exits the body primarily through urine, and its levels are regulated by aldosterone.

Potassium (K⁺)

• The most abundant intracellular cation.
• It supports neuromuscular and cardiac functions, fluid volume, and helps regulate pH by exchanging with H⁺.
• Potassium exits the body primarily through urine and is influenced by mineralocorticoids.

Acidosis and Alkalosis

• Normal blood pH is 7.35 - 7.45.
• Acidosis defined as blood pH lower than 7.35.
• Alkalosis defined as blood pH greater than 7.45.

Acidosis

• Clinically, the major effect is depression of the central nervous system.

Metabolic Acidosis

• Increased H⁺, thus pH is decreased.
• May be due to acid accumulation or loss of bases, symptoms may include increased ventilation and fatigue.

Respiratory Acidosis

• Increased CO₂, thus pH decreases.
• Due to factors that cause an increase in CO₂, accompanied by increased respiratory acid (carbonic acid)
• Symptoms may include decreased or nonexistent ventilation, weakness.

Alkalosis

• Clinically, the major effect is that the nervous system becomes overexcitable.

Metabolic Alkalosis

• Decreased H⁺, thus pH is increased.
• Due to excessive H⁺ loss or gain in base or bases, symptoms may include depressed breathing, confusion, and tetany.

Respiratory Alkalosis

• Decreased CO₂, thus pH is increased.
• Due to excessive loss of CO₂, thus loss of carbonic acid.
• Symptoms include increased depth and rate of breathing, lightheadedness, and numbness.

The Major Buffer Systems

• The major buffer systems for the body's fluids include bicarbonate, phosphate, and protein buffer systems.

Bicarbonate Buffer System

• Based on carbonic acid (H₂CO₃) and sodium bicarbonate (NaHCO₃).
• Acts as a regulator of blood pH via respiratory and urinary systems
• Kidneys reabsorb HCO₃⁻ and excrete H⁺ to maintain pH.

Phosphate Buffer System

• Primarily intracellular and based on H₂PO₄⁻ (NaH₂PO₄) and HPO₄²⁻ (Na₂HPO₄).
• Acts as a regulator of pH in RBCs and the kidney's fluids.

Protein Buffer System

• Found in pH of blood plasma and in cells using amino acid chemistry.
• The acid component is COOH, and the base component is the amino group NH₂.
• Histidine and cysteine are two important amino acid buffers.

Hormonal Regulation

• Hormonal regulation of fluid and electrolyte balance involves antidiuretic hormone (ADH), aldosterone, and atrial natriuretic peptide (ANP).

Antidiuretic Hormone (ADH)

• Originates in the hypothalamus and secreted in reference to ECF osmolarity.
• Results in water conservation in the kidney and stimulates the brain's "thirst center" to stimulate drinking of fluids.

Aldosterone

• Originates in the adrenal cortex and is stimulated by decreased blood pressure or plasma volume.
• It helps determine sodium absorption by the kidney, causing more sodium/water to be absorbed and results in water retention.

Atrial Natriuretic Peptide (ANP)

• Originates in cardiac muscle fibers and stimulated by atrial wall stretching due to increased blood volume or pressure.
• Results in both ADH and aldosterone being blocked, decreasing the desire to drink fluids.