1. Water Distribution and Balance in the Body
Composition of the Body: The body is made up of approximately 60% water, with minerals (7%), proteins (18%), and fats (15%) making up the rest. Women tend to have 10% less water due to more adipose tissue.
Role of Water: Acts as a solvent, enabling chemical transport and lubrication. High specific heat makes water ideal for maintaining stable body temperature.
Fluid Compartments:
Intracellular Fluid (ICF): Accounts for 40% of body weight (25 L).
Extracellular Fluid (ECF): 20% of body weight (15 L), subdivided into:
Plasma: 3 L, about 20% of ECF.
Interstitial Fluid (IF): 12 L, about 80% of ECF.
Body Fluid Composition: Includes organic substances (glucose, amino acids, fatty acids) and inorganic ions (Na+, K+, Ca2+, Cl-), which play roles in hydration, signaling, and cell function.
Water Balance: Water intake from beverages, food, and metabolic processes should equal output (~2500 ml/day), lost through urine, skin, lungs, and feces.
This balance can be disrupted by atmospheric vapor pressure (temperature), skin damage (burn victims), exercise, diarrheal/renal diseases
2. Osmosis and Water Movement
Driving Forces for Water Movement:
Osmotic Pressure: Driven by solute concentration differences across membranes, leading to water moving from areas of low to high solute concentration.
Isosmotic: Equal osmolarity as body fluids (300 mOsm)
Hyperosmotic: Higher osmolarity, causing water loss from cells (600 MOsm)
Hyposmotic: Lower osmolarity, causing water entry into cells (100 mOsm)
Hydrostatic Pressure: The pressure needed to stop osmosis; influenced by blood pressure and fluid levels.
Side note on moles to osmoles: glucose doesn’t dissociate in water, so 1 mole = 1 osmole. NaCl dissociates in water so 1 mole of NaCl = 2 osmoles (1 Na+, 1 Cl-)
Other side note: Ionic composition across intracellular and extracellular fluid can vary in a happy health cell, but osmotic concentration should be the same in both.
3. Estimate of body fat:
Method 1: Can measure the amount of total resistance when applying electrical current between points on the body. Fat is not a good conductor of electricity, so more fat = more resistance.
Method 2: subtract the lean body mass (H2O content is constant) from body mass to determine fat mass.
4. Disorders of Water Balance
Dehydration: Occurs with excessive ECF water loss, causing cells to shrink.
Hypotonic Hydration: Also known as water intoxication, leads to cell swelling and possible brain damage due to rapid water intake or renal failure. Swelling of RBC (increase volume) decreases their oxygen-carrying capacity and may prevent them from traveling thru capillaries. Can be treated with hypertonic saline to pull water out of cells.
Edema: Excess Interstitial fluid accumulation, leading to tissue swelling. Can result from increased fluid outflow (from increased BP) from blood or reduced fluid reabsorption (from lower albumin - renal problems)
5. Electrolyte Balance
Importance: assist in transmission of electrical impulses, stabilize protein structures in enzymes, aid in releasing hormones from endocrine glands, act as buffers, osmotic balance (movement of water between cells)
Ionic Composition of Fluid Compartments:
ECF: High Na+ and Cl-.
ICF: Low Na+ and Cl-, High K+ and PO4³⁻
Sodium (Na+): Primary extracellular cation affecting water distribution and blood pressure. High dietary sodium can lead to imbalances.
Potassium (K+): Predominant intracellular cation, essential for neuron and muscle cell function.
Elevated potassium blood levels (ECF) can result in Hyperkalemia and can impair skeletal, nervous and heart muscle function and result in fatal cardiac arrest due to inability to relax heart muscles between contractions
Chloride (Cl⁻): Predominant extracellular anion that maintains hydration, balance other cations in the extracellular fluid, and ensure electrical neutrality.
Excess chloride, known as hyperchloremia, can occur due to dehydration or excessive salt intake, leading to an imbalance that affects pH and fluid levels.
Bicarbonate (HCO₃⁻): second most abundant anion in the body, key in maintaining the body’s acid-base balance, acting as a buffer to stabilize pH by neutralizing excess acids in the blood and other fluids.
Calcium (Ca²⁺): mostly bound in bones/teeth. Are essential for muscle contraction, nerve function, enzyme activity, and blood clotting, with half of blood calcium bound to proteins and the other half in an ionized form readily available for physiological processes.
Phosphate (PO₄³⁻): major anion of intracellular fluid, mostly bound in bone/teeth and phospholipids (cell membrane, ATP, nucleotides, buffers)
6. Acid-Base Balance
pH Regulation:
Normal pH: Arterial blood (7.4), venous blood and IF (7.35), and ICF (7.0).
Acidosis: Blood pH < 7.35.
Alkalosis: Blood pH > 7.45.
Buffer Systems (System that prevents radical change in fluid pH):
Can be chemical (first line), Brain stem respiratory centers (1-3 mins), Renal mechanisms (most potent, hours to days)
Types:
Protein Buffers: Utilize amino acids which contain amino/carboxyl groups that can bind/release hydrogen/hydroxyl groups to neutralize acids/bases.
Phosphate Buffer: Found in blood as either a
Weak acid: reverts back to a weak base and water when coming into contact with a strong base.
Weak base: picks up hydrogen ion/NaCl when coming into contact with a strong acid to create a weak acid and salt
Stabilizes pH in cells and blood.
Bicarbonate-Carbonic Acid Buffer: Primary buffering system if Interstitial fluid. Balances acidic blood pH (from lactic acid/ketone bodies) by converting strong acids/bases to weak acids/bases and salt/water.
Respiratory and Renal Regulation:
Respiratory: Adjusts CO₂ levels to maintain pH. Hyperventilation reduces CO₂, while hypoventilation increases it.
Renal: Alters bicarbonate and hydrogen ion levels but requires longer to respond.
7. Disorders of Acid-Base Balance:
Metabolic acidosis occurs when there is an excess of acid or a loss of bicarbonate in the body, often due to increased acid production, acid ingestion, decreased renal acid excretion, or GI/renal HCO3- loss. Leads to decreased blood pH, which can impair organ function and cause fatigue and rapid breathing.
Ketoacidosis: occurs when there is an accumulation of acids (like ketone bodies from uncontrolled diabetes) or a loss of bicarbonate, leading to a drop in blood pH. This can impair organ function, cause fatigue, and induce rapid, deep breathing as the body attempts to expel CO₂ to restore balance.
Metabolic alkalosis is caused by a loss of acid, such as from prolonged vomiting or excessive bicarbonate intake, resulting in elevated blood pH that can lead to muscle twitching, confusion, and arrhythmias.
Respiratory acidosis happens when CO₂ levels rise due to hypoventilation (e.g., from lung disease or airway obstruction), causing excess carbonic acid and lowering blood pH, which can result in drowsiness, confusion, and respiratory distress.
Respiratory alkalosis is due to excessive CO₂ loss from hyperventilation, often triggered by anxiety, pain, or high altitude, raising blood pH and potentially causing dizziness, tingling, and muscle cramps.