Body Water Content and Fluid Compartments PDF

Body Water Content and the Fluid Compartments Body water content is a function of Age: in infants ~70%+; ~45+ in elderly Body mass Sex: young men ~60%; young women ~50% Relative amount of body fat: adipose tissue is the least hydrated, containing up to 20% water Skeleta...

Body Water Content and the Fluid Compartments Body water content is a function of Age: in infants ~70%+; ~45+ in elderly Body mass Sex: young men ~60%; young women ~50% Relative amount of body fat: adipose tissue is the least hydrated, containing up to 20% water Skeletal muscle is about 75% water Having greater muscle mass = proportionately more body water & T Water is considered a universal solvent: wide variety of solutes will Water occupies 2 main fluid compartments: dissolve Intracellular fluid (ICF): Two classifications of solutes: 2/3 of the volume Electrolytes Water within all body’s cells Nonelectrolytes Extracellular fluid (ECF): remaining 1/3; 2 sub-compartments: Plasma: fluid portion of the blood Interstitial fluid: fluid in microscopic spaces between the cells Non-plasma/non-IF fluids: lymph, CSF (cerebrospinal fluid), humors of the eye, synovial fluid, etc. tend to be considered IF  Composition of Body Fluids Nonelectrolytes: Usually composed of covalent bonds Generally do not dissociate in solution No electrically charged species are created when added to water Electrolytes: Chemical compounds that dissociate into ions in water Because electrolytes are charged, they can conduct an electrical current Examples: inorganic salts, many acids and bases, some proteins Have much greater osmotic power than nonelectrolytes: dissociation produces at least 2 ions e · SKIP nn Electrolyte concentrations are usually expressed in mEq/L-> number of electrical charges in 1 L of solution  Comparison of ECF vs ICF e Patterns to electrolyte compositions of different fluid compartments: Blood plasma contains high concentrations of both sodium and chloride The intracellular compartment contains a higher concentration of protein than the ECF Sodium and potassium concentrations in the ECF and ICF are nearly opposite Characteristic distribution of these ions Na+/K+ pumps: resting membrane potential · In terms of bulk/mass, (rather than concentration) electrolytes are eclipsed by proteins and certain nonelectrolytes: Phospholipids, cholesterols and triglycerides in addition to proteins, are large molecules 90% of the mass of solutes in plasma 60% of the mass of solutes in IF 97% of the mass of solutes in ICF Exchanges Between Fluid Compartments Water molecules generally freely between the compartments, which keeps osmolality in all body fluids the same Substances must pass through both the plasma compartment and the interstitial fluid to reach the intracellular compartment Plasma serves as a delivery mechanism for shuttling materials throughout the body Exchanges between plasma and IF occur across capillary walls Exchanges between the IF and ICF occur across plasma membranes > SKIP Water Intake and Output Water intake varies substantially from person to person Metabolism: water produced by cellular metabolic processes: metabolic water or water of oxidation Foods Beverages Water exits the body via Feces Sweat (perspiration) Vaporizing via lungs or diffusing through the skin: insensible water lose Urination In the body, water and Na+ are very closely tied together; Na+ acts as a ‘water magnet’  Regulation of Water Output -8 Obligatory water losses: - Water that must be lost Insensible water losses Sensible water loss of ~ 500 mL of urine/day Human kidneys must flush about 600 nmols/day of urine solutes that are the waste products of cellular metabolic processes Even is we consumed no water in a day’s time the body would still need to void water to flush out those normal waste products Beyond obligatory water lose, a number of factors affect water output: Fluid intake Diet Perspiration Disturbances in Water Balance ⑧ ⑤ Tonicity If the environment outside the cell has a higher concentration of solutes than inside the cell (hypertonicity), water will move out of the cell to dilute it: cell will shrink ISOTONIC: balanced solutes; water moves both into and out of cells If the environment inside the cell has a higher concentration of solutes than outside the cell (hypotonicity), water will move into the cell to dilute it: cell will expand/possibly burst  Regulation of Sodium - ‘Water follows Na+’ Balancing Na+ is one of the most important renal functions NaHCO3 and NaCl account for 90-95% of all solutes in the ECF Na+ is n The single most abundant cation in the ECF The ion exerting the most osmotic pressure Largely unable to cross plasma membrane and so much be actively pumped Na concentration AND content are both important + Concentration in ECF determines osmolality and influences electrical excitability of neurons and muscles Regulated by ADH and thirst mechanism Total content determines ECF volume and blood pressure Regulated by renin-angiotensin-aldosterone and atrial natriuretic peptide (ANP) DESPITE THE IMPORTANCE OF SODIUM ON WATER BALANCE, THERE IS NO IDENTIFIED SODIUM RECEPTOR - NaHCO3: Na Bicarb  Regulation of Physiological pH Nearly all biochemical reactions are influenced by pH Chemical Buffers: system of 1+ compounds that resist changes in pH; Optimal pH varies in different parts of the body can tie up excess acids or bases temporarily, but cannot eliminate Arterial blood is about pH 7.4 them If arterial pH rises > 7.45: alkalosis (alkalemia) Bicarbonate system (covered with respiratory system) If arterial pH falls < 7.35: acidosis (acidemia) Phosphate Buffer System H+ from strong acids gets tied up in weak acid Don't need to Almost all acidic substances originate as metabolic by-products remember this [ or end-products equation Break-down of phosphorus-containing proteins yields Strong bases are converted to weak bases phosphoric acid Anaerobic respiration of glucose produces lactic acid Fat metabolism will yield ketones, among other organic acids Protein Buffer System: some portions of amino acids can be Transport of HCO3- as part of CO2 transport will yield H+ proton donors (release H+) or proton acceptors (bind H+) H+ in blood is regulated by 1. Chemical buffers => 2. Brain stem respiratory centers 3. Renal mechanisms SKIP