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Week/Lecture 4: Fluids and Electrolytes.pdf

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BIOL 171

Fluids and Electrolytes Homeostasis: the ability of the body to maintain a relatively constant internal environment Homeostasis of cells depends on homeostasis of body fluids and electrolytes (if cells are not homeostatic, the rest of the body can't be either) Body fluid: all water and dissolved substances in the body's fluid compartments Mechanisms Regulate: total volume distribution concentration of solutes pH Water: fluids account for 60% of adult total body weight about 75% at birth may drop to 45% in elderly Female: 45% solid, 55% liquid Male: 40% solid, 60% liquid Major fluid compartments Intracellular Fluid: inside the 50 trillion cells in body cytoplasm Extracellular Fluid: fluid filling in extra/empty space around body Interstitial Fluid: around cells Intravascular Fluid: blood Trans-cellular Fluid: in specialized places (eyeballs, joints, serous fluids) Body Weight Distribution: ⅔ -- 40% bw (intracellular fluid) ⅓ -- 20% b.w. (extracellular fluid) 16% b.w. (interstitial fluid) 4% b.w. (plasma) Composition of Body Fluids: Blood: lots of sodium lots of chloride little potassium lots of protein* lots of HCO3- (bicarbonate ions) Cells: not much sodium (constantly trying to pump it out) not much chloride tons of potassium (constantly trying to gather more) lots of protein* lots of HPO42 Interstitial Fluid: lots of sodium lots of chloride little potassium no protein lots of HCO3- (bicarbonate ions) * plasma proteins; primary one is albumin ICF has more K+ and protein ECF has more Na+, Cl-, HCO3- Composition of plasma and interstitial fluid is similar Movement of water between fluid compartments: cells, interstitial fluid, blood all work together / affect each other exchange between compartments occurs in only 2 places: through cell membranes (between ICF [cells] and interstitial fluid) through capillary walls (between interstitial fluid and plasma [blood]) Starling's Law of the Capillary: Fluid moves continuously out of and into the capillary as a result of hydrostatic and osmotic pressures respectively pressure is pushing fluid into interstitial space (filtration) blood cells, albumin do not leave osmotic pressure: lower concentration of water inside, higher concentration of albumin inside arteriole end: fluids move from the blood to the tissues by the blood hydrostatic pressure: filtration (blood cells and albumin do not leave blood) blood hydrostatic pressure: the more content you are pushing in, the higher the pressure to release is (hose) venous end: fluids return to the blood, drawn by blood osmotic pressure (created by albumin): osmosis --> lots of albumin, not a lot of water (filtration on arteriole end into interstitial fluid) any tissue fluid not returned directly to the blood drains from tissues via lymphatic capillaries fluid doesn't return automatically because as gradient levels out, osmotic pressure weakens, until it cannot draw last tiny bit of water into capillary lymphatic capillaries transport ~3 Litres of water back into the blood every day Movement between the interstitial space and ICF: Transport Processes: active or passive passive: no metabolic energy required substances move along their concentration gradients driven by diffusion pressure simple diffusion: movement of molecules from an area of higher concentration to an area of lower concentration osmosis: movement of water through a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration facilitated diffusion: molecules diffuse down their concentration gradient and across a membrane only when attached to a carrier protein active transport: carrier molecules move solute through membranes against concentration gradient requires metabolic energy (ATP) Between the interstitial space and ICF: ions move by active transport (sodium/potassium pump) water moves by osmosis nutrients, gases, and wastes move by diffusion Fluid Balance Body water gain and loss: gain from ingestion, metabolic water (from converting glucose to ATP) ~ 2500 mL/day loss from urine, feces, sweat, breath Regulation of fluid intake and output: controlled by: thirst mechanism ADH RAAS: renin-aniotensin-aldosterone system regulation of water gain: formation of metabolic water is not regulated (consequence of ATP production) main regulator of water gain is intake adjustment thirst centre in hypothalamus stimulated by: not enough saliva (mouth is dry) increase in blood osmolarity (water volume getting lower; remaining blood is more and more concentrated with other dissolved substances) decrease in blood volume (water increases volume) drinking restores body water levels RAAS: liver makes angiotensinogen (floating in blood stream) drop in blood pressure kidney releases renin renin converts angiotensinogen to angiotensin1 (floating in blood stream to lungs) angiotensin converting enzyme (ACE) in lungs convert angiotensin1 to angiotensin2 angiotensin2 stimulates adrenal cortex to produce aldosterone (hormone) aldosterone causes kidneys to absorb more sodium (renal absorption), and causes hypothalamus to release ADH (antidiuretic hormone) ADH causes kidneys to retain water in blood stream sodium absorption causes water absorption water absorption increases blood volume which increases blood pressure Regulation of Water and Solute Loss excess water, solutes eliminated in urine water follows salt so urinary water loss regulates body fluid osmolarity urinary NaCl loss regulates body fluid volume urinary excretion of NaCl varies Hormone Effects on Solutes: increasing fluid volume: heart wall release atrial natriuretic peptide (ANP) decreased release of renin by kidneys ANP promotes renal excretion of Na+ (natriuresis) increased urine output, decreased blood volume decreased renin results in: decreased aldosterone production decreased water/Na+ reabsorption increased urine output (b/c Na+ increases water absorption into body), decreased blood volume water levels and sodium levels regulate each other output regulation increased consumption of salt results in increased blood volume results in increased ANP and decreased renin outputs results in more loss of NaCl + water in urine results in decreased blood volume Fluid Imbalance body is always fighting these imbalances (homeostasis) Edema: abnormal accumulation of fluid in interstitial spaces causes: increased venous pressure increased back pressure in veins results in increased blood hydrostatic pressure fluid leaking out, and not all of it returning to the vessel causes: venous obstruction (blood clot, high cholesterol) causes of edema hypoproteinemia not enough protein decreased plasma proteins causes: kidney disease liver disease malnutrition (large abdomen b/c of edema) decreased plasma osmotic pressure results in decreased return of fluid to capillaries lymphatic obstruction lymphatic capillaries not working impairs drainage of fluid from tissues extreme cases: elephantiasis inflammation permeability of capillary walls more fluid and albumin leaks out into interstitial fluid if albumin leaks out, no power to draw fluid back in results in "water blisters" types of edema ascites fluid accumulation in the abdominal cavity due to any cause of edema pleural effusion fluid accumulation in the thoracic cavity no room for lungs to expand b/c ribcage doesn't stretch Dehydration: net loss of fluid in all compartments causes: inadequate intake of water increased surface losses (sweating, hemorrhage) G.I. tract losses (vomiting, diarrhea) excessive urine output manifestations: thrust abrupt weight loss decreased blood pressure b/c of low blood volume dry skin/mucous membranes decreased skin turgor lacklustre, sunken eyes (body pulling water from any part of the body) decreased urine output Overhydration: net gain of fluid in all compartments hard to do to yo
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