BIOG 1440 Lecture Notes - Lecture 15: Oncotic Pressure, Fluid Compartments, Extracellular Fluid
27 Apr 2018
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Basic Objective: How do organisms maintain a constant body fluid composition?
Body Fluid Composition and Variation
1. What are the main fluid components?
a. 40% bone, lipid, protein, etc
b. 60% water
c. Where fluids reside?
i. Within the cell (most fluid is intracellular--25L)
ii. In the blood (plasma--3L)
iii. Interstitial fluid (solution between cells--14L)
2. Fluid Components within the body
a. Changes in fluid composition leads to water flowing between compartments
(osmosis)
b. Water is constantly…
i. Forced out of the blood by hydrostatic pressure
ii. Reabsorbed in plasma by oncotic pressure
c. Water flows b/n interstitial fluid and intracellular fluid (two way osmoregulation)
d. Ion fluxes are restricted and controlled
i. Especially between interstitial fluid and plasma
e. Movements of nutrients, gases and wastes occur in one direction
3. What are body fluids made of?
a. Body fluid = water + electrolytes + non-electrolytes
i. Non-electrolytes: organic molecules that do not dissociate (no electrical
charge)
ii. Electrolytes: dissociate in water (salts, acids, bases, some proteins)
b. Electrolytes AND non-electrolytes contribute to/influence osmosis
c. Extracellular fluid
i. Major cation is sodium
ii. Major anion is chloride
1. High levels outside of cell, very low levels within cell
iii. Reminder: polarization of cells
1. Na+ outside of cell, K+ inside of cell
2. Used to generate action potentials like in the heart or in neurons
4. What are body fluids for?
a. Body fluids are “internal milieu”=water and solutes
b. Required to maintain cell structure
i. Water alters volume of cells (think osmosis and RBC)
ii. Hydrostatic pressure maintains tissues and organs in place
c. Required to ensure cell function
i. Ions required for cell function (ex: enzymes)
ii. Maintain electrical gradients across membranes (and allows for action
potentials)
d. Vehicle for nutrients and chemicals
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e. Amount of fluid, and concentration of solutes are both important! Fluctuation is
dangerous!
5. Organisms face external and internal fluctuations in hydration and salinity
a. Example: daily water balance in a human being
i. You lose water in feces, sweat, evaporation, urine (~2.5 L/day)
ii. Must regain the water lost through metabolism, foods, and beverages
iii. Gain must = loss
Regulators and Conformers
1. What does the organism need to maintain in the face of very different environments?
a. Homeostasis of extracellular fluids. What parameters need to be maintained?
i. VOLUME of water in body fluids
1. → volume regulation
ii. CONCENTRATION of ions available in fluid
1. → ionic regulation
iii. OSMOTIC PRESSURE (concentration) of fluid
1. → osmotic regulation
2. Osmotic Pressure: the force of a solution to attract water based
on its total dissolved solutes
2. Conformers vs. Regulators
a. Conformers:
i. No homeostasis
1. Works when environment is very stable
ii. Cells are isoosmotic with the environment
iii. Stable environments, typically marine
b. Regulators:
i. Homeostasis
ii. Tissues maintain relatively stable internal conditions against unstable
environments or constantly inhospitable habitats
1. Unstable environments: estuarine/brackish
2. Inhospitable: super salty or terrestrial
c. Examples:
i. Shrimp=strong regulators
1. Maintain internal osmotic pressure despite ambient osmotic
pressure
ii. Mussels=conformers
1. Osmotic pressure increases with increase of ambient osmotic
pressure
iii. Green crab=regulators only at some ranges of osmolarity
3. General problem of living in freshwater
a. Challenge: living in a dilute environment
i. Salt loss by diffusion and water gained by osmosis
b. Example: freshwater fish
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