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CHAPTER 5: MEMBRANES

Mass Balance and Homeostasis

Human body is an open systemExchanges heat and materials with external environment

Principle of MASS BALANCE is used to maintain Homeostasis

Law of Mass Balance:If the amount of a substance in the body is to remain constant,any gain must be offset by an equal loss

Example: Water loss (output) must be balanced by water intake (from external environment and from metabolic water production)

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Figure 5-1


Law of Mass Balance:If the amount of a substance in the body is to remain constant,any gain must be offset by an equal loss

Total amount (load) of substance X in body =Intake + production – excretion - metabolism


Applies to:

WaterOxygenCarbon DioxideSalts (electrolytes)Hydrogen ions (pH)


Figure 5-2


Input:

Most substances enter the body from the outsideWater, nutrients, enter via digestive tract

Oxygen and other gases enter via lungs:

Some lipid-soluble molecules can enter through the skin


Output:

1. ExcretionThrough urine, feces, lungs, or skin

2. Metabolize the substance to a different one

Metabolite: a product created in a metabolic pathway

Clearance:The rate at which a molecule disappears from the body(via excretion or metabolism)


Clearance example:Volume of blood plasma cleared of the substance per unit of time

Mass Flow: a more direct way to measure this

Mass Flow = concentration x volume flow

Amount of substance/min = amount of substance/vol x vol/min


Mass Flow: example:A person is given an intravenous infusion of glucose solution (50 g glucose per liter), given at a rate of 2 ml per min

Mass flow is:

50 g glucose/1000 ml x 2 ml solution /min = 0.1 g glucose/min

Mass flow applies to entry, production, and removal of substancesAnd also to movement of substances from one functional compartment to another

Homeostasis and Equilibrium

Homeostasis: usually refers to stability of the internal environment

Usually refers to stability of the ECF (plasma, interstitial fluid)

Cells maintain a state of cellular homeostasisBut, lots harder to measure stability of ICF (inside environment of cells)

Homeostasis and Equilibrium

In a state of homeostasis, the composition of both body compartments is relatively stable

BUT

The composition of the compartments are different


Figure 5-3a

Electrolytes

These dissociate (come apart) in liquidsand form ions (charged particles)

Acids: HCl (hydrochloric acid)Dissociates to form H+ and Cl-

Bases: NaOH (sodium hydroxide)KOH (potassium hydroxide)

Salts: NaCl (sodium chloride or table salt)KCl (potassium chloride)

Electrolytes areChemically reactive in metabolism

– Major cations• Na+, K+, Ca2+, H+

– Major anions• Cl-, HCO3

-, PO43-

Cation: positive ionAnion: negative ion

ElectrolytesMain functions in body:

1. Many are essential minerals

2. As the most numerous solutes, they control osmosis of water between body compartments

ElectrolytesMain functions in body:

3. They help maintain the acid-base balance required for normal cellular activities

4. They can carry an electric current:

This allows Action Potentials, graded potentials to happen; controls secretion of some hormones and neurotransmitters

Distribution of Electrolytes in Extracellular Fluid1. Interstitial fluid contains:

Na+ (most abundant positive ion)Cl- (most abundant negative ion)

H2CO3 (Bicarbonate ion; abundant)K+

Ca++

Mg++

2. Plasma contains:Same ions as interstitial fluid and Lots of protein anions

Distribution of Electrolytes3. Intracellular fluid (ICF) contains:

K+ (most abundant positive ion)

Protein ions; phosphate ions (HPO42-)

(most abundant negative ions)

H2CO3 (Bicarbonate ion; not as abundant as in ECF's)Cl-Na+

Mg++


Figure 5-3b

Homeostasis and Equilibrium(p. 134-135)

4 phrases to learn:

Dynamic disequilibrium

Osmotic equilibrium

Chemical disequilibrium

Electrical disequilibrium


Dynamic disequilibriumECF and ICF contain different concentrations of various solutes, resulting in a state of dynamic disequilibrium

Osmotic equilibriumWater moves freely between ECF and ICF, so these can reach a state of osmotic equilibrium


Chemical disequilibriumCertain solutes are more concentrated in one compartment than in the other

Electrical disequilibriumBody as a whole is electrically neutral

But due to ion concentrations, ICF is slightly negative relative to ECF

Changes to the ionic imbalance create electrical signals (nerve cells, muscle cells)


Figure 5-3b


“The goal of homeostasis is to maintain the dynamic steady states of the body's compartments

Dynamic:Materials are constantly moving back and forth between compartments

Steady state:No net movement of materials between the compartments

Membrane Transport(p. 136-158)

“Humans are large complex organisms and the movement of materials within and between compartments is necessary for communication”

Movement of materials across selectively permeable membranes requires a variety of transport mechanisms

(Cell membranes are selectively permeable)

Membrane Transport(p. 136-158)

Transport Mechanisms:

Some require energy in the form of ATP(Active transport)

Some do not require ATP (Passive transport)This kind uses energy of molecular motion


Figure 5-4


Figure 5-5