Solutions Play a Big Role in Physiology

• Solution is a mixture
• Most abundant component = solvent (usually water in biology)
• Other components = solutes
• Solutions with ions conduct electricity
• Liquid solutions good for delivering food, removing wastes. Biological examples:
  • blood
  • urine
  • sea water
  • intracellular fluid (fluid within cells)
  • interstitial fluid (fluid between cells)

Your Body is Split Into 3 Solution-Filled Compartments

• Body 60-80% water by weight
• 70 kg man has ~ 49 kg water = ~49 liters
• 3 Main compartments:
  
  • Intracellular (inside cells) = ~ 34 liters
  • Interstitial (outside cells) = ~ 13 liters
  • Blood plasma = ~3 liters
    • 40% of blood is red blood cells (RBCs)
    • Note1: plasma is similar to interstitial fluid, but contains plasma proteins
    • Note 2: serum = plasma with clotting proteins removed
    • Note 3: intracellular fluid is very different from interstitial fluid (high K concentration instead of high Na concentration, for example)
  • Boundaries:
    • Capillary walls (1 cell thick) separate blood from interstitial fluid
    • Cell membranes separate intracellular and interstitial fluids
• Loss of about 30% of body water is fatal
  • This can be a problem in the desert
  • Many diseases involving diarrhea can dehydrate the body to this extent (i.e., cholera)

Concentrations of Substances Control the Rates of Chemical Reactions

• Concentration = amount/volume
• Usually given in moles/liter
• When chemicals react the rate of reaction is proportional to the concentrations
  • In general, if you double the concentration of a substance the rate of reaction with a 2nd substance will double
• The speed and effectiveness of reaction of hormones, drugs, poisons and other chemicals in the body is proportional to concentration ("The dose makes the poison")

Much of Physiology is Concerned With Regulation of Body Solutions

• Internal environment = " milleu interior"
• Constancy of internal environment essential for survival of cells & organs (homeostasis)
• Blood highly regulated, especially by kidney (kidney filters almost 200 liters of body fluids/day- about 60 times the plasma volume)
• Blood ion concentrations slightly too high or too low can be lethal
• Hormones involved in water and salt regulation: ADH, aldosterone, angiotensin, renin

Water is Very Special in Biology

• Most abundant chemical of life
• Special water properties of importance in physiology:
  

  Property	Biological Applications
  Liquid over wide temperature range	Chemical reactions take place readily in liquids. Useful for circulatory systems. Many microorganisms find body fluids an attractive place to live.
  Excellent solvent	Good for chemical reactions, waste removal, delivery of food materials.
  Ionizing solvent (high dielectric constant)	Ionizes salts, makes conductive solutions. Important for nerves and other excitable tissues.
  Low viscosity (flows readily)	Important for circulatory systems. Less work for the heart.
  High surface tension (surface acts as though coated with tough film)	Tends to make the lung alveoli collapse. Causes serious medical problems when lung surfactant is low.
  High heat capacity (large amount of heat required to raise water temperature)	Useful for moving large amounts of heat in the circulatory system.
  High heat of vaporization (large amount of heat removed when water evaporates)	Used by mammals for sweating (the only way the body can lose heat if the ambient temperature is above body temperature).
  High heat conduction	Useful for removing the heat produced by biological reactions. Prevents overheating of the body.

Ions (Charged Atoms or Molecules) Can Conduct Electricity

• Giving up electron leaves a + charge (cation)
• Taking on electron produces a - charge (anion)
• Ions conduct electricity
• Without ions there can be no nerves or excitability
  • Na+ and K+ cations (monovalent = 1 charge)
  • Ca2+ and Mg2+ cations (divalent = 2 charges) control metabolism and trigger muscle contraction and secretion of hormones and transmitters
  • 

Na+ & K+ are the Major Cations in Biological Fluids

• High K+ in cells, high Na+ outside
• Ion gradients maintained by Na pump (~1/3 of basal metabolism)
• Think of Na+ gradient as a Na+ battery- stored electrical energy
• K+ gradient forms a K+ battery
• Energy stored in Na+ and K+ batteries can be tapped when ions flow
• Na+ and K+ produce action potential of excitable cells

Biological Functions are Extremely Sensitive to pH

• H+ and OH- ions get special attention because they are very reactive
• Substance which donates H+ ions to solution = acid
• Substance which donates OH- ions to solution = base
• Because we deal with H ions over a very wide range of concentration, physiologists have devised a logarithmic unit, pH, to deal with it
  • pH = - log [H+]
  • [H+] is the H ion concentration in moles/liter
  • Because of the way it is defined a high pH indicates low H ion and a low pH indicates high H ion- it takes a while to get used to the strange definition
  • Also because of the way it is defined, a change of 1 pH unit means a 10X change in the concentration of H ions
    • If pH changes by 2 units the H+ concentration changes by 10 X 10 = 100 times
      • Suppose the pH changes from 4 to 8, what is the change in H+ concentration?
      • Suppose the H+ concentration goes up by a factor of 1000, how much will the pH change?
      • Click to check your answers
• Learn the pHs of common substances:
  

  	Approximate pH	Common Examples
  Strong Acids	0-2	Stomach acid (HCl), battery acid (H2SO4)
  Weak Acids	3-6	Lemon juice, vinegar, rainwater
  Neutral	7	Pure water
  Weak Bases	8-11	Bicarbonate solution
  Strong Bases	12-14	Solutions of lye (NaOH), oven cleaner (KOH)

• Human blood pH is 7.4
  • Blood pH above 7.4 = alkalosis
  • Blood pH below 7.4 = acidosis
• Body must get rid of ~15 moles of potential acid/day (mostly CO2)
  • CO2 reacts with water to form carbonic acid (H2CO3)
  • Done mostly by lungs & kidney
• In neutralization H+ and OH- react to form water
• If the pH changes charges on molecules also change, especially charges on proteins
  • This changes the reactivity of proteins such as enzymes
• Large pH changes occur as food passes through the intestines.

The Body Regulates pH in Several Ways

• Buffers are weak acid mixtures (such as bicarbonate/CO2) which minimize pH change
  • Buffer is always a mixture of 2 compounds
    • One compound takes up H ions if there are too many (H acceptor)
    • The second compound releases H ions if there are not enough (H donor)
  • The strength of a buffer is given by the buffer capacity
    • Buffer capacity is proportional to the buffer concentration and to a parameter known as the pK
  • Mouth bacteria produce acids which attack teeth, producing caries (cavities). People with low buffer capacities in their saliva have more caries than those with high buffer capacities. Why do you suppose this is true? Discuss your answer.
• CO2 gas (a potential acid) is eliminated by the lungs
• Other acids and bases are eliminated by the kidneys

Many Serious Medical Problems Involve Abnormalities of Salt, Water or pH

• Examples:
  • Hyperkalemia: caused by kidney disease & medical malpractice
    • High K+ in blood- can stop the heart in contraction (systole)
  • Dehydration: walking in desert- can lose 1-2 liters/hour through sweat
    • Blood becomes too viscous to circulate well -> loss of temperature regulation -> hyperthermia, death
  • Acidosis: many causes including diabetes mellitus and respiratory problems; can cause coma, death

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