• Distinguish between strong and weak acids and bases.

• Compare and contrast the three major acid-base buffering systems that maintain pH in humans.

• Discuss how the respiratory center is involved in maintaining the acid-base balance.

• Discuss the important function of the kidneys in maintaining  pH homeostasis in the human body.

I.          INTRODUCTION

Throughout this course we have stressed the importance of homeostasis, or the tendency of our body to maintain a stable internal environment.  Water and electrolytes are also included in this delicate balance or state or equilibrium, whereby their intake must equal their output.  In order to maintain stable levels of water and electrolytes, the body must utilize mechanisms that ensure that lost water and electrolytes will be replaced while excreting any excesses.

II.        Review from Chapter 2: Chemistry

A.        Inorganic compounds are small compounds that do not contain the atoms C and H; The most important ones in humans include carbon dioxide (CO₂) water, salts, acids & bases.

• CO₂ is a waste product of cellular respiration .

2.         Water is a polar molecule that demonstrates hydrogen bonding and therefore it possesses very unique characteristics.

                                                                        a.        Water is an excellent solvent (universal).

• Many solutes are dissolved in our body's water.
• Many ionic compounds (i.e. NaCl) dissociate or break apart in water.

• Water participates in many chemical reactions.
• Dehydration (synthesis) is when water is removed from adjacent atoms (of molecules) to form a bond between them.
• Hydrolysis (degradation) is when water is used to break bonds between molecules.

• Water is an excellent temperature buffer.
• absorbs and releases heat very slowly

• Water provides an excellent cooling mechanism.
• requires a lot of heat to change from a liquid to a gas;
• high heat of vaporization.

• Water serves as a lubricant
• mucus;
• internal organs;
• joints.

f.          Water is the most abundant component in cells.

II.        Review from Chapter 2: Chemistry

A.        Inorganic compounds

3.                     Salts:  See Fig 2.9, page 46.

a.         Salts dissociate (ionize) into ions when dissolved in water.

• an anion is formed and
• a cation is formed.
• Example = NaCl in water.

H₂O

NaCl               Na⁺  +   Cl^-     

• These ions are referred to as electrolytes (charged particles).
• Electrolytes must be maintained within a very narrow range in our blood and tissues (i.e. homeostasis);
• Needed for muscle contraction, nerve impulses, etc.;
• Examples include Na⁺, K⁺, Cl^-, Ca⁺, PO₄^-; HCO₃^-, etc.

• Acids/Bases

• Acids dissociate (ionize) in water into:
• a hydrogen cation, H⁺, and
• an anion.
• Example = HCl (hydrochloric acid).

H₂O                                                 

HCl                H⁺  +  Cl^-

• Bases dissociate (ionize) in water into:
• a hydroxyl anion, OH^-, and
• a cation.
• Example = NaOH (sodium hydroxide).

H₂O

NaOH                         Na⁺  +  OH^-.

II.        Review from Chapter 2: Chemistry

A.        Inorganic compounds

• Acids/Bases (continued)

c.         pH Scale:        See Fig 2.10, page 47.

• The relative concentrations of hydrogen ions and hydroxyl ions determine the pH in our blood, fluids, and tissues;
• pH in body = [H⁺]  +  [OH^-] .
• pH = -log[H⁺];
• pH Scale ranges from 0 to 14:

0 ---------------------------7---------------------------14

                                                       acid                               neutral                              basic

[H⁺] > [OH^-]                     [H⁺] = [OH^-]                    [H⁺] < [OH^-]

• Physiologic pH = 7.4

• Buffering Systems

Definition:     Buffers are compounds added to solutions to prevent abrupt change in pH.

• usually weak acids;
• function by donating H⁺ when needed and by accepting H⁺ when in excess;
• very important in biological systems!
• Example = the carbonic acid (H₂CO₃) buffering system.

when pH is rising

H₂CO₃                       HCO₃^-                +                     H⁺

when pH is falling

carbonic acid             bicarbonate ion           hydrogen ion

(H+ donor)                (H+ acceptor)

• Physiologic pH = 7.4.
• pH < 7.4 = acidosis; lethal below 7.0;
• pH > 7.4 = alkalosis; lethal above 7.8.

III.      DISTRIBUTION OF BODY FLUIDS:

•  
• Water composes the majority of a human’s body weight:

• Males = 63% water;
• Females = 52% water.

• B.        Two Major Fluid Compartments:

• 1.         Intracellular Fluid Compartment (ICF) = fluid within cells.
• a.         comprises about 63% of total water
•  
• 2.         Extracellular Fluid Compartment (ECF) = fluid outside of cells:
• a.         in interstitial spaces (interstitial fluid);
• b.        in blood vessels (plasma);
• c.         in lymphatic vessels (lymph).
• d.        comprises about 37% of total water.

See Figures 21.1 and 21.2 on page 809 to better understand the division of these compartments and the fluid in each.

• C.        Body Fluid Composition:    Also see Fig 21.3, page 810.

• D.        Movement of Fluid Between Compartments:       See Fig 21.4, page 811.

• Hydrostatic pressure and Osmotic pressure regulate fluid movement.
•  
• a.         Hydrostatic Pressure drives fluid out of plasma and drives fluid into lymphatic vessels;
•  
• b.         Osmotic pressure causes the return of fluid to plasma and regulates movement into and out of cells.
•  
• Sodium ion concentrations are especially important in fluid movement
•             regulation (i.e. water follows sodium through cell membranes).

IV.      WATER BALANCE

• A.        Water intake

• 1.         varies from person to person;
•  
• 2.         Majority comes from consumption of liquid or moist foods:

See Fig 21.5a, page 811.

•  
• 3.         Dehydration synthesis produces some water.

• B.        Regulation of water intake: See Table 21.1, page 812.

• 1.         The body loses less than 1% of its water.
•  
• 2.         An increase in the osmotic pressure of ECF due to water loss stimulates osmoreceptors in the thirst center (in hypothalamus).
•  
• 3.         Hypothalamic activity triggers thirst and the person seeks water.
•  
• 4.         Drinking fluids distends the stomach whose stretch receptors send nerve impulses that inhibit the thirst center.
•  
• 5.         Water is absorbed into ECF through walls of stomach and small intestine.
•  
• 6.         Osmotic pressure of ECF increases back to normal.

• C.        Water Output

• Water output occurs in a variety of ways:     See Fig 21.5b, page 811.
•  
•                         1.         excretion in:

                                    a.         urine;

                                    b.         feces;

                                    c.         skin.

•                        
•                         2.         evaporation from:

                                    a.         skin (sweating);

                                    b.         lungs (expiration).

• Urine production regulates water output through a variety of mechanisms (see below).

IV.      WATER BALANCE

• D.        Regulation of Water Output: (Review Renin-Angiotensin System in Chapter 20):

See Table 21.2, page 813.

• 1.         In dehydration:

• a.         ECF becomes more concentrated (i.e. increase in osmotic pressure).
•  
• b.         Osmoreceptors in hypothalamus detect this increase in osmotic pressure.
•  
• c.         Hypothalamus triggers posterior pituitary to release antidiuretic hormone (ADH).
•  
• d.         ADH targets distal convoluted tubules and collecting ducts in nephrons to increase their water reabsorption (i.e. back into the blood).
•  
• e.         Urine output decreases, returning osmotic pressure of ECF to normal.
•  
• 2.         In water excess:

• a.         ECF becomes less concentrated (i.e. decrease in osmotic pressure).
•  
• b.        Osmoreceptors in hypothalamus detect this decrease.
•  
• c.         Hypothalamus instructs posterior pituitary to decrease its release of ADH.
•  
• d.        Renal tubules decrease their water reabsorption.
•  
• e.         Urine output increases returning osmotic pressure of ECF to normal.

See Clinical Application 21.1 pages 814 & 815:  Water Balance Disorders.

V.        ELECTROLYTE BALANCE

Electrolyte balance exists when the intake of electrolytes from various sources equals the output of all electrolytes:    See Figure 21.6, page 813.

• A.        Electrolyte Intake
•  
• 1.         Important electrolytes needed for cellular function (which ones are these?) are primarily obtained from food;
•  
• 2.         Some may be obtained from water and other beverages.
•  
• 3.         Some are byproducts of metabolic reactions.

• B.        Regulation of Electrolyte Intake
•  
• 1.         Electrolytes are usually obtained in sufficient quantities in response to hunger and thirst;
•  
• 2.         Salt Craving may occur in severe electrolyte deficiency (rare!).

• C.        Electrolyte Output
•  
• 1.         Most due to kidney function and urine output (see below);
•  
• 2.         sweating;
•  
• 3.         defecation.

V.        ELECTROLYTE BALANCE

• D.        Regulation of Electrolyte Output

1.         The kidney plays a major role in maintenance of electrolyte concentrations. 

Recall calcium homeostasis from Chapter 6, and the renin-angiotensin system discussed in Chapter 20 to name just a few.

• 2.         The concentrations of some positively charged ions are particularly important:
•  
• These are vital for:

                                    a.         nerve impulse transmission

                                    b.         muscle fiber contraction

                                    c.         maintenance of Resting Membrane Potential (RMP).

•  
• d.        Regulation of Na⁺:
• aldosterone (secreted by what gland?) increases sodium reabsorption in kidney tubules.
•  
• e.         Regulation of K⁺:
• aldosterone also causes secretion (excretion) of excess  K⁺ into the urine.  See Fig 21.7, page 816.
•  
• f.         Regulation of Ca⁺⁺:   See Fig 21.8, page 816.
• calcitonin (what gland?) decreases calcium levels by targeting:
•  
• 1.        Distal kidney tubules to secrete excess Ca⁺⁺ into urine
• 2.        Inhibiting osteoclast activity.
•  
• parathyroid hormone (PTH by what glands?) increases calcium levels by targeting:
•  
• 1.        Kidney tubules to reabsorb Ca⁺⁺ back into blood &
• 2.        Activating osteoclasts to resorb bone matrix.
•  
• See Clinical Application 21.2 on page 818 concerning Sodium and Potassium Imbalances.