Anatomy & Physiology Lecture Outline: Week #4a

• Acid / Base Balance

LSc 107 Anatomy and Physiology Spring 1999

Thibodeau Ch 30

Acid: substance that adds H⁺ ions to solutions

Ex: HCl (hydrochloric acid), dissociates into H⁺ and Cl^- ions adding H⁺ to the solution, pH less than 7.0

Base: substance that removes H⁺ ions from solutions

Ex: NaOH (sodium hydroxide), dissociates into Na⁺ and OH^- ions, OH^- ions combine with H⁺ ions forming water (thereby removing the H⁺ ions from the solution), pH greater than 7.0

Regulation of H+ ion concentration in the body fluids

Interior of cells (cytoplasm) is neutral, therefore what is meant is H⁺ concentration of extracellular fluids (ECF: including plasma, interstitial fluid, lymph, CSF … )

pH = -log [H⁺] pH scale from 0-14

pH scale - Fig. 30-1

pH 0 has maximum H+ ions, no OH^- ions

pH 7 is balanced H⁺ = OH^-

pH 14 has max OH- ions, no H⁺

normal pH of blood = 7.36 - 7.41, venous blood more acidic than arterial blood

Source of ions which influence pH

Ions from metabolism

• carbonic acid - aerobic metabolism
• lactic acid - anaerobic metabolism
• sulphuric acid - from sulphur containing amino acids
• phosphoric acid - from nuclear proteins
• ketone bodies - breakdown of fats

Food which contribute acids

• high protein, meat (minerals S, Cl, P)
• cranberries, rhubarb

Foods which contribute bases

• vegetables, fruits ( minerals K, Na, Mg)
• antacids (Mg, OH-, Ca)

How is the pH of body fluids kept within a narrow range when, in most people, acids from metabolism are constantly being added? - Buffer mechanisms

Buffer mechanisms in the body Table 30-1

• Physiological buffer mechanisms (delayed action) Ex: lung blows off CO₂, kidney excretes H⁺
• Chemical buffers (rapid action buffers) act in ECF

Buffers – chemical substances that help regulate pH, protect against drastic shifts in pH, Buffers can bind both H⁺ and OH^- ions

Action of a buffer -

Composition of a buffer -

Buffer pairs in body fluids

• bicarbonate pairs: HCO₃-/ H₂CO₃ (everywhere in ECF) Fig. 30-3, 30-4
• plasma protein pair: proteinate-/protein (weak acids) (in plasma)
• hemoglobin pair: Hb-/Hb and HbO₂-/HbO₂ (in blood)
• phosphate buffer pair: Na₂HPO₄/NaH₂PO₄ (in red cell and kidney)

How do buffers work in resisting change in pH when H⁺ or OH^- are added?

Buffers act by replacing a strong acid or strong base (highly ionized fully dissociated, or split into ions) with a weak acid or weak base (not so likely to break down into ions, fewer H+ or OH- in solution)

Ex: NaHCO₃-/H₂CO₃ buffer system

With strong acid:
NaHCO₃ + HCl ® NaCl + H₂CO₃

HCl is a strong acid, H₂CO₃ is a weak acid, fewer H+ in solution

With a strong base:
H₂CO₃ + NaOH ® NaHCO₃ + H₂O
NaOH is a strong base, NaHCO₃ is a weak base, fewer OH- in solution

What if the acid added to the solution is already a weak acid (carbonic, produced in metabolism)? It is buffered by an even weaker one

Fig. 30-5 buffering of H₂CO₃ by K salt of Hb

What is important in maintaining the effectiveness of the bicarbonate buffer system is the ratio of the salt (bicarbonates) 20: to 1 carbonic acid. If the ratio drifts away from this then that pair is not so effective in maintaining appropriate pH (body pH changes)

Carbonic acid buffer is important because it is widespread and because the body can adjust the ratio easily (CO₂ blown off by lungs, H⁺ or HCO₃ excreted by kidney as necessary)

IMPORTANT - chemical buffers do not get rid of the extra H+ ions added by metabolism. They merely mask them, until something more permanent occurs. Permanent unloading of acid takes place through the lungs and kidneys.

 

Respiratory Mechanism of pH Control

Of the equation CO₂ + H₂O n H₂CO₃ n H⁺ + HCO₃-

In every breath CO₂ is blown off. In 24 hrs ~30 liters of carbonic acid are removed

The pH of arterial blood therefore is slightly higher than venous (less CO₂)

How does this mechanism adjust to different rates of production of acid? Fig. 30-8

Remember that the respiration center of the brainstem is sensitive to the pH or CO₂ content of the blood supplying it.

Chemoreceptors in carotid arch signal O₂ level. Fall in pH results in increase in rate and depth of breathing.

Increase in pH results in suppression of breathing rate

acidosis leads to hyperventilation to raise pH /alkalosis leads to hypoventilation to lower pH

Urinary Mechanism of Control of pH Fig. 30-11

Fig. 30-9

• CO₂ diffuses from the capillaries surrounding the distal convoluted tubule into the cells of its wall
• dissolves in H₂O to form carbonic acid (enzyme carbonic anhydrase)
• H₂CO₃ breaks down into H⁺ and HCO₃-
• H⁺ diffuses into the filtrate (urine) in exchange for Na⁺
• Na⁺ joins the bicarbonate and both absorbed back into the blood

Summary: kidney gets rid of the H+ into the urine, and regains the HCO₃-. Urine pH goes down, blood pH goes up.

Fig. 30-10

• secretion of ammonia into distal and collecting tubule
• NH₃ combines with H⁺ to form NH₄⁺ ion (ammonium)
• Sodium is exchanged for the H⁺, and together with HCO₃- is reabsorbed back into the blood
• Net buffering effect: H⁺ in urine, HCO₃- in blood.

What determines how much H+ is excreted into urine?

Blood pH: more H⁺ there is, more is excreted via aldosterone mechanism - Fig. 30-11

 

Acid/Base Imbalances

pH vital for enzymes function

pH control within narrow limits necessary for maintaining function

Metabolic acidosis (bicarbonate deficit) /Metabolic alkalosis (bicarbonate excess)

Respiratory acidosis (carbonic acid excess) /Respiratory alkalosis (carbonic acid deficit)

 

[ WNJ Home Page | A & P Home | Lecture Outlines | Syllabus | Announcements ]