Chapter 9 Carbon Dioxide Equilibrium and Transport

What is the average amount of CO2 produced by the body at rest?

200 mL/min

PvCO2

46 mm Hg

Average tissue PCO2

46 mmHg

Which of the following statements explain the alkalemia present at high altitudes? I.Lack of oxygen at high altitudes creates a chemical stimulus for ventilation. II.Increased ventilation drives the PACO2 and PaCO2 below 40 mm Hg. III.Low PaCO2 lowers the blood H2CO3 concentration. IV.Low PaCO2 lowers PaO2 and promotes buffer release.

A.I, II, III

What effect does removing CO2 molecules through increased ventilation have on the blood's H+ concentration?

Removing CO2 molecules from the blood pulls the hydration reaction to the left, causing blood hydrogen ions and bicarbonate ions to react to form carbonic acid and ultimately an increase in CO2. As long as the body continues to remove CO2 molecules, the process continues and blood H+ concentration falls.

During cardiac resuscitation, a 63-year-old man receives sodium bicarbonate for a profound metabolic acidosis. Why should the respiratory therapist increase ventilation at this point?

To eliminate the additional CO2 generated by lactic acid buffering.

Which of the following mechanisms is responsible for maintaining a normal PACO2 near 40 mm Hg during heavy exercise?

VA normally increases such that it eliminates CO2 at a rate proportional to its production.

Bohr effect

H+ + HbO2 <---> HHb + O2 Carbon dioxide and H+ are affecting the affinity of hemoglobin to bind to O2

Why does CO2 not normally constitute a net gain of H+?

H+ generated via CO2 is reincorporated into water when reactions are reversed during passage of blood through lungs

CO2 hydration reaction

H20 + CO2 --> H2CO3 --> HCO3- + H+ This reaction shows that CO2 in the blood is associated with the formation of H2CO3, which dissociates to produce hydrogen ions.

carbonic acid

H2CO3 a very weak acid formed in solution when carbon dioxide dissolves in water.

CO2+H20 ->

H2CO3 -> HCO3- + H+ (how to generate H+ ions during passage of blood through peripheral tissues)

dynamic equilibrium, the reaction between CO2 and H2O in plasma occurs only to a slight extent; in this state of chemical equilibrium, CO2 and H2O molecules far outnumber H2CO3 molecules (about 500 times greater). Therefore, at equilibrium, the CO2 reaction is shifted to the left:

H2O + CO2 <----- H2CO3 <----- HCO3- + H+ --> -->

Buffer + H+ ->

HBuffer: either a weak acid that can dissociate (Buffer H+) or an undissociated molecule (HBuffer)

Explain how increased CO2 production of heavy exercise affects the blood's volatile acid level.

Normally the ventilation keeps pace with increased CO2 production with exercise, maintaining a constant blood PCO2. The blood volatile acid H2CO3- content remains constant.

Hb and binding site for O2 and CO2

O2 ---> with heme CO2 ---> with amino groups of protein

What happens to H+ concentration during hypoventilation?

decreased arterial H+ concentration leads to inhibition of ventilation, which increases arterial PCO2 and increases H+ concentration

Hyperventilation

decreases the concentration of carbon dioxide in the blood and increases the blood's pH condition called respiratory alkalosis

carbonic anhydrase

enzyme that catalyzes the reaction between carbon dioxide and water to form carbonic acid, it speeds up the reaction 13,000x faster

What do kidneys do if a net loss of H+ ions occur?

hyperventilation or vomiting causes net loss of H+ so kidneys try to replenish body with H+

When does net loss of CO2 occur?

hyperventilation, which causes net loss of H+

When does the net retention of CO2 occur?

hypoventilation or respiratory diseases, which cause the net gain of H+

What do kidneys do if a net gain of H+ ions occurs?

hypoventilation or respiratory malfunction or loss of alkaline gastrointestinal secretion (diarrhea) cause net gain of H+; kidneys increase elimination of H+ from body to restore balance

Alkalosis

pH exceeds 7.4; plasma hydrogen ion concentration decreases

Acidosis

pH less than 7.4; plasma hydrogen ion concentration increases

What happens to H+ concentration during hyperventilation

increased arterial H+ concentration leads to stimulation of ventilation, which lowers arterial PCO2 and, by mass action, reduces H+ concentration

Hypoventilation

increases the concentration of carbon dioxide in the blood and decreases the blood's pH (a condition generally called acidosis)

What happens when HCO3- is gained?

reaction driven to the left, which loses H+ in body

What happens to the reaction if H+ increases?

reaction forced to the right and more H+ is bound to the buffer to form HBuffer, which does not increase H+ concentration as much as it would have without the buffer

What happens to the reaction if H+ decreases?

reaction proceeds to the left and H+ is released from HBuffer; these movements allow buffers to stabilize H+ concentration against changes in either direction

What happens when HCO3- is lost?

reactions driven to the right, which generates H+ within the body

Hyperventilation

the condition of taking abnormally fast, deep breaths; no way to effectively open lungs. Examples: stress, anxiety, fear, pain, head injury, stroke, asthma, pneumonia, pulmonary edema, sepsis, thyrotoxicosis, salicylate overdose, excessive mechanical ventilation

chloride shift

the movement of chloride ions into the red blood cells as hydrogen ions move out to maintain the electrochemical equilibrium.

Sources of hydrogen loss

utilize H+ in metabolism of organic anions; loss of H+ in vomitus or urine; hyperventilation

How does vomitus affect H+ concentration?

vomitus contains high H+ so it constitutes a source of net loss (vomitus is acidotic)

Dissolved CO2 and HCO3

- Dissolved CO2 is expressed in mmol/L - at 37 degrees C 0.03 mmol of CO2 dissolves in 1 L plasma for each mm Hg CO2. - increase PACO2 = increase Plasma PaCO2 = increase dissolved CO2 = increase H2CO3

High altitude

- Lack of O2 = increased ventilatory drive, mediated via hypoxic chemoreceptors; consequently hyperventilation occurs. - Decrease in : PACO2, PaCO2, H2CO3 - Left shift hydrolysis reaction - Blood alkalinity increases (pH increases because acid is eliminated)

Hyperventilation

- decrease in CO2 = decrease in PaCO2 = hydrolysis left shift - CO2 elimination momentarily exceeds CO2 production - H2CO3 decreased due to left-shifted reaction - More ventilatory work to eliminate CO2 and lower blood acid levels

hypoventilation

- deficient movement of air into and out of the lungs, causing hypercapnia - increase in CO2 = increase PaCO2 = hydrolysis reaction to the right shift - CO2 production momentarily exceeds CO2 elimination - Alveolar gas and blood eventually equilibrate and CO2 elimination matches CO2 production but at a higher PaCO2 - Result in lower ventilatory reserve, less ventilatory work to eliminate CO2, however higher blood acid levels

CO2 transported by

- dissolved CO2 = 8% - HCO3- = 80% - Carbamino compounds = 12%

Haldane effect

- hemoglobin that has released oxygen binds more readily to carbon dioxide than hemoglobin that has oxygen bound to it - decreased oxygenation of the Hb increases blood's capacity for CO2

Capillary PCO2

40 mm Hg

Alveolar PCO2

40 mmHg

What is the significance of the straight (linear) CO2 equilibrium curve versus the sigmoid-shaped HbO2?

A change in alveolar ventilation is much more effective in changing arterial CO2 content than O2 content.

volatile acid

An acid that can be eliminated by the lungs; carbonic acid is converted to CO2, which diffuses into the alveoli.

hypoventilation

Caused from breathing too shallow or too slowly. Low oxygen concentration but high CO2. Examples: emphysema, cystic fibrosis, neuromuscular disorders and bronchitis

Theoretically, what effect would a lack of carbonic anhydrase in the erythrocyte have on plasma PCO2?

If carbonic anhydrase were suddenly removed from blood the hydration reaction speed in the RBC would slow greatly as a result the CO2 diffusion rate into the RBC would slow and plasma PCO2 to increase

What effect does inadequate V·A have on plasma PCO2 and volatile acid content?

Inadequate ventilation means the lungs do not remove CO2 rapidly enough to keep up with its rate of production by tissue cells. Consequently blood CO2 builds up and PCO2 rises, this drives hydrogen to the right. Generating the volatile H2CO3- (increase)

Buffer

any substance that can reversibly bind H+; most H+ is bound by extracellular and intracellular buffers; buffers don't eliminate or add H+ to the body: they keep H+ locked up until balance can be restored

CO2 =

by-product of aerobic metabolism

How does diarrhea affect H+ concentration?

diarrhea is alkaline and has very little H+ but higher amounts of HCO3-; diarrhea leads to gain of H+

Sources of hydrogen gain

generation of H+ from CO2; production of nonvolatile acids from metabolism of proteins and other organic molecules; gain of H+ due to loss of HCO3- in diarrhea or in urine

What if the respiratory system is the actual cause of the H+ imbalance?

kidneys are sole homeostatic responder

How do kidneys affect H+ concentration?

kidneys can either remove H+ from plasma or add it

What do kidneys do to H+ ions from nonvolatile acids generated from metabolism?

kidneys excrete excess H+ from nonvolatile acid metabolism

What happens if kidneys malfunction?

kidneys might eliminate too little or too much H+ and create the imbalance so respiratory system takes control

How do the kidneys and respiratory system work together to maintain balance of H+?

respiratory response is very fast (minutes) to fix H+ concentration and keeps it from changing too much until the kidneys can respond (hours to days) and actually eliminate imbalance