Chapter 12: Gas Exchange and Transport

What does the movement of gases between the lungs and the body tissues mainly depend on?

Gaseous diffusion

Where would the lowest PO2 normally be found?

Cells

Where are the highest PCO2 levels found?

Cells

Which statements are true regarding PACO2?

1 and 3 only (Directly proportional to whole-body carbon dioxide production and Normally maintained at approximately 35 to 45 mm Hg)

What is the approximate normal level of carbon dioxide production (CO2) for an adult?

200 ml/min

Under what conditions will the alveolar PACO2 rise above normal?

If VA decreases relative to carbon dioxide production

If a 70-kg male patient has a CO2 of 200 ml/min and a VA of 9 L/min, what can you infer?

The patient will have a lower than normal PACO2.

What is the primary determinant of the PAO2?

PO2 in the inspired gas

What factors determine the PAO2?

1, 2, 3, and 4 (Ambient pressure, Fractional concentration of inspired O2, Level of VA, Types of fuels burned)

Calculate the approximate PAO2 given FiO2 = 0.40, PB = 770 mm Hg, PACO2 = 31 mm Hg.

250 mm Hg

What would the PAO2 be for a healthy person breathing 100% O2 at sea level?

670 mm Hg

What best represents the partial pressures of gases in a normally ventilated and perfused alveolus when breathing room air at sea level?

PO2 = 100 mm Hg; PCO2 = 40 mm Hg; PN2 = 573 mm Hg; PH2O = 47 mm Hg

If the alveolar PCO2 rises from 40 to 70 mm Hg, what would you expect?

PAO2 to fall by approximately 30 mm Hg

Assuming a constant FiO2 and carbon dioxide production, what is correct?

Increases in alveolar ventilation (VA) decrease the PACO2 and increase the PAO2.

What is the highest PAO2 one could expect to observe in an individual breathing room air at sea level?

110 to 120 mm Hg

What condition must exist for gas to move between the alveolus and pulmonary capillary?

Adequate alveolar ventilation (VA)

What is adequate alveolar ventilation (VA)?

The amount of air that reaches the alveoli per minute.

What drives the diffusion of gases across membranes?

The difference in partial pressures (pressure gradient).

What is the normal central nervous system (CNS) control mechanism for breathing?

It regulates the rate and depth of breathing.

What is required for sufficient gas exchange in the blood?

A sufficient amount of blood hemoglobin (Hb).

Which layers must gases traverse to move across the alveolar-capillary membrane?

Alveolar epithelial membrane, capillary endothelial membrane, interstitial space.

What is the primary barrier for gas diffusion in the lungs?

The alveolar epithelium, interstitial space, and capillary endothelium.

When is the rate of gaseous diffusion across a membrane decreased?

When the partial pressure gradient is low.

What primarily affects diffusion in the normal lung?

Gas pressure gradients.

What are the normal PO2 and PCO2 values in mixed venous blood?

PO2 = 40 mm Hg; PCO2 = 46 mm Hg.

Which gas diffuses fastest across the alveolar-capillary membrane?

Carbon dioxide.

How many times faster does carbon dioxide diffuse compared to O2?

Approximately 20 times.

What determines the time available for diffusion in the lung?

The rate of pulmonary blood flow.

What is the minimum time for blood transit in the pulmonary capillaries for O2 equilibration?

0.25 seconds.

What maintains the pressure gradient for O2 diffusion into cells?

Cellular consumption of O2.

What blood parameter is measured to assess tissue oxygenation?

Systemic mixed venous blood.

What is the normal range of PAO2 - PaO2 for healthy young adults?

5 to 10 mm Hg.

What factors account for the normal PAO2 - PaO2 difference?

Anatomical shunts and regional differences in ventilation and blood flow.

What happens if ventilation remains constant but perfusion decreases?

The PaCO2 should fall, and the PAO2 should rise.

What occurs if perfusion remains constant but ventilation decreases?

The PAO2 should fall.

What indicates that ventilation is less than normal?

A low ventilation/perfusion ratio.

What is true about an area of the lung with no blood flow but normal ventilation?

The alveolar gas is like air, and it represents alveolar dead space.

What happens at the extreme right of the graph regarding perfusion?

Perfusion is 0.

What do areas with ventilation but no blood flow represent?

Alveolar dead space.

What is the makeup of gases in areas of alveolar dead space?

Similar to inspired air (PO2 = 150 mm Hg; PCO2 = 0 mm Hg).

What occurs in a lung area with no ventilation but is normally perfused?

Blood exiting the pulmonary capillary will have a PO2 = 40 and a PCO2 = 46.

What does an area with no ventilation but normal perfusion represent?

An alveolar shunt.

What is the blood flow comparison between the apexes and bases of the lungs?

The bases receive approximately 20 times more blood flow than the apexes.

During normal inspiration, how do alveoli at the apexes compare to those at the bases?

Alveoli at the apexes expand less than those at the bases.

What is the respiratory exchange ratio in the bases of the lung when a person is standing upright?

The respiratory exchange ratio is lower than average.

Why is the PaO2 lower than the PAO2 in healthy subjects?

Most blood flows to the bases of the lung.

How is the vast majority of O2 carried in the blood?

Chemically combined with hemoglobin.

What is the relationship between the amount of O2 that dissolves in plasma and its partial pressure?

It is directly proportional.

At a PO2 of 40 mm Hg, how much O2 will physically dissolve in plasma?

0.12 ml/dl.

How many milliliters of O2 can combine with 1 g of hemoglobin?

1.34 ml.

What is the HbO2 saturation if the total hemoglobin content is 20 g/dl and the oxyhemoglobin content is 15 g/dl?

75%.

At a PaO2 of 65 mm Hg, what is the approximate saturation of hemoglobin with O2?

90%.

Why is it necessary to keep the patient's PaO2 greater than 60 mm Hg?

It marks the beginning of the steep part of the O2-Hb dissociation curve.

How do you compute the total O2 content of the blood?

Using the equation: CaO2 = (0.003 × PO2) + (Hbtot × 1.34 × SO2).

What is the total O2 content if Hb = 18, PO2 = 40 mm Hg, and SO2 = 73%?

17.7 ml/dl.

What is the total O2 content if Hb = 16, PO2 = 625 mm Hg, and SO2 = 100%?

23.3 ml/dl.

What is the equation to compute total O2 content?

CaO2 = (0.003 × PaO2) + (Hbtot × 1.34 × SaO2)

What is the approximate normal CaO2 - CO2 in a healthy adult at rest?

5 ml/dl

What is the cardiac output if a patient has a whole-body O2 consumption of 320 ml/min and a measured CaO2 - CO2 of 8 ml/dl?

4.0 L/min

According to the Fick principle, what happens if O2 consumption remains constant and cardiac output increases?

CaO2 - CO2 decreases

What is the Bohr effect?

It describes how changes in blood pH affect hemoglobin's affinity for O2.

What happens to Hb affinity for O2 when pH drops?

The affinity of Hb for O2 decreases.

What effect does a right shift in the HbO2 curve have?

The Hb saturation for a given PO2 falls.

What role does the Bohr effect play in O2 transport?

It enhances O2 delivery to tissues and O2 pickup at lungs.

What happens when the temperature of the blood rises?

The Hb saturation for a given PO2 falls.

What is the effect of an elevated intracellular 2,3-DPG concentration?

Increases the availability of O2 to the tissues.

In which condition will erythrocyte concentration of 2,3-DPG be decreased?

Banked blood

What results from the oxidation of the Hb molecule's iron ions to the ferric state?

Formation of methemoglobin (metHb) and inability to bind with O2.

How much greater is hemoglobin's affinity for carbon monoxide (CO) compared to O2?

200 times greater

What increases the affinity of Hb for O2?

Decreased 2,3-DPG, decreased PCO2, and increased pH.

What is true regarding fetal hemoglobin (HbF)?

It delivers more O2 to tissues at low PaO2 than normal Hb.

What does a P50 value of 29 mm Hg indicate?

Decreased affinity of Hb for O2.

In which forms is carbon dioxide transported by the blood?

Chemically combined with proteins, ionized as bicarbonate (HCO3-), and simple physical solution.

Approximately what percentage of total blood carbon dioxide is carried in the form of Prot-NH2 + CO2?

Approximately 20%.

What percentage of blood carbon dioxide is carried as bicarbonate?

Approximately 80%

What is the second most important form of carbon dioxide transport in blood?

Carbamino compounds

What is the primary function of carbonic anhydrase in red blood cells?

It drives the hydrolysis reaction that forms HCO3-.

What is the process called when HCO3- is expelled from the cell in exchange for Cl-?

Hamburger phenomenon

What is the relationship called when Hb saturation with O2 is high and less carbon dioxide is carried in the blood?

Haldane effect

What happens to blood carbon dioxide content when Hb is converted to deoxygenated Hb?

It enhances carbon dioxide loading on Hb.

What is true about the Haldane effect?

At high SaO2 levels, the capacity of blood to hold carbon dioxide decreases.

What equation best describes O2 delivery to the tissues?

Arterial O2 content times cardiac output.

What can maintain normal O2 delivery to tissues in the presence of hypoxemia?

Increasing the cardiac output.

What condition is defined as inadequate tissue O2 delivery to meet cellular needs?

Hypoxia.

What are potential causes of hypoxia?

Decrease in arterial PO2, decrease in available Hb, decrease in cardiac output.

What best describes an abnormal metabolic state where tissues cannot utilize O2?

Dysoxia.

What is the most likely cause of hypoxemia in a patient with a PaO2 of 62 mm Hg and PCO2 of 75 mm Hg?

Hypoventilation.

What is the most common cause of hypoxemia in patients with lung disease?

Ventilation-perfusion (V/Q) mismatch.

What happens when a patient breathing 40% O2 has a PaO2 of 50 mm Hg and then raises FiO2 to 0.7?

Hypoxemia is primarily due to a right-to-left shunt.

What is venous admixture?

Venous blood bypasses ventilated alveoli and mixes with freshly oxygenated arterial blood.

What is the expected PaO2 for an 80-year-old man in good health breathing room air?

75 mm Hg

What formula can estimate the expected PaO2 in older adults?

Expected PaO2 = 100.1 - (0.323 age in years)

What is the most likely cause of tissue hypoxia in a patient with normal PaO2 and cardiac output?

Hemoglobin deficiency

What is the most important component in the O2 transport system?

Hemoglobin (Hb)

What happens when O2 uptake by tissues is abnormally low?

Increased CvO2

What indicates O2 debt?

O2 demand exceeds O2 delivery.

Under which conditions may carbon dioxide removal be impaired?

When there is a mismatch, increased dead space ventilation, or inadequate minute ventilation.

What can you assume about a patient with a mismatch and hypercapnia?

The patient cannot sustain the high E to overcome the high VD.

What does a mismatch impact the most?

Oxygenation

What is the result of mixing blood from high and low PaO2 areas?

PaO2 lower than the average of the two.