Chapter 18: Gas Exchange and Transport – Anatomy & Physiology Study Notes

Gas Exchange and Transport

Overview of Gas Exchange

Gas exchange refers to the movement of oxygen (O2) and carbon dioxide (CO2) between the atmosphere, lungs, blood, and tissues. Efficient gas exchange is essential for cellular respiration and maintaining homeostasis.

• Hypoxia: A state of low tissue oxygen.

• Hypercapnia: Elevated concentrations of carbon dioxide.

• Normal arterial blood composition: Oxygen, carbon dioxide, and pH are tightly regulated.

	Arterial	Venous
O2	95 mm Hg	40 mm Hg
CO2	40 mm Hg	46 mm Hg
pH	7.4	7.37

18.1 Gas Exchange in the Lungs and Tissues

Partial Pressures and Diffusion

Gases in the body are measured by their partial pressures. Gas molecules move from regions of higher partial pressure to regions of lower partial pressure, following concentration gradients.

• Partial pressure: The pressure exerted by a single gas in a mixture.

• Diffusion: Movement of gases down their concentration gradients.

	Dry Air	Alveoli	Arterial	Cells	Venous
O2	160 mm Hg	100 mm Hg	95 mm Hg	<40 mm Hg	40 mm Hg
CO2	0.25 mm Hg	40 mm Hg	40 mm Hg	>46 mm Hg	46 mm Hg

Variables Influencing Gas Exchange Efficiency

• Low Alveolar PO2: Caused by low inspired oxygen (e.g., high altitude) or inadequate alveolar ventilation (decreased lung compliance, increased airway resistance, CNS depression).

• Diffusion: Efficiency depends on concentration gradient, surface area, thickness of exchange barrier, and diffusion distance.

• Gas Solubility: Movement of gas molecules into liquid is proportional to pressure gradient, solubility, and temperature. CO2 is much more soluble in water than O2.

18.2 Gas Transport in the Blood

Oxygen Transport

Oxygen is transported in the blood either dissolved in plasma or bound to hemoglobin in red blood cells.

• Total blood O2 content: 2% dissolved in plasma, 98% bound to hemoglobin (Hb).

• Oxyhemoglobin (HbO2): Hemoglobin bound to oxygen.

Oxygen Transport: Mass Flow and Mass Balance

Mass flow refers to the amount of a substance moving per unit time. Mass balance ensures that any gain in the body is offset by an equal loss.

• Fick Equation: Used to estimate cardiac output or oxygen consumption.

$\text{O}_2\,\text{consumption} = \text{Cardiac Output} \times (\text{Arterial O}_2 - \text{Venous O}_2)$

Oxygen Binding to Hemoglobin

Oxygen binding to hemoglobin follows the Law of Mass Action: the ratio of substrates to products at equilibrium is constant. The amount of oxygen transferred depends on the amount of hemoglobin available.

• PO2: The partial pressure of oxygen in plasma determines how much oxygen binds to hemoglobin.

• Percent saturation: The percentage of hemoglobin binding sites occupied by oxygen.

Oxygen-Hemoglobin Saturation Curve

The oxygen-hemoglobin saturation curve shows the relationship between PO2 and hemoglobin saturation. The curve's shape reflects hemoglobin's affinity for oxygen and its ability to act as an oxygen reservoir.

• Bohr Effect: A decrease in pH or increase in PCO2 shifts the curve, decreasing hemoglobin's affinity for oxygen.

Carbon Dioxide Transport

Three Routes of CO2 Removal

CO2 is a by-product of cellular respiration and is transported in the blood in three ways:

• 7% dissolved in plasma

• 23% bound to hemoglobin

• 70% converted to bicarbonate ion (HCO3-)

CO2 and Bicarbonate Ions

Bicarbonate formation serves two purposes: transporting CO2 and buffering blood pH. The conversion is catalyzed by carbonic anhydrase in red blood cells.

$\text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3 \leftrightarrow \text{H}^+ + \text{HCO}_3^-$

• Chloride shift: Process in which red blood cells exchange HCO3- for Cl- to maintain electrical neutrality.

• Buffering: Removal of free H+ from the cytoplasm prevents large changes in pH.

CO2 Removal at the Lungs

At the alveoli, CO2 diffuses out of the plasma into the alveolar air due to lower alveolar PCO2. HCO3- and H+ recombine to form CO2, which is then exhaled.

18.3 Regulation of Ventilation

Neural Control of Breathing

Breathing is controlled by respiratory neurons in the medulla, which regulate inspiratory and expiratory muscles. Breathing can be voluntary to a point, but is primarily automatic.

• Chemoreceptors: Detect changes in CO2, O2, and H+ to regulate ventilation.

• Peripheral chemoreceptors: Located in the carotid and aortic bodies, respond to low O2, low pH, or high CO2.

• Central chemoreceptors: Located in the medulla oblongata, monitor plasma PCO2.

CO2, Oxygen, and pH Influence Ventilation

CO2 and pH have a greater influence on ventilation than oxygen. Sensory input from chemoreceptors modifies the rhythm of breathing to maintain blood gas homeostasis.

Protective Reflexes

Protective reflexes help guard the lungs from damage or irritation.

• Bronchoconstriction: Narrowing of airways to prevent harmful substances from entering.

• Irritant receptors: Detect harmful particles and trigger protective responses.

• Hering-Breuer inflation reflex: Prevents over-inflation of the lungs.

Higher Brain Centers and Breathing Patterns

Higher brain centers, such as the limbic system, can alter respiration, but are not required for normal breathing. Breathing is closely linked to cardiovascular and emotional states.

Review Questions

• What is the normal range of oxygen, carbon dioxide, and pH in arterial and venous blood?

• What variables affect alveolar gas exchange?

• How does oxygen diffuse into the blood? How is oxygen transported in the blood?

• What is the Bohr effect?

• How is carbon dioxide transported in the blood?

• How do chemoreceptors regulate ventilation?

Additional info: The notes include expanded explanations of physiological mechanisms, relevant equations, and tables for blood gas values, as well as review questions for exam preparation.