Bio 2160 Ch.12

Respiratory Physiology

Internal Respiration vs External Respiration

Respiratory physiology studies the processes by which oxygen (O2) and carbon dioxide (CO2) are exchanged and utilized in the body. Two main types of respiration are distinguished:

• Internal Respiration: Refers to cellular respiration, the use of O2 and production of CO2 within mitochondria of cells.

• External Respiration: Involves the exchange of O2 and CO2 between the atmosphere and body tissues.

Steps of External Respiration

External respiration consists of several sequential steps that ensure efficient gas exchange:

1. Pulmonary Ventilation: The process of breathing, moving air into and out of the lungs.

2. Gas Exchange (Lungs → Blood): O2 and CO2 diffuse between alveoli and pulmonary capillaries.

3. Transport of Gases: O2 and CO2 are carried in the blood to tissues.

4. Gas Exchange (Blood → Tissues): Diffusion of gases between systemic capillaries and tissue cells.

Respiratory Anatomy

The anatomy of the respiratory system is specialized for efficient air conduction and gas exchange:

• Upper Airways: Air passages in the head and neck (nasal cavity, pharynx, larynx).

• Pharynx: A tube for both air and food, connecting nasal and oral cavities to the larynx and esophagus.

• Respiratory Tract: All air passages from the pharynx to the lungs.

Zones of the Respiratory Tract

The respiratory tract is divided into two functional zones:

• Conducting Zone: Moves air from the larynx to the lungs, humidifies and warms air; consists of thick-walled passages (trachea, bronchi).

• Respiratory Zone: Site of actual gas exchange; consists of thin-walled structures (respiratory bronchioles, alveoli).

Anatomy and Function of Alveoli

Alveoli are the primary sites of gas exchange in the lungs:

• Structure: Alveoli have extremely thin walls and are surrounded by a dense network of capillaries.

• Function: The thinness facilitates rapid diffusion of gases; high density provides a large surface area for exchange.

• Example: Type I alveolar cells form the wall, while Type II cells secrete surfactant to reduce surface tension.

Mechanics of Breathing

Breathing consists of two phases:

• Inspiration: Inhalation; air enters the lungs.

• Expiration: Exhalation; air leaves the lungs.

Mechanics of Pulmonary Ventilation

Breathing muscles (diaphragm, intercostals) contract and relax, changing lung and alveolar volume. These changes create air pressure gradients that drive air movement.

• Key Principle: Air flows from regions of higher pressure to lower pressure.

Boyle's Law

Boyle's Law describes the relationship between pressure and volume in gases:

• Equation: $P = \frac{nRT}{V}$

• When volume increases, pressure decreases (and vice versa), assuming temperature and amount of gas are constant.

Air Flow in the Lungs

The driving force for air movement is the difference between atmospheric pressure and alveolar pressure:

• Equation: $\text{Air flow} = \frac{\text{Atmospheric pressure} - \text{Alveolar pressure}}{\text{Resistance}}$

• During inspiration, alveolar pressure drops below atmospheric pressure; during expiration, it rises above atmospheric pressure.

Gas Exchange in the Lungs

Gas exchange is governed by partial pressures:

• Total Pressure: The sum of partial pressures of all gases in a mixture.

• Partial Pressure: $\text{Partial pressure of a gas} = \text{Fractional concentration of that gas} \times \text{Total pressure}$

• Gases diffuse down their partial pressure gradients (from high to low).

Direction of Gas Movement

• O2 moves from alveoli (high PO2) to blood (low PO2).

• CO2 moves from blood (high PCO2) to alveoli (low PCO2).

Gas Exchange in Systemic Tissues

In systemic tissues, the process is reversed:

• O2 diffuses from blood (high PO2) into tissues (low PO2).

• CO2 diffuses from tissues (high PCO2) into blood (low PCO2).

Ventilation Patterns

Hyperpnea

• Physiological increase in alveolar ventilation to match increased tissue demand (e.g., during exercise).

• Achieved by increasing breath volume, frequency, or both.

Hypoventilation

• Ventilation is insufficient to meet tissue demands.

• Results in decreased arterial PO2 and increased PCO2.

Hyperventilation

• Ventilation exceeds tissue demands.

• Results in increased arterial PO2 and decreased PCO2.

Summary Table: Key Concepts in Respiratory Physiology

Concept	Definition	Key Equation
Boyle's Law	Pressure inversely related to volume	$P = \frac{nRT}{V}$
Partial Pressure	Pressure exerted by individual gas	$P_{\text{gas}} = \text{Fractional concentration} \times \text{Total pressure}$
Air Flow	Movement of air in/out of lungs	$\text{Air flow} = \frac{P_{\text{atm}} - P_{\text{alv}}}{\text{Resistance}}$
Hyperpnea	Increased ventilation to match demand	—
Hypoventilation	Insufficient ventilation	—
Hyperventilation	Excessive ventilation	—

Additional info: These notes provide foundational knowledge for understanding the physiological basis of breathing and gas exchange, which is essential for courses in biological psychology and health psychology.