Human Respiratory System Functions

The Human Respiratory System

By Marco Rios

Ventilation: Air Movement in the Lungs

Pulmonary Ventilation

The total volume of air breathed in and out per minute.

Alveolar Ventilation

The volume of air exchanged between the atmosphere and the alveoli per minute.

The Lungs: Core Organs of Respiration

A pair of organs consisting of the lower part of the respiratory airways, pulmonary circulation, and connective tissue.

Pleural Sacs and Cavity

Thin, fluid-filled membranes that enclose the lungs. The space between the pair is known as the pleural cavity.

Alveolar Cell Types

Type I Cells

Form the walls of the alveoli, facilitating gas exchange.

Type II Cells

Secrete pulmonary surfactant.

Pulmonary Surfactant

A substance that reduces the surface tension of water inside the alveoli, making it easier for the lungs to expand and preventing alveolar collapse.

Alveoli: Sites of Gas Exchange

Tiny air sacs that facilitate gas exchange, the transfer of oxygen from inhaled air into the bloodstream and carbon dioxide from the blood into the alveoli to be exhaled.

Elastic Recoil of the Lungs

The ease with which the lungs rebound after being stretched, crucial for passive expiration.

Pneumothorax: Lung Collapse

An abnormal condition of air in the pleural cavity, which leads to the collapse of a lung.

The Respiratory Pathway

Air travels through a specific sequence of structures to reach the alveoli:

Nasal Passages → Pharynx → Larynx → Trachea → Bronchus → Bronchiole → Alveolus

Pressure Gradients in Respiration

Pressure gradients are essential for preventing lung collapse and for air movement. Air always moves from a region of higher pressure to a region of lower pressure.

• During Exhalation: Pressure is greater in the lungs than in the atmosphere, forcing air out.
• During Inhalation: Atmospheric pressure is greater than the pressure in the lungs, drawing air in.

Boyle's Law and Lung Mechanics

This law states that the pressure of a gas varies inversely with its volume. For example, if volume decreases to 0.5 units, pressure increases to 2 units; conversely, if volume increases to 2 units, pressure decreases to 0.5 units. This principle is fundamental to how lungs inflate and deflate.

The Bohr Effect

In the presence of excess carbon dioxide or lower pH (more acidic conditions), hemoglobin's affinity for oxygen decreases, promoting oxygen release to tissues.

The Haldane Effect

This effect promotes the unloading of carbon dioxide in the lungs due to oxygen binding to hemoglobin. It is the opposite of the Bohr effect, facilitating CO2 removal where oxygen levels are high.

Muscles Involved in Respiration

Muscles of Inspiration

• Diaphragm: Contracts and flattens, increasing the vertical dimensions of the thoracic cavity.
• External Intercostal Muscles: Contract, elevating the rib cage and increasing the anterior-posterior and lateral dimensions of the thoracic cavity.

Muscles of Expiration

• Passive Expiration: Occurs when inspiratory muscles relax, allowing the ribs, sternum, and diaphragm to return to their resting positions due to elastic recoil.
• Active Expiration: Involves the contraction of abdominal muscles, which push the diaphragm upward, further reducing thoracic cavity dimensions and forcing more air out.

Key Respiratory Pressures

Atmospheric Pressure

Approximately 760 mmHg; the pressure exerted by the weight of air in the atmosphere on objects on Earth's surface.

Intra-Alveolar Pressure

The pressure within the alveoli; it changes dynamically during inspiration and exhalation to facilitate airflow.

Intrapleural Pressure

The pressure within the pleural sac, typically about 4 mmHg less than intra-alveolar pressure, which helps keep the lungs inflated.

Hemoglobin: The Oxygen Carrier

A soluble cytoplasmic protein found in red blood cells that reversibly binds up to four oxygen molecules. Hemoglobin transports approximately 98.5% of oxygen in the blood, indicating an almost direct association between hemoglobin and oxygen levels. If hemoglobin levels fall, the oxygen-carrying capacity of the blood will significantly decrease.

Carbon Monoxide Poisoning

Hemoglobin's affinity for carbon monoxide (CO) is approximately 240 times greater than its affinity for oxygen. This explains why even small levels of CO can be deadly, as CO readily displaces oxygen from hemoglobin.

Carbon Dioxide Transport in Blood

Carbon dioxide is transported in the blood through three primary mechanisms:

• ~10% dissolved directly in the blood plasma.
• ~30% bound to hemoglobin (as carbaminohemoglobin).
• ~60% transported as bicarbonate ions (HCO3-) in the plasma, a process facilitated by carbonic anhydrase in red blood cells.

Hemoglobin-Oxygen Dissociation Curve

The hemoglobin-oxygen dissociation curve illustrates the relationship between the amount of oxygen bound to hemoglobin and the partial pressure of oxygen (PO2) present in the blood. As the partial pressure of oxygen increases, hemoglobin's affinity for oxygen generally increases, leading to greater oxygen saturation.

Respiratory Volumes and Capacities

These measurements quantify the amount of air that can be inhaled, exhaled, and stored within the lungs.

Tidal Volume (TV)

The amount of air inhaled or exhaled during one normal, quiet breath (~500 mL).

Inspiratory Reserve Volume (IRV)

The amount of air that can be inhaled in excess of a normal tidal inspiration with maximum effort (~3000 mL).

Expiratory Reserve Volume (ERV)

The amount of air that can be exhaled in excess of a normal tidal expiration with maximum effort (~1200 mL).

Residual Volume (RV)

The amount of air remaining in the lungs after a maximum expiration (~1200 mL). This volume allows the alveoli to remain inflated and prevents lung collapse.

Vital Capacity (VC)

The maximum amount of air that can be exhaled with maximum effort after a maximum inspiration (~4700 mL). Calculated as TV + IRV + ERV.

Inspiratory Capacity (IC)

The maximum amount of air that can be inhaled after a normal tidal expiration (~3500 mL). Calculated as TV + IRV.

Functional Residual Capacity (FRC)

The amount of air remaining in the lungs after a normal tidal expiration (~2400 mL). Calculated as ERV + RV.

Total Lung Capacity (TLC)

The maximum amount of air the lungs can contain (~5900 mL). Calculated as VC + RV or TV + IRV + ERV + RV.