Fundamentals of the Respiratory System and Gas Exchange Processes

Functional Overview of the Respiratory System

The primary function of the respiratory system is to exchange gases with the cardiovascular system. This coordination enables the blood to transport oxygen from the lungs to the cells and carry carbon dioxide as a waste product from the cells back to the lungs.

Types of Respiration

Respiration occurs at two distinct levels within the body:

External Respiration: This is the exchange of gases between the atmosphere and the blood.
Internal Respiration: This is the exchange of gases between the blood and the individual cells of the body.

Cellular Purpose of Oxygen

Once oxygen reaches the cells through internal respiration, it is utilized to break down glucose and produce adenosine triphosphate ( $ATP$ ) via the process of aerobic respiration. This process is critical for survival, as the body cannot obtain sufficient energy from food without it. A byproduct of this energy production is carbon dioxide ( $CO_2$ ), which is toxic to cells if allowed to accumulate in excess. It is removed from the cells and transported away through internal respiration.

Anatomical Structure and the Path of Air

The Lungs and Thoracic Cavity

The lungs serve as the primary site for gas exchange between the atmosphere and the blood. They are located within the thoracic cavity, which is bounded by the rib cage and the diaphragm.

Lobe Structure: The right lung is divided into three sections or lobes and is slightly heavier than the left lung, which consists of only two lobes.
Pleura: These are membranes that line the entire thoracic cavity and cover the lungs. They secrete a slippery fluid designed to decrease friction during the physical movements of the lungs during breathing.

The Sequential Path of Inhaled Air

To reach the capillaries in the lungs, air follows a specific biological pathway:

Mouth and Nose: External respiration begins here. Tiny hairs in the nose filter incoming air.
Nasal Cavity: Located above the roof of the mouth, this cavity is lined with mucous membranes that warm and moisten the air to prevent damage to delicate respiratory tissues. Cilia in the nasal cavity trap inhaled particles and sweep them toward the throat to be swallowed.
Pharynx (Throat): A tube at the back of the nasal cavities and mouth that serves as a shared passageway for both food and air.
Epiglottis: A flap of cartilage that acts as a valve. When swallowing food, the epiglottis presses down to cover the air passage. When inhaling, it remains upright to allow air into the trachea.
Trachea (Windpipe): A cartilaginous tube approximately $10$ to $12\,cm$ in length. Its walls are lined with ciliated cells that trap inhaled particles and sweep mucus away from the lungs toward the throat.
Larynx (Voicebox): Located at the upper end of the trachea. It contains two ligaments called vocal cords. Sounds are produced when air is forced past these cords; the pitch and volume are regulated by the tension on the cords and the volume of air passing through.
Bronchi: The trachea branches into two large tubes called bronchi (singular: bronchus), each leading to one lung. Their walls consist of smooth muscle and cartilage and are lined with cilia and mucus.
Bronchioles: Within the lungs, the bronchi branch into increasingly smaller tubes called bronchioles, which maintain the lining of cilia and mucus.
Alveoli: These are clusters of tiny air sacs at the end of the bronchioles. These serve as the functional units of the respiratory system.

Surface Area for Exchange

The alveoli are surrounded by a dense network of capillaries. A healthy lung contains nearly $300$ million alveoli, providing a total surface area of approximately $70\,m^2$ . This is roughly $40$ times the surface area of the human skin, a vast expanse that greatly increases the rate of gas diffusion.

Physiological Mechanisms of Gas Exchange

Gas exchange occurs due to concentration gradients, where substances diffuse from an area of higher concentration to an area of lower concentration.

Exchange in the Lungs

Air entering the alveoli is rich in oxygen and low in carbon dioxide. Conversely, blood in the surrounding capillaries is low in oxygen and high in carbon dioxide.

Oxygen Diffusion: Oxygen crosses the thin alveolar membranes and capillary walls to dissolve into the blood.
Carbon Dioxide Diffusion: Carbon dioxide moves from the blood into the alveoli to be exhaled.

Hemodynamic Transport of Respiratory Gases

Oxygen Transport

Only a small fraction of oxygen remains dissolved in the plasma. The majority ( $95$ to 98\,\% \t) enters red blood cells to bind with hemoglobin, an iron-containing protein. Each hemoglobin molecule contains four iron atoms, and each iron atom can bind one oxygen molecule ( $O_2$ ). The resulting reaction is:

$Hb + 4O_2 \to HbO_8$

When oxygenated blood reaches body cells where oxygen concentration is lower than in the blood, the reaction reverses to release the oxygen:

$HbO_8 \to Hb + 4O_2$

Carbon Dioxide Transport

Carbon dioxide diffuses from cells into the blood due to its higher concentration in the tissue. It is transported in three ways:

Plasma: Approximately 7\,\% \t is dissolved directly in the plasma.
Hemoglobin: Approximately 23\,\% \t binds to hemoglobin.
Bicarbonate Ions: The remaining 70\,\% \t is carried as bicarbonate ions ( $HCO_3^-$ ).

The Bicarbonate Reaction

In the blood near the cells, $CO_2$ reacts with water to form carbonic acid ( $H_2CO_3$ ), which then dissociates into bicarbonate and hydrogen ions:

$H_2O + CO_2 \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+$

When the blood returns to the lungs, the reaction is reversed to reform $CO_2$ and water, allowing the carbon dioxide to diffuse into the alveoli for exhalation:

$HCO_3^- + H^+ \rightleftharpoons H_2CO_3 \rightleftharpoons H_2O + CO_2$

Mechanics and Regulation of Breathing

Breathing is the physical process of moving air into (inspiration) and out of (expiration) the lungs.

Inspiration (Inhalation)

Inspiration is an active process involving muscular contraction:

Rib Muscles: Contract to move the ribs up and outward, expanding the chest.
Diaphragm: This large skeletal muscle flattens and pushes down on the abdomen.
Result: These actions increase the volume of the thoracic cavity, which reduces the internal air pressure. Because the internal pressure is lower than the atmospheric pressure, air is pulled into the lungs.

Expiration (Exhalation)

Expiration involves the relaxation of the muscles used in inspiration:

Muscular Action: The diaphragm and rib muscles relax.
Elastic Recoil: The elastic tissues of the lungs recoil, deflating the lungs.
Result: The volume of the lungs decreases, causing the internal air pressure to become greater than the outside atmospheric pressure. This pressure differential forces air out until the pressures are equalized.

Regulation of Breathing Rate

The body adjusts the rate and depth of breathing based on the level of cellular activity and oxygen requirement. This is controlled subconsciously by the brain and brain stem.

Monitoring: The brain stem monitors the concentration of carbon dioxide in the blood.
Stimulation: High levels of $CO_2$ stimulate nerve cells in the brain, prompting the brain stem to signal the diaphragm to increase the breathing rate and depth.
Return to Homeostasis: Once $CO_2$ levels drop, sensors signal the respiratory muscles to return to a slower rate.
Conscious Override: Humans can temporarily control breathing voluntarily (e.g., holding breath while swimming). However, if a person loses consciousness from holding their breath, the brain stem automatically resumes control to restore normal breathing.

Questions & Discussion

1. How does internal respiration differ from external respiration? External respiration is the exchange of gases between the atmosphere and the blood (at the lungs), while internal respiration is the exchange of gases between the blood and the body's cells.

2. Outline the path of oxygen from the atmosphere to the bloodstream. Oxygen travels through the nose/mouth $\to$ nasal cavity $\to$ pharynx $\to$ larynx $\to$ trachea $\to$ bronchi $\to$ bronchioles $\to$ alveoli, where it diffuses across the alveolar and capillary walls into the blood.

3. Explain the process of gas exchange in the lungs. Gas exchange is driven by diffusion. Oxygen-rich air in the alveoli moves into the oxygen-poor blood of the capillaries. Carbon dioxide-rich blood moves into the carbon dioxide-poor alveoli to be exhaled.

4. Differentiate between oxygen transport and carbon dioxide transport in the bloodstream. Oxygen is mostly transported bound to hemoglobin (95\text{-}98\,\% \t). Carbon dioxide is mostly transported as bicarbonate ions (70\,\% \t), with smaller amounts bound to hemoglobin (23\,\% \t) or dissolved in plasma (7\,\% \t).

5. Sequence the skeletal and muscular changes that take place when a person inhales. First, the rib muscles contract to lift the rib cage up and out. Simultaneously, the diaphragm contracts and flattens, moving downward. This expands the thoracic cavity and lowers internal pressure, drawing air in.

6. What factors regulate the rate of breathing? The primary regulator is the concentration of carbon dioxide in the blood, which is monitored by the brain stem.

7. Why does a single-celled organism not need a respiratory system? A single-celled organism has a high surface-area-to-volume ratio and can rely on direct diffusion across its cell membrane to exchange gases with its environment.

8. What are two conditions that could result in lower oxygen saturation than the normal 98 percent? Conditions that impair gas exchange, such as lung disease (damaged alveoli) or environments with very low atmospheric oxygen (high altitudes), could lead to lower saturation.