What Is Included In The Process Of External Respiration

External respiration, a vital physiological process, encompasses the series of events involved in the exchange of oxygen and carbon dioxide between an organism and its environment. This process is essential for cellular respiration, which provides the energy required for life. Unlike internal respiration (cellular respiration), which occurs within the cells of an organism, external respiration involves the intake of oxygen from the environment and the release of carbon dioxide back into it.

Components of External Respiration

External respiration can be broken down into several key components, each playing a crucial role in ensuring efficient gas exchange:

1. Ventilation (Breathing):

   • Ventilation is the mechanical process of moving air into and out of the lungs. It involves the coordinated action of respiratory muscles, such as the diaphragm and intercostal muscles, to create pressure gradients that drive airflow.

2. Pulmonary Gas Exchange:

   • This is the exchange of oxygen and carbon dioxide between the air in the alveoli (tiny air sacs in the lungs) and the blood in the pulmonary capillaries.

3. Gas Transport:

   • Gas transport is the movement of oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs via the bloodstream.

4. Tissue Gas Exchange:

   • This is the exchange of oxygen and carbon dioxide between the blood in the systemic capillaries and the tissue cells.

Let's delve into each of these components in detail to understand the intricacies of external respiration.

Ventilation (Breathing)

Ventilation, often referred to as breathing, is the process of moving air into and out of the lungs. It is a mechanical process that depends on pressure gradients created by the respiratory muscles. Ventilation can be divided into two phases: inspiration (inhalation) and expiration (exhalation).

Inspiration (Inhalation)

Inspiration is the process of drawing air into the lungs. It is an active process that requires the contraction of several muscles, primarily the diaphragm and the external intercostal muscles.

• Diaphragm: The diaphragm is a large, dome-shaped muscle located at the base of the thoracic cavity. When the diaphragm contracts, it flattens and moves downward, increasing the volume of the thoracic cavity.
• External Intercostal Muscles: The external intercostal muscles are located between the ribs. When these muscles contract, they lift the rib cage up and out, further increasing the volume of the thoracic cavity.

The increase in thoracic volume during inspiration leads to a decrease in pressure within the lungs, creating a pressure gradient between the atmosphere and the alveoli. Air flows from the area of higher pressure (the atmosphere) to the area of lower pressure (the alveoli), resulting in the inflation of the lungs.

Expiration (Exhalation)

Expiration is the process of expelling air from the lungs. Under normal conditions, expiration is a passive process that does not require muscle contraction. It relies on the elastic recoil of the lungs and the relaxation of the respiratory muscles.

• Elastic Recoil: The lungs contain elastic fibers that allow them to stretch during inspiration. During expiration, these elastic fibers recoil, causing the lungs to return to their original size.
• Relaxation of Respiratory Muscles: As the diaphragm and external intercostal muscles relax, the thoracic cavity decreases in volume. This decrease in volume leads to an increase in pressure within the lungs, creating a pressure gradient between the alveoli and the atmosphere. Air flows from the area of higher pressure (the alveoli) to the area of lower pressure (the atmosphere), resulting in the deflation of the lungs.

During forceful expiration, such as during exercise or coughing, the internal intercostal muscles and abdominal muscles may contract to further decrease the volume of the thoracic cavity and increase the pressure within the lungs.

Pulmonary Gas Exchange

Pulmonary gas exchange is the exchange of oxygen and carbon dioxide between the air in the alveoli and the blood in the pulmonary capillaries. This process occurs via diffusion, driven by the difference in partial pressures of oxygen and carbon dioxide between the alveolar air and the blood.

Alveoli: The Site of Gas Exchange

The alveoli are tiny air sacs in the lungs that provide a large surface area for gas exchange. The alveolar walls are very thin, consisting of a single layer of epithelial cells called type I pneumocytes. These cells are closely associated with pulmonary capillaries, allowing for efficient diffusion of gases.

Type II pneumocytes are also present in the alveolar walls. These cells secrete surfactant, a substance that reduces surface tension in the alveoli, preventing them from collapsing.

Partial Pressure Gradients

The partial pressure of a gas is the pressure exerted by that gas in a mixture of gases. Gases diffuse from areas of high partial pressure to areas of low partial pressure.

• Oxygen: The partial pressure of oxygen in the alveolar air is higher than that in the pulmonary capillary blood. This pressure gradient drives the diffusion of oxygen from the alveoli into the blood.

• Carbon Dioxide: The partial pressure of carbon dioxide in the pulmonary capillary blood is higher than that in the alveolar air. This pressure gradient drives the diffusion of carbon dioxide from the blood into the alveoli.

Factors Affecting Pulmonary Gas Exchange

Several factors can affect the efficiency of pulmonary gas exchange:

• Surface Area: The greater the surface area of the alveoli, the more efficient the gas exchange. Conditions that reduce the surface area, such as emphysema, can impair gas exchange.
• Thickness of the Respiratory Membrane: The thinner the respiratory membrane (the barrier between the alveolar air and the blood), the more efficient the gas exchange. Conditions that thicken the respiratory membrane, such as pulmonary edema, can impair gas exchange.
• Partial Pressure Gradients: The larger the partial pressure gradients, the more efficient the gas exchange. Conditions that reduce the partial pressure gradients, such as high altitude, can impair gas exchange.
• Ventilation-Perfusion Matching: Ventilation-perfusion matching refers to the coordination of airflow (ventilation) and blood flow (perfusion) in the lungs. Efficient gas exchange requires that areas of the lungs with high ventilation also have high perfusion. Conditions that disrupt ventilation-perfusion matching, such as pulmonary embolism, can impair gas exchange.

Gas Transport

Gas transport is the movement of oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs via the bloodstream. Oxygen and carbon dioxide are transported in the blood in different ways.

Oxygen Transport

Oxygen is transported in the blood in two forms:

• Dissolved in Plasma: A small amount of oxygen (about 1.5%) is dissolved directly in the plasma, the liquid component of blood. However, the solubility of oxygen in plasma is low, so this is not a very efficient way to transport oxygen.
• Bound to Hemoglobin: The majority of oxygen (about 98.5%) is transported bound to hemoglobin, a protein found in red blood cells. Hemoglobin is composed of four subunits, each containing a heme group that can bind one molecule of oxygen.

The binding of oxygen to hemoglobin is influenced by several factors, including:

• Partial Pressure of Oxygen: The higher the partial pressure of oxygen, the more oxygen binds to hemoglobin.
• pH: The lower the pH (more acidic), the less oxygen binds to hemoglobin (Bohr effect).
• Temperature: The higher the temperature, the less oxygen binds to hemoglobin.
• 2,3-Bisphosphoglycerate (2,3-BPG): 2,3-BPG is a molecule produced by red blood cells that decreases the affinity of hemoglobin for oxygen. Higher levels of 2,3-BPG result in less oxygen binding to hemoglobin.

Carbon Dioxide Transport

Carbon dioxide is transported in the blood in three forms:

• Dissolved in Plasma: A small amount of carbon dioxide (about 7%) is dissolved directly in the plasma. Carbon dioxide is more soluble in plasma than oxygen, so this is a more efficient way to transport carbon dioxide than oxygen.
• Bound to Hemoglobin: About 23% of carbon dioxide is transported bound to hemoglobin. Carbon dioxide binds to a different site on hemoglobin than oxygen, so the binding of carbon dioxide does not interfere with the binding of oxygen.
• As Bicarbonate Ions: The majority of carbon dioxide (about 70%) is transported as bicarbonate ions (HCO3-). Carbon dioxide reacts with water in the red blood cells to form carbonic acid (H2CO3), which then dissociates into hydrogen ions (H+) and bicarbonate ions. The bicarbonate ions are then transported out of the red blood cells into the plasma.

The conversion of carbon dioxide to bicarbonate ions is catalyzed by the enzyme carbonic anhydrase, which is found in high concentrations in red blood cells.

Tissue Gas Exchange

Tissue gas exchange is the exchange of oxygen and carbon dioxide between the blood in the systemic capillaries and the tissue cells. This process occurs via diffusion, driven by the difference in partial pressures of oxygen and carbon dioxide between the blood and the tissue cells.

Partial Pressure Gradients

• Oxygen: The partial pressure of oxygen in the blood is higher than that in the tissue cells. This pressure gradient drives the diffusion of oxygen from the blood into the tissue cells.

• Carbon Dioxide: The partial pressure of carbon dioxide in the tissue cells is higher than that in the blood. This pressure gradient drives the diffusion of carbon dioxide from the tissue cells into the blood.

Factors Affecting Tissue Gas Exchange

Several factors can affect the efficiency of tissue gas exchange:

• Blood Flow: Adequate blood flow to the tissues is essential for efficient gas exchange. Conditions that reduce blood flow, such as vasoconstriction or blockage of blood vessels, can impair gas exchange.
• Capillary Density: The higher the capillary density in a tissue, the more efficient the gas exchange. Tissues with high metabolic rates, such as muscle tissue, have high capillary densities.
• Metabolic Rate: The higher the metabolic rate of a tissue, the greater the demand for oxygen and the greater the production of carbon dioxide. This leads to larger partial pressure gradients and more efficient gas exchange.

Regulation of External Respiration

External respiration is regulated by several mechanisms to ensure that the body receives an adequate supply of oxygen and eliminates carbon dioxide efficiently. These mechanisms involve both neural and chemical controls.

Neural Control

The neural control of respiration is primarily located in the brainstem, specifically in the medulla oblongata and the pons. These areas contain respiratory centers that control the rate and depth of breathing.

• Medullary Respiratory Center: The medullary respiratory center contains two main groups of neurons: the dorsal respiratory group (DRG) and the ventral respiratory group (VRG).

  • The DRG is primarily involved in inspiration. It receives sensory input from various sources, including chemoreceptors and stretch receptors, and sends signals to the diaphragm and external intercostal muscles to initiate inspiration.
  • The VRG is involved in both inspiration and expiration. It contains neurons that control the internal intercostal muscles and abdominal muscles, which are used during forceful expiration.

• Pontine Respiratory Center: The pontine respiratory center, located in the pons, influences the activity of the medullary respiratory center. It helps to smooth out the transitions between inspiration and expiration and to regulate the rate and depth of breathing during activities such as speech and exercise.

Chemical Control

The chemical control of respiration is primarily mediated by chemoreceptors that detect changes in the levels of oxygen, carbon dioxide, and pH in the blood. These chemoreceptors are located in the brainstem and in the carotid and aortic bodies.

• Central Chemoreceptors: Central chemoreceptors are located in the medulla oblongata and respond to changes in the pH of the cerebrospinal fluid (CSF). Changes in the partial pressure of carbon dioxide in the blood can affect the pH of the CSF. When the partial pressure of carbon dioxide increases, it diffuses into the CSF and is converted to carbonic acid, which lowers the pH. The central chemoreceptors detect this decrease in pH and stimulate the respiratory centers to increase the rate and depth of breathing, which helps to eliminate carbon dioxide from the body.
• Peripheral Chemoreceptors: Peripheral chemoreceptors are located in the carotid and aortic bodies and respond to changes in the partial pressure of oxygen, carbon dioxide, and pH in the blood. When the partial pressure of oxygen decreases, the partial pressure of carbon dioxide increases, or the pH decreases, the peripheral chemoreceptors stimulate the respiratory centers to increase the rate and depth of breathing.

Clinical Significance

Understanding the process of external respiration is crucial in clinical settings for diagnosing and treating respiratory disorders. Conditions such as asthma, chronic obstructive pulmonary disease (COPD), pneumonia, and pulmonary embolism can significantly impair external respiration and lead to hypoxia (low oxygen levels) and hypercapnia (high carbon dioxide levels).

• Asthma: Asthma is a chronic inflammatory disease of the airways that causes bronchoconstriction (narrowing of the airways), mucus production, and difficulty breathing.
• COPD: COPD is a group of lung diseases, including emphysema and chronic bronchitis, that cause airflow obstruction and difficulty breathing.
• Pneumonia: Pneumonia is an infection of the lungs that causes inflammation of the alveoli and fluid accumulation, impairing gas exchange.
• Pulmonary Embolism: Pulmonary embolism is a blockage of one or more pulmonary arteries by a blood clot, which can impair blood flow to the lungs and disrupt ventilation-perfusion matching.

Conclusion

External respiration is a complex process involving ventilation, pulmonary gas exchange, gas transport, and tissue gas exchange. Each component plays a crucial role in ensuring that the body receives an adequate supply of oxygen and eliminates carbon dioxide efficiently. The process is regulated by neural and chemical mechanisms to maintain homeostasis and meet the metabolic demands of the body. Understanding the intricacies of external respiration is essential for comprehending respiratory physiology and for diagnosing and treating respiratory disorders.