Breathing and Exchange of Gases

Learning Outcomes:

1. Understanding the respiratory organs and their mechanisms in different species.
2. Detailed exploration of the human respiratory system and its anatomy.
3. Comprehension of breathing mechanisms, including inspiration and expiration.
4. Gaining insight into respiratory volumes and capacities.
5. Understanding gas exchange processes at different body sites.
6. Examining the regulation of respiration and associated disorders.

Breathing is essential for the survival of all organisms as it allows the exchange of oxygen (O₂) and carbon dioxide (CO₂), which are critical for cellular respiration. Oxygen is necessary for breaking down glucose and other molecules, while CO₂, a harmful byproduct, must be eliminated. The respiratory process involves the intake of oxygen and the release of carbon dioxide through diffusion and circulation, commonly known as respiration.

Respiratory Organs

Various organisms possess different respiratory organs, which have evolved depending on their habitats and levels of biological organization.

1. Lower invertebrates like sponges, coelenterates, and flatworms exchange gases via simple diffusion across their entire body surfaces.
2. Earthworms utilize their moist cuticles for gas exchange, while insects use a system of tracheal tubes.
3. Aquatic arthropods and molluscs rely on specialized structures called gills for breathing underwater, while terrestrial forms such as mammals, birds, and reptiles use lungs.
4. Amphibians like frogs can breathe through both their lungs and moist skin (cutaneous respiration).

Human Respiratory System

The human respiratory system consists of the following structures:

1. External nostrils above the upper lips lead to the nasal chamber, which then connects to the pharynx.
2. The pharynx opens into the larynx, also known as the soundbox. During swallowing, the epiglottis, a cartilaginous flap, prevents food from entering the larynx.
3. The trachea extends to the mid-thoracic cavity, dividing into the primary bronchi, which further branch into secondary and tertiary bronchi and then bronchioles, ultimately ending in alveoli, where gas exchange occurs.
4. Alveoli are thin-walled, vascularized, and essential for the diffusion of O₂ and CO₂.
5. The lungs are protected by pleural membranes, with pleural fluid between them to reduce friction during breathing.

Important Note: The respiratory system is divided into a conducting part, which filters and humidifies air, and a respiratory part, where gas exchange occurs.

Steps of Respiration

Respiration involves several steps:

1. Breathing (pulmonary ventilation): The intake of atmospheric air and expulsion of CO₂-rich air.
2. Gas diffusion: O₂ and CO₂ are exchanged across the alveolar membrane.
3. Gas transport: O₂ and CO₂ are transported through the blood.
4. Diffusion at tissues: O₂ is delivered to tissues, and CO₂ is taken up by the blood.
5. Cellular respiration: Cells utilize O₂ for metabolic reactions, producing CO₂.

Mechanism of Breathing

Breathing occurs in two stages—inspiration and expiration—facilitated by the creation of a pressure gradient between the lungs and the atmosphere.

1. Inspiration: Air enters the lungs when the intra-pulmonary pressure is lower than the atmospheric pressure. This is achieved by the contraction of the diaphragm and external intercostal muscles, which increase the thoracic volume.
2. Expiration: Air is expelled when the intra-pulmonary pressure exceeds the atmospheric pressure. This occurs when the diaphragm and intercostal muscles relax, reducing the thoracic volume.
3. Humans breathe approximately 12-16 times per minute, and the volume of air moved can be measured using a spirometer.

Respiratory Volumes and Capacities

Understanding respiratory volumes is crucial for assessing lung function:

1. Tidal Volume (TV): The amount of air inhaled or exhaled during a normal breath (~500 mL).
2. Inspiratory Reserve Volume (IRV): The extra volume of air that can be inhaled beyond normal inspiration (2500-3000 mL).
3. Expiratory Reserve Volume (ERV): The additional air that can be forcibly exhaled after a normal breath (1000-1100 mL).
4. Residual Volume (RV): The air that remains in the lungs even after forced expiration (1100-1200 mL).

From these volumes, various pulmonary capacities can be derived:

Important Concept: Lung capacities are critical in diagnosing respiratory disorders. Vital capacity measures the maximum air a person can expel after a full inspiration and is vital for assessing respiratory health.

Exchange of Gases

The primary sites for gas exchange are the alveoli in the lungs and the tissues.

1. Diffusion: O₂ and CO₂ move across membranes based on their partial pressure gradients. The partial pressure of oxygen (pO₂) in the atmosphere is higher than in the alveoli, which promotes the diffusion of O₂ into the blood.
2. Solubility and membrane thickness: CO₂ is 20-25 times more soluble than O₂, which allows for faster diffusion despite lower partial pressure gradients.
3. Gas exchange follows the principles of pressure gradient and solubility, with O₂ diffusing into the blood at the alveoli and CO₂ diffusing out of the blood.

Transport of Gases

Transport of Oxygen

Oxygen is primarily transported by hemoglobin (Hb) in red blood cells. 97% of O₂ is carried as oxyhemoglobin, and the remaining 3% dissolves in plasma. Hemoglobin’s affinity for oxygen is influenced by several factors, including:

1. Partial pressure of O₂ (pO₂): Higher pO₂ in the alveoli promotes oxygen binding to hemoglobin.
2. Partial pressure of CO₂ (pCO₂), H⁺ concentration, and temperature: Higher levels of these factors in tissues promote oxygen release from hemoglobin.

The relationship between O₂ binding and its partial pressure is represented by the oxygen dissociation curve, a sigmoidal shape indicating that hemoglobin binds O₂ efficiently at higher pO₂ (in the lungs) and releases it at lower pO₂ (in the tissues).

Transport of Carbon Dioxide

CO₂ is transported through three main mechanisms:

1. Dissolved CO₂: About 7% of CO₂ is transported in the dissolved state in plasma.
2. Bicarbonate ions (HCO₃⁻): The majority, around 70%, is carried in the form of bicarbonate. The enzyme carbonic anhydrase facilitates the conversion of CO₂ and H₂O into bicarbonate ions and H⁺.
3. Carbaminohemoglobin: About 20-25% of CO₂ binds to hemoglobin as carbaminohemoglobin. In tissues where pCO₂ is high, CO₂ binds to hemoglobin, and at the alveoli where pCO₂ is low, CO₂ is released for exhalation.

Regulation of Respiration

The medulla oblongata houses the respiratory rhythm center, responsible for regulating the breathing rate and rhythm.

1. The pneumotaxic center in the pons modulates the respiratory rate by reducing the duration of inspiration.
2. A chemosensitive area near the rhythm center detects changes in pCO₂ and H⁺ concentration. An increase in these levels triggers adjustments in breathing to expel excess CO₂.
3. Peripheral chemoreceptors in the aortic arch and carotid arteries also detect changes in gas concentrations and relay signals to the brain for adjustments in respiration.

Important Note: The role of oxygen in regulating breathing

is minor compared to CO₂ and H⁺ concentrations.

Disorders of the Respiratory System

Several respiratory disorders can impair breathing:

1. Asthma: Characterized by difficulty in breathing due to inflammation of the bronchi and bronchioles, often accompanied by wheezing.
2. Emphysema: A chronic condition where alveolar walls are damaged, reducing the surface area available for gas exchange. Smoking is a significant cause.
3. Occupational respiratory disorders: Exposure to dust and other particles in industries such as stone-breaking can lead to lung damage and fibrosis. Workers should use protective gear to prevent inhalation of harmful substances.

MCQ: What is the major factor that affects the binding of oxygen to hemoglobin in the lungs?
Answer: High pO₂