Chapter 17: Breathing and Exchange of Gases Revision Notes
- Breathing is the process of taking in oxygen and giving out carbon dioxide.
- Respiration is a catabolic process of the breakdown of energy-rich molecules to produce the energy needed for the survival of the organism.
- Different organisms have different types of respiratory organs depending upon the habitat and level of their organization.
- The earthworm has moist skin that participates in respiration. This type of respiration is known as Cutaneous Respiration.
- Insects have tracheal tubes that are respiratory in function.
- Simple organisms can exchange gasses directly with the environment by general body surface.
- Aquatic animals have gills as respiratory organs.
- Higher animals, including humans, have lungs for respiration.
Human Respiratory System
- Humans have a pair of nostrils that lead into the nasal passage.
- The nasal chamber then leads to the pharynx, which is a common passage for food and air.
- The pharynx opens through the larynx region into the trachea.
- The larynx is responsible for sound production and is commonly called the sound box.
- The trachea is a straight tube that divides into the right and left bronchi.
- Primary bronchi are further divided into secondary and tertiary bronchi and bronchioles.
- Each terminal bronchiole gives rise to a thin vascularized bag-like structure known as Alveoli.
- Humans have pair of lungs that are covered by a membrane known as the Pleural Membrane.
- The lungs are situated in the thoracic chamber which is anatomically an air-tight chamber.
Respiration involves the following steps:
• Pulmonary ventilation involves taking in atmospheric oxygen and carbon dioxide.
• Diffusion of oxygen and carbon dioxide across the alveolar membrane.
• Transport of gasses by the blood.
• Diffusion of O2 and CO2 occurring in blood and tissues.
• Utilization of oxygen by the cells for catabolic reactions and release of carbon dioxide.
There are two processes of Breathing – Inspiration and Expiration.
• Process of taking atmospheric air in is known as inspiration.
• It is an active process.
• Pressure inside the lungs is less than atmospheric pressure.
• External intercostal muscles contract, which raises the ribs to increase the volume of the thoracic cavity.
• Process of giving out carbon dioxide is known as expiration.
• It is a passive process.
• Pressure inside the lungs is more than the atmospheric pressure.
• External intercostal muscles relax, which lowers the ribs to decrease the volume of the thoracic cavity.
Source: Inspiration and Expiration
Respiratory Volumes and Capacities
- Tidal Volume (TV): Volume of air inspired or expired during a normal respiration.
- Inspiratory Reserve Volume (IRV): Additional volume of air, a person can inspire by a forcible inspiration.
- Expiratory Reserve Volume (ERV): Additional volume of air, a person can expire by a forcible expiration.
- Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration.
- Inspiratory Capacity (IC): Total volume of air a person can inspire after a normal expiration.
- Expiratory Capacity (EC): Total volume of air a person can expire after a normal inspiration.
- Functional Residual Capacity (FRC): Volume of air that will remain in the lungs after a normal expiration. This includes ERV+RV.
- Vital Capacity (VC): The maximum volume of air a person can breathe in after a forced expiration.
- Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of a forced inspiration.
Exchange of gasses
- The exchange of gasses occurs in alveoli.
- Two important parameters that affect the rate of diffusion are –solubility of gases and the thickness of the membrane.
- Pressure contributed by each gas from a mixture of gas is known as partial pressure.
- The partial pressure of oxygen and carbon dioxide is represented by pO2 and pCO2, respectively.
- The partial pressure of oxygen in the alveoli is 104 mmHg, whereas in the blood it is 40 mmHg.
- Similarly, the partial pressure of carbon dioxide is 40 mmHg in alveoli and 45 mmHg in blood.
- This creates a concentration gradient between the blood and the alveoli.
- The diffusion membrane comprises 3 layers – the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance in between them.
Source: Exchange of gases
How are oxygen and carbon dioxide transported in blood?
- The transport of oxygen and carbon dioxide occurs via blood.
- About 97% of the transport of oxygen occurs by blood.
- And the remaining 3% is transported by plasma.
- Similarly, about 70% of carbon dioxide is transported in the form of bicarbonate in deoxygenated blood around 25% is transported via red blood cells, and around 7% is transported in a dissolved state via plasma.
Transport of oxygen
- Oxygen transport within the human body occurs through both convection and diffusion.
- Oxygen diffuses from the alveoli into the pulmonary capillaries and the systemic capillaries into the tissues.
- Red blood cells contain an iron-containing red-colored pigment known as Hemoglobin.
- Hemoglobin reversibly binds oxygen to form oxyhemoglobin.
- A single hemoglobin molecule can bind 4 oxygen molecules.
- The partial pressure of oxygen determines the binding of oxygen with hemoglobin.
- Oxygen binds to hemoglobin in the lungs and gets dissociated in tissues.
Transport of carbon dioxide
- CO2 is carried by hemoglobin as carbamino-hemoglobin.
- This is due to the partial pressure of CO2 .
- When pCO2 is high and pO2 is low as in the tissues, more binding of carbon dioxide occurs.
- When the pCO2 is low and pO2 is high as in the alveoli, dissociation of CO2 from takes place.
Regulation of Respiration
- Our neural system gives us the ability to maintain and moderate our respiratory rhythm based on the needs of the different tissues.
- The respiratory rhythm center present in the medulla region is responsible for this regulation.
- The pneumotaxic center present in the pons region moderates the functions of the respiratory rhythm center.
Disorders of the Respiratory System
- Asthma is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.
- Emphysema is a chronic disorder in which alveolar walls are damaged due to which the respiratory surface is decreased.