Unit 3: Respiration
Respiration involves two main processes: breathing (the physical act of moving air in and out of the lungs) and cellular respiration (the chemical reaction in cells that uses oxygen to release energy from food).
1. Respiratory system of Human
The human respiratory system is a network of organs and tissues responsible for gas exchange. It is divided into the upper and lower respiratory tracts.
The Path of Air:
- Nostrils & Nasal Cavity: Air enters, where it is filtered by hairs, warmed by blood, and moistened.
- Pharynx: A shared pathway for food and air.
- Larynx (Voice Box): A cartilaginous structure that contains the vocal cords. The epiglottis, a flap, prevents food from entering the trachea.
- Trachea (Windpipe): A tube held open by C-shaped rings of cartilage.
- Bronchi: The trachea splits into two primary bronchi (one for each lung).
- Bronchioles: The bronchi branch into smaller and smaller tubes called bronchioles.
- Alveoli (Air Sacs): Tiny, grape-like clusters at the end of the bronchioles. This is the site of gas exchange. They have a huge surface area and are surrounded by blood capillaries.
2. Mechanism of breathing in human
Breathing is a physical process controlled by the diaphragm (a large, dome-shaped muscle below the lungs) and the intercostal muscles (between the ribs).
Inhalation (Breathing In)
- This is an active process.
- The diaphragm contracts and moves down (flattens).
- The intercostal muscles contract, pulling the rib cage up and out.
- Result: The volume of the chest cavity (thorax) increases. This causes the pressure inside the lungs to decrease (becoming lower than atmospheric pressure).
- Air rushes into the lungs to equalize the pressure.
Exhalation (Breathing Out)
- This is normally a passive process.
- The diaphragm relaxes and moves up (returns to its dome shape).
- The intercostal muscles relax, allowing the rib cage to move down and in.
- Result: The volume of the chest cavity decreases. This increases the pressure inside the lungs (making it higher than atmospheric pressure).
- Air is forced out of the lungs.
3. Transport of gases
Once air is in the alveoli, oxygen needs to be transported to all body cells, and carbon dioxide needs to be transported back to the lungs.
Transport of Oxygen (O₂)
- About 97% of O₂ is transported by Red Blood Cells (RBCs).
- It binds to the iron-containing pigment Hemoglobin (Hb) to form a temporary, reversible compound called Oxyhemoglobin (HbO₂).
Hb + O₂ ↔ HbO₂- About 3% of O₂ is transported by being dissolved in the blood plasma.
Transport of Carbon Dioxide (CO₂)
CO₂ is transported in three ways:
- As Bicarbonate Ions (approx. 70%): This is the main method. CO₂ enters RBCs, reacts with water to form carbonic acid (
H₂CO₃), which then splits into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻). The bicarbonate ions are then transported in the plasma.
- Bound to Hemoglobin (approx. 20-25%): CO₂ binds to a different part of the hemoglobin molecule (not the iron) to form Carbaminohemoglobin.
- Dissolved in Plasma (approx. 7%): A small amount is carried dissolved in the liquid blood plasma.
4. Respiratory volumes and capacities
These are measurements used to assess lung function, often measured with a spirometer.
- Tidal Volume (TV): The volume of air breathed in or out during a normal, quiet breath (approx. 500 ml).
- Inspiratory Reserve Volume (IRV): The additional volume of air one can forcefully inhale *after* a normal inhalation.
- Expiratory Reserve Volume (ERV): The additional volume of air one can forcefully exhale *after* a normal exhalation.
- Residual Volume (RV): The volume of air that always remains in the lungs even after a maximum forceful exhalation. This prevents the lungs from collapsing.
Respiratory Capacities (Combinations of volumes)
- Vital Capacity (VC): The maximum amount of air a person can exhale after a maximum inhalation.
VC = TV + IRV + ERV
- Total Lung Capacity (TLC): The total volume of air the lungs can hold.
TLC = VC + RV
Key Concept: You cannot measure Residual Volume (RV) with a simple spirometer, because you can never breathe it out. Therefore, Total Lung Capacity (TLC) cannot be measured by simple spirometry either.
5. Some respiratory disorders
- Asthma: An allergic reaction causing inflammation and spasms of the bronchi and bronchioles, leading to difficulty in breathing (wheezing).
- Emphysema: A chronic disease, often caused by smoking, where the alveolar walls are damaged and break down. This reduces the surface area for gas exchange, leading to shortness of breath.
- Bronchitis: Inflammation of the lining of the bronchial tubes, often causing a persistent cough with mucus.
- Pneumonia: An infection that inflames the alveoli, which may fill with fluid, making breathing painful and limiting oxygen intake.