Breathing and Exchange of Gases
Class 11 Biology · Biology · Chapter 14
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Summary
This chapter describes how animals, especially humans, take in oxygen and remove carbon dioxide. Mechanisms vary with habitat: lower invertebrates use diffusion, earthworms use moist skin, insects use tracheal tubes, aquatic arthropods and molluscs use gills, and terrestrial vertebrates use lungs. The human respiratory system consists of external nostrils, nasal chamber, pharynx, larynx (the sound box with the epiglottis), trachea, bronchi, bronchioles and alveoli, the thin vascularised sacs where gas exchange occurs; the lungs lie in an airtight thoracic chamber bounded by ribs, sternum and the diaphragm. Breathing has two stages: inspiration, when contraction of the diaphragm and external intercostal muscles increases thoracic and pulmonary volume and lowers intra-pulmonary pressure to draw air in, and expiration, when their relaxation reverses this. Respiratory volumes and capacities such as tidal volume, vital capacity and residual volume are measured with a spirometer. Exchange of oxygen and carbon dioxide at alveoli and tissues occurs by diffusion along partial-pressure gradients. Oxygen is carried mainly as oxyhaemoglobin and described by the sigmoid oxygen dissociation curve, while most carbon dioxide is transported as bicarbonate with the help of carbonic anhydrase. The respiratory rhythm is regulated by centres in the medulla and pons, and disorders include asthma and emphysema.
Respiratory organs in animalsHuman respiratory systemMechanism of breathingRespiratory volumes and capacitiesExchange and transport of gasesRegulation and disorders of respiration
Key terms
Alveoli
The thin, irregular-walled, vascularised bag-like structures of the lungs where gas exchange takes place.
Tidal volume
The volume of air inspired or expired during a normal respiration, about 500 mL.
Vital capacity
The maximum volume of air a person can breathe in after a forced expiration, including ERV, TV and IRV.
Oxygen dissociation curve
The sigmoid curve obtained by plotting percentage saturation of haemoglobin with oxygen against partial pressure of oxygen.
Partial pressure
The pressure contributed by an individual gas in a mixture, written pO2 for oxygen and pCO2 for carbon dioxide.
Carbonic anhydrase
The enzyme in red blood cells that catalyses the interconversion of carbon dioxide and bicarbonate.
Extra questions & answers
Try one from this chapter
The primary sites of exchange of gases in the lungs are the: