Breathing and respiration
Breathing is the exchange of O₂ from the atmosphere with CO₂ produced by cells. Cellular respiration is the use of that oxygen to break down food and release energy. Breathing serves respiration.
Respiratory organs in animals
Animal | Organ |
|---|---|
Sponges, coelenterates, flatworms | simple diffusion over the whole body surface |
Earthworm | moist cuticle (skin) |
Insects | network of tubes called tracheae |
Most aquatic arthropods and molluscs; fish | gills (branchial respiration) |
Amphibians | lungs and moist skin |
Reptiles, birds, mammals | lungs (pulmonary respiration) |
The human respiratory system
Air passes through:
external nostrils → nasal chamber → pharynx → larynx → trachea → primary bronchi → secondary and tertiary bronchi → bronchioles → alveoli
The larynx (sound box) is a cartilaginous box that produces sound. During swallowing, a flap called the epiglottis covers the glottis to stop food entering the larynx.
The trachea, bronchi and early bronchioles are kept open by incomplete cartilaginous rings.
The alveoli are tiny, very thin-walled, vascularised sacs where gases are exchanged.
The two lungs are covered by a double-layered pleura, with pleural fluid between the layers that reduces friction.
The lungs sit in an airtight thoracic chamber, bounded by the vertebral column, sternum, ribs and, below, the dome-shaped diaphragm.
The part from the nostrils to the terminal bronchioles is the conducting part: it filters, warms and humidifies air. The alveoli and their ducts form the respiratory (exchange) part.
The mechanism of breathing
Breathing works by creating a pressure difference between the lungs and the atmosphere.
Inspiration (active):
The diaphragm contracts and flattens, increasing the chest volume from top to bottom.
The external intercostal muscles contract, lifting the ribs and sternum, increasing the volume from front to back.
The lung volume rises, the pressure inside falls below atmospheric pressure, and air rushes in.
Expiration (normally passive):
The diaphragm and intercostal muscles relax, and the chest returns to its smaller size.
Pressure in the lungs rises above atmospheric, and air is pushed out.
During forceful breathing, the abdominal muscles and internal intercostals help.
A healthy adult breathes 12–16 times a minute. Breathing volumes are measured with a spirometer.
Respiratory volumes and capacities
Term | Meaning | Typical value |
|---|---|---|
Tidal volume (TV) | air inspired or expired in a normal breath | about 500 mL |
Inspiratory reserve volume (IRV) | extra air that can be inspired by a forceful inspiration | 2500–3000 mL |
Expiratory reserve volume (ERV) | extra air that can be expired by a forceful expiration | 1000–1100 mL |
Residual volume (RV) | air remaining in the lungs even after forceful expiration | 1100–1200 mL |
Inspiratory capacity (IC) | TV + IRV | |
Expiratory capacity (EC) | TV + ERV | |
Functional residual capacity (FRC) | ERV + RV | |
Vital capacity (VC) | ERV + TV + IRV: the maximum air that can be breathed out after a maximum breath in | |
Total lung capacity (TLC) | VC + RV |
A healthy person moves about 6000–8000 mL of air per minute (500 mL × 12–16 breaths).
Exchange of gases
Gases move by simple diffusion, from higher to lower partial pressure (pO₂ and pCO₂).
Atmospheric air | Alveoli | Deoxygenated blood | Oxygenated blood | Tissues | |
|---|---|---|---|---|---|
pO₂ (mm Hg) | 159 | 104 | 40 | 95 | 40 |
pCO₂ (mm Hg) | 0.3 | 40 | 45 | 40 | 45 |
So O₂ moves from the alveoli into the blood and from the blood into the tissues; CO₂ moves the other way.
CO₂ is about 20–25 times more soluble than O₂, so it diffuses faster across the membranes for each unit of pressure difference. The diffusion membrane is very thin (less than a millimetre), made of the alveolar wall, the capillary wall and the basement substance between them.
Transport of gases
Oxygen: about 97% is carried by haemoglobin in red blood cells as oxyhaemoglobin; about 3% is dissolved in plasma. Each haemoglobin molecule can carry up to four O₂ molecules.
The oxygen dissociation curve, a plot of % saturation of haemoglobin against pO₂, is sigmoid (S-shaped). In the alveoli (high pO₂, low pCO₂, lower H⁺ concentration, lower temperature), oxygen binds. In the tissues (low pO₂, high pCO₂, higher H⁺, higher temperature), oxygen is released. Every 100 mL of oxygenated blood delivers about 5 mL of O₂ to the tissues under normal conditions.
Carbon dioxide:
about 20–25% as carbaminohaemoglobin (bound to haemoglobin),
about 70% as bicarbonate ions, made with the help of the enzyme carbonic anhydrase in red blood cells: CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺,
about 7% dissolved in plasma.
Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli.
Regulation of breathing
The respiratory rhythm centre in the medulla controls breathing. The pneumotaxic centre in the pons can moderate it. A chemosensitive area next to the rhythm centre responds to rising CO₂ and H⁺, increasing the breathing rate. Receptors in the aortic arch and carotid arteries also detect changes. Oxygen plays only a small role in regulating rhythm.
Disorders
Asthma: inflammation of the bronchi and bronchioles makes breathing difficult and causes wheezing.
Emphysema: alveolar walls are damaged, reducing the surface area for exchange. Cigarette smoking is a major cause.
Occupational lung disorders: in industries producing dust, such as stone grinding and breaking, long exposure causes inflammation and fibrosis (scarring). Workers should wear protective masks.
Summary
Air passes from the nostrils to the alveoli, where gases are exchanged by diffusion.
Inspiration: diaphragm and external intercostals contract; expiration is normally passive.
Vital capacity = TV + IRV + ERV; residual volume always stays in the lungs.
97% of O₂ travels as oxyhaemoglobin; about 70% of CO₂ travels as bicarbonate.
The medulla's rhythm centre controls breathing, responding mainly to CO₂ and H⁺.