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Blood and the respiratory system
Blood and the respiratory system

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1.2 Muscles of ventilation

The expansion and contraction of the lungs is controlled mechanically by the diaphragm and the intercostal muscles (Figure 1, repeated).

Described image
Figure 1 (repeated) Components of the respiratory system.
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Pink silhouette of the lower head and the thorax of a human.

The thoracic cavity is shown bounded by yellow ribs (in cross-section) with intercostal muscles between the ribs. Inside the ribs is the thoracic cavity in pale blue. At the base of the thoracic cavity is the diaphragm, coloured purple, and displayed as an inverted U-shape, with the abdominal cavity below.

Two lungs, coloured pink are on the right and left sides of the thoracic cavity; the lungs are covered with pleurae, shown as a white layer covering the lungs. The lobes of the lung are shown on the right lung.

The mouth and nasal cavities proceed to the pharynx in the neck; this leads to the oesophagus, all coloured pink. From the top of the oesophagus proceeds the trachea (coloured yellow) with darker yellow cartilage rings stiffening and supporting it; at the top of the trachea is shown the box-like larynx in pale yellow. The trachea branches to lead to two bronchi, one leading into each of the right and left lungs. The left bronchus is traced into the left lung, showing extensive branching into ramifying bronchioles, stiffened with cartilage.

The heart is shown in dark red between the base of the lungs; the trunks of major arteries are shown proceeding from the heart.

Figure 1 (repeated) Components of the respiratory system.
  • The diaphragm is a dome-shaped muscle that sits underneath the lungs and separates the thoracic cavity from the abdominal cavity. It is innervated by the phrenic nerve, which originates in the medullary respiratory centre in the medulla of the brain.
  • The intercostal muscles are located in the ribcage. They receive neuronal inputs from intercostal nerves that arise from the thoracic nerves of the spinal cord.

The bronchi and bronchioles are also surrounded by smooth muscle cells that contract and dilate to regulate the amount of air that passes down to the alveoli.

Activity 2 Diaphragm and intercostal muscles

Timing: Allow about 20 minutes

Part 1

Described image
Figure 5 Diaphragm and intercostal muscles.
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Diagram of thoracic muscles featuring the diaphragm. From Gray's Anatomy 1918.

Figure 5 Diaphragm and intercostal muscles.

To explore the location and function of the diaphragm and intercostal muscles, take a look at the anatomical information and diagrams on this site (open the links in a new tab/window so you can easily return to this page):

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Part 2

Now try matching each of these statements with the correct muscles.

Two lists follow, match one item from the first with one item from the second. Each item can only be matched once. There are 4 items in each list.

  1. flatten(s) when contracted to expand the size of the thoracic cavity and decrease the size of the abdominal cavity, generating a pressure gradient within the thoracic cavity

  2. draw(s) the ribs upwards and outwards; most active during inhalation

  3. draw(s) the ribs downwards and inwards; most active during forced exhalation

  4. stiffen(s) the chest wall during respiration; most active during forced exhalation

Match each of the previous list items with an item from the following list:

  • a.external intercostal muscles

  • b.diaphragm

  • c.internal intercostal muscles

  • d.innermost intercostal muscles

The correct answers are:
  • 1 = b,
  • 2 = a,
  • 3 = c,
  • 4 = d