Muscles of breathing
- Diaphragm – this is the main muscle of inspiration. It flattens out. During normal quiet breathing it is only really the diaphragm that does any work (other muscles are often not involved). It is controlled by the phrenic nerve which has nerve roots in C3-5 [Remember: C3,C4,C5 keep the diaphragm alive])
- External intercostals – they raise the ribcage, and also pull the sternum outwards slightly, which increases the volume of the thorax in a different dimension to the diaphragm (horizontal and vertical vectors)
- Sternocleidomastoid – this will lift the sternum up slightly
- Anterior serrate – this lifts up many of the ribs. It attaches to the inside of the scapula, and its other ends attach to the lateral surfaces of the ribs.
- Scalene – these lift the first two ribs. They attach to the front of the first two ribs, and their other attaches to the transverse processes of the C2-C7 vertebrae.
- Pectoralis minor – this lifts ribs III-IV
- However – it is important to remember that this surface tension is only present when there is an air fluid interface in the lungs. I guess the excess mucus produced in COPD will have an adverse effect on this surface tension effect.
- 78% nitrogen
- 21% oxygen
- 0.5% water
- 0.04% carbon dioxide
- 75% nitrogen
- 13.6% oxygen
- 6.2% water
- 5.3% carbon dioxide
- 75% Nitrogen
- 15.7% oxygen
- 3.6% carbon dioxide
- 6.2% water
- Ondine’s curse – this is a condition where there is damage to the autonomic nervous system such that a person may ‘forget’ to breathe, usually during sleep. It occurs in 1 in 200,000 live births, but can also occur as a result of trauma to the brainstem, or poliomyelitis. These patients will require mechanical ventilation for the rest of their lives (but usually only during sleep). It is also known as primary alveolar hypoventilation.
- The DRG (Dorsal respiratory group) is a group of neurons in the medulla. They are ‘I’ (inspiratory) neurons. They are active in every breath, whether quiet or forced.
- The VRG (ventral respiratory group) are also found in the medulla. This contains both I and E (expiratory) neurons, and it is active in forced breathing.
- When blood pressure falls, respiratory rate increases
- When blood pressure rises, respiratory rate decreases
The Hering-Breur Reflexes
Accessory muscles of respiration
Ellicit drugs and the respiratory system
- Cannabis – increases the risk of COPD. A very heavy cannabis smoker could possibly get COPD in their 30’s or 40’s even without an α-1 antitrpysin deficiency. Some evidence suggests that one spliff is equivalent to smoking 20 cigarettes
- Cocaine – can cause MI in young people, as well as emphysema
- Heroin – can cause severe respiratory depression – for which you would give naloxone in the acute situation. This drug can be used to treat respiratory depression in any opioid overdose.
Definitions & Terminology
- Kussmaul breathing – this is deep, rapid breathing that is induced by acidosis
- Orthopnoea – this is dyspnoea (shortness of breath) that occurs whilst lying down
- Eupnoea – normal breathing
- Respiratory distress syndrome sometimes exists in new born infants – and it is caused by a lack of surfactant – it makes it very difficult for the baby to breathe because the high fluid tension makes it difficult for the lungs to expand.
- Bronchodilation – is stimulated by adrenaline and the sympathetic nervous system
- Bronchoconstriction – is stimulated by histamine and the parasympathetic nervous system. Cold air and chemical irritants can also have a similar effect
- Respiratory rate – the normal respiratory rate in an adult is 12-18 breaths per minute – this is roughly one for every 4 heartbeats. Children will breathe more rapidly at approximately 18-20 breaths per minute.
- Anatomical dead space – about 150ml of every 500ml of inhaled air (so 30%) will fill the bronchi and not the alveoli – and this air is obviously not available for gaseous exchange. This space that the air fills is known as the anatomical dead space.
- Physiological dead space – this is the sum of the anatomical dead space, and any extra dead space caused by alveolar damage. In healthy individuals the anatomical dead space = physiological dead space