Concepts: Regulation of respiration, disorders and exercise practice

4.1 The nervous control of breathing

Breathing goes on without conscious effort because a group of neurons in the medulla oblongata sets the basic rhythm. This respiratory rhythm centre keeps producing the impulses that drive inspiration and expiration. A second centre in the pons, the pneumotaxic centre, moderates the rhythm: when it is stimulated it reduces the duration of inspiration, so the breathing becomes faster and shallower. A chemosensitive area lies close to the rhythm centre. It is sensitive mainly to carbon dioxide and to hydrogen ions, and a rise in either of them stimulates it and increases the rate and depth of breathing. Carbon dioxide is therefore the main chemical regulator of breathing, and the textbook stresses that oxygen plays a relatively small role in this control. This is why rebreathing into a paper bag makes a person breathe faster: the extra carbon dioxide in the bag raises the pCO2 of the blood and stimulates the chemosensitive area.

4.1.1 Disorders of the respiratory system

Asthma is an inflammation of the bronchi and bronchioles that narrows the airway, causes wheezing and makes breathing out difficult. Emphysema is a condition in which the walls of the alveoli are damaged and the total surface available for exchange falls, so the person becomes short of breath on exertion; cigarette smoking is the major cause. Occupational respiratory disorders result from long exposure to dust and fumes in mines, factories and construction sites, and the dust collects in the lungs and reduces their working capacity.

4.1.2 Exercise practice

The chapter has one exercise section of fourteen questions and all are represented across the pages. The remaining items are collected here.

Exercise 7. Respiration is regulated by the respiratory rhythm centre in the medulla, which sets the basic rhythm, the pneumotaxic centre in the pons, which modifies the rhythm, and the chemosensitive area, which responds to carbon dioxide and hydrogen ions and adjusts the rate and depth of breathing.

Exercise 9. At high altitude the partial pressure of oxygen in the air falls, so less oxygen diffuses into the blood. The body responds by increasing the breathing rate and depth, and over a longer stay the red blood cell count and haemoglobin level rise so that the blood can carry more oxygen. A person who climbs too fast can develop breathlessness, headache and nausea, which are the signs of altitude sickness.

Exercise 10. In insects the site of gaseous exchange is the tracheal system, with air entering through spiracles and travelling along the tracheae and tracheoles directly to the tissues.

Exercise 12. Hypoxia is a condition of insufficient oxygen supply to the tissues. It can be caused by high altitude, lung disease, anaemia or a blocked airway, and its symptoms include breathlessness, headache, confusion and a bluish tinge to the skin in severe cases. The textbook asks the student to gather information about it and discuss it with friends.

4.1.3 What the chemosensitive area does

The chemosensitive area is not the rhythm generator; it is a sensor that reports the chemical state of the blood to the rhythm centre. When carbon dioxide rises the area fires more, and the rhythm centre responds by increasing both the rate and the depth of breathing. The extra ventilation blows off the excess carbon dioxide and the stimulus falls, so the whole arrangement is a negative feedback loop. This loop is why breath holding becomes uncomfortable after a minute or two: carbon dioxide accumulates, the drive to breathe rises and the subject has to breathe again.

4.1.4 When the rhythm changes

The resting rate is not fixed for all conditions. During exercise the muscles produce more carbon dioxide, the chemosensitive area is stimulated and both rate and depth of breathing rise, which supplies more oxygen and removes the extra carbon dioxide. At high altitude the air has a lower partial pressure of oxygen, so the body increases ventilation and, after some days, raises the red blood cell count. A person who goes up too quickly experiences breathlessness, headache and nausea. The same centres also respond to factors such as pain and emotion, which is why breathing becomes faster when a person is frightened.

4.1.5 The control loop in one line

The rhythm centre provides the timing, the chemosensitive area provides the feedback, and the pneumotaxic centre adjusts the finish of each breath; together they keep the alveolar pCO2 near a set value without any conscious decision. That is why breathing continues during sleep and why it changes automatically the moment the body starts producing more carbon dioxide.

Exercise 3 (restated). The reason diffusion happens only in the alveolar region is that only there the wall is one cell thick, the surface is moist and the blood supply is rich; the rest of the tract is a conducting passage.