Homeostasis: Breathing Rate Flashcards
1
Q
Regulation of Gas Concentration
A
- The muscles that cause air to move in and out of the lungs are skeletal muscles and require stimulation from nerve impulses to initiate contraction.
- The diaphragm is stimulated by impulses from the phrenic nerve, while impulses from the intercostal nerves stimulate the intercostal muscles.
- These spinal nerves have their origin in the spinal cord at the level of the neck and thorax.
- The nerve impulses that travel to the diaphragm and intercostal muscles are controlled by a respiratory centre located in the medulla oblongata of the brain.
- There are two regions within the respiratory centre:
- one that controls expiration (breathing out) and one that controls inspiration (breathing in).
- To coordinate breathing, messages need to pass back and forth between the neurons in these two regions.
2
Q
Control of Breathing
A
- Both oxygen and carbon dioxide are carried in the blood and their concentrations have an effect on breathing rate.
- In addition, the concentration of carbon dioxide in the blood plasma affects the concentration of hydrogen ions.
- When carbon dioxide dissolves in water it forms carbonic acid (H2CO3), which readily breaks down to form hydrogen ions (H+) and bicarbonate ions (HCO3–), as shown in the following chemical equation:
- Body measures CO2 to breathe
- Decrease in pH increases acidity (CO2)
- Oxygen, carbon dioxide and hydrogen ions all have some effect on the regulation of breathing activity.
3
Q
Concentration of Oxygen
A
- If the concentration of oxygen falls below normal while other factors are held constant, then there is an increase in breathing rate.
- However, within the normal range of blood oxygen concentration, the effect on breathing rate is only slight.
- The concentration has to fall to very low levels before it has a major stimulatory effect on the rate of breathing.
- Thus, under normal circumstances, oxygen plays little part in the regulation of breathing.
- There are groups of cells within the walls of the aorta and carotid arteries that are sensitive to changes in the concentration of oxygen in the blood plasma.
- These groups of chemoreceptors are known as the aortic and carotid bodies.
- Besides these peripheral chemoreceptors, there are central chemoreceptors in the medulla oblongata.
- A large decrease in oxygen concentration stimulates the chemoreceptors and nerve impulses are transmitted to the respiratory centre.
- Aorta is a good location for regulating the concentration of oxygen as it measures the blood when it has the least concentration of Oxygen and the most, allowing it to regulate the concentration as needed.
4
Q
Concentration of Carbon Dioxide
A
- The concentration of carbon dioxide in the blood plasma is a major factor in the regulation of breathing rate.
- A relatively small increase in the concentration of carbon dioxide is enough to cause a marked increase in the rate of breathing.
- The increase in the concentration of carbon dioxide and hydrogen ions (H+)in the blood results in the stimulation of the central and peripheral chemoreceptors.
- These in turn transmit nerve impulses to the respiratory centre, resulting in an increase in breathing rate.
- The chemoreceptors most sensitive to changes in the concentration of carbon dioxide in the plasma are those located in the medulla oblongata.
- The neurons making up these central chemoreceptors are separate from, but communicate with, the neurons of the respiratory centre.
- These chemoreceptors are responsible for 70–80% of the increase in breathing rate that results from an increase in the carbon dioxide concentration of the blood.
- However, this response takes several minutes.
- The immediate increase in breathing rate that occurs following an increase in the carbon dioxide concentration of the plasma is produced by the stimulation of the aortic and carotid bodies.
- These are stimulated by the associated increase in hydrogen ion concentration.
5
Q
Concentration of Hydrogen Ions
A
- As the hydrogen ion concentration of the blood increases, the pH decreases, causing an increase in the breathing rate.
- A decrease in the pH directly stimulates chemoreceptors in the aortic and carotid bodies, which then transmit impulses to the respiratory centre, resulting in an increase in the breathing rate.
6
Q
Voluntary Control of Breathing
A
- The voluntary control comes via connections from the cerebral cortex to descending tracts in the spinal cord.
- Voluntary control thus bypasses the respiratory centre in the medulla oblongata.
- This is a protective device as it enables us to prevent irritating gases and water from entering the lungs. (For example: holding your breath)
- The build-up of carbon dioxide in the plasma stimulates the inspiratory centre to send impulses to the inspiratory muscles.
- Thus, we are eventually forced to take a breath whether we want to or not.
- Rapid, deep breathing can provide more oxygen than required and remove more carbon dioxide than necessary.
- This is called hyperventilation. It can occur voluntarily or may be stimulated by physical stress such as severe pain, or emotional stress such as extreme anxiety.
- Hyperventilation usually corrects itself because the reduction in carbon dioxide concentration means that the chemoreceptors are not stimulated and there is no urge to breathe until carbon dioxide levels return to normal.
7
Q
Exercise and Breathing
A
- During exercise the contracting muscle cells require large amounts of oxygen and produce large amounts of carbon dioxide.
- In responding to this increased demand for gas exchange, the respiratory system increases both the rate of breathing and the depth of breathing.
- During heavy exercise, the volume of air going into and out of the lungs each minute may increase ten- to twentyfold.
- The same factors that influence breathing at rest appear to be involved in this increase; that is, the concentrations in the blood plasma of carbon dioxide, hydrogen ions and, to a lesser extent, oxygen.
