Factors Affecting Breathing, General Guidelines
We breathe to get oxygen to all the cells in our body. The more metabolic activity in a cell the more oxygen it will need (more ATP/energy needed). So thinking about oxygen demand will help you understand what factors effect breathing and whether they increase or decrease activity in the medullary respiratory centers and therefore breathing rate, depth, and rhythm.
When we exercise, we have an increased oxygen demand. This means that not only do the muscle cells need more oxygen, but they also release more carbon dioxide. The carbon dioxide is released from the muscle cells and travels to the blood following its partial pressure gradient. Once in the blood it is combine with water by carbonic anhydrase to form carbonic acid. This makes the blood more acidic which means more H+ ions and a lower pH. So breathing would increase if oxygen levels decrease to or below dangerously low levels ~50-60mmHg (end of plateau region of oxygen-hemoglobin dissociation curve). Also if blood has too much carbon dioxide (measured as too much acidity).
There are many factors that can increase our metabolic rate, or metabolism. Anything that increases this will increase breathing. Anything that is a by product of an increased metabolism also indicates an increase in breathing. For example, our muscles when we exercise also produce lots of heat, so an increase in temperatures would increase breathing rate. An increase in blood acidity or acidosis (due to increased carbon dioxide, or perhaps increased H+ ions such as in diabetes mellitus) would also increase breathing. An interesting factor is increased proprioceptor discharge. Before you exercise, your body prepares for the exercise by increasing signals from proprioceptors in your muscle cells (letting body know position) this leads to an increased breathing rate, so you will have more oxygen and less carbon dioxide in your blood in preparation for the large oxygen demand to come!
Another set of factors that effect breathing have to do with fight or flight type responses (think sympathetic system). Things like pain, cold, and extreme emotions (limbic system, RAS, etc) can cause an increase in breathing rate if chronic (prolonged) or can cause a temporary stopping of breathing if sudden. This has to do with survival instincts.
A similar set of factors that effect breathing have to do with more local control and maintenance of the lungs. For example if you try to breathe in as hard as you can and manage to take in more than 1L of air, your lungs become overstretched, and stretch receptors in your lungs send signals via the vagus nerve (CN10, sends mainly sensory info about viscera to CNS) to the medulla to inhibit inspiration so you don't take in any more air. This prevents you from over-inflating your lungs.
Factors Affecting Breathing, List
- Herring-Breuer Reflex - Stretch of bronchi/bronchiole smooth muscle -> stretch receptors->vagus nerve->inhibition of inspiratory neurons in medulla->diaphragm+ external intercostals relax-> exhale
- Voluntary Control - higher brain centers->primary motor cortex->corticospinal pathway->skeletal muscles of breathing->inhale/exhale
- Increased acidity in CSF (cerebrospinal fluid) -> due to increased carbon dioxide which can cross blood brain barrier and once there through carbonic anhydrase become carbonic acid then bicarbonate and H+ -> H+ detected by central chemoreceptors in medulla (easy to detect b/c CSF not buffered well -> increase ventilation
- Increased acidity in arterial blood -> due to increased carbon dioxide in blood, but just as in CSF, H+ ions produced and detected by chemoreceptors, but this time in aortic and carotid bodies of the aorta and carotid arteries (these can also weakly detect actual carbon dioxide levels)-> increase ventilation
- Increased acidity due to diabetes mellitus -> increase ventilation
- Severe decrease in oxygen levels -> described above -> increase ventilation
- Carbon monoxide poisoning -> CO binds to oxygen binding sites on hemoglobin producing carboxyHb, and since it binds 210 times more readily than oxygen it leads to very little oxygen binding. CO is an odorless, colorless gas and since it binds to oxygen binding sites, it doesn't effect the amount of oxygen dissolved in the blood, only the amount of oxygen saturation of Hb (i.e. no change in partial pressure of oxygen or carbon dioxide which are both measures of how much of each gas is dissolved in solutions). This is very dangerous since, there will be no obvious signs of CO poisoning and it is the leading cause of death from fires!
- Temperature, emotion, cold, pain, anal sphincter stretch, proprioceptor discharge -> all of these if increased increase ventilation rate
- Blood pressure -> if BP increases then there is lots of oxygen getting to blood and lots of carbon dioxide being removed, so ventilation would decrease (specifically if sudden increase in BP). Also if BP suddenly decreases, ventilation increases.
- Breath holding -> after 4-5 min (amount of time before the 75% saturation of Hb decreases so that no oxygen can be unloaded) -> medulla overrides voluntary control _>breathe
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