If you were going to “install” gas detection sensors (called chemoreceptors) in the body, where would you put them?  What cells in the body should have top priority to sufficient oxygen levels?  Let’s think for a moment – hey, what about the big guy upstairs?  Though our brain only makes up 3% of our body weight, it burns 20% of our energy.  Why is it such a pig?  We have to remind ourselves that day and night it never stops giving orders to the rest of the body to keep us alive,  maintaining essential life functions such as breathing and heart rate.  We can’t ignore  its demands.   

Cars traveling south on I-5 heading from Oregon into California are routed through a checkpoint where a fruit inspection takes place. In a similar way, blood carrying oxygen to the brain traveling up the right and left carotid arteries is routed through a gas sensor checkpoint. The Engineer of our bodies strategically installed one set of sensors along each of these arteries so that oxygen-rich blood traveling up through the carotids passes through them.   In fact, this tiny sensor handles the highest volume of blood flow per tissue weight of any other tissue in the body. (2) This strategic placement of the gas sensors along the carotid arteries resembles the placement of oxygen sensors in the exhaust pipe of cars, and even more so when you consider the similarity between cells and car engines. Both rely on a continuous supply of oxygen to burn their respective fuels, and both utilize input from the sensors to adjust oxygen levels.

Located within this tiny sensor (called carotid body) are glomus cells whose purpose is to detect changes in partial pressures of oxygen (PO2).  Normal arterial blood has a PO2 of around 95-100 mm Hg.  Should this level decrease these sensors begin firing messages through a branch of the glossopharayngeal nerve up to the respiratory center in the pons of the brain stem. The respiratory center responds by issuing an order to the diaphragm and other respiratory muscles through the phrenic nerve to speed up the breathing rate.  These muscles begin firing at a faster pace, bringing more precious oxygen to be distributed to cells whose levels are running low.  If oxygen levels decrease to 80 mmHg PO2 these glomus cells rapidly increase their signals, as the need for oxygen is more urgent.

In addition to their sensitvity to PO2 the carotid bodies are also tracking carbon dioxide (CO2) and blood pH (acid) levels. Chasing that ball down the soccer field means an increase in the production of carbon dioxide (CO2) because for every oxygen molecule used to make ATP’s a carbon dioxide molecule is produced. These CO2 molecules created in our cells are picked up by the blood and taken to the lungs where we can get rid of them by exhaling.

Our standard equipment includes two more sets of gas detection sensors. One set, located in the respiratory center in the brain stem itself, specializes in sensing changing pH levels both in blood and in spinal fluid. Another set is located in the aorta, the large artery where fresh oxygenated blood is pumped from the heart. These sensors have the capability to detect changing levels of PO2, PCO2, and blood pH levels.

          How carotid and aortic bodies detect changing O2 and CO2 levels is incredibly complicated and still being explored.  Our current understanding is that glomus cells in the carotid body generate an enzyme which results in the production of hydrogen sulfide (H2S) gas. When oxygen levels in blood decrease this induces the glomus cells to increase their production of this enzyme leading to an increase in H2S being generated.  The H2S gas is partially responsible for stimulating these glomus cells to begin sending the message (1).  

As you can see in the diagram above, the respiratory centers in our brain stem along with the three sets of gas detection sensors create a closed loop feedback system similar in design to the adaptive cruise control of cars and the air-fuel control by the PCM in cars.  These car systems are the result of teams of engineers who have benefited from hundreds of years of scientific research, whereas our built-in breathing controls assembled themselves from instructions found on our DNA.  What do you think? Is it reasonable to believe that the breathing feedback loop with all its components is the result of natural selection acting on random chance mutation, or does it make more sense to believe it is the result of a designer?

We have the best of both worlds. Most of the time our breathing is fully automatic, controlled by some amazing circuitry in our brain stem, constantly adjusting our breathing rate to whatever we're doing. But,we can override these controls whenever we want, as in holding our breath while under water. No buttons to push, just a thought from our brain will give us control of breathing when we need it. Then, it automatically switches back to the default, which is 24/7 worry-free oxygen delivery perfectly suited for the moment. The Designer of this wonderful system wants us to realize our life-breath is in His hands

Take a deep breath and let it out.  Relax.  Your life-giving oxygen levels are constantly being monitored by perfectly placed sensors, and adjustments are being made every second of your day.  Go out and play some hoops or soccer.  Run up a hill and rejoice that the Engineer of your body has installed fully automatic control centers in your brain stem that with each step you take are receiving a constant stream of feedback from some sophisticated sensors, and are simultaneously firing messages to speed up the  respiratory and circulatory systems to deliver the oxygen you need to continue running. 

Questions to Consider:  

What does evolution “know”?  Did it “know” we needed a way to breathe without having to think about it, especially at night? Did natural selection and blind chance ‘design” this feedback system with all its components, built-in settings, ability to detect gas and pH levels and then to send and interpret messages?   How did our respiration come under the automatic control of the medulla?  How was survival possible before this?  How did the automatic controls get “set” to around 15 breaths per minute? How does this control system “know” to respond to messages from the gas detection sensors by sending messages to speed up breathing? Why is all the wiring (nerves) going to and from the correct places?

Review Questions:

1. What are the 3 main components of the automatic breathing system and what do they do?

2. What are the 3 changing conditions being analyzed by the chemoreceptors?

3. What happens to a liquid when it absorbs CO2? What happens to the CO2?

4. Discuss: Would this system work if it was missing one of its components?

5. Where does CO2 in our body come from?

Activity 1

Materials: Candle; glass jar; matches

1. Light a candle and place it inside the glass jar which is upside down. towards the bottom of the jar  (candle keeps burning).  After 30 seconds look to see what is happening to the glass.  Do you see moisture? Where did it come from?

2. Now push the lit candle up towards the top of the upside down jar (this is the bottom of the jar). What happens and why?  What else was the burning candle giving off besides water and energy? (Hint: it’s a gas that we breathe out, that is also used in fire extinguishers)

Activity 2

Materials: red cabbage, clear soda like Sprite

1. Make some red cabbage juice by cutting up 1 cup of red cabbage and covering with water. Heat for 5 minutes over stove and let sit. Strain out the leaves.

2. Place the an equal amount of purple cabbage juice in soda. What color change happens? Soda has carbonic acid in it, which lowers the pH.

3. Optional: Place an alka seltzer tablet in the cabbage juice and watch as the tablet dissolves and gives off CO2 into the liquid.

Endnotes

1.  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4294136/