why do we breathe harder when we run

ponytayne wrote: You're off a little on your understanding of oxygen uptake kinetics. You can produce various metabolic wastes and excess CO2 without driving down PO2 in the blood. Your breating will increase to get the CO2 out, and while more O2 will be coming into the alveoli of the lungs, it will not all necessarily be taken into the blood. Only if there is a driving force, meaning a concentration or pressure difference, will more oxygen than normal go into the blood. The blood has a massive store of O2 in the form of oxyhemoglobin inside the red blood cells. The sensors in the carotid bodies and hypothalamus do not detect Hb oxygen saturation. They detect PO2, which is the partial pressure of O2 dissolved in the fluid portion of the blood. On the other hand, the primary method of CO2 transport in the blood is via dissolution into the fluid.

Actually CO2 will dissolve into the water, but will react with water for form carbonic acid via the carbonic anhydrase reaction. The CA then dissociates into H+ (acid) and HCO3- (bicarbonate). The acid from this reaction is usually kept withing the RBCs while the bicarb is let out to serve as the primary acid buffer of the blood. Release of any metabolic acid will create H+, which binds to the bicarb, and by simple thermodynamic law pushes that reversible reaction back to the left, favoring the production of CO2 in the blood. Then, breathing will increase to get rid of it. Anyway, the point I'm making is that CO2 and its intimate partner pH are very closely monitored by the chemosensors in the body, and they are the driving force for breathing rate and depth.

Interesting fact about carbon monoxide (CO). The reason it kills you so easily and painlessly is that it binds to the Fe atom at the center of hemoglobin irreversibly, displacing O2. However, it does not affect the PO2 in the fluid portion of the blood becuase usually when you're inhaling CO, you have plenty of available O2 also. Therefore, breathing rate does not increase, you do not sense distress, and you simply "fall asleep" to die. So, hang on a sec. given the primary function is elimination of waste (~C02), and the sensing mechanism is triggered by reduction of P02 (not hemoglobic 02). But P02 would effectively decrease as hemoglobic 02 is utilized, correct? So there is a distinct relationship between C02 removed and 02 intake (via P02 absorption and eventually hemoglobic 02), with hemoglobic 02 creating a buffer (which is why you probably don't start breathing hard immediately, and continue to breath hard for a period afterwards).

Exercise helps keep you fit and healthy by strengthening your muscles and making your heart beat faster. Your muscles, lungs, and heart all work together to move your body and make sure you are getting enough oxygen. This results in an increase in your breathing rate, or rate of ventilation. In order to reap the benefits of exercise, your heart and breathing rates must increase. Your muscles are pushing your blood back to your heart at a faster rate, so your heart must increase its rate of pumping to match. Some of the blood pumped by the heart travels to the lungs to get rid of carbon dioxide and pick up oxygen.

The increase in heart rate stimulates your breathing rate. With an increase in heart rate, your blood pumps through your muscles at a faster rate, leaving less time for oxygen uptake. Having more oxygen available in the blood, from a faster breathing rate, helps the muscles get the amount they need. Your muscles are working harder during exercise and that means their demand for oxygen increases. This happens because oxygen is needed to burn calories more efficiently. Since the blood picks up oxygen in the lungs, and the demand for oxygen increases during exercise, the lungs must work harder. With a faster breathing rate, more oxygen is picked up at the lungs for delivery to the working muscles. A by-product of metabolism is carbon dioxide. Part of the lungs function is to rid the blood of carbon dioxide.

As exercise continues, or exercise intensity increases, more carbon dioxide is produced and needs to be removed. Increased breathing rate allows carbon dioxide to be expired more rapidly. Another by-product of metabolism is heat. As your body temperature rises, signals are sent to the nerves and muscles of the respiratory system to increase breathing rate. The mechanism of this response is unknown, but increased ventilation is also often present accompanying fever. At the onset of exercise, the brain signals increases in heart and breathing rates in anticipation of the increased need for oxygen and carbon dioxide exchange of exercise. Once exercise begins, circulating levels of the hormone epinephrine also referred to as adrenaline increase. This increase stimulates ventilation as well.