Rocket is halfway up the stairs when Raven calls, "Say a whole sentence!"
"I am climbing the stairs and I am fine," Rocket says, a little puffed, and keeps going.
"Now sing it," Raven calls. Rocket tries. The tune falls apart by the fourth word.
He sits on the top step, laughing. "What was that for?"
"The talk test," Raven says. "What do you notice? Talking worked, singing did not."
Nova rises to the landing. "Your muscles asked for something, and two systems answered," she says. "Can you name all three?"
"Muscles ask. Lungs bring it in. Heart delivers it," Rocket says. "One demand, two systems."
Cells need energy to do work, such as contracting muscle fibers to create movement. That energy comes from sugar.
The reaction that releases the energy of a sugar molecule consumes oxygen. It releases carbon dioxide as a waste product.
So a running muscle uses more oxygen and makes more carbon dioxide than a resting one.
The lungs answer by moving more air. Breathing rate and the size of each breath are both regulated when oxygen demand increases.
The heart answers too. Aerobic exercise is any activity that uses the large muscles in your body and gets your heart beating faster.
If the brain, muscles or other organs do not get enough oxygen, a feeling of breathlessness can occur. That is the puff after a race.
Breathing is both a voluntary and an involuntary event. A breathing center in the brain regulates how often you breathe.
It responds to signals about the carbon dioxide content in the body.
You can override this automatic control for a while, for speaking, singing or swimming under water.
That is why the talk test works. During moderate activity you can talk but not sing. During vigorous activity you can say only a few words before pausing for a breath.
So a third system, the nervous system, joins the lungs and the heart. Three systems, one job.
| Week review | True or false? |
|---|---|
| The lungs bring in oxygen and remove carbon dioxide. | ? |
| Gas exchange happens only in the alveoli. | ? |
| The heart pumps oxygen-rich fluid to the organs and muscles. | ? |
| Breathing is completely under your control at all times. | ? |
| Working muscles use less oxygen than resting muscles. | ? |
Rocket noticed his muscles asked for oxygen before he thought about it. Something sensed the need and acted.
Next week you will study how the body senses the world, sends signals to the brain, and acts or remembers.
Strong week, scientist. Tomorrow is Nature Day: a family walk with the talk test and a breath count.