Raven makes a fist and holds it against her chest. "About this big," she says. "That is the size of a heart."
Rocket makes his own fist and squeezes it, open and shut, open and shut. "It is a pump."
"Two pumps, side by side," Raven says. "What do you notice when you listen?" She taps a steady lub-dub on the table.
Nova dims her light and plays a soft recording of a heartbeat. "What kind of tissue could squeeze like that all day?"
"Muscle," Rocket says at once. "But I cannot make my heart stop and start like my arm."
"Then it is not the same muscle as your arm," Raven says, and writes three kinds of muscle in the Log.
The heart is a complex muscle that consists of two pumps. One pumps toward the lungs. The other pumps to the rest of the body.
In humans, the heart is about the size of a clenched fist. It is divided into four chambers: two atria and two ventricles.
The main purpose of the heart is to pump in a repeating sequence called the cardiac cycle.
Each cycle has a contraction phase and a relaxation phase. Valves closing inside the heart make the "lub" and "dub" sounds.
The pumping is done by cardiac muscle cells. Cardiac muscle tissue is found only in the heart.
The body contains three types of muscle tissue: skeletal muscle, cardiac muscle and smooth muscle.
Skeletal muscle attaches to bones and controls movement you can consciously control. It is called voluntary muscle.
Smooth muscle is found in the walls of hollow organs such as the intestines and stomach. It is not under voluntary control.
Cardiac muscle pumps rhythmically and involuntarily, like smooth muscle. You cannot consciously make it stop and start.
That is why Rocket could squeeze his fist on purpose but not his heart.
| Muscle tissue | Where it is found | Can you control it on purpose? |
|---|---|---|
| Skeletal | Attached to bones, and sometimes skin | Yes (voluntary) |
| Smooth | Walls of hollow organs such as the intestines and stomach | No (involuntary) |
| Cardiac | Only in the heart | No (involuntary) |
Cardiac muscle cells are cells. Grouped together, they make cardiac muscle tissue.
That tissue, with other tissues, builds the heart, an organ. The heart and the vessels form the circulatory system.
The system is part of you, an organism. Every rung of the ladder is in one heartbeat.
| Statement | True or false? |
|---|---|
| Cardiac muscle can be controlled on purpose, like an arm muscle. | ? |
| Skeletal muscle attaches to bones. | ? |
| The heart has two atria and two ventricles. | ? |
| Smooth muscle is found only in the heart. | ? |
Strong work. Tomorrow is Windowsill Lab: you will count your pulse at rest and after twenty jumps.