[00:00.00] Every minute of every day, blood flows through every corner of your body. [00:04.28]The flow of blood brings oxygen and other nutrients to your tissues. [00:08.58]Then it loads up the waste products of your metabolism and carries them away. [00:13.58]This system of nutrient exchange is what keeps your organs and tissues alive. [00:17.85]It’s made possible only because of the steady beating of your heart—the driving force of your circulatory system.
[00:25.17]How can your heart keep beating and beating, never stopping, and rarely getting out of sync?[00:31.68] It’s possible because of the specialized type of muscle—cardiac muscle—that makes up the heart tissue, and also because of the action of specialized electrical pathways that run through the heart’s walls.
[00:44.11]Before I go into cardiac muscle, let’s back up a bit. [00:48.79]You may recall from last time that the action of a muscle is always to contract. [00:54.09]Muscles can extend only passively. [00:56.71]They extend as they relax between contractions. [01:00.30]You’ll also recall that the human body has different types of muscles. [01:04.78]For example, skeletal muscle controls body movements, and cardiac muscle controls the body’s blood flow. [01:11.88]Skeletal muscle is found throughout the body, but cardiac muscle is found only in one place—the heart.
[01:18.38]Skeletal muscles get their name from the fact that their movement is based on the contraction of muscles working against the skeleton. [01:25.94] Skeletal muscles are attached to the bones, and they move the bones by contracting.
[01:31.95]A skeletal muscle consists of a bundle of long fibers running the length of the muscle. [01:39.01]The bundles of fibers are arranged in a regular pattern—a repeating pattern of light and dark bands. [01:46.07]This pattern is called striation, and skeletal muscle is also known as striated muscle. [01:52.18]When a muscle contracts, the length of each bundle is reduced. [01:57.10]A skeletal muscle contracts only when it’s stimulated by a motor neuron.
[02:01.74]Cardiac muscle, on the other hand, contracts rhythmically all on its own. [02:06.36]Cardiac muscle cells can generate impulses on their own, without any input from the nervous system. [02:12.58]They can do this because of complex chemical and electrical changes involving sodium, calcium, and potassium, each of which carries an electrical charge.
[02:22.34]Like skeletal muscle, cardiac muscle is striated. [02:26.46]However, the membranes between cardiac muscle cells contain specialized regions that provide direct electrical contact among cells. [02:35.45]What this means is, an electrical impulse generated in one part of the heart will spread to all the cardiac muscle cells. [02:43.24]Thus, the whole heart will contract. [02:46.07]Cardiac muscle cells can generate impulses without any input from the nervous system. [02:51.50]In contrast to this, skeletal muscles don’t contract unless the nervous system tells them to.
[02:57.34]A single cardiac muscle cell will “beat” even when it’s isolated from the heart and placed in cell culture in the laboratory. [03:05.63]If you removed a little piece of muscle from your heart and put it in a dish, you would see it continue to contract. [03:12.94]That’s because cardiac muscle cells have something called an action potential. [03:18.04]Electrically, they’re set up so, so that sodium and calcium—positive ions—change places with potassium— a negative ion—across the cell membrane. [03:30.76]This happens automatically, over and over again. [03:34.03]When sodium, calcium, and potassium ions move into or out of a cardiac cell, it causes the muscle to contract.
[03:42.10]What we think of as a heartbeat is really a series of contractions in the cardiac muscle. [03:47.87]When the heart’s walls contract, they force blood out of the chambers and into the next chamber or the large blood vessels that lead to the rest of your body. [03:57.08]Every day, your heart contracts 100,000 times or more, pumping more than 2,000 gallons of blood.
[04:04.12]But, unlike the skeletal muscles in your body, your cardiac muscle can never stop and take a rest. [04:10.10] If your heart stops beating, you have very little time—only a matter of minutes—for it to start up again before your other body systems are permanently damaged or stop functioning altogether.
托福14天冲刺微信战团