And that's how we get to regulate our temperature. So let's talk about it here. When it's hot, our smooth muscle then is going to relax. And in doing so, the arterioles then are going to become wider or larger. And this process is called vasodilation. Your arterioles are dilating. They're becoming wider. And so the way you could see that happen is that this blood vessel that was about yay big, is now going to become that big in the skin.
And so that means that you're going to have a lot more blood flow going in here to the skin. You're going to have tons of red and white blood vessels and protein just kind of merrily tumbling about, going like this.
That means you're going to have a ton more heat or energy that's present here, that can then dissipate and leave. And so that helps you cool off. You're losing all of this energy or this heat that you have by diverting blood flow to your skin. Now, what does skeletal muscle do here? They don't do anything when it's hot. They'll respond in a second-- and we'll talk about that-- when it gets cold. What do smooth muscle do when we're in the cold?
Well, you can imagine that if we relaxed when it was hot, it's fair to think that we're going to contract when it's cold. And so this process then in the arterioles is going to be called vasoconstriction, which just means the narrowing of our arterioles. And so if I were to draw that on a diagram right here, you would see that this blood vessel, that looked like this in our normal state, is now going to turn into something more like that, kind of a pipsqueak, really tiny.
So you're going to have red blood cells, white blood cells, and protein just kind of shifting through here like this. But there's not a lot of opportunity to kind of tumble around and be naughty. They just got to go forward. They can only go in one direction because there's not a lot of space here for blood flow. And so in the skin, you're going to have less energy or less heat that's present at this very external or superficial part of your body.
The more blood that you have that gets away from the skin, the more that's going to go towards your core. Your core will then have more energy or more heat to help you stay warm when it's cold outside. So that's what your smooth muscle does.
What do skeletal muscle do? Well, skeletal muscle will also contract when it's cold. But there's a very different purpose for why that happens. When skeletal muscle contracts, it's going to take ATP, or adenosine tri-- or three-- phosphate and break that to make adenosine diphosphate, ADP. So that's two phosphates. And we just snapped off a little phosphate group.
I'll write an "I" here to show that it just snapped off. But these aren't the only two things that are made. You actually will produce energy as well. And we call this reaction an exothermic reaction, "exo" meaning exiting or the leaving of. And so we're actually producing energy here by contracting our skeletal muscle. This happens in our core muscle groups that cause more of the heat that's being produced from this reaction to be stored there, to help us respond to these cold environments.
In addition to movement, muscle contraction also fulfills some other important functions in the body, such as posture, joint stability, and heat production. Posture, such as sitting and standing, is maintained as a result of muscle contraction. The skeletal muscles are continually making fine adjustments that hold the body in stationary positions.
The tendons of many muscles extend over joints and in this way contribute to joint stability. The decline in muscle mass causes a loss of strength, including strength required for posture and mobility. This may be caused by a reduction in the proportion of FG fibers that hydrolyse ATP quickly to produce short, powerful contractions.
Muscles in older people sometimes possess greater numbers of SO fibers, which are responsible for longer contractions and do not produce powerful movements.
There may also be a reduction in motor units, resulting in fewer fibers being stimulated and less muscle tension being produced. Some athletes attempt to boost their performance by using various agents that may enhance muscle performance.
Anabolic steroids are one of the more widely known agents used to boost muscle mass and increase power output. Anabolic steroids are a form of testosterone, a male sex hormone that stimulates muscle formation, leading to increased muscle mass. They have been used by athletes in many sports, but sprinting is one sport in which the effects of steroids are readily apparent. Because a meter dash can last less than 10 seconds, incredible amounts of power need to be created by the muscles.
Increasing the muscle mass increases the numbers of actin and myosin cross-bridges, increasing the power that can be produced by a muscle, which provides a competitive advantage in a sport measured in hundredths of seconds. Similarly, creatine has become a substance used by some athletes to increase power output.
Because creatine phosphate provides quick bursts of ATP to muscles in the initial stages of contraction, increasing the amount available to cells is thought to produce more ATP and therefore increase explosive power output.
However, both creatine and steroids are banned in sports and they can be extremely harmful to other systems of the body as well as to long-term muscle health. Slow fibers are predominantly used in endurance exercises that require little force but involve numerous repetitions. The aerobic metabolism used by slow-twitch fibers allows them to maintain contractions over long periods. Endurance training modifies these slow fibers to make them even more efficient by producing more mitochondria to enable more aerobic metabolism and more ATP production.
Endurance exercise can also increase the amount of myoglobin in a cell, as increased aerobic respiration increases the need for oxygen. Myoglobin is found in the sarcoplasm and stores oxygen. Endurance training can also trigger the formation of more extensive capillary networks around the fiber, a process called angiogenesis, to supply oxygen and remove metabolic waste.
To allow these capillary networks to supply the deep portions of the muscle, muscle mass does not greatly increase, maintaining a shorter distance for the diffusion of nutrients and gases. All of these cellular changes result in the ability to sustain low levels of muscle contraction for greater periods of time without fatiguing.
Endurance athletes also engage in drug use, but instead of trying to add muscle mass or produce power, they focus on substances that increase muscle endurance and reduce fatigue. This includes trying to boost the availability of oxygen to muscles to increase aerobic respiration by using substances such as erythropoietin, or EPO.
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