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In this lesson, we'll look at how the body uses hormones to maintain homeostasis of blood glucose levels, what happens in people who have diabetes, and how diabetics can manage their disease.
According to the Centers for Disease Control and Prevention, there are almost 26 million people in the United States alone that have diabetes, which is 8.3% of the total U.S. population. With so many Americans suffering from diabetes, how do we treat all of them? Do all of these people now need insulin shots, or are there other ways to treat, or prevent, diabetes? In order to answer these questions, we must first understand the fundamentals of blood glucose regulation.
As you may remember, homeostasis is the maintenance of a stable internal environment within an organism, and maintaining a stable internal environment in a human means having to carefully regulate many parameters including glucose levels in the blood. There are two major ways that signals are sent throughout the body. The first is through nerves of the nervous system. Signals are sent as nerve impulses that travel through nerve cells, called neurons. These impulses are sent to other neurons, or specific target cells at a specific location of the body that the neuron extends to. Most of the signals that the human body uses to regulate body temperature are sent through the nervous system.
The second way that signals can be sent throughout the body is through the circulatory system. These signals are transmitted by specific molecules called hormones, which are signaling molecules that travel through the circulatory system. In this lesson, we'll take a look at how the human body maintains blood glucose levels through the use of hormone signaling.
Glucose is the main source of fuel for the cells in our bodies, but it's too big to simply diffuse into the cells by itself. Instead, it needs to be transported into the cells. Insulin is a hormone produced by the pancreas that facilitates glucose transport into cells. By facilitating glucose transport into cells from the bloodstream, insulin lowers blood glucose levels. It also inhibits glucose production from amino acids, fatty acids and glycogen, which, you may remember, is a carbohydrate composed of many glucose subunits.
In fact, insulin actually stimulates glycogen formation from glucose. All of these functions of insulin help to lower glucose levels in the blood. But insulin isn't the only hormone that regulates glucose levels in the blood. Glucagon is a hormone produced by the pancreas that raises blood glucose levels by stimulating the breakdown of glycogen into glucose, stimulating glucose production from amino acids and fatty acids, and stimulating the release of glucose from the liver. Glucagon and insulin have antagonistic effects, with glucagon promoting glucose production and release into the bloodstream, and insulin promoting the transport of glucose into cells from the bloodstream and inhibiting glucose production.
Glucose levels in the blood are usually measured in terms of milligrams per deciliter (mg/dl), with a normal range of 70 to 110 mg/dl. Generally speaking, if glucose levels stray out of this range, the amounts of insulin and glucagon produced by the pancreas will be adjusted to bring glucose levels back into this range. It should be noted here that insulin and glucagon signaling are not all-or-nothing responses in normal individuals. When the system is functioning properly, there is always some insulin and some glucagon being produced by the pancreas that is trying to find a balance between glucose release into the blood, and glucose uptake into cells.
This chart shows the concentrations of glucose, insulin, and glucagon in the blood, both before and after a meal that is high in carbohydrates. You may remember that carbohydrates are broken down into monosaccharides, like glucose, before they are absorbed into the bloodstream by the small intestine. In this graph, the black line is glucose, the blue line is insulin, and the red line is glucagon.
Before the meal, glucagon levels represented by the red line are high, because no glucose is being absorbed by the small intestine, so it must be released by the liver, and stimulating glucose synthesis and release from the liver are glucagon's main functions. However, very soon after a high-carbohydrate meal, glucose levels in the blood spike higher, and as you can see in the graph here, insulin levels spike higher, too. The insulin spike is in response to the higher glucose levels, but it happens so quickly that the two spikes happen almost simultaneously. In fact, you can see by this graph that insulin levels rise whenever glucose levels rise and fall whenever glucose levels fall.
Glucagon is a different story, though. Notice that at the 1.5 hour time point on the graph, when the glucose levels reach the high end of the normal glucose range of 70-110 mg/dl, glucagon levels start to fall, and they continue to fall until glucose levels return back to the normal range, at which point the glucagon concentration levels off and then slowly begins to rise again as glucose levels drift back down to where they were before the meal was consumed.
Diabetes is a disease where people have trouble regulating their blood glucose. Actually, there are two main forms of diabetes: type I diabetes, and type II diabetes. Type I diabetes is also known as juvenile-onset or insulin-dependent diabetes, and is actually a disease where the immune system suddenly attacks and irreversibly destroys the insulin-producing cells of the pancreas. This chart shows what glucose and glucagon levels might look like in a person who has no insulin. Notice that when glucose levels rise, there is no insulin response, and glucose levels remain dangerously high for a long period of time.
People with type I diabetes cannot produce insulin, and must receive frequent insulin injections. They also must carefully regulate when and what they eat, and monitor their blood glucose levels closely. If they do not, their blood glucose levels could easily fall too low or rise too high. In either case, the person could then suffer a diabetic reaction and become confused or behave irrationally. If left untreated, the person could become comatose and die.
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Type II diabetes is also known as adult-onset or non-insulin-dependent diabetes, and usually occurs when a person's cells become resistant to the effects of insulin. Type II diabetes develops over a long period of time, and accounts for over 90% of all diabetes cases in the US. There is no single known cause of type II diabetes. Instead, there are several well-documented risk factors, including genetics, obesity, high blood pressure, high cholesterol levels, and a sedentary lifestyle. The good news for type II diabetics is that exercise increases cell sensitivity to insulin. Because of this, type II diabetes can often be managed through diet and exercise, and sometimes, it can even be reversed if some of the underlying risk factors are eliminated.
So, to answer the question at the beginning of this lesson, most people with diabetes do not need insulin shots, but all diabetes should carefully monitor what they eat. And the best treatment for type II diabetes is a healthy diet, and exercise.
So in review, hormones are signaling molecules that travel through the circulatory system. There are lots of different hormones in the human body that serve lots of different functions, but some are used to maintain homeostasis of various physiological parameters, like blood glucose levels.
Blood glucose levels are regulated by two opposing hormones: insulin and glucagon. Insulin is a hormone produced by the pancreas that facilitates glucose transport into cells. Insulin also inhibits glucose production from amino acids, fatty acids, and glycogen. All of these functions of insulin help to lower glucose levels in the blood. Opposing these blood glucose-lowering actions is glucagon, which is a hormone produced by the pancreas that raises blood glucose levels by stimulating the breakdown of glycogen into glucose, stimulating glucose production from amino acids and fatty acids, and stimulating the release of glucose from the liver.
Diabetes is a disease where people have trouble regulating their blood glucose. When diabetes is caused by an absence of insulin-producing cells, the person cannot make insulin, and therefore must have regular insulin injections to regulate glucose levels and make sure that their cells can import glucose from the blood. This type of diabetes is called type I, or insulin-dependent diabetes.
However, most cases of diabetes in the United States aren't caused by a lack of insulin. Instead, these cases are a result of cells becoming insulin-resistant, meaning that they do not respond to normal concentrations of insulin. These people have type II, or non-insulin-dependent diabetes, and they also have difficulty lowering blood glucose levels. But they don't need insulin. Instead, these people must increase the sensitivity of their cells to insulin through diet and exercise. The good news for type II diabetics is that with a healthy diet and exercise, many cases of type II diabetes are reversible.
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