1. Our body is composed of different organ systems, working together to maintain homeostasis and keep us alive.

  2. Hormones facilitate communication between the various organs. 

  3. Lifestyle factors such as lack of movement, poor food choices, stress, and insufficient sleep, act as stressors that disrupt efficient hormone signaling. 

Like a bustling city, our bodies are composed of interdependent systems that are responsible for essential parts of the body's operation. To work properly, these systems must communicate with one another and send resources to where they are needed most. There are a couple of ways that our bodies can achieve this level of cooperation.

Just like the phone networks in real cities, our nervous system allows for fast point-to-point communication between our brain and the various organs. Through the nervous system, the brain has direct, almost instantaneous, control over functions that are essential to life such as our breath and heartbeat. The nervous system utilizes electrical impulses to quickly (on the order of milliseconds) respond to the environment. While this type of centralized control is important, it is also useful to allow the various organ systems to communicate with one another without involving the brain. The body makes use of the endocrine and circulatory systems for this type of decentralized communication.

The endocrine system has glands spread throughout the body. These glands secrete chemical messengers called hormones into the circulatory system (bloodstream) to send messages to other parts of the body. Using the circulatory system for communication is analogous to sending bike or car messengers out on the roads of the city. The circulatory system is not point-to-point like the nervous system but is instead a shared resource, accessible to almost every cell in our body. As long as the cell has a door (receptor) for a specific signaling molecule (hormone), it can detect the concentration of that molecule in the bloodstream. While this method of communication between organs may not seem as fast or efficient as that of the nervous system, it allows for prolonged physiological responses on the target cells. Another major difference between the nervous and endocrine systems is that hormone messengers are regulated by chemical feedback loops. For example, when high blood glucose triggers the release of insulin in the beta cells of the pancreas, the insulin will act to lower the glucose levels in the bloodstream, and in response, the beta cells will gradually stop the production of insulin. This is just one of many types of feedback loops that help control our hormone levels.

There are many hormones in the human body. Melatonin is a hormone produced by the pineal gland in the brain. It promotes sleep and helps control our circadian rhythm. Epinephrine and norepinephrine are produced by the adrenal glands and control our blood pressure, heart rate, glycogenolysis (turning the storage form of glucose into usable glucose) and lipolysis (turning stored fat into free fatty acids). They also play an important role in the fight-or-flight response, exercise, and fasting. Triiodothyronine (T3) and Thyroxine (T4) are produced by the pituitary gland and affect our metabolic rate, body temperature, and heart rate.

Because hormone levels are controlled by long-acting feedback loops, they are powerfully affected by certain environmental factors and behaviors. For example, altering the sleep or wake-up time can lead to circadian rhythm disruption. Because our circadian rhythm affects the timing and release of other hormones, this can lead to downstream effects such as fatigue, weight gain, and mental health issues. The overabundance of processed, carbohydrate-rich foods in the Standard American Diet is another example of our environment affecting our hormones. By constantly elevating our blood sugar, we force our pancreas to release a higher than normal amount of insulin in order to absorb the excess sugar. After a while, the constant overproduction of insulin causes our cells to become less responsive to the insulin signal, a condition called insulin resistance. If this cycle continues, it can lead to Type 2 diabetes and other chronic diseases. Finally, every-day stress can stimulate our fight-or-flight response and flood our bodies with adrenaline and cortisol. This can be healthy in moderation, but if the stress is not addressed, it can lead to anxiety, depression, weight gain, and other chronic issues.

While hormones play an essential function in the body, environmental factors and certain behaviors can cause these well-tuned systems to malfunction. In the next section, we will focus on hormones that affect our metabolism and energy levels - leptin and insulin - and will discuss situations that may cause them to misbehave.

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