Overview of Physiology. Homeostasis

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Clyde Edgar Griffith
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1 Overview of Physiology Homeostasis

2 Life s Challenges Nutrients Oxygen Water Temperature regulation Atmospheric pressure

3 Life s Challenges

4 Homeostasis Responding to life s challenges (Maintaining a stable internal environment)

5 Homeostasis

6 Homeostasis Negative feedback loop: Reverses an internal change Positive feedback loop: Enhances an internal change

7 Homeostasis

8 Overview of Physiology Homeostasis 1

9 Life s Challenges Nutrients Oxygen Water Temperature regulation Atmospheric pressure 2 There are several challenges that organisms have to meet in order to survive. Five of the most important include: Nutrients: This includes carbohydrates, lipids, proteins, vitamins and minerals that organisms have to get in their food. Since these nutrients are used up by life s processes, organisms need to constantly absorb more nutrients and have a way of storing excess nutrients so that they are available even when food is not. Too little or too many of most nutrients disrupts life s processes. The digestive system is primarily responsible for absorption of nutrients, while the cardiovascular system is responsible for transporting the nutrients to all of our tissues and the urinary system is responsible for removing waste products after nutrients have been metabolized. Oxygen: Humans require oxygen in order to metabolize energy-rich nutrients like carbohydrates. Without an adequate supply of oxygen, not enough ATP can be made to power the chemical reactions necessary for life. The respiratory system is responsible for getting oxygen into the blood stream and the cardiovascular system carries oxygen to every cell in the body. Water: Water has many functions in the body. It dilutes toxic substances in the blood and urine, it contributes to blood volume and blood pressure, it provides a medium for diffusion of molecules, etc. Too much or too little water can be harmful, so humans have ways of carefully regulating the body s water balance. The digestive system is responsible for water absorption while the urinary system plays a major role in regulating the amount of water in the body at any given time. Temperature regulation: All of the enzymes in the human body work optimally at normal body temperatures (around 37 degrees Fahrenheit). If the temperature gets much lower than that, chemical reactions occur too slowly to support life. If the temperature gets too high, chemical reactions may occur to fast. If the temperature gets very high, proteins in the human body may denature (change shape and stop working altogether). The muscular system generates most of our body heat while the integumentary system (through sweating) and cardiovascular system (by routing blood flow deeper or more superficial) regulate body temperature. Atmospheric pressure: Ventilation (breathing) requires the ability to change the atmospheric pressure in the lungs and thoracic cavity. Without this ability, we would be unable to move air into and out of the lungs. The muscular and skeletal system work together to regulate thoracic pressure by changing the size of the thoracic cavity.

10 Life s Challenges Click to add an outline 3 It is important to understand that although we well be talking about organ systems as separate units, they always interact together. If one system becomes damaged, all the other systems are affected as well. Metabolism is a good example of this interaction: food is absorbed through the digestive system while oxygen is brought into the blood via the respiratory system. The cardiovascular system carries the nutrients and oxygen to every cell in the body, where metabolism occurs. Metabolism creates waste products (such as carbon dioxide and nitrogenous wastes), though, so the urinary system removes these byproducts from the cardiovascular system. If any of these systems suffers damage or disease, metabolism cannot occur normally.

11 Responding to life s challenges (Maintaining a stable internal environment) Homeostasis 4 The standard definition for homeostasis is maintaining a stable internal environment, but a simpler way of putting it is that homeostasis is organs responding to life s challenges. Some conditions that we need to keep in homeostasis include temperature, water/electrolyte balance, nutrient levels, blood pressure, blood ph, hormone levels, etc. It is important to realize that homeostasis is not something that starts only when the body gets out of whack, then stops when things return to normal. Homeostasis is always making minor corrections to make sure the internal conditions remain the same. For example, if body temperature starts to drop, muscle activity may increase slightly, which starts to raise the body temperature. Eventually, though, the body temperature gets a little above its set point, so other mechanisms (reduced muscle activity, sweating, blood flow changes) start to reduce body temperature. This change triggers a response that increases body temperature again, etc. It s an ongoing, neverending cycle. Major point: Every organ and every organ system in the body are involved in some way in keeping the body in homeostasis.

12 Homeostasis Click to add an outline 5 The basic mechanisms that keeps the body in homeostasis are called feedback loops. A feedback loop has three main structures: Receptors constantly monitor specific internal body conditions (temperature, blood ph, blood pressure, sodium levels, etc.) They send information about these changing conditions to the Control centers (almost always located in the brain), which decide what to do about the changing conditions and then send messages to Effectors, which trigger a response usually designed to reverse a move away from homeostasis. Effectors, by the way, include muscles and glands. Here s an example with blood pressure. Receptors in some of the body s largest arteries constantly monitor blood pressure and send this information to a control center in the medulla oblongata of the brain. When the control center sees that blood pressure has dropped, it sends signals out to the heart muscles telling them work harder as well as to certain endocrine glands which release hormones that cause the body to retain more water.

13 Homeostasis Negative feedback loop: Reverses an internal change Positive feedback loop: Enhances an internal change 6 There are two different types of feedback loops. One maintains homeostasis and one does not. Negative feedback loops maintain homeostasis. When an internal change is detected, the response is to reverse the change. For example, when body temperature drops, the response is to try to bring the body temperature back up. Positive feedback loops do not maintain homeostasis. In fact, they work against it! When an internal change is detected, the response is to make the change worse. For example, an increase in hormone production might cause even more hormone to be released. Positive feedback loops are usually the result of damage or disease for example, a big drop in blood pressure can damage the heart, which results in an even bigger drop in blood pressure. There are a few instances when positive feedback loops are important (but not homeostatic), though. For example, a positive feedback loop ensures that once childbirth begins, labor contractions do not stop until the child is born.

14 Homeostasis Click to add an outline 7 This figure shows an example of a negative feedback loop involving a particular hormone. When the hypothalamus (a part of the brain) detects a drop in the level of thyroid hormone, it sends signals that ultimately cause more thyroid hormone to be released. When the hypothalamus detects the rise in thyroid hormone levels, it is inhibited and stops stimulating the release of more thyroid hormone.

The Human Body: An Orientation

1 The Human Body: An Orientation Yong Jeong, MD, PhD Department of Bio and Brain Engineering The Human Body An Orientation Anatomy Study of the structure and shape of the body and its parts Physiology