Info

Factor

Characteristic

Use

Factor

Characteristic

Use

Water

A chemical substance

For metabolic processes, as a medium for metabolic reactions, to transport substances, and to regulate body temperature

Oxygen Heat

A chemical substance A form of energy

To help release energy from food substances To help regulate the rates of metabolic reactions

Food

Various chemical substances

To supply energy and raw materials for the production of necessary substances and for the regulation of vital reactions

Pressure

A force

Atmospheric pressure for breathing; hydrostatic pressure to help circulate blood

Homeostasis

Some organisms exist as single cells, the smallest living units. Consider the amoeba, a simple, one-celled organism found in lakes and ponds (fig. 1.3). Despite its simple structure compared to a human, an amoeba has very specific requirements that must be met if it is to survive. As long as the outside world—its environment— supports its requirements, an amoeba flourishes. As environmental factors such as temperature, water composition, and food availability become unsatisfactory, the amoeba's survival may be threatened. Although the amoeba has a limited ability to move from one place to another, environmental changes are likely to affect the whole pond, and with no place else to go, the amoeba dies.

In contrast to the amoeba, we humans are composed of about 70 trillion cells that surround themselves with their own environment inside our bodies. Our cells interact in ways that keep this internal environment relatively constant, despite an ever-changing outside environment. The internal environment protects our cells (and us!) from changes in the outside world that would kill isolated cells such as the amoeba. The body's maintenance of a stable internal environment is called homeostasis, (hoSme-o-stacsis) and it is so important that most of our metabolic energy is spent on it. Many of the tests performed on Judith R. during her hospitalization (as described in the opening vignette) assessed her body's return to homeostasis.

To better understand this idea of maintaining a stable internal environment, imagine a room equipped with a furnace and an air conditioner. Suppose the room temperature is to remain near 20° C (68° F), so the thermostat is adjusted to a set point of 20° C. Because a thermostat is sensitive to temperature changes, it will signal the furnace to start and the air conditioner to stop whenever the room temperature drops below the set point. If the temperature rises above the set point, the thermostat will

Figure

The amoeba is an organism consisting of a single cell (50x micrograph enlarged to 100x).

Figure

The amoeba is an organism consisting of a single cell (50x micrograph enlarged to 100x).

cause the furnace to stop and the air conditioner to start. As a result, a relatively constant temperature will be maintained in the room (fig. 1.4).

A similar homeostatic mechanism regulates body temperature in humans (fig. 1.5). The "thermostat" is a temperature-sensitive region in a control center of the brain called the hypothalamus. In healthy persons, the set point of this body thermostat is at or near 37° C (98.6° F).

If a person is exposed to a cold environment and the body temperature begins to drop, the hypothalamus senses this change and triggers heat-conserving and heat-generating activities. For example, blood vessels in the skin constrict so that blood flow there is reduced and deeper tissues retain heat. At the same time, small groups of muscle cells may be stimulated to contract

Shier-Butler-Lewis: I I. Levels of Organization I 1. Introduction to Human I I © The McGraw-Hill

Human Anatomy and Anatomy and Physiology Companies, 2001

Physiology, Ninth Edition

Response

Room temperature 1

Thermostat detects change '

Response

Room temperature 1

Room Air Condition Anatomy

Normal room ^ temperature range

Room temperature i decreases

Heater turns off; air conditioner turns on

Heater turns off; air conditioner turns on

Normal room ^ temperature range

Return to normal

Thermostat set point

Thermostat set point

Room temperature i decreases

Heater turns on; air conditioner turns off

Room temperature returns toward set point

Room temperature returns toward set point _

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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