The Science of Life and Living
By Ron Hammond, PhD
Department Head, Physiology & Lab Safety
Carolina Biological Supply Company
What makes you get that chills-up-your-spine, goose bumps feeling when something frightens you? What keeps you from shriveling like a raisin from lost body fluid? Or what makes the sound heard through a stethoscope positioned over your heart?
Everybody is interested in our bodies’ construction and function. The life science classroom is an ideal environment for students to learn how to make healthy lifestyle choices. For example, the standard method for measuring blood pressure and the significance of the readings easily incorporate into a study of the circulatory system. Use an inexpensive stethoscope to hear the sounds of the heart valves closing and give tips for taking good care of the heart while teaching about the cardiac cycle. Provide Snellen eye charts and let students measure their own visual acuity while you help them understand the process and importance of clinical vision examinations.
The biology instructor also has opportunity to teach about evolutionary adaptations that enable us to survive as terrestrial homeotherms in a generally hostile environment. Consider our integument (skin), the largest of the human organs. Although we tend to regard it as a “bag” containing various body parts, this remarkable tissue performs functions essential to our survival.
Keeping body fluids balanced
Most obvious is the skin’s role as a covering or barrier between the environment and our sensitive underlying structures. The skin protects against uncontrolled body fluid loss to the terrestrial environment, keeping us from shriveling like raisins. This enables us to maintain the internal balance of fluid and electrolytes necessary for our many biochemical processes. At the same time, this puncture- and tear-resistant barrier restricts harmful microorganisms from unlimited access to our tissues.
Regulating body heat
Equally important, skin regulates the heat exchange—between the body and the environment—caused by blood flow through the capillary beds near the body surface combined with sweat glands flooding the skin surface with moisture to promote evaporative cooling. In a cold environment, superficial vascular beds constrict, reducing the amount of heat-laden blood that reaches the surface. This helps conserve heat within core tissues and often exhibits as a blanching of the skin. Simultaneously, sweat gland activity decreases, restricting the amount of heat lost to evaporation. When the environmental temperature becomes excessively warm, these processes work in reverse.
What makes a dog’s hair rise along its hackles?
Exposure to cold, shock, or fright activates the pilomotor reflex. The reflex occurs when sympathetic nerve impulses initiate contraction of the arrectores pilorum muscles attached to hair follicles. The hair shafts pull away from the skin surface, increasing surface insulation to conserve body heat. Bristling hair also makes an animal appear larger and more formidable in a defensive situation. A porcupine’s quills spreading and a dog’s back hair rising during unfriendly encounters are examples of this reflex. Modern humans, having sparse body hair, derive limited practical value from this ancestral reflex. (The spine-tingling goose bumps sensation that sometimes occurs when humans experience fright or cold is a pilomoter reflex.)
What’s under your skin?
The skin contains a vast network formed by sensory receptors located at varying depths beneath the skin surface. These include mechanoreceptors, sensing pressure and vibrations; nociceptors (free nerve endings), sensing pain; and thermoreceptors, responding specifically to heat and cold. Via an extensive network of sensory nerve fibers, the receptors convey sensory information to the brain for interpretation. An area of skin served by a single nerve fiber is a receptive field.
Distribution of the sensory receptors varies widely between regions of the body. The acuity with which we perceive a specific sensation depends upon the concentration of type-appropriate receptors in the stimulated area of skin. For example, fingertips require a more refined sense of touch than is needed at the shoulders, so touch receptor population is far denser in fingertips.
The two-point threshold
Receptive fields are smallest in areas of skin richly endowed with sensory receptors. Demonstrate this phenomenon in the classroom using the two-point discrimination test, a measure of the two-point threshold—the smallest distance between 2 points of stimulation that produce separate sensations of touch. For 2 distinct contact points to be perceived, a non-stimulated receptor must reside between 2 stimulated receptors. The Carolina™ Two-Point Discriminator is an excellent means of applying the touch stimuli. The instrument contains a built-in ruler for direct measurement of the two-point threshold.
The Carolina™ Cutaneous Sensations Kit features this activity and others that explore the sensory functions of skin. Use additional Carolina physiology products for introducing your students to other aspects of human physiology, including respiration, visual function, and the sense of smell.
Incorporating physiology activities into the life science curriculum heightens your students’ interest and enables them to relate subject material to their own lives. After all, biology truly is the science of life and living!