Clinical Applications of Human Anatomy and Physiology for Healthcare Professionals. Jassin M. Jouria

level of functioning. These complex interactions facilitate compensatory changes accommodating the physical and psychological functioning needed for survival.

All homeostatic mechanisms have at least three separate but codependent modules for regulating and controlling the variables involved in their respective homeostatic processes.5

      The “receptor” is the sensing module that oversees and reacts to changes in the internal environment. When the receptor senses a stimulus, it responds by sending the appropriate information to a second module, the “control center” (the brain in humans).

      The control center sets the scope of which a variable is maintained and regulates an appropriate response to the stimulus, which signals the third module, an “effector”, to correct the abnormality by either augmenting it with positive feedback or diminishing it through negative feedback.

      Figure 1-2 Homeostasis.

      For example, when body temperature rises due to external environment, the nervous system triggers blood vessels to dilate and sweat glands to secrete. Under comfortable condition, sweat glands might secrete half a liter of sweat daily. On a hot day, sweat glands can secrete as much as 12 liters per day. It’s the evaporation of sweat from the surface of the skin that cools the body through dissipation of body heat.

      Mechanisms involved in homeostatic controls, such as body temperature controls, are known as positive or negative feedback.

      Positive feedback

      Positive feedback mechanisms are intended to promote or enhance the body’s response to a stimulus that has already been activated. Contrary to negative feedback mechanisms, which initiate a response to return physiological functions within the body’s set and normal range, the positive feedback mechanisms are actually designed to force and keep physiologic functions out of normal ranges.

      Figure 1-3 Wound healing.

      In order to accomplish this, a sequence of events triggers a respective physiological process through a cascading progression that enhances the effect of the stimulus.

      One example of a positive feedback loop mechanism that can be observed in the body is platelet accumulation during a blood clotting episode in response to a break or cut in the lining of blood vessels.

      Another example is the release of the hormone oxytocin, which triggers uterine contractions in a female in order to promote the delivery of a newborn that takes place during childbirth. Oxytocin also influences breast milk secretions.

      Negative feedback

Negative feedback mechanisms exist and operate to decrease activity of any organ or organ system in an effort to revert it to its normal range of functioning.6 A great example and common method of doing this is the regulation of blood pressure. Stretch receptors in blood vessels can sense an increase in the resistance of blood flow against the walls during a period of increased blood pressure.

      The blood vessels, acting as receptors, receive the increased pressure as a stimulus and signal this message to the brain, the control center. The brain then transmits a message to the heart and blood vessels, both of which then respond as effectors. The heart rate decreases and vasodilation (expansion in blood vessel diameter) occurs. The combination of this physiologic response would cause the blood pressure to decrease and return within its normal range.

      The opposite occurs during a sudden decrease in blood pressure; blood vessels now sense the decrease in resistance and signal the brain, which relays the message back to the heart and blood vessels. The heart rate would increase, and vasoconstriction (narrowing of the blood vessels) will occur – ultimately raising blood pressure back to its normal physiological range.

      Another excellent example of the negative feedback loop mechanism is seen when the human body is deprived of nutrition. The body, in protective mode, will reset the metabolic rate to a point much lower than its norm. This approach allows the body to continue to function and complete all of its normal necessary physiological functions, albeit at a slower rate, even though the body is starving.

      This is why people who drastically reduce their caloric intake while trying to lose weight find it easy to lose the weight initially, but notice it becomes much harder to lose more weight after some time passes. This is due to the body readjusting itself to function at a lower metabolic set point in order to allow for survival than with a lower than normal supply of energy. Exercise increases the body’s caloric expenditure, and can alter this effect by exogenously increasing the metabolic demand.

      One simpler, yet effective example of the negative feedback mechanism is temperature regulation. The hypothalamus, which monitors the body’s temperature, is highly proficient at detecting even the slightest deviation of normal body temperature (37°C/98.6°F). The response to such deviation would be stimulation of sweat glands to produce sweat in an effort to reduce temperature by the cooling effect of evaporation, or signaling various muscles in the body to contract rapidly, or shiver, in an effort to produce heat and increase body temperature.

      Both feedback mechanisms are correspondingly essential for normal healthy functioning of the human body. Complications, disease, aging processes, and even death can occur if either of the two feedback mechanisms are distorted in any way.

       ■Organization of the Human Body

      The human body contains two major body cavities – the ventral cavity and the dorsal cavity.

These two cavities are further subdivided and structured to organize and classify the body’s internal organs.7

      Body cavities

      The dorsal cavity contains two sub-cavities, the cranial cavity and the spinal cavity. Technically speaking there is no anatomical division as the cranial cavity flows directly into the spinal cavity. However, they are separated into two distinct cavities to assist in study and analysis.

      Figure 1-4 Muscle, nerve, and blood vessel connections.

      The cranial cavity is a fluid-filled spaced inside the skull occupied by the brain. The spinal cavity encompasses the spinal cord, which travels down the posterior aspect of the body and is continuous with the cranial cavity (the spinal cord is attached directly to the brainstem), extending down toward the base of the spine.

      The ventral cavity also includes two sub-cavities, the thoracic cavity and the abdominopelvic cavity. These two cavities are separated anatomically by the diaphragm, with the thoracic cavity residing above the abdominopelvic cavity.

      The thoracic cavity holds the heart, lungs, thymus, lower one-third of the esophagus, and an irregularly shaped, central compartment called the mediastinum.

      The abdominopelvic cavity is further subdivided in some texts into the abdominal cavity and the pelvic cavity, with the caveat that there is no real anatomical partition separating the two cavities.

      The abdominal portion contains both the main and accessory organs of digestion (stomach, small and large intestines, liver, pancreas, and gallbladder), as well as the kidneys and ureters. The pelvic portion is surrounded and guarded by the pelvic girdle, and encloses mainly the reproductive organs and the bladder.

      Serous membranes

At this point, discussion of serous membranes, also called serosa, of the ventral cavity is appropriate. A membrane is a thin, flexible lining of tissues that secretes a lubricating fluid. Serous membranes form the lining that insulates each body cavity and all internal organs.8 The membrane itself is composed of two types of tissue – a superficial epithelial layer,