and all of the cell’s functions. The human body cannot produce all of the required nutrients organically, and therefore, must ingest nutrition from other sources for this purpose.
All living organisms grow and develop.3 Each cell undergoes a cell cycle, where it grows and divides to form another indistinguishable, duplicate cell. Following specific instructions from the organism’s DNA, or genetic code, differentiation transforms an organism’s cells into different types, making a more complex organism. Development is the growth, maturation, and transformation of an organism.
All living organisms reproduce. Reproduction is essential to the prolongation of a species existence. In sexual reproduction, there is a joining of the DNA of two organisms at the cellular level. Sexual reproduction is the form in which the human species reproduce.
All living organisms must possess a normal and stable internal environment; this process is called homeostasis.4 The internal environment is a matter of physiologic components that include temperature, water regulation, pH balance, heartbeat, sleep, energy, and blood pressure, as well as other conditions. Homeostasis is preserved through a complex system of checks and balances in human beings which is discussed further.
Homeostasis
Homeostasis is defined as maintaining a normal, stable, internal environment. The human body directs a multitude of highly complex interactions in order to sustain balance or to return its operating organ systems to their normal, standard 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,
