What is physiology?

The Fundamentals of Physiology

Physiology is a branch of science that applies to all living things. The goal of the physiologist is to understand the mechanisms leading to the proper functioning of organisms such as bacteria, plants, animals, and humans. Physiology is an important aspect of many of the medical sciences. In immunology, researchers seek to understand the functioning of the immune system. Cardiovascular scientists study the workings of the heart. Knowledge of the normal physiologic functioning of an organism is important in identifying the diseases that cause a deviation from the normal state.

Essential to the discovery of new data is the application of the scientific method of research. The first step in this method is observation. This involves examining a particular system or organism of interest, such as the transport of nutrients in a plant stem or the flow of air into the lungs of an animal, and initially observing a specific event or phenomenon. Asking why and how the event occurs leads to the next step—forming a hypothesis, or scientific question. A hypothesis is a possible explanation of the observation, which can be tested further to see if it is true. Testing, or experimentation, is the third step in the scientific method. The experiment involves setting up a controlled situation that directly tests the hypothesis in order to determine the cause-and-effect relationship between the hypothesis and the initial observation. As the experiment is conducted, the investigator makes further observations of the system’s functioning and collects these findings as data. Following data collection and analysis comes the fourth step, making conclusions. The conclusion may or
may not support the researcher’s original hypothesis. To confirm the results further, more testing is often done. If, upon additional testing, similar conclusions are reached, the researcher may move on to the final step of the scientific method: publication. The experimental design, results, and conclusions are written down in a paper that is then submitted to a scientific journal. Publication permits other researchers to be informed of and evaluate the experiment and possibly to use the results to further their own research endeavors.

Applying this scientific method to the human model, it becomes apparent that the physiological conditions of the human body are in a state of dynamic equilibrium. While variables such as pH, ion and nutrient concentrations, and water content are constantly changing, they never differ significantly from their optimum level, unless the body is in a state of disease. Sensors, an integrating center, and effectors are important contributors to the regulation of these variables. Sensors are located throughout the body and are designed to monitor specific physiologic conditions. For example, baroreceptors monitor arterial blood pressure, chemoreceptors monitor the concentrations of hydrogen ions and carbon dioxide in the extracellular fluid, and thermoreceptors monitor body temperature. The conditions of the body are then transmitted via nerve impulses to a particular integrating center located in the brain, spinal cord, or endocrine glands. The integrating center interprets the nerve impulses and determines if a response is required. To respond to a particular condition, the
integrating center activates certain effectors. An effector is usually a muscle or a gland that, when activated, performs a specific function to return the body to normal physiologic conditions.

Extracellular fluid is also important in the maintenance of the above-mentioned variables. Two basic types of fluid compose about 60 percent of the human body. Two-thirds of the body’s fluid is intracellular fluid (cytoplasm) and is found inside cells. The other one-third of the fluid is extracellular fluid and is located in the spaces between cells (interstitial fluid), in blood vessels (plasma), and in the lymphatic vessels (lymph). This fluid is constantly circulated throughout the human body in the blood and lymphatic vessels. The ions, gases, and nutrients in the fluid can easily diffuse out of the capillaries and into the adjacent cells, where they can be utilized in normal cell activities. Because the estimated one hundred trillion cells of the body are exposed to basically the same composition of extracellular fluid, it has been termed the body’s internal environment.

The extracellular fluid is the medium of exchange of biologically important molecules and materials. Their concentration and availability is under the control of various organs and systems.
For example, the kidneys play an important role in regulating the water content, ion and waste concentrations, and acid-base balance of the extracellular fluid. The gastrointestinal tract is responsible for digesting and making food available for absorption into the bloodstream. The liver maintains the normal glucose concentration in the blood through processes that store glucose, remove glucose from storage, or produce new glucose from other nutrients. The lungs aspirate to provide adequate oxygen and carbon dioxide exchange in the blood. An important job of the physiologist is to discover the mechanisms used by these and other organs to perform their respective functions.

Homeostasis and Feedback Systems

Each of the aforementioned variables has an optimum physiologic range over which the body can function normally. The systems of the body work together in an attempt to keep the body’s internal environment within these normal ranges. This process is called homeostasis. Stress placed on the body—whether from the outside environment (heat, cold) or from within (disease, emotional reactions)—can lead to fluctuations in the internal environment. Significant deviations from the normal can lead to a state of disease in the body. To prevent these types of changes, the body has incorporated a number of control devices that are governed by the nervous and endocrine systems. The nervous system is constantly evaluating the state of the body and is able to detect when something is awry. When the body strays too far from its balanced condition, it responds in one of two ways. First, it may send nervous impulses to the proper organs that counteract the stress and return it toward its original state. Second, it may activate the endocrine system to release its chemical messengers or hormones that will bring the body back
into balance.

The nervous and endocrine systems are also important components in the feedback systems that regulate the body’s internal environment. The two basic types of feedback systems are called
negative feedback and positive feedback. Negative feedback is a homeostatic control mechanism that responds to a stress-related change by returning a condition to its normal range. For example, an increase in the blood glucose level induces specific steps that reduce the glucose to its normal level. Positive feedback is designed to amplify the response given to a certain stress. For example, during childbirth, uterine contractions intensify as a result of a positive feedback system, thus enabling the birth of a baby. The majority of the feedback systems in the human body are negative, since in most instances (with a few, specific exceptions) a positive system is detrimental to the body.

Negative feedback. Often, several systems work together to monitor and regulate a particular physiologic condition. An excellent example of this can be seen in the negative feedback regulation of the mean arterial blood pressure. Baroreceptors are the monitoring devices for arterial pressure. They are composed of nerve endings located in the arterial wall of specific blood vessels, such as in the arch of the aorta and the neck area where the common carotid artery divides into the internal and external carotid arteries. Baroreceptors respond to the stretching of the arterial wall. These arteries undergo greater-than-normal stretching during times of high blood pressure and are more relaxed in times of low blood pressure.

Baroreceptors are constantly informing the brain of the status of the mean arterial pressure by sending out nervous impulses to the cardiovascular control center, located in the medulla of the brain. When the body is subject to stress that causes the blood pressure to rise, the elastic arterial walls experience an increase in the amount that they are stretched. The baroreceptors, being stretch receptors, respond to this change by increasing their output of nervous impulses to the brain. In contrast, a drop in blood pressure decreases arterial wall stretching, causing the baroreceptors to send out fewer impulses to the brain.

The nervous impulses from the baroreceptors travel to the cardiovascular control center in the brain via afferent neurons. The cardiovascular control center is a network of neurons that receive and integrate nervous impulses from a variety of other control centers. The cardiovascular control center is connected to the heart by both sympathetic and parasympathetic nerves and to the blood vessels primarily by sympathetic nerves. An increase in the rate of impulses from the baroreceptors (in response to an increase in arterial pressure) results in an increase in the amount of parasympathetic stimulation and a decrease in sympathetic stimulation. On the contrary, a decrease in the rate of impulses from the baroreceptors (in response to a decrease in arterial pressure) results in a decrease in the amount of parasympathetic stimulation and an increase in sympathetic stimulation.

To understand the significance of the stimulation of parasympathetic-versus-sympathetic nervous stimulation, a brief comment must be made as to the organization of the nervous system. The
central nervous system (CNS) is made up of the brain, the brain stem, and the spinal cord. The
peripheral nervous system (PNS) comprises the nerves that go into and leave the brain stem and spinal cord. The peripheral nervous system is further divided into a somatic portion and an autonomic portion. The somatic nervous system’s sensory receptors transmit impulses to the CNS, which then sends impulses through motor neurons to the skeletal muscles. The
autonomic nervous system monitors the condition of the internal
organs via sensory nerves to the CNS and responds via motor nerves to glands and involuntary muscles.
The parasympathetic and sympathetic nerves are the two types of motor nerves that make up the autonomic nervous system. In general, sympathetic nerves initiate the body’s fight-or-flight response to stressful situations. Parasympathetic nerves are responsible for more vegetative functions, such as the ingestion and digestion of food.

The combinations of particular nerve activations and deactivations have particular effects on the state of the heart and peripheral blood vessels. As stated above, an increase in mean arterial pressure, as detected by the baroreceptors, invokes a response that increases parasympathetic stimulation of the heart and decreases sympathetic stimulation of the heart and blood vessels. This response serves to reduce the heart rate and the contractility of the heart, thus reducing cardiac output. In addition, blood vessels in the skin and muscles are allowed to dilate, reducing the total peripheral resistance of the blood. The direct relationship of the mean arterial pressure (MAP) to the cardiac output (CO) and total peripheral resistance (TPR) is represented by the formula “MAP = CO TPR.” It can be observed that decreasing both CO and TPR results in a decrease in the MAP. This occurs to the point at which the MAP returns to its normal range. If the applied stress decreases the MAP as detected by the baroreceptors, parasympathetic stimulation decreases and sympathetic stimulation increases. This causes an increase in CO and an increase in TPR, resulting in an overall increase in mean arterial pressure.

Positive feedback. An example of positive feedback regulation can be seen in the process of childbirth. Weak, periodic uterine contractions begin during the third trimester of a human pregnancy. As the pregnancy progresses and the time comes for delivery of the baby, these contractions become stronger and increase in frequency. Such contractions are necessary for the expulsion of the baby and the placenta from the mother.

Contractions of the uterine muscles begin at the top, or apex, of the uterus and move down toward its base, at the location of the cervix. These contractions force the head of the baby onto the cervix and stretch it open in a process termed cervical dilation. Cervical stretching sends nerve impulses to the hypothalamus in the brain, which then stimulates the posterior
pituitary gland to release the hormone oxytocin. Oxytocin makes the uterine contractions stronger and more rhythmic. Continued stretching of the cervix leads to an increase in the amount of oxytocin released by the pituitary. This positive feedback cycle continues until the baby is born, at which point the pressure on the cervix is removed and it can relax.

Perspective and Prospects

Physiology began more than two thousand years ago during the time of the ancient Greeks. The well-known philosopher Aristotle (384-322 b.c.e.) was also a physiologist who made biological observations and described the blood vessels as part of a system with the heart at its center. He also believed that the heart was a furnace that heated the blood and that it was the body’s seat of intellect. In the city of Alexandria, Herophilus (335-280 b.c.e.) believed that the seat of intellect was the brain, not the heart. He studied arteries and veins and determined that arteries have thicker walls. Erasistratus (310-240 b.c.e.) began his training under Herophilus. He believed that arteries served as air vessels and that the veins carried the blood, which was made in the liver from food. Several hundred years later, Galen (129-c. 199 c.e.) conducted an experiment that showed that blood, not air, flowed through arteries. Though some of his other ideas were later disproved, he left behind a considerable number of writings on physiology, medicine, and philosophy.

Galen’s ideas were taught for many hundreds of years until the Renaissance. During this time, physiologists made important discoveries and observations that challenged the findings of Galen. Andreas Vesalius (1514-1564) was trained as a doctor and became a professor of surgery and anatomy at a medical school in what is now Italy. Vesalius’s style of teaching was to dissect a cadaver as he lectured, and he included anatomical drawings in his written texts. He wrote what has become known as the first modern anatomy textbook, De Humanis Corporis Fabrica (the structure of the human body), published in 1543.

A few decades later, William Harvey (1578-1657) was born in England and later trained as a doctor. He also did some lecturing and wrote a study of circulation, De Motu Cordis et Sanguinis in Animalibus (on the motion of the heart and of blood in animals), published in 1628. Harvey viewed the heart as a pump that contracted to expel blood and discovered that the blood moved in a circular path in the body. He believed that the blood traveled from the right ventricle to the lungs and then to the left ventricle, not directly from the right to the left ventricle. He also theorized that air stayed in the lungs and did not move to the heart to meet the blood.

Joseph Priestley (1733-1804), an English chemist, investigated the processes of combustion and breathing. Priestley’s experiments used a bell jar, a candle, a green plant, and a mouse. He observed that a candle placed under a bell jar went out, but could later be lit after a green plant was placed under the jar for several days. He also observed that a mouse placed under the jar died after some time. When placed under the jar with a green plant, however, the mouse lived for a longer period of time.

Using an early microscope, Robert Hooke (1635-1703) studied a piece of cork and coined the term“cells” for he compartments that he observed. Two hundred years later, German biologists Matthias Schleiden(1804-1881) and Theodor Schwann (1810-1882) proposed their cell theory. Their theory encompassed some of the essential ideas in biology and physiology, including that all organisms are made of cells which have similar metabolic processes and chemical components, that an organism’s functions result from different cells working together, and that all cells have their origin in preexisting cells.

The invention of the thermometer allowed scientists to gain further insights into human physiology. With this investigation came the discovery that the human body maintains a relatively constant internal body temperature, despite changes in the temperature of its environment. The French physiologist Claude Bernard (1813-1878) used the words “milieu intérieur” to describe this constant internal environment of the human body. American physiologist Walter Cannon (1871-1945) later coined the term “homeostasis” to describe this condition.

Essential to investigations and discoveries that advance the field of physiology is the ever-changing nature of technology. Early research was conducted simply with observations of the unaided eye. Later discoveries developed glass lenses to form a light microscope which magnified images up to 1,000 times and opened up a world of tissues and cells that were previously indiscernible. Newer technology led to the formation of the electron microscope, which provided magnification of specimens greater than 100,000 times and revealed the intricacies of subcellular structures.

Technology has advanced to the growth of living cells and tissues in laboratory dishes. Cell culture allows the investigator to change or regulate the environment of the biological material and to determine the effect of such alterations on growth and reproduction. Techniques such as cytochemistry, autoradiography, and immunochemistry have been developed to stain or localize certain regulatory molecules or cellular structures, enabling the scientist to quantify their effects on cell physiology. Cell fractionation has been developed to separate cells into their various components or organelles, thus providing a way that a specific organelle can be studied in isolation. Genetic engineering is a powerful technology that allows the researcher to manipulate and alter the genes of cells that control their functions. All these techniques are frequently used in physiology research and are powerful forces that have expanded scientific understanding.

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