PRINCIPLES OF PHYSIOLOGY

1.2.1 Homeostasis

Homeostasis is the basic principle of animal physiology. Physiologists use the term homeostasis to mean the mainte­nance of nearly constant conditions in the internal environ­ment.

The preservation of equilibrium around a certain value of an element of the body or its cells is referred to as a set point, and it is the aim of homeostasis. The body’s systems often try to return to the established point, even though there are occasional variations from it. A receptor detects a change in the internal or external environment, which is referred to as a stimulus. The system’s response is to mod­ify its operations to bring the value back closer to the set point. For example, if the body temperature rises too high, modifications are made to cool the body. If blood glucose levels rise after a meal, adjustments are made to lower them and deliver the nutrition into the tissues that require it, or to store it for future use.

To maintain a stable internal environment for the body and cells, an animal must adapt when its surroundings change.

1.2.2 Feedback Loops

Animals must: (1) detect external conditions; (2) initiate compensating reactions as needed; and (3) keep vital parts insulated against unfavorable changes to preserve homeo­stasis. To maintain homeostasis, animals typically use a reflex control mechanism.

A stimulus is provided by a change in the internal or external environment. The stimulus is subsequently fol­lowed by a response. Animals use antagonistic controls to fine-tune physiological reactions.

Animals maintain their body temperature by controlling the generation and dissipation of heat. Numerous antago­nistic controls are mediated by hormones. The antagonistic regulators of glucose levels are glucagon and insulin.

1.2.2.1 Negative Feedback Loops

A negative feedback loop occurs when the stimulus’s strength is decreased by the reaction sending a signal back to it. For instance, the food that enters the stomach causes it to expand when animals eat. A negative feedback loop that lowers appetite is triggered by the shift in stomach volume and early-digestion products. This loop operates through the brain. A set point is a desired physiological state that is pro­tected in many physiological systems via feedback loops.

The body temperature of animals has a fixed point that is roughly 37°C. Animals’ bodies sweat to regulate body temperature when it rises, and they shiver to return to their set point when their body temperature drops. While the average animal body temperature is approximately 37°C, each animal’s precise body temperature set point differs and fluctuates during the day.

A negative feedback system consists of three basic com­ponents: a sensor, also called a receptor, a component of a feedback system that monitors a physiological value and reports it to the control center, which is the component in a feedback system that compares the value to the normal range and activates an effector if the value deviates too much from the set point. Negative feedback is a mecha­nism that reverses a deviation from the set point, therefore maintaining body parameters within their normal range. The maintenance of homeostasis by negative feedback occurs throughout the body at all times, so comprehension of negative feedback is essential to understanding human physiology. In a feedback system, an effector is the part that initiates a change to reverse the circumstance and return the value to the usual range.

A stimulus must drive a physiological parameter (i.e., go beyond homeostasis) beyond its typical range to initiate the system. A particular sensor “hears” this stimulus. For instance, in the regulation of blood glucose, the pancreas’s unique endocrine cells identify excess glucose, or the stim­ulus, in the blood. In response to the elevated blood glucose level, these pancreatic beta cells release the hormone insu­lin into the bloodstream. Insulin instructs the liver, adipo­cytes, and skeletal muscle fibers to absorb extra glucose and eliminate it from the bloodstream. Insulin release ceases when pancreatic alpha cells detect a decline in glucose con­centration in the bloodstream, which is a negative feedback loop. By doing this, blood sugar levels are kept from falling below the usual range.

Similar feedback mechanisms govern temperature in humans, whereby heat gain or loss is encouraged. The brain’s “heat-loss center” is a group of brain cells that is stimulated when the temperature regulation center receives information from the sensors showing that the body tem­perature is higher than normal. Three main outcomes result from this stimulation:

• The skin’s blood vessels start to widen, allowing more blood to come from the body’s core to the skin’s surface, radiating heat into the surrounding area.

• Sweat glands are activated to produce more sweat when blood flow to the skin increases. Heat is released into the surrounding air when sweat evaporates from the skin’s surface. An animal’s depth of respiration increases, and it may breathe through its mouth rather than its nasal passages. As a result, the lungs lose more heat.

On the other hand, blood flow to the skin is decreased when the brain’s heat-gain center is activated by cold, and blood returning from the limbs is redirected into a system of deep veins. This configuration limits heat loss and traps heat closer to the body’s center. Severe heat loss causes the brain to send more random messages to the skeletal muscles, which makes them contract and shiver. Shaking releases heat through the contraction of muscles while depleting ATP. Thyroid hormone is released by the thyroid gland in response to a brain signal. This hormone raises heat production and metabolic activity in all of the body’s cells. Additionally, the brain triggers the adrenal glands to release the hormone known as adrenaline, or epinephrine, which breaks down glycogen into glucose, which the body may utilize as fuel. The conversion of glucose from glyco­gen also raises metabolism and produces more heat.

The body’s water content is essential for healthy opera­tion. Without conscious effort on the part of the individual, the body maintains extremely strict control over hydration levels.

1.2.2.2 Positive Feedback Loops

Positive feedback loops regulate a few physiological sys­tems. Positive feedback loops increase the number of changes in the regulated variable as opposed to negative feedback, which minimizes those changes. For instance, the stomach’s muscles are generally controlled to contract and relax predictably to gently combine food. On the other hand, strong contractions caused by a positive feedback loop are activated upon detection of a toxin, pushing food back up the esophageal tube and causing vomiting.

Positive feedback loop pathways start gently but quickly gain intensity.

Rather than undoing a physiological state change, posi­tive feedback amplifies it. More change and a further depar­ture from the typical range are experienced by the system when there is a divergence from it. The body only responds positively when there is a clear end goal in sight. Positive feedback loops that are typical but only active when neces­sary include parturition and the body’s reaction to blood loss.

A circumstance when maintaining the current physical state is not desired is the birth of a child at full term. To evacuate the baby at the end of the pregnancy, the mother’s body must undergo enormous changes. Furthermore, once parturition has started, it must proceed quickly to its end to save both the mother’s and the child’s lives. Positive feed­back systems are responsible for the intense physical exer­tion involved in labor and delivery.

The fetus is forced towards the cervix, the lowest point of the uterus, by the stimulus of the first labor contractions. Stretch-sensitive nerve cells (the sensors) in the cervix track the amount of stretching. The pituitary gland at the base of the brain releases the hormone oxytocin into the blood­stream as a result of the messages these nerve cells transmit to the brain. Stronger contractions of the uterine smooth muscles, or effectors, triggered by oxytocin push the baby into the delivery canal. The cervix stretches considerably more as a result. Only when the baby is born does the cycle of stretching, oxytocin release, and progressively stronger contractions come to an end. At this moment, the cervix stops expanding, preventing the release of oxytocin.

Reversing severe bodily harm is the subject of a second illustration of positive feedback. Excessive bleeding is the most immediate danger after a penetrating wound. Reduced blood pressure and blood perfusion - the amount of blood that reaches the brain and other important organs - are the results of having less blood moving.

Because clotting proteins are strictly regulated in their availability, clotting is confined to a small region. This sequence of events is adaptive and life-saving.

1.2.2.3 Thermoregulation

Animals can be categorized into two groups: those whose body temperature remains constant despite changes in the surrounding temperature and those whose body tempera­ture is environment-dependent, meaning it fluctuates with changes in the surrounding temperature. Ectotherms are animals without an internal temperature regulation system. These species’ body temperatures are typically comparable to those of their surroundings, while some may take actions to maintain their body temperatures somewhat above or below that of their surroundings. This can involve tak­ing a daytime nap in the sun on a chilly day or digging a hole beneath the surface on a hot day. Although the phrase “cold-blooded” may not apply to an animal in the desert with a very warm body temperature, the ectotherms have been described as such.

An endotherm is an animal that maintains a fixed body temperature despite changes in its surroundings. Because these creatures produce internal heat that keeps their cellu­lar processes running smoothly even in cold environments, they can maintain an activity level that an ectothermic spe­cies cannot.

In many different ways, animals can dissipate or store heat. Endothermic animals have insulation of some kind. They might be bald, plump, or have feathers. Thick-furred or feathered animals produce an air cushion between their skin and internal organs. Even though they swim and live in a subfreezing climate, polar bears and seals can maintain a steady, warm body temperature. When it rolls up to sleep in chilly weather, the Arctic fox, for instance, uses its fluffy tail as additional insulation. By shivering, or convulsively contracting their muscles, mammals can generate more heat within their bodies. A person’s arrector pili muscles can also contract when they get chilly, which causes indi­vidual hairs to stand up. The insulating power of the hair is increased as a result. The reaction that is still present in humans results in “goosebumps” rather than the desired outcome on our comparatively hairless bodies. Animals also insulate with layers of fat. The capacity to retain heat will be hampered by a substantial reduction in body fat.

Circulatory systems are used by ectotherms and endo­therms to help regulate body temperature. Vasodilation, or the widening of arteries to the skin due to the relax­ation of their smooth muscles, increases the amount of blood and heat that reaches the skin’s surface, allowing the body to cool through radiation and evaporative heat loss. By driving blood towards the core and important organs, vasoconstriction - the narrowing of blood vessels to the skin by the contraction of their smooth muscles - reduces blood flow in peripheral blood vessels and preserves heat. Certain animals’ circulatory systems have evolved to allow them to move heat from arteries to veins that are next to one another, warming blood that is returning to the heart. This keeps the heart from being cooled by the icy venous blood; it’s known as a countercurrent heat exchange. This countercurrent adaptation is shared by bony fish, humming­birds, sharks, and dolphins.

Certain ectothermic animals employ behavioral adjust­ments to control their body temperature. To avoid overheat­ing during the warmest portion of the day in the desert, they merely look for cooler spots. On a chilly desert night, the same animals might scale rocks in the evening to gain some heat before heading into their burrows.

The neurological system regulates body temperature. In the sophisticated animal brain, the hypothalamus is the primary location for temperature regulation functions. The hypothalamus regulates body temperature through reflexes that result in shivering or sweating, vasodilation or constric­tion, and other physiological responses. The hypothalamus, which regulates the sympathetic nervous system, controls the reactions that cause temperature variations and bring the body back to the set point. In certain cases, the set point might be changed. Pyrogens are substances that are cre­ated during an infection and travel to the hypothalamus, where they raise the temperature. This permits the body’s temperature to rise to a new homeostatic equilibrium point, resulting in what is usually known as fever. The increase in body heat reduces the body’s optimal conditions for bacte­rial development and enhances cell activity, allowing it to better fight the illness.

1.3