CHAPTER I HOMEOSTASIS

Homeostasis is the ability to preserve oneself in the face of a constantly changing external environment is the most basic trait of all living beings. The human body, like all other living bodies, is kept alive by a series of biochemical events (called metabolism) that capture, transform, and deliver energy for important life processes and activities (e.g., reproduction, growth, movement). Normal body metabolism takes place under a specific set of physical and chemical parameters (e.g., temperature, pH, osmolality). Homeostasis is the ability of the human body to maintain such steady internal circumstances in the face of changing external conditions.

One of the most essential aspects of homeostasis is that it necessitates energy expenditure; for example, keeping a constant amount of glucose in the blood relies on biochemical activities that are powered by chemical energy obtained from adenosine triphosphate (ATP). Homeostasis is frequently confused with equilibrium, however, the two terms are not interchangeable. Molecules are widely scattered in a condition of equilibrium, resulting in a highly disordered, low-energy state (i.e., low potential energy). On the other hand, homeostasis is a highly ordered condition that necessitates the expenditure of energy to maintain. A comparison of how a dam works demonstrates the difference between these two concepts.

When a dam is built across a stream, water collects behind it to form a reservoir. The majority of water is retained in the reservoir, while less flows on the streambed downstream of the dam, resulting in a highly organized arrangement of water molecules. The dam’s construction and upkeep take a lot of energy, while the water in the reservoir has a lot of potential energy. In contrast, consider the unrestrained flow of water in an undammed stream. Gravity (not energy) drives the flow of water in the stream, and water molecules meander in a very disorganized fashion without creating potential energy.

Blood glucose control is similar to our stream scenario. For most cells, glucose is the principal source of energy. Following a meal, glucose is absorbed into the bloodstream and delivered to various tissues, where it is stored or used to fuel metabolism. Blood glucose levels remain steady between meals, owing to the liver’s ability to store glucose and release it into the bloodstream when dietary glucose is unavailable. The mechanisms of glucose absorption, storage, and mobilization are all energy-dependent. These systems are similar to a dam. Similar to how the dam prevents water from spreading at random, glucose regulation mechanisms prohibit glucose molecules from dispersing at random throughout the body.

THE INTERNAL ENVIRONMENT OF THE BODY

Approximately 60% of the human body is made up of fluids, which include water and other solutes. Ions, nutrients, proteins, and other molecules are examples of major solutes (e.g., hormones). Intracellular water makes up the majority of the body’s water (63 percent) (i.e., within cells). Extracellular fluids make up the remaining 37% of bodily fluids. Interstitial fluid (the fluid between cells) makes up 80% of extracellular fluid, whereas blood plasma makes up 20%. (the fluid portion of blood). At the cellular level, homeostasis entails keeping the extracellular and intracellular fluids steady in terms of oxygen, ions, nutrients, and other solutes.

The plasma membrane separates a cell’s intracellular fluid from the extracellular fluid. Because this barrier is selectively permeable, the constituents of the intracellular and external fluids differ. Regardless of these distinctions, changes in one fluid can have an impact on the other; for example, a loss of body water affects both intracellular and extracellular fluid quantities.

THE ROLE OF ORGAN SYSTEMS ON HOMEOSTASIS

The interaction of numerous organs and organ systems to maintain homeostasis within the body’s internal environment is a primary focus of physiology. Five fundamental processes are required for homeostasis to occur: 1) fluid and solute transport; 2) nutrition procurement; 3) waste removal; 4) protection from infectious organisms and hazardous circumstances; 5) organ communication. One or more of these processes is served by each of the body’s organ systems.

TRANSPORT OF FLUIDS

The capacity to maintain enough extracellular fluid quantities is essential for normal bodily function. Fluid transmission is governed by the circulatory system (heart and blood vessels). Extracellular fluid is transported in two steps. Blood is first distributed throughout the body by blood vessels. Fluid passes between capillaries (the tiniest blood vessels) and the interstitial space in the second step. The two stages are propelled by opposing forces. The pumping motion of the heart creates a pressure differential that drives blood movement within blood arteries. Osmosis is the transport of fluid over the permeable walls of capillaries.

PROCUREMENT OF NUTRIENTS

Nutrients are a type of nutrient that provides nourishment to cells. Cells cannot complete the metabolic activities required to maintain homeostasis without these substances. Nutrients are obtained through the respiratory, gastric, muscular, and skeletal systems. The gastrointestinal tract is frequently the first organ system that comes to mind when thinking about getting nutrients. This system’s organs break down (digest) foods into their chemical components (nutrients) and transfer them to places where they can be absorbed into the bloodstream. These nutrients flow directly to the liver after entering the bloodstream, where they can be stored or delivered to other body tissues.

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There is some debate on whether oxygen should be considered a nutrient. It is, nevertheless, essential for the aerobic metabolism of glucose, fatty acids, and amino acids, which produce ATP. In this sense, oxygen is a vitamin since it offers sustenance. The respiratory system is in charge of absorbing and transporting oxygen from the atmosphere into the bloodstream. Body movements are created by combining the muscular and skeletal systems. Obtaining food necessitates a number of physical motions. Locating and gathering food necessitates locomotion, whereas eating necessitates a variety of actions.

REMOVING WASTES

End products of nutrition metabolism are poisonous or ineffective for the body to use. Preventing the buildup of these chemicals in the extracellular fluid is essential for maintaining homeostasis. As a result, waste disposal is an important part in maintaining homeostasis. Carbon dioxide is a waste product of aerobic metabolism that can easily be exhaled out of the body. Nongaseous wastes, such as urea, are taken from the bloodstream and expelled in urine. The liver, a gastrointestinal system auxiliary organ, is also involved in waste elimination. The liver detoxifies various waste materials, which are then released into the bile, carried to the colon, and expelled with the feces.

PROTECTION

Pathogens such as viruses, bacteria, and parasites can disrupt homeostasis by interfering with cell activities. To protect itself from infections by these organisms, the body relies on its immunological and integumentary systems. The immune system defends the body through cellular and chemical mechanisms, in which intruder cells are attacked and destroyed by cells and chemicals. The skin and accompanying structures (e.g., hair, nails) form the integumentary system, which protects the body by acting as a physical barrier between the exterior and interior environments.

COMMUNICATION

Internal cell communication is frequently required to maintain internal stability. This sort of intercellular communication is enabled by the neurological and endocrine systems. Nerve cells (neurons) transfer electrical impulses from one area to another, allowing the nervous system to communicate. Hormones, chemical messengers that pass between cells via the extracellular fluid, are involved in endocrine communication.

CONTROL SYSTEMS

To maintain internal homeostasis, the human body relies on thousands of control mechanisms. Some of these works within cells to keep them functioning normally. Others are extracellular, coordinating distinct cells within an organ or interrelationships between cells of separate organs.

NEGATIVE FEEDBACK

Negative feedback systems are the most common way to keep things stable. The following components make up a negative feedback control system: 1) a central comparator (control center) that compares actual levels of a variable to a “set point,” or the level at which the variable is to be maintained; 2) a sensor that detects the variable of interest; 3) effectors that cause physiological changes to correct deviations in the variable from the setpoint; 4) afferent pathways that carry information from the sensor to the central comparator; 5) efferent pathways that carry information from the sensor to the central comparator; The system that regulates carbon dioxide levels in the blood can be used to explain this concept.

A negative feedback loop including the aforementioned components strictly regulates the amount of carbon dioxide in blood: 1) specialized sensors (nerve cells called chemoreceptors) that detect carbon dioxide levels in the blood; 2) an afferent neuronal pathway conveying information from the chemoreceptors to a central comparator, in this case, the brain’s respiratory centers; 3) an efferent neuronal pathway conveying information¬†from the respiratory centers to effectors (i.e., muscles that control breathing).¬†To keep blood carbon dioxide levels within a small range, these components interact in the following way.

  • Sensors in the major arteries of the neck detect an increase in carbon dioxide levels in the blood.
  • These cells deliver impulses to the brain’s respiratory regions via afferent neurons when triggered.
  • Efferent neurons are activated by the control center in the respiratory centers of the brain in response to the input.
  • The effector muscles that govern breathing get signals from the efferent neurons, which causes the breathing rate to increase.
  • The lungs’ ability to dispose of carbon dioxide improves as the breathing rate rises.
  • Carbon dioxide levels in the blood decline to pre-stimulus levels when the gas is removed from the body, removing the stimulus for the increased breathing rate (negative feedback).

Negative feedback mechanisms, it should be noted, prohibit substantial variations in physiologic variables. However, there are times when the body must react to stimuli quickly and efficiently. Positive feedback systems are responsible for such developments.

POSITIVE FEEDBACK

Positive feedback differs from negative feedback in that the reaction to a stimulus is enhanced rather than inhibited in positive feedback systems. A system like this has the potential to be exceedingly harmful, if not lethal. Consider what would happen if the link between blood pressure and heart rate was driven by positive feedback. A decline in blood pressure in this situation would force the heart to beat at a slower rate, lowering blood pressure. In a negative feedback loop, a drop in blood pressure would inhibit the brain areas that govern heart activity, preventing a further decline in heart rate and, as a result, a significant drop in blood pressure. In a negative feedback loop, a drop in blood pressure would inhibit the brain areas that govern heart activity, preventing a further decline in heart rate and, as a result, a significant drop in blood pressure. This drop in blood pressure would cause an even bigger drop in heart rate in a positive feedback system. This interaction would eventually become a vicious cycle, resulting in cardiac activity ceasing and death.

Because the feedback link between stimulus and response rapidly terminates, the few positive feedback systems that help sustain normal physiological function do not become vicious loops. This principle is illustrated through the regulation of birth. The fetus is forced into the cervix after the uterus’ contractions reach a specific strength and frequency (birth canal). Stretch receptors are activated, which convey afferent signals to the brain and trigger the release of oxytocin from the pituitary gland. Oxytocin is a hormone that causes the fetus to progress further down the birth canal by increasing the strength of uterine contractions. More oxytocin is released when the cervix is stretched further, which increases uterine contractions.

HOMEOSTASIS AND DISEASE

Understanding homeostasis can help you understand the idea of disease. The disease is basically a pathological (bad) condition that affects physiological function and causes particular indications or symptoms. Disturbances in homeostasis, or the failure of the body’s control systems, cause disease. Fever is a common sign of several viral or bacterial infections, for example. A fever is an excessively high body temperature induced by substances (pyrogens) that disrupt the body’s temperature regulation negative feedback processes.

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