HOMEOSTATIC PRINCIPLES − Rakyat Biologi

Physiology - is the study of biological function, which is a generic definition. Physiology can be defined more specifically in several ways:

1. It is the study of life processes that occur inside the body at all levels of organization: cells, tissues, organs, organ systems, and the interactions among these.

• It is very closely related to chemistry and physics.

2. It is the study of “cause and effect” with an emphasis on the mechanisms of “how does this work.”

• A generalized example would be “what causes the heart to pump and what is its effect on the circulatory system.”

• Or more specifically we could say “what causes the pancreas to secrete insulin and what effect does this have on blood glucose levels.”

3. It is based upon careful observation and experimentation from which conclusions may be drawn. The data collected from these experiments and observations are always open to interpretation and change.

• Everything we are going to study this semester, from cells to reproduction, is based on careful scientific investigation, as produced by what is called the scientific method. It is not infallible, but is in fact, designed to be open to change and modification—self-correcting philosophy.

• Example: Can test whether athletes have lower resting heart rates than sedentary people.

4. The ultimate goal of physiological research is to understand the normal functioning of cells, organs, & systems. YOU CANNOT MODIFY (MUCH) WHAT YOU DO NOT UNDERSTAND.

5. There are also different types of physiology:

 Pathophysiology is the study of physiological process as affected by injury or disease.

 Exercise physiology is the study of physiological processes as affected by exercise.

 Comparative physiology is the study of the physiology of different invertebrate and vertebrates groups.

Much of the knowledge gained from comparative phys. has benefited mankind, because many other animals, (particularly our closer relatives among Mammalia) have the same or similar physiology. The small differences in physiology between human and other mammals can be of crucial importance in the development of pharmaceutical drugs, but these differences are relatively slight in the overall scheme of things.

Requires an extensive knowledge of anatomy. If you are in this class I'm going to assume you have a working knowledge of anatomy. Review Tissues, Organ Systems, and Cell Organelles.

The study of Physiology generally revolves around the concept of Homeostasis.

 It is a term coined by the father of modern physiology, French physiologist Claude Bernard (1813-1878).

 “homeo” = Greek for like or similar.

 “stasis” = Greek for fixed or stationary.

Homeostasis is the maintenance of the conditions in the cell or within the body that maintain life, despite changes that may be occurring in or outside the body.

1. Your body is going to maintain a constant internal temperature of 98.6˚F (37˚C) no matter what the temperature is on the outside, whether it’s 120^◦F or -20^◦F.

2. Because the body is an aggregate of about 75 trillion cells, the whole body’s survival is based on the survival of its cells. All bodily processes work together to maintain homeostasis of the internal environment of the cells which keeps you alive.

3. All of the needs of the these cells are supplied by the external environment (e.g. food, water, minerals, oxygen), but because the cells are not in contact with the external environment there must be a means of exchange to get what they need to survive needs and to get rid of their wastes.

Diagram: cells make up systems, systems maintain homeostasis, which is essential for cell survival

Bodily Fluid Compartments: accomplish this task

1. The body can be thought of as mostly water. Approximately 60% of its weight is water. The fluids contain various mineral ions (e.g. sodium, potassium, chloride) and organic substances (e.g. proteins, glucose) dissolved in the body water. These fluids are grouped into 2 major divisions

• Intracellular fluids (ICF) are the fluids within the cell part of the cytoplasm (water, proteins, enzymes).

• Extracellular fluids (ECF) are all the fluids outside the cell; composed of interstitial or tissue fluid (fluid between cells), plasma (fluid portion of blood, & lymph (fluid in lymphatic vessels)).

• Together these extracellular fluids make up the internal environment of the body, whose constant regulation is the purpose of homeostasis.

Homeostasis: Control of the Internal Environment

1. To reiterate, most of the living cells of the body are not directly exposed to the gaseous external environment (atmosphere), but exist in ECF.

There is a constant flow or exchange of materials between the ECF & ICF.

2. Because of this constant exchange, conditions in this ECF must be maintained at a relatively constant level (volume & compositions) in order to permit our cells to live & function = homeostasis.

Factors to be maintained in the ECF

a) Conditions such as O₂ and CO₂ conc.: cells use O₂ for energy & produce CO₂ as a waste

b) pH: correct pH is needed for enzymes to work correctly.

c) conc. of H₂O & electrolytes, such as Na, K, Cl

d) temperature: important for enzyme action

e) concentrations of various nutrients, hormones, & waste products

f) volume & pressure: important for transfer of materials between plasma and interstitial fluid

3. All are closely regulated by the cooperative workings of body’s tissues, organs, & systems.

• Digestive system transfers nutrients to plasma to cells.

• Respiratory system transfers O₂ from external environment to cells, & carries CO₂ away.

• Circulatory & lymphatic systems transport nutrients & wastes throughout body.

4. If homeostasis of the internal environment is disrupted, the result is often disease or sickness, which sometimes has a snowball effect. In many diseases, the composition and/or volume of the internal environment becomes abnormal.

• Consider a person who has kidney failure and cannot get rid of metabolic waste products that are toxic to cells at an adequate rate.

• In such a person, toxic waste materials will accumulate in the ECF and the body will be poisoned by its own internal environment. The normal functioning of body cells will be disturbed under such conditions.

• What if someone has respiratory failure? Heart failure? Iron deficiency? Sterility? Some you can live without

5. So the study of physiology revolves around the concept of homeostasis and the regulatory processes by which the body maintains this constancy of the internal environment.

• Some are simple, some are extremely difficult, but every process in the body has a common goal: to maintain homeostasis of the internal environment of the body.

Homeostasis has been achieved when:

1. the internal environment has the best (optimum) concentrations of gases, nutrients, ions, & water.

2. the internal environment has the optimal temperature, it varies a little but normal body T is 37°C = 98.6°F.

3. The internal environment has optimal pressure: gas, water, & osmotic pressure.

4. Goldilocks analogy: "everything has to be just right."

Requirements for Maintaining Homeostasis

1. Communication within the body: accomplished by endocrine & nervous systems. How does each communicate? The factor being regulated is the variable (temp, blood sugar levels, etc.). All homeostatic control mechs. have at least 3 interdependent components (receptor, integrating center, effector).

2. Input from sensory receptors: sensory receptors respond to stimuli (detect changes) to the body, an awareness of what is happening internally & externally.

3. Control / Integrating center: brain, spinal cord, or endocrine glands. They receive sensory information and bring about a response (efferent signal) via nerve impulses or hormones, intended to change conditions back to normal. The control center determines a set point (the level or range) at which a variable is to be maintained; it analyzes afferent signals & responds with an efferent signal.

4. Effectors bring about the change specified by the integrating center (e.g., skeletal muscle, glands, & organs). Effectors provide the means for acting on the control center’s response to a stimulus, either depressing it (negative feedback) or enhancing it (positive feedback).

*Military Analogy: field troops detect enemies (sensor) ® send message to HQ (afferent signal to control center) ®HQ issues a command & sends message to jets (efferent signal) ®jets (effectors) bomb the enemy (boom boom)

Diagram: Homeostasis Pathway

To maintain homeostasis, we must have communication networks & mechanisms for regulating this network.

Homeostatic Control Systems: a collection of interconnected cells that function to maintain a physical or chemical property of the internal environment relatively constant.

• Each individual cell exhibits some degree of self-regulation, but the existence of a multitude of cells organized into tissues, which are further combined to form organs, imposes the requirement for overall regulatory mechanisms to coordinate the activities of all cells.

• Example: If you start sprinting, you require more O₂, so to maintain O₂ in the ECF, we must have a way of detecting O₂ concentration and alter respiratory activity so that O₂ levels are optimum. Vessels dilate/constrict on a local level before the nervous system is aware.

• As you keep sprinting, other control mechanisms come into play.

Control Systems of the Body: the are 2 general classes of control systems in the body.

1. Local Control: built in or inherent to the organ or tissue itself.

• When you exercise skeletal muscle (as in running), you immediately decrease or use up the available oxygen & nutrients available to those cells. There is also an increase of CO₂ & lactic acid (waste products) in the cells.

• This directly triggers smooth muscle relaxation in arterioles in muscle, which increases blood flow (vessel dilation) to the area for more nutrients, O₂, and to transport wastes away.

• So, we have specific control of a local area (tissue, organ, etc.) w/o the nervous system being "aware."

2. Reflex Controls: regulatory mechanism outside the organ system that involves the nervous & endocrine systems.

• most widely used means of control.

• initiated by an internal change that is detected by an "external control system" (nervous/endocrine systems). Detect intrinsic response.

• Motor nerve impulses or hormones are released from outside of the tissue/organ to produce a more widespread effect in the body

• Example: So if we keep running for a long period of time, the extrinsic controls will kick in to also help regulate the changes going inside the body: Temp, glucose levels, etc.

3. So if the internal environment is disrupted like in running, the external & internal processes mediate reactions that try to return the body to a desired level = set point. Body wants to keep itself at this relatively constant level = homeostasis.

• A set point is really an average of values within the normal range. The values are constantly changing between the high & low points.

• blood pH varies between 7.35-7.45.

• body T can range from 96.0^◦F in the morning to 99.0^◦F at night. Organs even have different temperatures. No "normal" T—what we call normal is an average of many people. Remember that 98.6^◦F is an oral temperature reading—your normal core T is greater.

• Analogy of T set on a house thermostat compared to body T maintenance

There are 2 Types of Extrinsic Control Mechanisms: Negative & Positive Feedback.

Negative Feedback: regulatory mechanism in which a change in a controlled variable triggers a response that opposes the change, thus maintaining a steady set point for the regulated factor. This is the most common extrinsic control system to maintain homeostasis (i.e. it is homeostatic).

Negative Feedback Involves: Remember our requirements!

1. In order for the internal constancy to be maintained, the body must have sensors that are able to detect deviations from a set point.

• The set point is analogous to the temperature set of a house thermostat.

• In a similar manner, there is a set point for body temp, blood glucose conc., tension on the tendon, etc.

2. Integrator receives info. from the sensors and increases or decreases the activity of the effectors.

3. Effectors carry out the instructions from the Integrating Center to maintain internal constancy.

Because the activity of the effectors is influenced by the effects they produce, and because this regulation is in a "negative" or "reverse/opposite" direction, this type of control system is known as a negative feedback loop.

Antagonistic Effectors: Most factors (O₂, glucose, ions, etc.) in the ECF/Internal environment are controlled by several effectors, that often have opposite effects. These are called antagonistic effectors and they have "push-pull" effects. Tonic control refers to ongoing control that’s adjusted up & down.

• tonic control: increasing the activity of one effector in turn decreases the activity of an antagonistic effector.

• Example: Blood glucose levels are regulated by negative feedback loops involving hormones that have opposite effects. While insulin lowers blood glucose levels, glucagon raises blood glucose levels.

Positive Feedback: is the opposite of negative feedback!! The action of the effector amplifies the changes that first stimulated the effectors. There are no antagonistic factors.

• If the change in the homeostatic condition were to increase, positive feedback increases it even further.

• Back to the thermostat example, usu. it maintains a constant T by increasing heat production when it is cold and decreasing heat production when it is warm = negative feedback. A thermostat that works by positive feedback, would increase heat production in response to a rise in temperature.

A reflex action is not homeostatic—the response amplifies/reinforces the stimulus, rather than having an opposite effect. It can only be stopped by removing the stimulus. Shine light in someone’s eyes, pupil contracts & stays contracted until the stimulus is removed. Hit someone on the patellar ligament & keeping hitting them, body will keep jerking knee.

• Blood clot: initial damage caused by a cut, sets off a series of reactions to form a clot. The activation of one clotting factor results in the activation of other clotting factors in a positive feedback avalanche-like manner.

• Giving Birth. Birth initiation starts with uterine contraction, which causes oxytocin release. This stimulates contraction of more uterine smooth muscle which stimulates more oxytocin release. Contractions keep getting stronger and stronger. Production stops only after baby and placenta have been completely delivered.

• A single change is amplified to produce a blood clot or to give birth. Aid in the completion of negative feedback loops.

• Essentially uncommon, because it tends to move the conditions further away from homeostatic levels. Prolonged positive feedback is generally detrimental and usually pathogenic.

FEEDFORWARD CONTROL

Negative feedback loops can stabilize a function and maintain it within a normal range, but are unable to prevent the change that triggered the reflex in the first place. A few reflexes have evolved that allow the body to predict that a change is about to occur and start the response loop in anticipation of the change. These responses are called feedforward controls (example = salivation reflex: sight, smell, or thought of food is enough to start the response loop, which also trigger the release of HCl in the stomach).

Others examples:

· erection in human males (visual or tactile stimuli)

· response to exercise (visualization of an event in your mind)

· relaxation techniques.

CIRCADIAN RHYTHMS

Unlike some reflex loops which require a stimulus, some occur spontaneously, e.g., hormones are secreted at different levels at different times of the day. Most occur in a predictable manner and are often timed to coincide with a predictable environmental change such as light-dark cycles or the seasons. Circadian rhythms are regular rhythms of growth or activity that occur on a 24 hour cycle.

· controlled by the hypothalamus, which receives information from an ascending pathway from the retina via the suprachiasmatic nucleus.

· studying at night ® get cold not because of drop in environmental temp, but because your thermoregulatory reflex has turned down your internal thermostat

· others that vary are cortisol (steroid hormone that regulates metabolism; secreted from the adrenal gland); levels are at their lowest at 4 AM and peak around 9 AM in the morning.

· growth hormone

· sex hormones (testosterone levels peak at midnight and in the morning hours)

· morning people (temps rise before they wake up and drop off early in the evening) vs. night owls (tough time waking up but maintain body temps longer)

· Effects vary with age and state of well-being.

· People who work night shifts never really quite adjust, because even though their body wants to rest, the control centers of the brain tell them it’s time to get up.

Biological reflexes are mediated by the nervous system, the endocrine system, or both. Reflexes can be quite complex, passing through many integrating centers before reaching the effector. There is so much overlap between reflexes controlled by the nervous & endocrine systems that they should be considered as a continuum rather than two discrete systems.

CONTROL SYSTEMS vary in speed, specificity, duration, signal type, and intensity because tissues require different means of responses for different functions.

Specificity

· nerves are specific, can trace a pathway from origin to its target cells

· endocrine control is more general: hormone is released into the blood; however, receptors on the cell membrane ensure the response is cell/tissue specific.

Nature of the Signal

· Nervous system uses both a electrical (depolarization travel long distances) and chemical signals (NT’s travel short distances)

· Endocrine system uses chemical signals only

Speed

· nerves are much faster than endocrine reflexes because hormones must be distributed through the circulatory system

Duration of Action

· Nerve control has a much shorter duration than endocrine

· short-term functions mostly performed by the nervous system

· long-term by the endocrine system

Intensity

· frequency of nervous impulses relays stimulus intensity (i.e. how fast)

· hormone concentration relays stimulus intensity (i.e. how much)

Dynamic Constancy and the Steady State

• Because negative feedback loops respond after deviations from the set point have stimulated sensors, the internal environment is never absolutely constant.

• Homeostasis is best described as a state of dynamic constancy, in which conditions hover above and below the set point.

• In other words, feedback systems do not maintain a complete constancy. The internal environment varies around the set point. (i.e. it oscillates around set point)

• Feedback systems just try to minimize the changes taking place.

• Example: If you are exposed to extremely cold temperatures, as long as the exposure to cold continues, some decrease in body temperature must persist to serve as a signal to maintain the responses of the effectors & antagonists.

Homeostatic imbalance

1. Homeostasis is so important that most disease is regarded as a result of its disturbance, a condition called homeostatic imbalance.

2. As we age, our organs & control systems become less efficient, placing us a greater risk for illness.

3. Another source of homeostatic imbalance occurs in certain pathological situations when the usual negative feedback mechanisms are overwhelmed and destructive positive feedback mechanisms take over, such as in heart failure.

Important Generalizations about Feedback Mechanisms:

1. Stability of an internal environmental variable is achieved by balancing inputs and outputs. The balance between variables - inputs & outputs is the concern.

2. In negative feedback systems, a change in the variable being regulated brings about responses that tend to push the variable in the direction opposite to the original change back toward the original value.

3. Homeostatic control systems cannot maintain complete constancy of any given feature of the internal environment - regulated values will have a narrow range of normal values.

4. It is not possible for everything to be maintained constant by homeostatic control mechanisms—some controls are more important than others, and therefore, are regulated more.