homeostasis

Physiology In the distant past, humans thought that good health was somehow associated with a "balance" among the multiple life-giving forces ("humours") in the body –Today we know that living tissue is composed of trillions of small cells, all are packaged to permit movement of certain substances, but not others, across the cell membrane. –also we know that cells are in contact with the interstitial fluid. The interstitial fluid is in a state of flux, with chemicals, gases, and water moving it in two directions between the cell interiors and the blood.

Fluid compartments of the body

most of the common physiological variables found in normal, healthy organisms are maintained at relatively steady states. –i.e. blood pressure, body temperature, blood oxygen, and sodium. –This is true despite external conditions that are not constant.

Homeostasis defined homeostasis is simply defined as a state of reasonably stable balance between the physiological variables –NO variable is constant over time. Blood glucose can have dramatic swings. Homeostasis is in DYNAMIC balance, not static. It is relatively stable, if disturbed mechanisms can restore it to normal values.

What does it mean to be relatively constant? It depends on what is being monitored. –Arterial oxygen must be tightly controlled –Blood glucose can vary wildly A person can be in homeostasis for one variable but not for another. –You could be in sodium homeostasis but have abnormally high levels of CO 2. This is a life threatening condition. Just one variable out of homeostasis can have life-threatening consequences.

Physiology vs. Pathophysiology If all your major organ systems are in homeostasis, then you are in good health. diseases take one or more systems out of homeostasis. Physiology:When homeostasis is maintained Pathophysiology: homeostasis is not maintained.

How do you know if a variable is in homeostasis? You have to observe a person over time to find out what is “normal.” Not usually possible because you only go to a doctor when you are sick (out of homeostasis). –Usually, doctors rely on normal values for large populations of people. Body temperature –Normal values are useful, but not if a person has been exercising. –There are rhythms to a person’s body temperature.

Many variables are cyclical Examples –Body temperature, –sleep/wake, –levels of certain hormones –If you took one measurement, they may be normal, but might not detect when they are abnormally high or low. Measure over 24 hour period to get a better picture of homeostasis.

Characteristics of homeostatic control systems cells, tissue and organ activity must be integrated so that changes in the ECF initiate a reaction to correct the change. Homeostasis, then, denotes the relatively stable conditions of the internal environment –These conditions result from compensating regulatory responses controlled by homeostatic control systems.

Regulation of body temperature Man w/ body temp. of 37 0 C is in room at 20 0 C –He is losing heat to the environment –Chemical reactions in his cells are releasing heat at a rate = to loss –Body is in a steady state but state is maintained by input of energy Steady state is not equilibrium Steady state temperature is the set-point

Lower room temp to 5 0 C This increases loss of heat from skin and body temp starts to fall –What responses will occur? Blood vessels to skin constrict Person curls to reduce skin surface area Shivering occurs producing large amounts of heat

Negative Feedback Defined –an increase or decrease in the variable being regulated brings about responses that tend to move the variable in the direction opposite ("negative" to) the direction of the original change –It can occur at the organ, cellular, or molecular level

negative feedback example

Negative feedback in an enzyme pathway When energy is needed by a cell, –glucose is converted into ATP. The ATP that accumulates in the cell inhibits the activity of some of the enzymes involved in the conversion of glucose to ATP As ATP levels increase within a cell, production of ATP is slowed down

Not all feedback is negative Positive feedback is less common but does occur –In nerve cells, when a stimulus is received, pore-like channels open letting Na + in –In childbirth The baby’s head presses against the uterus stimulating the release of oxytocin Oxytocin causes uterine contractions, pushing the baby’s head against the uterine wall releasing more oxytocin.

Feedforward regulation While your body can respond to changes in external temperatures AFTER the body’s internal temperature changes, it can also respond to changes BEFORE your body temp. starts to fall. –Nerve cells in the skin detect changes and send information to the brain. –Often this response is a result of LEARNING

Parts of homeostatic control systems- Reflexes reflex is a specific involuntary,unlearned "built­ in" response to a particular stimulus –The stimulus is a detectable change in the internal or external environment. –Detected by a nerve receptor –The stimulus causes the receptor to send a signal to the integrating center (afferent) Reflex Arc

Reflex part 2 Integrating center receives signals from many receptors –Receptors may be for different kinds of stimuli –Output from center (efferent) goes to effector to alter its activity

Reflex for minimizing decrease in body temperature

Reflexes are not just part of the nervous system We usually think of reflexes are part of the nervous system (hand on a hot stove), but now we include many other systems as part of reflexes. –Hormone-secreting glands serve as integrating centers –Chemical messengers travel through the blood.

Intercellular chemical messengers reflexes and other responses depend on the ability of cells to communicate w/ each other. –Most often occurs with chemical messengers. Hormones- allow hormone secreting cell to communicate with target cells. –Blood delivers the hormone to the cell. Neurotransmitters- allow nerve cells to communicate with each other –One nerve cell can alter the activity of another cell. –Neurotransmitters released into the area around effector cells can alter their activity. Paracrine agents- chemical messengers in local responses

Categories of chemical messengers

Paracrine/autocrine agents Paracrine agents are made by cells (given a stimulus) and released into the ECF. –Agents diffuse to neighboring cells which are their target cells. Autocrine agents are made by a cell, released and the target cell is the one that released it. (?)

Why do you care about these agents? We are finding many different paracrine/autocrine agents that have many diverse effects. –They are not just proteins. –Secreted by many cell types in many kinds of tissues –So many that they can be organized into families i.e. Growth factor family has 50 distinct molecules that can cause cells to divide/differentiate.

Processes related to homeostasis Some seemingly unrelated processes have implications for homeostasis –Adaptation and acclimatization –Biological rhythms –Apoptosis

Adaptation/ acclimatization Adaptation is a characteristic that favors survival in specific environments. –Your ability to respond to a specific environmental stress isn’t fixed, but it can be enhanced by prolonged exposure to the stress. –Acclimatization: A specific type of adaptation- the improved functioning of an existing homeostatic system.

Acclimatization is reversible (usually) If daily exposure to the stress is eliminated, then acclimatization is reversible… Some acclimatizations that happen early in life may become permanent. –Natives of the Andes Mountains Low oxygen levels cause increased chest sizes, wide nostrils, broad dental arches

Biological rhythms Many body functions are rhythmic –Occur in 24 hour (circadian rhythm) cycles –Sleep/wake, body temp., hormone levels, etc… –Are anticipatory (kind of like feedforward systems without detectors)

Rhythms allow responses to occur automatically Remember that most homeostatic responses are corrective, they occur after homeostasis is perturbed –Rhythms cause responses to occur when a challenge is likely but before it actually does. Urinary excretion of potassium is high during the day and low at night.

Body rhythms are internally driven Environmental factors don’t drive the rhythms, but provide timing cues. –Sleeping experiment (no light cues) –Sleep/wake cycle is a free-running rhythm –Sleep/wake cycles can vary between hours but not more or less than that.

Other environmental cues Light/dark cycle is very important, but not the only one. External environmental temperature Meal timing Social cues –Sleep experiment people are separated, their cycles are each different. –Put them together and their cycles synchronize

Jet Lag Environmental time cues can phase-shift rhythms. –Going from LA to Atlanta and staying for a week. –Circadian rhythm will adjust, but it takes time –In the meantime, you suffer jet lag Sleep disruption, gastrointestinal trouble, decreased vigilance and attention span, general malaise

Neural basis of body rhythms In the hypothalamus –A group of nerve cells (suprachiasmatic nucleus) –Acts as the pacemaker for rhythms Pacemaker receives input from the eyes and other senses. Then it sends signals to other parts of the brain that control other systems, activating some and inhibiting others. Not well understood

Sleep and the Pineal gland Pacemaker sends signal to pineal gland –Gland releases melatonin –Pineal secretes during darkness, not daylight –Melatonin influences other organs –Makes you sleepy

Apoptosis Defined- –The ability to self-destruct by activation of an intrinsic program within the cell Important for –sculpting a developing organism or –Eliminating undesirable cells (cancerous)

Importance of Apoptosis Crucial for regulating the number of cells in a tissue or organ. –Control of cell number is determined by a balance between cell proliferation (addition of new cells by mitosis) and cell death (apoptosis) Neutrophils (cells alive)cells alive

How does it occur? Controlled autodigestion of cell organelles. –Enzymes breakdown the nucleus and then other organelles The cell membrane isn’t digested. The cell sends out chemical signals that recruit phagocytic cells (cells that “eat” other cells). –This is different than what happens when a cell is injured (necrosis)

How is it kept off? Virtually all cells have the apoptosis enzymes. –Why aren’t they turned on? A large number of molecules called “survivor signals” keep the cell from activating the enzymes. So most cells are programmed to commit suicide UNLESS they receive a signal to stay alive. –Prostate gland cells will die if testosterone is not present

What about cancer? Degenerative diseases? Cancer cells undergo uncontrolled cell proliferation. –So the apoptosis enzymes are always turned off. In degenerative diseases (osteoporosis) –The rate of cell death is higher than that of cell proliferation. Drugs that reduce rate of apoptosis

Balance in the homeostasis of chemicals Most homeostatic systems control the balance of specific chemicals.

3 states of total body balance Negative balance –Loss exceeds gain, total amount of substance in body is decreasing. Positive balance –Gain exceeds loss Stable balance –Gain equals loss

Water, sodium balance Water –Stable balance is upset with excessive sweating. –Restored by? Sodium (Na + ) –Kidneys excrete Na + into urine in approx. = amounts of ingested daily. –If intake were to increase dramatically, kidneys will excrete more in urine, but only so much can be excreted. –If the increase is continued, it can have effects on other systems –A small change in blood sodium has been linked to hypertension.

A quick summary Homeostasis is a complex, dynamic process. –It regulates the adaptive responses of the body to changes in external and internal environments. – homeostatic systems require a sensor to detect changes and a means to produce a response. Responses can include: muscle activity, synthesis of chemical messengers (hormones) and behavioral changes. All responses require energy. You get energy to respond from the food you eat.