Homeostasis and Adaptation

Homeostasis

any self-regulating process by which biological systems tend to maintain stability while adjusting to conditions that are optimal for survival

• feedback loops

• generally describes resting conditions

Example parameters of homeostasis

• blood pressure

• oxygen and carbon dioxide tensions

• blood concentrations of glucose and other metabolites

• blood and tissue osmotic pressures

• blood pH

• core temperature

what is homeostasis challenged by

• external and internal stimuli

• ex) heat, cold, lack of oxygen, hydration, food intake, psychological stresses, exercise

• most disruptions are mild and temporary

steady state

• internal environment is relatively constant, but the parameter is not necessarily at resting or “normal” levels

  • energy expenditure may be required

  • balance has been achieved

components of feedback loops

1. receptor (sensor)

2. control center

3. effector

receptor - feedback loops

monitors changes, sends input in the form of nerve impulses or chemical signals to a control center

control center - feedback loops

sets the range of values within which a controlled condition should be maintained, evaluates input it receives from receptors, and generates output commands

effector - feedback loops

body structure that receives output from the control center and produces a response or effect that changes the controlled condition

negative feedback loops

how most of our physiological control systems operate

• response that reverses the original stimulus

• the activity of the effector produces a result that opposes the stimulus

example of negative feedback loop

blood pressure regulation by heart rate

feed-forward control

• anticipatory response of a physiological parameter to prepare the body for change

  • often a learned anticipatory response to a known cue

• usually acts in concert with negative feedback systems

examples of feed-forward control

• increase in HR and ventilation in anticipation of exercise

• increase in salivation and digestive enzyme secretion in anticipation of a meal

• ability to use appropriate force and smooth movements when picking up an object

• anticipatory postural adjustments

positive feedback loop

• response that enhances the original stimulus

• positive feedback system reinforces a change in a controlled condition

examples of positive feedback loop

1. labor and delivery

2. action potentials

3. urge to urinate

exercise as a challenge to homeostasis

• body must quickly respond to prevent drastic alterations in the internal environment

• steady state may not be possible when demands of exercise are very large

examples of how exercise can challenge homeostasis

1. increased demands on regulating core temperature because of heat production

2. increased demands on regulating arterial blood gases because of increased cellular respiration

3. increased demands on regulating blood glucose levels because of increased cellular energy demand

4. increased production of CO2 during exercise which the body must get rid of

adaptation

• cellular changes that occur in response to an altered environment

  • cells are constantly exposed to changes in their environment

• cells can adapt to acceptable changes in their environment by modifying metabolism or growth pattern

importance of discussing physiology of adaptation

• how the body adapts to a stressor (exercise training adaptations, chronic disease)

• certain adaptations in growth act as a fertile ground for the later development of neoplasia

• nomenclature is used in clinical work

physiological stimuli for adaptation

those which are within an acceptable range

examples of physiological stimuli for adaptation

• exercise training

• change in the female reproductive system during pregnancy

• age related changes in skeletal muscles

pathological stimuli for adaptation

those that cause a severe disturbance to cell function

examples of pathological stimuli for adaptation

• nutritional: severe deprivation of nutrients or calories

• immune: allergic reactions

• endocrine: over or under production of hormones

• physical agents: cigarette smoke, asbestos

• chemical agents: drugs

• infections: viruses, bacteria, fungi, parasites

• anoxia: poor blood flow

• genetic: chromosomal disorders

exercise training results in cellular adaptation

• change in the structure or function of a cell or organ system that results in an improved ability to maintain homeostasis and steady states during stressful conditions

• occurs through cellular signaling mechanisms

example: exercise training results in cellular adaptation

skeletal muscle adaptations to aerobic and resistance exercise training

acclimations

ability to adapt to an environmental stressor

ex) ability to acclimatize to exercise in the heat

skeletal muscle adaptations to aerobic exercise training: changes in cellular contents and structure

• increase in the number and size of mitochondria

• increase in myoglobin content and capillary density

  • myoglobin carries and stores oxygen in muscle cells

• increase in glycogen and triglyceride content

skeletal muscle adaptations to aerobic exercise training: changes in cellular biochemical properties

• increased ability to oxidize fats and carbohydrates

• improved H⁺ ion buffering capacity

• down-regulation of myosin ATPase to slow-twitch isoform

skeletal muscle adaptations to aerobic exercise training: physiological implication

improved aerobic endurance, particularly during submaximal aerobic exercise

• improved delivery of O₂ rich blood to muscles

• improved ability to utilize fats and carbs to produce ATP using aerobic metabolism pathways

• decreased production of lactic acid during exercise

• improved ability to buffer acids produced during exercise

overtraining continuum

fatigue → overreaching (functional or non-functional) → overtraining syndrome

fatigue - overtraining continuum

recovery is rapid, usually within 24-48 hours

functional overreaching - overtraining continuum

recovery takes longer (up to 2 weeks) but is all part of a planned program to improve performance longer term (ex training camps)

non-functional overreaching - overtraining continuum

recovery takes even longer (weeks, sometimes months). the negatives outweigh the positives, there is no long term gain

overtraining syndrom - overtraining continuum

recovery takes a very long time, sometimes many months

difference between overreaching and overtraining

amount of time needed for performance restoration (not the type or duration of training stress or degree of impairment)

training adaptations

occur because we have overloaded the body with a repeated stimulus, thus inducing adaptation

• happens as long as the overload is appropriate

• when we overtax the body with an inappropriate stimulus, we run into the risk of non-functional overreaching and overtraining

cellular adaptation to an increased functional demand

1. hypertrophy

2. hyperplasia

these can occur independently or together and are reversible processes, reflected by an increase in size and weight of an organ

hypertrophy

increase in cell size

hypertrophy example

skeletal muscle hypertrophy in response to exercise

• increased synthesis of proteins → size and volume of individual fibers increase

• number of fibers does not increase

hyperplasia

increase in cell number

cellular adaptation to an decreased functional demand

1. atrophy

2. hypoplasia

3. mixture of the two

reflected in a reduced size and mass of an organ

atrophy

reduction in the cell volume of a tissue by elimination of cytoplasms and structural proteins

hypoplasia

reduction in cell number through apoptosis

common causes of atrophy

• denervation

• immobilization

• reduced endocrine stimulation

• ischemia

• aging

cell components removed by degradative systems