Homeostasis and Biological Control Systems

Homeostasis, Steady State, and Circadian Rhythm

• Homeostasis is the maintenance of a stable internal environment.
• Steady state is a dynamic equilibrium where the body maintains a relatively constant internal environment despite changes in the external environment.
• Circadian rhythm is the natural, internal process that regulates the sleep-wake cycle and other biological processes over a 24-hour period.

Biological Control Systems

• Thermoregulation, blood pressure control, and blood glucose maintenance are examples of biological control systems in the human body.
• Biological control systems consist of a receptor, control center, and effector.

Feedback Loops

• Negative feedback loops maintain homeostasis by reversing a change in a regulated variable.
• Feed-forward mechanisms anticipate a change in a regulated variable and initiate a response before major deviation occurs.
• Positive feedback loops amplify a change in a regulated variable.

Hormesis and Exercise

• Hormesis is a phenomenon where low-dose exposure to a stressor can have beneficial effects.
• Exercise is an example of hormesis, where low to moderate levels of stress trigger adaptation and improvement in health and function.

Cell Signaling Mechanisms

• Five cell signaling mechanisms exist, although the lecture did not list them.

Gene Expression Regulation

Transcription

• Increase: Transcription factors that enhance, histone acetylation, increased mRNA, increased ribosomal number, presence of HSPs to assist with protein folding, transport of the protein to its target location.
• Decrease: Transcription factors that repress, DNA methylation, increased microRNAs, inability to transport mRNA from the nucleus, insufficient amino acids to build a protein, ubiquitin tagging, lack of protein transport to its target location.

Translation

• Increase: Increased rRNA activity, increased ribosomal numbers, increased tRNA availability.
• Decrease: Lack of genetic material, insufficiency of amino acids.

Post-Translational Modification

• Increase: Addition of chemical groups, protein cleavage, formation of complex structures.
• Decrease: Removal of regulatory groups, or inability to undergo important steps.

Cellular Adaptation

• The body adapts to homeostatic challenges through regulated gene expression.
• For example, beta cells increase insulin production when exposed to high glucose levels.

Heat Shock Proteins (HSPs)

• HSPs protect cells from damage by assisting in the proper folding of proteins.

Cell Membranes and Receptors

• Cell membranes are composed of a phospholipid bilayer with embedded proteins.
• Membrane fluidity is increased when needed by altering the lipid composition.
• Cell junctions include tight junctions, desmosomes, and gap junctions.

Membrane Transport

• Passive transport (diffusion and osmosis) does not require energy.
• Active transport requires energy, like the sodium-potassium pump, to maintain ion gradients.

Signal Transduction

• Receiving, converting (transducing), and reacting to external signals are all part of signal transduction.
• Cellular responses can include activation of enzymes or gene expression.

ATP Production and Metabolic Pathways

• Enzymes catalyze cellular chemical reactions.
• Macromolecules used as fuels during exercise include carbohydrates, fats, and proteins.
• ATP is the primary energy currency of the cell.
• ATP production varies in different cell types.
• Aerobic and anaerobic ATP production interact during exercise.

Apoptosis and Oxidative Stress

• Oxidative stress occurs when reactive oxygen species (ROS) exceed the body's antioxidant defense capacity.
• Common ROS examples are hydrogen peroxide, superoxide radicals, and hydroxyl radicals.
• Apoptosis (programmed cell death) is different from necrosis (uncontrolled cell death).
• Apoptosis benefits the organism by eliminating damaged cells.
• Mitochondria play a crucial role in apoptosis.
• Environmental stressors can initiate apoptosis.
• ROS contribute to apoptosis.

Metabolism: Rest and Exercise

• Resting metabolic rate is influenced by factors like age, sex, and activity level.
• Maximal oxygen consumption (VO2 max) is the maximum rate of oxygen uptake by the body during exercise.
• VO2 max can be calculated from heart rate and VO2 data, using formulas provided in the course.
• Respiratory Exchange Ratio (RER) reflects the macronutrient being used.
• Exercise intensity and duration influence the bioenergetic pathways used for ATP production.

Hormone Regulation and Action

• Hormone concentration is determined by release rate, transport (free vs. bound), and metabolism.
• Hormones act on cells through direct gene activation or second messenger systems.
• The hypothalamic-pituitary axis is a key endocrine control center.
• The anterior pituitary releases and regulates growth hormone and thyroid hormone.
• Negative feedback loops regulate hormone release.

Hormonal Control of Glucose Homeostasis

• Glucose homeostasis is maintained differently in the absorptive state, postabsorptive state, and during exercise.