Homeostasis and Feedback Mechanisms Quiz

Homeostasis

• Homeostasis is a steady state, requiring energy
• Equilibrium is a state without energy consumption
• A vital parameter (e.g., blood glucose) is well-regulated when in a steady state
• The body carefully manages actions that lower and raise the vital parameter, maintaining a constant value

Negative Feedback

• Negative feedback is the most common feedback mechanism, self-limiting
• It reverses any deviation of a controlled variable from its stable point
• A control system initiates a series of changes that return the factor towards its mean value, maintaining homeostasis
• Key components include a stimulus initiating event, a variable, a receptor (detects change in variable), a control center (compares input to reference value), and effectors (make adjustments to the variable to return the variable to its original set point)

Positive Feedback

• Positive feedback loops maintain the direction of the stimulus
• The cycle tends to exaggerate deviations of the controlled variable from its stable point
• Each cycle of this feedback leads to more of the same variable, possibly accelerating the process
• It may cause instability and death

Electrolyte Content of Body Fluids

• The table provides the electrolyte content in plasma, interstitial fluid, and intracellular fluid measured in mEq/L
• The electrolytes listed include cations (sodium, potassium, calcium, magnesium) and anions (chloride, bicarbonate, sulfate, phosphate, protein).

Osmotic Pressure and Reflection Coefficient

• Osmotic pressure and reflection coefficient are related
• Isotonic solutions are always iso-osmotic. However, an iso-osmotic solution may not be isotonic when the reflection coefficient is 0.
• Substances with a reflection coefficient of 0 move across the membrane, rapidly equilibrates between the compartments
• The impermeable solutes determine the osmotic pressure and water movement

Tonicity of Solutions

• Isotonic: No change in cell volume
• Hypotonic: Cell swells
• Hypertonic: Cell shrinks
• Tonicity depends on the concentration of impermeable solutes, some solutes can permeate cell membrane

Review of Effects of Volume Changes on Osmolarity of Body Fluids

• The table (Table 1) summarizes volume changes and body osmolarity following changes in body hydration

Transport Function of the Plasma Membrane

• Simple diffusion follows the electrochemical gradient, through the phospholipid layer. Examples include O2, CO2, glycerol.
• Facilitated diffusion follows the electrochemical gradient, through protein channels or pores. Examples include Na+, K+, H2O, fructose(GLUT5)
• Primary active transport goes against the electrochemical gradient, using ATP directly. Examples include Na+-K+ ATPase, Ca++ pump
• Secondary active transport goes against the electrochemical gradient, uses the electrochemical gradient created by primary active transport. Examples include Na+-glucose (SGLT1), Na+-Ca++ exchanger, Na+-amino acid exchanger
• Bulk transport goes against the electrochemical gradient, includes endocytosis (bulk intake of substances) and exocytosis (bulk secretion of substances). WBC, neurotransmitters, (Ach), and hormones are examples.

Fick's Law of Diffusion

• Fick's Law relates diffusive flux to the gradient of concentration across a membrane.
• J = P × (ΔC/ΔX). More precisely written as J = (K×D×ΔC)/ΔX where J is the flux, P is permeability coefficient, K is partition coefficient, D is diffusion coefficient, ΔC is concentration difference and ΔX is the thickness of the membrane

Saturation in Carrier-Mediated Transport

• Carrier proteins have a limited number of binding sites for solutes
• At low solute concentrations, the rate of transport increases as more binding sites are available
• At high concentrations, the rate of transport levels off (transport maximum or Tm)

Stereospecificity in Carrier-Mediated Transport

• Binding sites on transport proteins are stereospecific (stereoselective)
• This means the transporter for glucose, in the kidney, recognizes and transports the natural isomer, D-glucose, but not L-glucose.

Competition in Carrier-Mediated Transport

• Binding sites for transported solutes are specific, but they may also recognize and bind chemically related solutes
• D-galactose may compete with D-glucose for binding sites, reducing glucose uptake

Ion Channel Characteristics

• Conductance (g) of a channel depends on the probability that it is open, the higher the probability, the higher its conductance. Conduction speeds up the rate of diffusion.
• Ion channel gates are controlled by sensors: voltage-gated channels respond to changes in membrane potential, ligand-gated channels respond to changes in ligands (hormones, neurotransmitters), second-messenger-gated channels respond to changes in signaling molecules, and mechanically-gated channels respond to changes in membrane tension.

Mechanisms Responsible for the Resting Membrane Potential

• Chemical gradients are generated by active transport pumps (e.g., Na+/K+ ATPase)
• Selective permeability of the membrane, particularly to potassium (K+) ions, creates a negative intracellular charge
• Electrical gradients arise from potassium leak (K2P channels) which attracts potassium ions back into the cell, opposing the chemical gradient
• Electrochemical equilibrium develops when the electrical and chemical forces balance for each ion species

Equilibrium Potential

• The equilibrium potential is the membrane potential where the diffusive forces (chemical gradient) equal the electrical forces(electrical charges).
• The ion always diffuses in a direction that brings the membrane potential toward its equilibrium potential.
• The overall current flow of the ion is directly proportional to the net force and conductance of the membrane for the ion.

Driving Force for Diffusion: Equilibrium Potential vs. RMP

• A cell's resting membrane potential (RMP) affects the diffusion of ions based on the driving force of the ion's equilibrium potential.
• The net driving force is the difference between the RMP and the ion's equilibrium potential.

Characteristics of Action Potentials

• Action potentials are stereotyped (specific size and shape) for a given cell type
• They propagate down the entire length of the axon without decrement
• They are all-or-none responses, requiring a certain threshold to initiate

Refractory Periods

• Absolute refractory period is the time where no stimulus can cause another AP to occur. Inactivation gates of Na+ channels are responsible.
• Relative refractory period is the time after the ARP, when an AP can fire but only if a larger than usual stimulus is applied. Higher K+ conductance is present during this period.

Propagation of Action Potentials

• Action potentials propagate along a nerve or muscle fiber by the spread of local currents, from active regions to inactive regions
• APs are initiated in the axon's initial segment (hillock)and propagated down the axon by local current spread

Factors Affecting Conduction Velocity in Nerves

• Factors that influence nerve conduction velocity include myelination (myelinated nerves conduct faster), axon diameter (wider axons conduct faster), and temperature (warmer temperatures conduct faster)

Receptors, Signaling Pathways, and Messengers

• Receptors include ligand-gated ion channels (e.g., GABA, acetylcholine), G protein-coupled receptors (e.g., acetylcholine, peptides), enzyme-linked receptors (e.g., insulin, growth factors), and nuclear receptors (e.g., steroid hormones)
• Signaling pathways involve second messengers (e.g., cAMP, DAG, IP3).
• Note: information about neurotransmitter pathways, second messengers, and associated receptors

Good Study Habits

• Good study habits are important to optimize understanding and retention
• Consistently cultivating good study habits leads to better comprehension, improved performance, and reduced stress.

Test your understanding of homeostasis and feedback mechanisms with this quiz. Explore the concepts of equilibrium, negative feedback loops, and positive feedback in physiological processes. Challenge yourself with questions that delve into these critical biological principles.