Cardiac Function and Membrane Transport Quiz

Digoxin and Cardiac Function

• Digoxin inhibits Na+/K+ ATPase, leading to increased intracellular sodium (Na+) and decreased sodium gradient.
• This results in a decreased Na+/Ca2+ counter-transport, ultimately increasing intracellular calcium (Ca2+).
• It has been a foundational treatment for heart failure and is currently the only oral inotropic agent in clinical use.

Cell Membrane Transport

• Carriers are enzyme-like proteins that create passive pathways for solutes to cross membranes along concentration gradients.
• Channels are ion-selective pores that open and close transiently, facilitating ionic movement based on electrical and chemical gradients.
• Signals in excitable cells are produced through channel activity that alters membrane potential.

Excitable Cells and Membrane Potential

• The transmembrane potential (Em) is the voltage difference across the cell membrane, typically negative inside relative to the outside.
• Em ranges from -10 mV to -100 mV in living cells, indicating high intracellular potassium (K+) and anionic protein levels, while sodium (Na+) and chloride (Cl-) are relatively low.
• Diffusion potential depends on ion charge, membrane permeability, and ion concentration differences.

Excitable Cells: Action Potential

• Action potentials are initiated by depolarization reaching a threshold, following a passive process dependent on membrane properties.
• The local circuit current maintains depolarization spread, increasing Na+ permeability as the threshold is met.
• Refractory regions help ensure action potentials propagate away from each other.

Nerve Fibers and Action Potential Propagation

• The speed of action potential conduction in nerve fibers is influenced by axon diameter and myelination.
• Larger diameter unmyelinated fibers conduct signals faster than smaller ones, while myelination significantly increases conduction velocity.
• Absolute refractory period prevents subsequent action potentials regardless of stimulus strength.

Cellular Physiology and Organization

• The human body is formed from ~100 trillion differentiated cells organized into tissues, organs, and organ systems.
• Hormones are chemical messengers acting on distant cells, while paracrine and autocrine signals target nearby and self cells, respectively.

Body Fluid Composition

• Body water content is approximately 57%, with variations (65-70% in infants).
• Total body water distribution: intracellular fluid (ICF) at ~63%, extracellular fluid (ECF) at ~37%, with interstitial fluid and plasma volumes noted.

Body Mass Index (BMI)

• BMI is calculated as weight (kg) divided by height (m²) and serves as a key metric for assessing obesity.
• Categories include underweight, normal weight, overweight, and obesity.

Cell Membrane Functions and Solute Movement

• Cell membranes regulate solute movement, with Fick’s First Law detailing solute diffusion across membranes.
• Lipid bilayers serve as barriers to water and water-soluble substances, while specific channels facilitate transport.
• Reflection coefficient indicates solute permeability through membranes, ranging from impermeable (s = 1) to highly permeable (s = 0).

Epithelial Transport Mechanics

• Epithelia act as barriers between internal and external environments, regulating fluid balance, nutrient absorption, and waste excretion.
• Na+ transport in epithelial cells can be intrinsic or extrinsic, adjusting to maintain electrolyte homeostasis.
• Epithelial water transport can vary significantly between regions, displaying high, low, or regulated permeability depending on specific needs.

Excitable Cells: Muscle Types

• Skeletal muscle exhibits isometric contractions without motion, characterized by passive and total tension measurements at various lengths.
• Active tension stems from cross-bridge cycling, while passive tension accumulates from muscle stretching.

Cell Volume Regulation and Na-K Pump

• Cells swell and burst without the Na-K pump, which transports 3 Na+ out for every 2 K+ in.
• Proteins and organic compounds inside cells are anionic, leading to cation accumulation and water influx.
• The Na-K pump is activated by a 15% increase in cell size.

Body Fluid Distribution: Tonicity

• Isotonic: No net fluid movement in or out of red blood cells.
• Hypotonic: Causes net fluid influx into red blood cells, leading to swelling or lysis.
• Hypertonic: Causes net fluid outflow, resulting in cell shrinkage.
• Isosmotic: Equal concentration of osmotically active particles across a semipermeable membrane.

Osmolarity vs Tonicity

• Osmolarity indicates concentration of solutes and varies with solute type (NaCl, Urea, CaCl2).
• NaCl concentrations and their corresponding osmolarity impact tonicity classifications (isotonic, hypertonic, hypotonic).

Cell Membrane Transport

• Diffusion: Passive transport, movement down a concentration gradient without energy.
• Active Transport: Involves energy and a "carrier" to move solutes against a concentration gradient.

Pumps and Membrane Carriers

• Pumps, like ATP-dependent Na-K pump, establish ion gradients and utilize energy (ATP).
• Cardiac glycosides inhibit Na-K ATPase, leading to increased intracellular Na+ and Ca2+ for heart function.

Reflection Coefficient

• Measures permeability of solute through membranes (ranges from 0 to 1), where 1 indicates complete impermeability.

Membrane Structure and Solute Movement

• Cell membranes limit solute movement, maintaining concentration gradients.
• Fick’s First Law describes solute movement across membranes, emphasizing permeability and concentration differences.

Ion Concentration Gradients in Excitable Cells

• High intracellular potassium (K+) and anionic proteins (Pr-); low sodium (Na+) and chloride (Cl-).
• Extracellular fluid is rich in Na+ and Cl-, almost devoid of K+.

Membrane Potential in Excitable Cells

• Transmembrane potential (Em) is negative inside relative to outside, typically between -10 mV to -100 mV.
• Changes in permeability to ions regulate Em; Na-K ATPase maintains homeostasis.

Graded and Action Potentials

• Graded Potential: Local depolarization that may not reach threshold; proportional to stimulus intensity.
• Action Potential: A rapid and all-or-nothing signal.
• Depolarizing phase caused by increased Na+ permeability; repolarizing phase by K+ efflux.

Excitable Cell Functionality

• Types include neurons, skeletal, cardiac, and smooth muscle.
• Action potentials are crucial for signaling and muscle contraction.

Membrane Potential Dynamics

• Depolarization increases Na+ permeability; hyperpolarization decreases it.
• Overshoot occurs when Em surpasses 0 mV, returning to repolarization.

Refractory Periods

• Absolute Refractory Period: No new action potential can be generated regardless of stimulus strength.

Factors Influencing Nerve Conduction

• Conduction velocity in nerve fibers is influenced by axon diameter and myelination.
• Myelinated fibers offer faster conduction due to insulated segments (internodal lengths).

Overall Functional Properties of Axons

• Large diameter axons enable fast conduction and high excitability, while smaller diameters result in slower conduction.

Test your knowledge on digoxin's effects on cardiac function as well as the principles of cell membrane transport. Explore how ion channels and carriers facilitate solute movement and understand the significance of membrane potential in excitable cells. This quiz covers essential concepts in cardiovascular physiology and cellular biology.