General Physiology: Body Fluids, Homeostasis, and Membrane Transport

A complete set of vocabulary flashcards covering fluid compartments, homeostatic systems, ion channel gating, and passive, active, and bulk transport mechanisms.

Total Body Water (TBW)

The total volume of water in the body, accounting for approximately $60\%$ of body weight ($42\,\text{L}$) in a standard $70\,\text{kg}$ adult male.

Intracellular Fluid (ICF)

Fluid located inside body cells, representing $\frac{2}{3}$ of total body water ($28\,\text{L}$ in a $70\,\text{kg}$ male) with $\text{K}^+$ as its major cation and $\text{PO}_4^{3-}$ and proteins as major anions.

Extracellular Fluid (ECF)

Fluid located outside body cells, representing $\frac{1}{3}$ of total body water ($14\,\text{L}$ in a $70\,\text{kg}$ male) with $\text{Na}^+$ as its major cation and $\text{Cl}^-$ and $\text{HCO}_3^-$ as major anions.

Interstitial Fluid (ISF)

The sub-compartment of extracellular fluid surrounding tissue cells, making up $\frac{3}{4}$ of ECF ($10.5\,\text{L}$ in a $70\,\text{kg}$ male) with very low to negligible protein concentration.

Plasma (Intravascular Fluid)

The liquid portion of blood contained within blood vessels, making up $\frac{1}{4}$ of ECF ($3.5\,\text{L}$ in a $70\,\text{kg}$ male) and containing a high concentration of plasma proteins such as albumin.

Homeostasis

The maintenance of a relatively constant or stable internal environment within the body necessary for cellular function and survival.

Stimulus (Homeostatic)

A change in the internal or external environment that disrupts homeostasis.

Receptors (Sensors)

Structures that detect a specific change or stimulus in the environment and transmit input signals to the control center via afferent pathways.

Afferent Pathways

Routes through which input signals are transmitted from receptors or sensors to the homeostatic control center.

Control Center (Set Point)

Component of a homeostatic control system that evaluates input signals against an established physiological set point and determines the appropriate response.

Efferent Pathways

Routes through which output signals are transmitted from the control center to the effectors.

Effectors

Organs, muscles, or glands that carry out the physiological command received from the control center.

Homeostatic Response

The action executed by an effector that corrects a physiological alteration and brings the parameter back toward the set point.

Negative Feedback Mechanism

The primary homeostatic mechanism that reverses or opposes the direction of an initial change to return a physiological parameter to its baseline set point.

Positive Feedback Mechanism

A homeostatic mechanism that amplifies or reinforces an initial change in the same direction, pushing a variable further away from baseline until a self-terminating process completes.

Non-Gated (Leakage) Channels

Ion channels that remain continuously open, permitting passive ion movement down electrochemical gradients (e.g., $\text{K}^+$ leakage channels).

Voltage-Gated Channels

Gated ion channels regulated by changes in membrane electrical potential.

Ligand-Gated Channels

Gated ion channels regulated by the binding of a chemical messenger (ligand) to a specific receptor.

Resting State (Voltage-Gated $\text{Na}^+$ Channel)

The closed gating state of voltage-gated sodium channels dominating at $-70\,\text{mV}$, where the activation gate ($\text{M gate}$) is closed and the inactivation gate ($\text{H gate}$) is open.

Activated State (Voltage-Gated $\text{Na}^+$ Channel)

The open gating state triggered when threshold potential ($-55\,\text{mV}$) is reached, opening both $\text{M}$ and $\text{H}$ gates to allow rapid $\text{Na}^+$ influx up to $+30\,\text{mV}$.

Inactivated State (Voltage-Gated $\text{Na}^+$ Channel)

The refractory closed state occurring at peak potential ($+30\,\text{mV}$) where the inactivation gate ($\text{H gate}$) closes rapidly to block further $\text{Na}^+$ influx until the membrane repolarizes.

Simple Diffusion

Passive transport of substances directly through the lipid bilayer or non-gated channels down concentration gradients without energy, carrier proteins, saturation ($\text{T}_\text{m}$), or competitive inhibition.

Facilitated Diffusion

Passive transport of substances across a membrane down their concentration gradient using a specific carrier protein, exhibiting transport maximum ($\text{T}_\text{m}$), saturation, and competitive inhibition.

Glucose Transporter-4 (GLUT-4)

A carrier protein that translocates to the cell membrane upon insulin binding to allow glucose uptake down its concentration gradient via facilitated diffusion.

Osmosis

The net diffusion of water molecules across a semi-permeable membrane from a region of high water concentration (low solute) to a region of low water concentration (high solute).

Primary Active Transport

Active transport mechanism where energy is derived directly from the hydrolysis of ATP by ATPase enzymes to move substances against concentration gradients.

Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+\text{ ATPase}$)

Primary active transport pump that extrudes $3\,\text{Na}^+$ and imports $2\,\text{K}^+$ per hydrolyzed ATP molecule to maintain low intracellular $\text{Na}^+$, high intracellular $\text{K}^+$, resting membrane potential, and cellular volume.

Calcium Pump ($\text{Ca}^{2+}\text{ ATPase}$)

Primary active transport pump that pumps $\text{Ca}^{2+}$ out of the cytosol into extracellular fluid or intracellular organelles against a steep concentration gradient.

Proton Pump ($\text{H}^+/\text{K}^+\text{ ATPase}$)

Primary active transport pump in parietal cells of stomach mucosa that pumps $\text{H}^+$ into the gastric lumen to generate gastric acid ($\text{HCl}$).

Secondary Active Transport

Active transport mechanism driven indirectly by energy stored in an ion concentration gradient (typically $\text{Na}^+$) previously created by primary active transport.

Co-Transport (Symport)

Secondary active transport process where the transported substance moves in the same direction as the driving ion (e.g., Sodium-Glucose Co-transporter SGLT).

Counter-Transport (Antiport / Exchanger)

Secondary active transport process where the transported substance moves in the opposite direction of the driving ion (e.g., $\text{Na}^+/\text{H}^+$ exchanger and $\text{Na}^+/\text{Ca}^{2+}$ exchanger).

Exocytosis

Bulk transport process in which intracellular secretory vesicles fuse with the plasma membrane to release contents into the extracellular fluid.

Endocytosis

Bulk transport process involving the infolding of the plasma membrane to engulf external materials into intracellular vesicles.

Phagocytosis

A form of endocytosis involving the internalization of large particulate matter ("cell eating").

Pinocytosis

A form of endocytosis involving the internalization of extracellular fluid droplets and dissolved solutes ("cell drinking").