General Physiology: Cell Membrane, Body Fluid Compartments, and Regulatory Systems

Human Physiology

The scientific study of the normal physical and biochemical functions of living human tissues, organs, and organ systems, and their integrated regulation in a body comprising approximately $75\text{--}100\text{ trillion}$ ($10^{12}$) cells.

Total Body Water (TBW)

The total volume of water distributed throughout the intracellular and extracellular fluid compartments, comprising approximately $60\%$ of total body weight ($\approx 42\,\text{L}$) in an average adult male, $70\text{--}75\%$ in infants, and approximately $54\%$ in adult females.

Intracellular Fluid (ICF)

The fluid compartment contained within cells, accounting for approximately two-thirds ($2/3$) of total body water or $40\%$ of total body weight ($\approx 28\,\text{L}$ in an average adult male), characterized by high concentrations of $\text{K}^+$, $\text{Mg}^{2+}$, and phosphate ions.

Extracellular Fluid (ECF)

The fluid compartment outside cell membranes, constituting one-third ($1/3$) of total body water or $20\%$ of body weight ($\approx 14\,\text{L}$), which is subdivided into interstitial fluid and plasma and characterized by high concentrations of $\text{Na}^+$, $\text{Cl}^-$, and $\text{HCO}_3^-$.

Interstitial Fluid (ISF)

The extravascular portion of extracellular fluid that bathes cells and serves as their immediate internal environment, accounting for three-quarters ($3/4$) of the ECF ($15\%$ of body weight, $\approx 11\,\text{L}$).

Plasma (Intravascular Fluid)

The fluid component of blood circulating within the cardiovascular system, accounting for one-quarter ($1/4$) of the ECF ($5\%$ of body weight, $\approx 3\,\text{L}$).

Heavy Water ($\text{D}_2\text{O}$)

Deuterium oxide; an isotopic indicator that distributes evenly throughout all water compartments of the body, used via Fick's dilution principle to determine total body water volume.

Inulin

An inert polysaccharide that freely distributes throughout the extracellular space but cannot penetrate cell membranes, used to measure extracellular fluid (ECF) volume.

Evans Blue Dye

A synthetic dye that binds tightly to plasma albumin and remains confined to the vascular compartment, used to determine total plasma volume.

Homeostasis

The maintenance of a relatively constant physical and chemical composition within the internal environment (the interstitial fluid) surrounding the cells of the body.

Negative Feedback Control

A primary homeostatic regulatory mechanism in which the effector response opposes and reverses the direction of the initial stimulus, restoring the variable toward its normal set point (e.g., thermoregulation).

Positive Feedback Control

A regulatory mechanism in which the output or response of a system accelerates, amplifies, or reinforces the original stimulus in the same direction (e.g., oxytocin release during parturition).

Cell Membrane

A thin, elastic, semipermeable barrier approximately $7.5\text{--}10\,\text{nm}$ ($75\text{--}100\,\text{\AA}$) thick surrounding the cell, composed of proteins ($55\%$), phospholipids ($25\%$), cholesterol ($13\%$), other lipids ($4\%$), and carbohydrates ($3\%$).

Amphipathic Molecule

A chemical structure that possesses both a hydrophilic, polar, water-soluble region (e.g., the phosphate head of a phospholipid) and a hydrophobic, non-polar, water-insoluble region (e.g., two fatty acid tails).

Membrane Carbohydrates

Sugars present on the external surface of the cell membrane conjugated as glycoproteins or glycolipids that function in immune responses, cell-to-cell adhesion, self-differentiation, and cellular recognition.

Tight Junctions (Zonula Occludens)

Intercellular junctional complexes that fuse adjacent cell membranes together to seal intercellular spaces and prevent paracellular movement of substances.

Gap Junction

An intercellular junction formed by aligned connexons in adjacent plasma membranes that establishes a direct $3.5\,\text{nm}$ aqueous channel allowing the transfer of ions and small signaling molecules between cells.

Simple Diffusion

The passive movement of molecules or ions across a membrane down their concentration gradient without binding to carrier proteins or expending metabolic energy.

Aquaporins

Specialized transmembrane protein water channels that allow rapid passage of water molecules across cell membranes in addition to direct diffusion through the lipid bilayer.

Voltage-Gated Channels

Transmembrane protein channels whose opening and closing gates are regulated by alterations in the electrical transmembrane potential.

Ligand-Gated Channels

Membrane ion channels whose gating mechanism is triggered by the binding of a specific chemical substance (ligand) to an extracellular or intracellular receptor domain.

Facilitated Diffusion

Carrier-mediated passive transport of large or polar molecules across the cell membrane down their concentration gradient, which exhibits chemical specificity, competitive inhibition, and a maximum transport rate ($V_{\max}$) due to carrier saturation.

Osmosis

The net diffusion of solvent particles down their chemical gradient across a selectively permeable membrane toward an area of higher solute concentration.

Osmotic Pressure

The exact hydrostatic pressure required to completely stop the movement of solvent across a semipermeable membrane, determined solely by the number of solute particles per unit volume rather than their mass.

Osmole

A unit equal to the number of osmotically active particles liberated into solution by one mole of a substance ($1\,\text{mole}$ of non-ionizing glucose = $1\,\text{Osmole}$; $1\,\text{mole}$ of completely dissociating $\text{NaCl}$ = $2\,\text{Osmoles}$).

Osmolarity

The concentration of osmotically active solute particles expressed as the number of osmoles per liter of water ($\text{mOsm/L}$, which in normal plasma equals approximately $290\,\text{mOsm/L}$ or $5597\,\text{mmHg}$ of osmotic pressure).

Tonicity

The effective osmolality of a solution relative to the osmolality of normal blood plasma ($290\,\text{mOsm/L}$), determining the direction of water movement and cell volume changes.

Primary Active Transport

The transport of a solute across a cell membrane against an electrochemical gradient powered directly by the hydrolysis of adenosine triphosphate (ATP) via the carrier's intrinsic ATPase activity.

$\text{Na}^+/\text{K}^+\,\text{ATPase}$ Pump

An electrogenic primary active transport pump heterodimer whose $\alpha$ subunit pumps $3\,\text{Na}^+$ out of the cell and $2\,\text{K}^+$ into the cell per ATP hydrolyzed; it is specifically inhibited by ouabain and digitalis glycosides.

Secondary Active Transport

The carrier-mediated transport of a substance against its concentration gradient coupled to the downhill movement of an ion (typically $\text{Na}^+$), drawing energy from the ionic concentration gradient established by primary active transport.

Symport

A form of co-transport in which two different molecules or ions are transported across the plasma membrane simultaneously in the exact same direction (e.g., sodium-glucose linked transporters SGLT1 and SGLT2).

Antiport

A carrier-mediated transport mechanism in which two substances are moved across the cell membrane simultaneously in opposite directions (e.g., the $\text{Na}^+/\text{K}^+$ pump or $\text{Na}^+/\text{H}^+$ exchanger).

Pinocytosis

A form of endocytosis ('cell drinking') wherein cells take up small volumes of extracellular fluid and dissolved macromolecules by pinching off small invagination vesicles from the plasma membrane.

Phagocytosis

A specialized vesicular endocytic process ('cell eating') in which certain specialized cells (e.g., white blood cells) engulf large particulate matter, dead cellular debris, or microorganisms.

Exocytosis

The vesicular transport process by which intracellular membrane-bound vesicles fuse with the plasma membrane to expel their contents into the extracellular fluid.

Reflex Arc

The fundamental functional unit of the nervous system, composed of a stimulus, a receptor (sense organ), an afferent neuron, a central integrating synapse, an efferent neuron, an effector organ, and an ultimate response (SRACEER).

Neurons

Excitable nerve cells that serve as the primary structural and functional units of nervous tissue, specialized for the generation and rapid propagation of action potentials.

Neuroglia

Supporting, non-conducting cells of the nervous system (from the Greek word for 'glue') that surround, insulate, and protect neurons.

Soma

The main cell body of a neuron containing the nucleus, organelles, and cytoplasm, which functions as the primary metabolic and processing center of the nerve cell.

Dendrites

Antenna-like branching cytoplasmic processes that project from the neuronal soma, functioning to increase the surface area available for receiving synaptic inputs from other neurons.

Axon Hillock

The distinct, conical, thickened zone of the neuronal soma from which the axon originates and gives rise to the unmyelinated initial segment.

Myelin Sheath

A multi-layered sphingomyelin lipid coating produced by Schwann cells in the PNS that insulates the axon, decreasing membrane ion flow approximately $5000\text{-fold}$ to markedly accelerate conduction velocity.

Nodes of Ranvier

Uninsulated periodic gaps in the myelin sheath along the length of an axon where the axolemma is exposed to the extracellular fluid, enabling saltatory nerve impulse conduction.

Terminal Buttons (Synaptic Knobs)

The bulbous terminal expansions of axon branches that contain granules or membrane-bound vesicles storing synaptic neurotransmitters.

Paracrine Signaling

A mode of local chemical intercellular communication in which regulatory molecules secreted into the interstitial fluid act upon adjacent target cells via local diffusion.

Autocrine Signaling

A form of local chemical signaling in which a cell secretes a chemical messenger that binds to and activates specific receptors located on that same cell's own membrane.

Endocrine Gland

A ductless secretory organ that synthesizes chemical messengers (hormones) and secretes them directly into circulating blood fluids to regulate distant target organs.

Hypothalamo-Hypophyseal Portal System

A specialized vascular circulation connecting the hypothalamus to the anterior pituitary gland (pars distalis of adenohypophysis), conveying releasing and inhibiting hormones to control pituitary secretion.

Hypothalamo-Hypophyseal Tract

A nerve bundle originating from the paraventricular and supraoptic hypothalamic nuclei that conducts and stores antidiuretic hormone (ADH) and oxytocin in nerve endings of the posterior pituitary (neurohypophysis).

Steroid Hormones

Lipid-soluble chemical messengers derived from cholesterol, synthesized and secreted by the adrenal cortex (cortisol, aldosterone), ovaries (estrogen, progesterone), testes (testosterone), and placenta.

Monoamine Hormones

Hormones derived from the amino acid tyrosine, including the catecholamines (epinephrine, norepinephrine, dopamine) secreted by the adrenal medulla and iodinated thyronines ($\text{T}_3$, $\text{T}_4$) from the thyroid gland.

Second Messenger

An intracellular molecule (e.g., $\text{cAMP}$, diacylglycerol) produced inside a target cell following the binding of an extracellular primary messenger (hormone) to its surface receptor, mediating downstream cellular cascades.

Receptor Down-Regulation

A cellular regulatory process wherein prolonged exposure to elevated hormone concentrations decreases the total number or responsiveness of target cell receptors via lysosomal degradation, internalization, or reduced synthesis.

Receptor Up-Regulation

An increase in target cell receptor expression or sensitivity that occurs secondary to prolonged exposure to deficient or diminished amounts of a stimulating hormone.

G Protein-Coupled Receptor (GPCR)

A seven-transmembrane cell-surface receptor protein that activates heterotrimeric G protein subunits ($\alpha$, $\beta$, $\gamma$) by exchanging GDP for GTP to regulate intracellular enzymes or ion channels.