Physiology - Unit 1 Fluids and Homeostasis

Extracellular Fluid (ECF)

All body water outside cells, divided into plasma and interstitial fluid. It serves as the compartment where nutrients, gases, and wastes are exchanged.
Exam tie‑in: ECF = 1/3 of Total Body Water (TBW, 60% of Body Weight).

Intracellular Fluid (ICF)

Fluid inside cells, containing most of the body’s water. It supports metabolism, enzyme activity, and ion gradients.
Exam tie‑in: ICF = 2/3 of TBW.

Interstitial Fluid (ISF)

The portion of ECF that lies between cells. It is the immediate environment for most cells.
Exam tie‑in: ISF = 80% of ECF.

Plasma

The fluid portion of blood (excluding RBCs and WBCs). Found inside the cardiovascular system.
Exam tie‑in: Plasma = 20% of ECF.

Cell (Plasma) Membrane

A selective barrier that separates ICF from ECF, maintaining very different compositions.
Concept tie‑in: ECF is often compared to the “internal sea” bathing cells.

Capillary Wall

Separates plasma from interstitial fluid. These compartments are similar except plasma contains plasma proteins that normally cannot cross the capillary wall.

Edema

Excess interstitial fluid accumulation. Often occurs when the capillary wall is damaged, allowing plasma proteins to leak into ISF and pull water with them.

Homeostasis

The body’s ability to maintain relative constancy of internal variables (e.g., osmolarity, fluid volume) despite external changes.

Relative Constancy / Dynamic Constancy

A regulated variable fluctuates within preset limits, but control systems return it toward normal. Not perfectly constant — controlled variation.

Reflex Arc

The basic biological control pathway linking a stimulus to a response:
Stimulus → Receptor → Afferent Pathway → Integrating Center → Efferent Pathway → Effector → Response
Exam tie‑in: You must be able to fill in this diagram.

Stimulus

A detectable change in a regulated variable (e.g., drop in blood pressure).

Receptor

A sensor that detects the stimulus and sends information to the integrating center.

Afferent Pathway

Neural or hormonal route carrying information to the integrating center.

Integrating Center

Usually the brain or spinal cord. Compares input to the set point and generates output signals.

Efferent Pathway

Neural or hormonal route carrying commands away from the integrating center to effectors.

Effectors

Cells or organs that carry out the corrective response (e.g., sweat glands, kidneys, smooth muscle).

Response

The change in effector activity that moves the variable back toward the set point.

Set Point / Equilibrium

The normal value the body aims to maintain (e.g., plasma osmolarity ~290 mOsm).
Note: Homeostasis ≠ equilibrium — homeostasis requires energy.

Negative Feedback

A response that reduces the initial disturbance, restoring the variable toward normal.
Example: Shivering when cold increases heat production.

Positive Feedback

A response that amplifies the initial change until a major event stops the loop.
Examples: Childbirth (oxytocin), blood clotting.

Feedforward Mechanism

A response that anticipates a change and activates corrective actions before the variable is altered.
Example: Salivating before eating; increased heart rate before exercise.

Negative Balance

Loss > gain of a substance.
Example: Sweating more water than you drink.

Positive Balance

Gain > loss of a substance.
Example: Rehydrating after exercise.

Hormone

Released by endocrine cells into the bloodstream to act on distant targets.
Example: ADH regulating water balance.

Neurotransmitter

Released by neurons into a synaptic cleft to act on another neuron, muscle, or gland.
Example: Acetylcholine at the neuromuscular junction.

Paracrine Agent

Acts on nearby cells within the same tissue.
Example: Histamine released during inflammation.

Autocrine Agent

Acts on the same cell that released it.
Example: Immune cells releasing cytokines that regulate their own activity.