Body organization · Topic 2 of 18

Levels of Organization & Homeostasis

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Anatomy · organization & homeostasis

Levels of Organization & Homeostasis

The body builds up from chemicals to organism and holds its internal conditions steady, mostly by negative feedback.

01 · LevelsChemical, cell, tissue, organ, system, organism

02 · Most commonNegative = reverses the change

03 · RarePositive = amplifies until done

What it is

Chemical → cell → tissue → organ → organ system → organism must know

Homeostasis = keeping internal conditions steady in a normal range must know

Negative feedback reverses a change back to the set point must know

Positive feedback amplifies a change until an end point must know

Cell = the smallest unit that can function on its own

Tissue = similar cells working together on one job

Organ = a structure made of two or more tissue types

Organ system = organs working together (OpenStax lists 11)

Parts & jobs

Most body water is inside cells: ICF ≈ 2/3, ECF ≈ 1/3

ECF = interstitial fluid (between cells) + plasma (~20% of ECF)

Step by step

Loop: sensor (receptor) → control center → effector

Know by heart

Set point = the target value; body temperature ≈ 37 °C (98.6 °F)

Worked examples

Temperature: too hot → sweat, skin vessels dilate; too cold → shiver

Glucose: high → insulin → glucose falls → insulin stops

Childbirth: stretch → oxytocin → stronger contractions, until birth

Don't confuse

Inside cells: K⁺ high; outside cells: Na⁺ and Cl⁻ high

Negative or positive feedback?

Commonly confused

	Negative feedback	Positive feedback
Effect on the change	Reverses it	Intensifies it
Result	Back to the set point (homeostasis)	Moves further away until an end point
How common	Most body regulation	Rare; normal only with a definite end
Examples	Body temperature, blood glucose	Childbirth contractions, blood clotting

Mix-ups to avoid

"Negative" = opposes the change, not "harmful"; both kinds are normal

More detail

Show 6 more facts

Chemical level: atoms join into molecules (water, sugars, proteins)

Many organs serve more than one organ system

Clotting: each step speeds the next until the vessel is sealed

Body water: ~50–60% of adult mass (~75% infant, ~45% elderly)

CSF, lymph, synovial and serous-cavity fluids are also ECF

Plasma → interstitial fluid → excess drains as lymph → back to blood

Pearl

Negative feedback pushes back against a change; positive feedback pushes it further until the job is done.

Sources · OpenStax Anatomy and Physiology 2e §1.2 Structural Organization of the Human Body · OpenStax Anatomy and Physiology 2e §1.1 Overview of Anatomy and Physiology · OpenStax Anatomy and Physiology 2e §1.5 Homeostasis · OpenStax Anatomy and Physiology 2e §26.1 Body Fluids and Fluid Compartments · Achievable — Intracellular vs. extracellular fluid (ICF ≈ two-thirds of body water; 60-40-20 rule) · Union Test Prep — HESI Anatomy and Physiology study guide, page 1 (fluid distribution, interstitial fluid) · NurseHub — HESI A2 Anatomy & Physiology course: General Terminology lessons (levels of organization; homeostasis) · OpenExamPrep — HESI A2 Anatomy & Physiology study guide §1.3 Homeostasis & Feedback Mechanisms · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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