Week 1 · Chapter 1

Introduction to Anatomy & Physiology

This week sets the vocabulary and mental map you'll use for the entire course — anatomical position, directional terms, body planes, body cavities, and the concept of homeostasis. Every later chapter assumes you're fluent in this language.

By the end of this week, you will be able to:

1Distinguish anatomy from physiology and explain how structure informs function
2Use anatomical position and directional terms correctly
3Identify the three body planes and what each one divides
4Locate and describe the major body cavities and their subdivisions
5Explain the levels of structural organization from chemical to organism
6Explain homeostasis and contrast negative vs. positive feedback loops

Big Picture

Structure and function are the same story, told two ways

Anatomy is the study of structure — what the body's parts look like and how they're arranged. Physiology is the study of function — how those parts actually work. You cannot separate them: the shape of a structure exists because of the job it does. A bone is dense and rigid because it needs to bear weight. A capillary wall is one cell thick because it needs to let gases pass through quickly.

Every chapter this semester will ask you to hold both questions at once: what does this look like, and why does it look that way? That's the habit this course is training.

Why this chapter comes first

Before you can describe any structure in the body, you need a shared vocabulary for location and direction — the same way you'd need to agree on "north" and "east" before giving someone driving directions. That's exactly what this week builds.

Vocabulary & Word Parts

Directional roots — decode, don't memorize

Most anatomical direction terms are built from a small set of Latin roots. Once you know the root, you can often guess the meaning of a word you've never seen before.

anter/o-front

Anterior = toward the front of the body.

poster/o-back

Posterior = toward the back of the body.

super/o-above

Superior = toward the head/upper part.

infer/o-below

Inferior = toward the feet/lower part.

medi/o-middle

Medial = toward the midline of the body.

later/o-side

Lateral = away from the midline, toward the side.

Vocabulary & Word Parts

More roots you'll see all semester

proxim/o-nearest

Proximal = closer to the point of attachment (e.g., the elbow is proximal to the wrist).

dist/o-far

Distal = farther from the point of attachment.

-logystudy of

Physiology = study of function. Histology = study of tissue.

homeo-same, similar

Homeostasis = keeping internal conditions the same/stable.

-stasisstanding still, staying constant

Homeostasis = a stable, steady internal state.

viscer/o-internal organs

Visceral = relating to the internal organs (e.g., visceral pleura covers the lungs directly).

Structure & Function

Anatomical position and directional terms

Anatomical position: standing upright, facing forward, arms at sides, palms facing forward. Every directional term is defined relative to this position — not relative to how the body happens to be oriented in front of you.

Superior / Inferior — toward head vs. feet
Anterior / Posterior — toward front vs. back

Medial / Lateral — toward midline vs. side
Proximal / Distal — nearer vs. farther from attachment point

Structure & Function

The three body planes

A plane is an imaginary flat surface used to section the body for description or imaging (like a CT scan slice).

Sagittal — divides body into left and right
Frontal (Coronal) — divides body into front and back
Transverse — divides body into top and bottom (a "cross-section")

Structure & Function

The dorsal body cavity

Both parts are continuous with each other and both contain parts of the central nervous system — this is why a spinal injury and a head injury are discussed in the same anatomical "compartment."

Structure & Function

The ventral body cavity

The diaphragm is the muscular floor of the thoracic cavity and the physical boundary that separates it from the abdominopelvic cavity below.

Structure & Function

Mapping the abdomen: quadrants vs. regions

Two different systems for the same space. Quadrants are used for quick clinical description ("pain in the RLQ"); the 9-region system is used for precise anatomical location.

Structure & Function

Levels of structural organization

This is the roadmap for the entire course. Chapters 2–3 cover the chemical and cellular levels; Chapter 4 covers tissues; the rest of the semester moves through organs and organ systems.

Structure & Function

Homeostasis: the body's balancing act

Homeostasis is the body's ability to maintain a relatively stable internal environment — a steady body temperature, blood pH within a narrow range, stable blood glucose — even while the outside world is constantly changing.

Homeostasis is not a single fixed number — it's a narrow, dynamic range. Body temperature isn't locked at exactly 98.6°F; it fluctuates slightly and is constantly being nudged back toward that target.

Every feedback loop in the body exists to defend some variable's normal range. There are two kinds, and they behave in opposite ways.

Structure & Function

Negative feedback: the body's most common strategy

In a negative feedback loop, the response works to reverse the original change — moving the variable back toward normal. This is how most of the body's systems work, most of the time.

1

1 · Stimulus

Something pushes a variable away from its normal range — for example, body temperature rises above normal.

2

2 · Receptor

A sensor detects the change (e.g., temperature receptors in the skin and hypothalamus).

3

3 · Control Center

The brain (often the hypothalamus) compares the reading to the normal range and sends a command.

4

4 · Effector

An organ or tissue carries out the command — e.g., sweat glands activate, blood vessels near the skin dilate.

5

5 · Response

The variable moves back toward normal, which then reduces the original stimulus — the loop shuts itself off.

The defining feature: the response works AGAINST the direction of the original change. That's what "negative" means here — not bad, just opposing.

Structure & Function

Negative feedback loop — visualized

Body temperature regulation is the textbook example. Trace the loop clockwise: stimulus → receptor → (control center, not shown separately here) → effector → response reverses the stimulus.

Structure & Function

Positive feedback: the exception, not the rule

In a positive feedback loop, the response amplifies the original change instead of reversing it. These loops are rare in the body precisely because an unchecked amplifying loop is dangerous — they only appear where a process needs to be pushed to a fast, decisive completion.

Classic examples: childbirth (uterine contractions stretch the cervix → stretching signals the brain to release more oxytocin → oxytocin causes stronger contractions → repeat until birth occurs and the stretching stimulus ends) and blood clotting (platelets release chemicals that attract more platelets, rapidly building a clot).

Negative vs. Positive — side by side

Feature	Negative Feedback	Positive Feedback
Direction of response	Opposes/reverses the stimulus	Amplifies the stimulus
Frequency in the body	Very common — most systems	Rare — only specific processes
Ends when...	Variable returns to normal range	The triggering event is complete
Examples	Body temp, blood glucose, blood pressure	Childbirth, blood clotting, action potentials

Your Turn — Guided Practice

Apply it before the quiz

A patient reports pain that started near the navel and has now moved to the lower right side of the abdomen — a classic pattern for appendicitis.

Question 1 — Which quadrant would a clinician use to describe this final pain location?

The pain has moved to the lower right side of the abdomen — that's the Right Lower Quadrant (RLQ), which is also where the appendix sits. LUQ describes the upper-left abdomen, nowhere near this pain's location.

Question 2 — A fever spikes, and the body responds with sweating and vessel dilation to cool back down. Is this negative or positive feedback?

The response (sweating, vessel dilation) works to cool the body back down — it opposes the original rise in temperature. That reversal is the defining signature of negative feedback. Positive feedback would mean the response pushed the fever even higher, which isn't happening here.

Clinical Correlation

Why directional terms are a patient-safety issue

This isn't academic vocabulary for its own sake — imprecise language in a medical chart can cause real harm.

🩺 In practice

A surgical note that says "incision on the left side" is dangerously vague — left according to whom, looking from where? Clinical documentation always uses anatomical position as the fixed reference: the patient's own left and right, as if they were standing in anatomical position, regardless of how they're actually lying on the table or how the provider is standing. This is exactly why "RLQ pain" and "proximal fracture" are the standard, not casual shortcuts.

Clinical Correlation

When positive feedback goes wrong

Positive feedback loops are powerful precisely because they don't self-correct — which makes them dangerous when they activate inappropriately.

🩺 In practice

Septic shock is a clinical example of a normally protective process (the inflammatory response, and the clotting cascade) spiraling into a harmful positive feedback loop — inflammation triggers more inflammation, clotting triggers more clotting, faster than the body's negative feedback systems can rein it back in. This is part of why septic shock can deteriorate so quickly and why it's treated as a medical emergency.

Mini Knowledge Test

Show what you know — 7 questions

Question 1

Which term means "toward the head"?

Superior means toward the head/upper part of the body. Inferior is its opposite (toward the feet). Distal isn't a head-vs-feet term at all — it describes distance from a point of attachment on a limb, a completely different axis.

Question 2

A sagittal plane divides the body into:

Sagittal = left and right. Front/back is the frontal (coronal) plane. Top/bottom is the transverse plane. Picture a sagittal cut as a vertical slice running front-to-back through the body — it's that front-to-back blade that splits left from right.

Question 3

The heart sits within which cavity?

The heart's own compartment is the pericardial cavity, nested inside the larger thoracic cavity. The cranial cavity holds the brain; the pelvic cavity holds reproductive organs, bladder, and rectum — neither one holds the heart.

Question 4

Which is the correct order of organization, smallest to largest?

Each level is built entirely FROM the level directly below it: atoms/molecules (chemical) form cells (cellular), similar cells group into tissues, tissues combine into organs, organs sharing a job form an organ system, and all systems together make the organism. Any order that skips a step or runs backward breaks that build-up chain.

Question 5

Homeostasis is best described as:

Homeostasis is dynamic — the body defends a narrow RANGE, not one frozen number (body temperature drifts in a small band around 98.6°F rather than sitting locked on it). And homeostasis is about function/regulation (physiology), not structure — that's anatomy's territory.

Question 6

In a negative feedback loop, the response:

By definition, a negative feedback response counteracts the stimulus, nudging the variable back toward its normal range. A response that instead amplifies the stimulus is positive feedback. And every true feedback loop responds directly to its stimulus — "no relationship" would mean it isn't a feedback loop at all.

Question 7

Which of these is a positive feedback example?

Labor contractions are positive feedback: each contraction triggers MORE oxytocin release, which makes contractions even stronger, until birth ends the loop. Sweating and shivering are both negative feedback — they work to push body temperature back toward normal, opposing the original change instead of amplifying it.

Score: 0 / 7

Debrief

Week 1 Quick-Reference Recap

One page to glance at before Exam 1 — everything from this week, condensed.

Directional Terms

Superior/Inferior — head/feet
Anterior/Posterior — front/back
Medial/Lateral — midline/side
Proximal/Distal — near/far from attachment

Body Planes

Sagittal — left/right
Frontal (Coronal) — front/back
Transverse — top/bottom

Body Cavities

Dorsal — Cranial + Vertebral
Ventral — Thoracic (2 Pleural + Pericardial) + Abdominopelvic (Abdominal + Pelvic)

Levels of Organization

Chemical → Cellular → Tissue → Organ → Organ System → Organism

Homeostasis

Stable internal range, actively defended by feedback loops.

Feedback Loops

Negative = opposes change (common)
Positive = amplifies change (rare, self-limiting event)

Key Terms This Week

AnatomyPhysiologyAnatomical PositionDirectional TermsBody PlanesBody CavitiesHomeostasisNegative FeedbackPositive Feedback

Next — Week 2: The Chemical Level of Organization. You'll move one level down the organization ladder — atoms, bonding, and the biochemistry that everything cellular is built from.

Next: Week 2 →