The Human Body: An Orientation
The language of anatomy, levels of organization and homeostasis
By the end of this unit you can…
- ✓Distinguish anatomy from physiology and name their subdivisions
- ✓List the levels of structural organization from chemical to organismal
- ✓Explain homeostasis and compare negative vs positive feedback
- ✓Use anatomical position, directional terms, regional terms and body planes
- ✓Locate the body cavities, serous membranes and abdominopelvic regions
Key terms
Practice →1Anatomy vs physiology
Anatomy studies the structure of body parts and their relationships. Physiology studies function — how those parts work and keep you alive.
The unifying idea of the whole course is the principle of complementarity of structure and function: what a structure can do depends on its specific form. Bones support because they contain hard minerals; the thin walls of lung alveoli let gases diffuse quickly.
| Subdivision | Focus |
|---|---|
| Gross (macroscopic) anatomy | Structures visible to the naked eye — studied regionally, systemically or as surface anatomy |
| Microscopic anatomy | Cytology (cells) and histology (tissues) |
| Developmental anatomy | Structural changes across the lifespan; embryology covers changes before birth |
| Physiology | Usually organized by organ system: renal, neurophysiology, cardiovascular… |
2Levels of structural organization
The body is built in six levels, each made from the one below it:
- Chemical — atoms combine into molecules (water, proteins, DNA).
- Cellular — cells are the smallest units of life; organelles do the work inside them.
- Tissue — groups of similar cells with a common function. There are only four basic tissue types: epithelial, connective, muscle and nervous.
- Organ — two or more tissue types performing a specific function (e.g. the stomach).
- Organ system — organs working closely together (e.g. digestive system).
- Organismal — all the organ systems combined: the living human.
| Organ system | Key job |
|---|---|
| Integumentary | External protective covering; vitamin D synthesis; temperature regulation |
| Skeletal | Support, protection, blood cell formation, mineral storage |
| Muscular | Movement, posture, heat production |
| Nervous | Fast-acting control via electrical impulses |
| Endocrine | Slower control via hormones (growth, reproduction, metabolism) |
| Cardiovascular | Heart pumps blood carrying O₂, nutrients, hormones and wastes |
| Lymphatic / Immune | Returns leaked fluid to blood; houses white blood cells; defense |
| Respiratory | Supplies O₂ and removes CO₂ |
| Digestive | Breaks food into absorbable units; eliminates indigestible residue |
| Urinary | Removes nitrogenous wastes; regulates water, electrolytes and acid–base balance |
| Reproductive | Produces offspring |
3Life functions & survival needs
Every living human must carry out these necessary life functions:
- Maintaining boundaries
- Keeping the inside distinct from the outside — the plasma membrane for cells, the skin for the body.
- Movement
- Of the whole body (skeletal muscle) and of substances inside it (blood, food, urine).
- Responsiveness (excitability)
- Sensing a change (stimulus) and reacting to it — e.g. withdrawing a hand from a hot pan.
- Digestion
- Breaking food down to simple molecules that can be absorbed into the blood.
- Metabolism
- All chemical reactions in the body: catabolism (breaking down) + anabolism (building up).
- Excretion
- Removing wastes (urea, CO₂, feces).
- Reproduction
- At the cellular level (new cells for growth and repair) and the organismal level (making a new person).
- Growth
- An increase in size of a body part or the whole organism, usually by more cells.
Survival needs: nutrients, oxygen, water (the most abundant chemical in the body — about 60–80 % of body weight), normal body temperature (≈ 37 °C / 98.6 °F) and appropriate atmospheric pressure (needed for breathing and gas exchange).
4Homeostasis & feedback
Homeostasis is the ability to maintain relatively stable internal conditions even though the outside world keeps changing. It is a dynamic equilibrium — values fluctuate within a narrow normal range, they don't sit still.
Every homeostatic control mechanism has three parts:
- Receptor
- A sensor that monitors the environment and detects changes (stimuli). Sends info along the afferent pathway (toward the control center).
- Control center
- Sets the range (set point), analyzes input and decides the response. Sends output along the efferent pathway (away from the control center — think E for Exit).
- Effector
- Carries out the response that feeds back to change the stimulus.
Negative feedback (by far the most common) shuts off or reduces the original stimulus. Examples: body-temperature regulation by the hypothalamus, blood glucose regulation by insulin and glucagon, blood pressure control.
Positive feedback enhances the original stimulus so the activity accelerates — it is used only for events that must finish quickly and have a natural endpoint. Examples: labor contractions (oxytocin), blood clotting (platelet plug), and the breast-milk let-down reflex.
5Anatomical position & directional terms
All directions assume anatomical position: body erect, feet slightly apart, palms facing forward, thumbs pointing away from the body. Right and left always refer to the patient's/specimen's sides, not yours.
| Term | Meaning | Example |
|---|---|---|
| Superior (cranial) | Toward the head | The forehead is superior to the nose |
| Inferior (caudal) | Away from the head; toward the lower body | The navel is inferior to the chin |
| Anterior (ventral) | Toward the front | The breastbone is anterior to the spine |
| Posterior (dorsal) | Toward the back | The heart is posterior to the breastbone |
| Medial | Toward the midline | The heart is medial to the arm |
| Lateral | Away from the midline | The arms are lateral to the chest |
| Intermediate | Between a medial and a lateral structure | The collarbone is intermediate between the breastbone and shoulder |
| Proximal | Closer to the origin of a limb / attachment to the trunk | The elbow is proximal to the wrist |
| Distal | Farther from the origin of a limb | The knee is distal to the thigh |
| Superficial (external) | Toward the body surface | The skin is superficial to the skeletal muscles |
| Deep (internal) | Away from the body surface | The lungs are deep to the skin |
6Regional terms
The body has two major divisions: the axial part (head, neck and trunk) and the appendicular part (the limbs, or appendages).
| Term | Region | Term | Region |
|---|---|---|---|
| Cephalic | Head | Cervical | Neck |
| Thoracic | Chest | Abdominal | Abdomen |
| Axillary | Armpit | Brachial | Upper arm |
| Antecubital | Front of elbow | Antebrachial | Forearm |
| Carpal | Wrist | Pollex | Thumb |
| Coxal | Hip | Inguinal | Groin |
| Femoral | Thigh | Patellar | Anterior knee |
| Crural | Leg (anterior) | Tarsal | Ankle |
| Hallux | Great toe | Sural | Calf (posterior leg) |
| Occipital | Back of head | Vertebral | Spine |
| Lumbar | Lower back (loin) | Gluteal | Buttock |
| Popliteal | Back of knee | Calcaneal | Heel |
| Olecranal | Back of elbow | Scapular | Shoulder blade |
7Body planes & sections
A section is a cut along a plane. The three most common planes lie at right angles to each other:
- Sagittal plane
- Vertical; divides the body into right and left parts. A midsagittal (median) plane runs exactly down the midline; a parasagittal plane is offset from it.
- Frontal (coronal) plane
- Vertical; divides the body into anterior and posterior parts.
- Transverse (horizontal) plane
- Runs horizontally; divides the body into superior and inferior parts. A cut along it is called a cross section.
- Oblique section
- A cut at an angle between the horizontal and vertical planes.
8Body cavities & membranes
The dorsal body cavity protects the nervous system: the cranial cavity (brain) is continuous with the vertebral (spinal) cavity (spinal cord).
The ventral body cavity houses the internal organs (viscera) and is split by the diaphragm into:
- Thoracic cavity — two lateral pleural cavities (each around a lung) and the medial mediastinum, which contains the pericardial cavity (around the heart), esophagus, trachea and great vessels.
- Abdominopelvic cavity — the superior abdominal cavity (stomach, intestines, spleen, liver) and the inferior pelvic cavity (urinary bladder, reproductive organs, rectum). The two are continuous — there is no wall between them.
Serous membranes (serosae) line the ventral cavity and cover its organs. The parietal serosa lines the cavity wall; the visceral serosa covers the organ. The thin space between them is filled with lubricating serous fluid so organs can slide without friction.
| Serosa | Surrounds |
|---|---|
| Pericardium | Heart |
| Pleura | Lungs |
| Peritoneum | Abdominopelvic organs |
Smaller cavities: oral (mouth), nasal, orbital (eyes), middle ear and synovial cavities (joints).
9Abdominopelvic regions & quadrants
To describe locations precisely, the abdominopelvic cavity is divided either into nine regions (used by anatomists) or four quadrants (used by clinicians).
- Top row: right hypochondriac, epigastric, left hypochondriac.
- Middle row: right lumbar, umbilical, left lumbar.
- Bottom row: right inguinal (iliac), hypogastric (pubic), left inguinal (iliac).
Quadrants are formed by a midsagittal and a transverse plane through the umbilicus: right upper (RUQ), left upper (LUQ), right lower (RLQ) and left lower (LLQ).