Chapter 2: Anatomical Organization — From Cells to Organ Systems

Human anatomy describes the structure of the body, while physiology explains how those structures function. Together they form the foundation of clinical medicine. Before examining any organ system, you must understand the organizational hierarchy and the spatial language anatomists use.

Anatomical Terminology

Clinicians and anatomists use standardized planes, directions, and positions to describe the body without ambiguity. All descriptions assume the anatomical position: standing upright, facing forward, palms facing anteriorly.

Directional terms:

• Superior / Inferior — toward the head / toward the feet
• Anterior (Ventral) / Posterior (Dorsal) — front / back
• Medial / Lateral — toward the midline / away from midline
• Proximal / Distal — closer to / farther from the trunk
• Superficial / Deep — toward the surface / toward the core

Anatomical planes:

• Sagittal — divides body into left and right
• Coronal (Frontal) — divides into anterior and posterior
• Transverse (Axial) — divides into superior and inferior

The Structural Hierarchy

The body is organized from simplest to most complex:

1. Chemical level — atoms and molecules (water, proteins, DNA)
2. Cellular level — the basic structural unit of life
3. Tissue level — groups of similar cells with a shared function
4. Organ level — two or more tissue types performing a specific function
5. Organ system level — organs working together toward a common goal
6. Organism level — all systems integrated into a living whole

The Four Primary Tissue Types

Tissue	Function	Example
Epithelial	Covering, lining, secretion	Skin epidermis, gut lining
Connective	Support, binding, transport	Bone, blood, adipose tissue
Muscle	Contraction and movement	Cardiac, skeletal, smooth muscle
Nervous	Signal transmission	Neurons, glial cells

The Eleven Organ Systems

System	Primary Functions
Integumentary	Protection, temperature regulation, sensation
Skeletal	Support, movement, blood cell production, mineral storage
Muscular	Movement, posture, heat production
Nervous	Control, communication, integration
Endocrine	Hormonal regulation of metabolism, growth, reproduction
Cardiovascular	Transport of O₂, nutrients, hormones, waste removal
Lymphatic/Immune	Fluid balance, immunity, fat absorption
Respiratory	Gas exchange (O₂/CO₂)
Digestive	Nutrient digestion and absorption
Urinary	Filtration, waste excretion, fluid/electrolyte balance
Reproductive	Continuation of species

Homeostasis

Homeostasis is the body’s ability to maintain a stable internal environment despite external changes. It is achieved through negative feedback loops: a deviation from the set point triggers a corrective response that returns the variable toward normal (e.g., body temperature regulation, blood glucose control). Positive feedback amplifies deviations and is used in specific situations such as childbirth and blood clotting.

Key Checklist

• Can correctly use directional anatomical terms (superior, medial, proximal, etc.)
• Describes the structural hierarchy from cell → tissue → organ → organ system
• Explains homeostasis and the difference between negative and positive feedback loops