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Anatomy and Physiology Unit 1: Body organization and homeostasis

The body is organized from cells to systems, and it constantly adjusts to keep internal conditions stable.

Covers: Levels of organization, anatomical terms and planes, body cavities, homeostasis, feedback loops

Study guide

The body is organized in levels: chemicals make up cells, similar cells form tissues, tissues form organs, organs work together in organ systems, and the systems make up the organism. There are eleven organ systems, including the integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary and reproductive systems.

Anatomical terms describe locations precisely, always from the anatomical position: standing upright, facing forward, arms at the sides, palms forward. Directional terms come in pairs: superior and inferior (above and below), anterior and posterior (front and back), medial and lateral (toward and away from the midline), proximal and distal (closer to and farther from where a limb attaches), and superficial and deep. Regional terms name body areas, such as brachial (arm) and femoral (thigh).

Body planes divide the body for viewing sections. The sagittal plane divides left and right (midsagittal into equal halves), the frontal (coronal) plane divides front and back, and the transverse plane divides top and bottom.

Body cavities hold and protect organs. The dorsal cavity contains the cranial cavity (brain) and vertebral cavity (spinal cord). The ventral cavity contains the thoracic cavity (heart and lungs) and the abdominopelvic cavity (digestive, urinary and reproductive organs), separated by the diaphragm. Membranes like the pleura and pericardium line them.

Homeostasis is keeping internal conditions such as temperature, blood sugar and blood pressure stable. A feedback loop has a receptor that detects change, a control center (often the brain) that compares it to a set point, and an effector that responds. Negative feedback reverses the change and is the most common type. Positive feedback amplifies it, as in childbirth and blood clotting.

The integumentary system (skin, hair, nails and glands) is the body's largest organ system. The skin has an outer epidermis of stratified squamous epithelium, a dermis of connective tissue with blood vessels, nerves and sweat glands, and an underlying fatty hypodermis. It protects against water loss and pathogens, makes vitamin D with sunlight, senses touch, and regulates temperature by sweating and by widening or narrowing its blood vessels. A basic principle of A&P is that structure fits function: the form of a body part suits the job it does. Thin alveolar walls allow fast diffusion, valves keep blood flowing in one direction, and the folded lining of the small intestine increases absorption.

Each organ system has a main role. The integumentary system protects. The skeletal system supports and makes blood cells, and the muscular system moves the body. The nervous and endocrine systems control the body. The cardiovascular system transports materials, the lymphatic system returns fluid and supports immunity, and the respiratory system exchanges gases. The digestive system breaks down food, the urinary system removes wastes and balances water, and the reproductive system produces offspring. Serous membranes have two layers: the parietal layer lines the cavity wall, and the visceral layer covers the organ, with lubricating fluid between them. The abdomen is divided into four quadrants (right upper, left upper, right lower, left lower), or into nine regions for more precise descriptions.

Key ideas

• Levels: chemical → cell → tissue → organ → organ system → organism.
• Anatomical position: standing, facing forward, palms forward.
• Directional terms: superior/inferior, anterior/posterior, medial/lateral, proximal/distal.
• Homeostasis keeps temperature, blood sugar and pH in narrow ranges.
• Negative feedback reverses changes; positive feedback amplifies them.

Formulas and rules

Common mistakes

• Mixing up proximal (closer to the trunk) and distal (farther).
• Thinking positive feedback is 'good'. It means amplifying.

Words to know

Anatomical position

Standing upright, facing forward, palms forward.

Proximal

Closer to where a limb attaches to the trunk.

Homeostasis

Keeping internal conditions stable.

Feedback loop

Receptor, control center, effector.

Sagittal plane

Divides the body into left and right.

Frontal and transverse planes

Frontal divides front and back; transverse divides top and bottom.

Levels of organization

Chemical → cell → tissue → organ → organ system → organism.

Superior / inferior

Above / below.

Anterior / posterior

Front / back.

Medial / lateral

Toward / away from the midline.

Distal

Farther from where a limb attaches.

Superficial / deep

Toward / away from the body surface.

Dorsal cavity

Holds the brain (cranial) and spinal cord (vertebral).

Thoracic cavity

Holds the heart and lungs.

Abdominopelvic cavity

Holds digestive, urinary and reproductive organs.

Negative feedback

Reverses a change; the most common homeostatic loop.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.