Chp 1

1. Foundations and Anatomical Terminology

Homeostasis

Definition: Homeostasis is the maintenance of a relatively stable internal environment despite external changes. It's dynamic, not static — variables fluctuate within a narrow, physiologically acceptable range (a set point ± normal range). It matters because cells (and therefore tissues, organs, and the organism) can only function properly within narrow ranges of temperature, pH, ion concentration, glucose, O2/CO2, etc. Loss of homeostasis = disease or death.

Feedback loop components

Component	Role
Receptor (sensor)	Detects a change (stimulus) in a variable and sends info (afferent) to the control center
Control center (integrator)	Usually brain/endocrine gland; compares input to a set point and determines the response
Effector	Muscle, gland, or organ that carries out the response (efferent), driving the variable back toward the set point

Negative vs. positive feedback

	Negative feedback	Positive feedback
Effect on stimulus	Reverses/opposes the change (shuts the response off)	Amplifies/enhances the change (drives it further)
Result	Stabilizes variable near set point; most common mechanism	Pushes toward a defined endpoint, then stops
Self-limiting?	Yes	No (needs an external cutoff event)

Examples

● Negative feedback: thermoregulation (shivering/sweating to return body temp to 98.6°F); blood glucose regulation (insulin lowers glucose, glucagon raises it); blood pressure regulation (baroreceptor reflex)

● Positive feedback: childbirth (oxytocin → stronger contractions → more oxytocin, until baby is delivered); blood clotting cascade (platelet activation reinforces itself until clot forms); fever cycle onset; action potential depolarization (Na⁺ channels reinforcing opening)

Anatomical Language

Anatomical position: Body erect, facing forward, feet flat and forward, arms at sides, palms facing forward (anterior), thumbs pointing away from body. All directional terms are defined relative to this position, regardless of the body's actual orientation.

Directional terms (pairs)

Term	Meaning	Term	Meaning
Superior (cranial)	Toward the head	Inferior (caudal)	Toward the feet
Anterior (ventral)	Toward the front	Posterior (dorsal)	Toward the back
Medial	Toward the midline	Lateral	Away from midline
Proximal	Closer to trunk/origin	Distal	Farther from trunk/origin
Superficial	Toward body surface	Deep	Away from surface, more internal
Ipsilateral	Same side	Contralateral	Opposite side

Regional terms

Know major surface regions — cephalic (head), cervical (neck), thoracic (chest), abdominal, pelvic, brachial (arm), antebrachial (forearm), carpal (wrist), manual (hand), femoral (thigh), crural (leg), pedal (foot), gluteal, lumbar, axillary, inguinal, patellar, popliteal (back of knee), sural (calf), cubital (front of elbow).

Body planes and sections

● Sagittal plane — vertical, divides body into left/right (midsagittal = equal halves; parasagittal = unequal)

● Frontal (coronal) plane — vertical, divides body into anterior/posterior

● Transverse (horizontal/axial) plane — divides body into superior/inferior; produces a "cross section"

Body cavities

● Dorsal cavity

○ Cranial cavity — contains brain

○ Vertebral (spinal) canal — contains spinal cord

● Ventral cavity

○ Thoracic cavity — divided by mediastinum; contains two pleural cavities (lungs) and the pericardial cavity (heart), within the mediastinum

○ Diaphragm separates thoracic from abdominopelvic cavity

○ Abdominopelvic cavity — subdivided into abdominal cavity (stomach, liver, intestines, spleen, kidneys) and pelvic cavity (bladder, reproductive organs, rectum). Often mapped with 4 quadrants (RUQ, LUQ, RLQ, LLQ) or 9 regions (hypochondriac, epigastric, lumbar, umbilical, iliac/inguinal, hypogastric).

Serous membranes

● Parietal layer — lines the wall of the cavity

● Visceral layer — covers the surface of the organ itself

Between them: serous fluid, which reduces friction during organ movement (e.g., lung expansion, heart beating, gut peristalsis).

Named serous membranes

Membrane	Location
Pleura	Surrounds lungs (parietal pleura = thoracic wall; visceral pleura = lung surface)
Pericardium	Surrounds heart (parietal pericardium; visceral pericardium = epicardium)
Peritoneum	Surrounds abdominal organs (parietal peritoneum = abdominal wall; visceral peritoneum = organ surface)