Chapter 1: The Human Body – An Orientation (Marieb Human Anatomy & Physiology, 12th Edition)

The Human Body: An Orientation

Introduction to Anatomy and Physiology

Anatomy and physiology provide the foundational framework for understanding the structure and function of the human body. These disciplines are essential for careers in health and biological sciences.

• Anatomy: The study of the structure of body parts and their relationships to one another.

• Physiology: The study of the function of the body; how body parts work and carry out life-sustaining activities.

Example: Understanding the anatomy of the heart (its chambers and valves) helps explain its physiological function (pumping blood).

Principle of Complementarity of Structure and Function

Form Determines Function

The structure of a body part determines its function. This is known as the principle of complementarity of structure and function.

• Example: Bones are rigid and provide support because of their hard mineralized structure.

Reference Standards in Anatomy

• Reference male: Healthy, young male weighing about 155 lb (70 kg).

• Reference female: Healthy, young female weighing about 125 lb (57 kg).

Anatomical Variability

• Over 90% of anatomical structures match textbook descriptions, but variations exist (e.g., nerves or blood vessels may be out of place, small muscles may be missing).

• Extreme anatomical variations are rare and usually incompatible with life.

Sex and Gender

• Sex: Set of biological attributes based on chromosomes, gene expression, and hormone action; reflected in reproductive anatomy and physiology (male or female).

• Gender: Psychosocial construct including behaviors, expressions, and identities (e.g., man, woman, transgender, non-binary).

Divisions of Anatomy

Gross (Macroscopic) Anatomy

• Regional anatomy: Study of all structures in a particular area of the body.

• System anatomy: Study of one body system (e.g., cardiovascular, nervous, muscular).

• Surface anatomy: Study of internal structures as they relate to the overlying skin (e.g., visible muscle masses or veins).

Microscopic Anatomy

• Cytology: Study of cells.

• Histology: Study of tissues.

Developmental Anatomy

• Traces structural changes throughout the lifespan.

• Embryology: Study of developmental changes before birth.

Essential Tools and Techniques in Anatomy

• Observation, manipulation, palpation (feeling organs with hands), and auscultation (listening with a stethoscope).

• Medical imaging technologies: X-ray, MRI, CT, ultrasound.

Divisions of Physiology

• Based on organ systems (e.g., renal physiology, neurophysiology, cardiovascular physiology).

• Often focuses on cellular or molecular events and chemical reactions in cells.

• Requires understanding of basic chemical and physical principles (e.g., electrical currents, pressure, lever systems).

Levels of Structural Organization

The human body is organized from the simplest to the most complex levels:

• Chemical level: Atoms and molecules.

• Cellular level: Cells and their organelles.

• Tissue level: Groups of similar cells.

• Organ level: Two or more types of tissues.

• Organ system level: Organs working closely together.

• Organismal level: All organ systems combined to make the whole organism.

Body's Organ Systems

The human body has 11 organ systems, each with specific functions:

• Integumentary

• Skeletal

• Muscular

• Nervous

• Endocrine

• Cardiovascular

• Lymphatic

• Respiratory

• Digestive

• Urinary

• Reproductive

Requirements for Life

Necessary Life Functions

• Maintaining boundaries: Separation between internal and external environments (e.g., plasma membranes, skin).

• Movement: Of body parts (skeletal muscle), substances (cardiac muscle, smooth muscle), and cells (e.g., white blood cells).

• Responsiveness (excitability): Ability to sense and respond to stimuli (e.g., withdrawal reflex, control of breathing rate).

• Digestion: Breakdown of ingested foodstuffs to simple molecules for absorption.

• Metabolism: All chemical reactions in body cells, including catabolism (breakdown) and anabolism (synthesis). $\text{Metabolism} = \text{Catabolism} + \text{Anabolism}$

• Excretion: Removal of wastes (e.g., urea, carbon dioxide, feces).

• Reproduction: Cellular division for growth/repair; production of offspring at the organismal level.

• Growth: Increase in size of a body part or organism.

Survival Needs

• Nutrients: Chemicals for energy and cell building (carbohydrates, proteins, fats, minerals, vitamins).

• Oxygen: Essential for metabolic reactions; survival without oxygen is limited to a few minutes.

• Water: Most abundant chemical in the body; necessary for chemical reactions and as a fluid base.

• Normal body temperature: Required for proper rates of chemical reactions (about 37°C or 98.6°F).

• Appropriate atmospheric pressure: Needed for adequate breathing and gas exchange.

Homeostasis

Definition and Importance

Homeostasis is the maintenance of relatively stable internal conditions despite continuous environmental changes. It is a dynamic equilibrium, with internal conditions kept within narrow limits.

• Law of mass balance: The total amount of a substance in the body must equal the amount taken in minus the amount lost.

Homeostatic Control Mechanisms

• Involves three components: receptor (sensor), control center, and effector.

• Receptor: Monitors environment and responds to stimuli.

• Control center: Determines set point and appropriate response.

• Effector: Provides the means to respond; response reduces (negative feedback) or enhances (positive feedback) the effect of the stimulus.

Types of Feedback

• Negative feedback: Most common; response reduces or shuts off the original stimulus, returning the variable to its set point. Examples: Regulation of body temperature, blood glucose by insulin.

• Positive feedback: Response enhances or exaggerates the original stimulus; usually controls infrequent events. Examples: Enhancement of labor contractions by oxytocin, platelet plug formation and blood clotting.

• Feedforward (anticipatory) response: Occurs in anticipation of a change (e.g., salivation before eating).

Homeostatic Imbalance

• Disturbance of homeostasis increases risk of disease and contributes to aging.

• If negative feedback is overwhelmed, destructive positive feedback may occur (e.g., heart failure).

Anatomical Terminology

Standard Anatomical Position

• Body erect, feet slightly apart, palms facing forward, thumbs pointing away from the body.

Directional Terms

• Describe the location of one body structure relative to another.

• Always based on standard anatomical position.

• Right and left refer to the subject's right and left, not the observer's.

• Examples: The ears are lateral to the nose. The heart is posterior to the breastbone.

Term	Definition	Example
Superior (cranial)	Toward the head or upper part of a structure	The head is superior to the abdomen.
Inferior (caudal)	Away from the head or toward the lower part	The navel is inferior to the chin.
Anterior (ventral)	Toward the front of the body	The breastbone is anterior to the spine.
Posterior (dorsal)	Toward the back of the body	The heart is posterior to the breastbone.
Medial	Toward the midline of the body	The heart is medial to the arm.
Lateral	Away from the midline of the body	The arms are lateral to the chest.
Proximal	Closer to the origin of a body part	The elbow is proximal to the wrist.
Distal	Farther from the origin of a body part	The knee is distal to the thigh.
Superficial	Toward or at the body surface	The skin is superficial to the skeletal muscles.
Deep	Away from the body surface; more internal	The lungs are deep to the skin.

Body Planes and Sections

• Sagittal plane: Divides the body into right and left parts. A midsagittal (median) plane lies exactly in the midline; a parasagittal plane is offset from the midline.

• Frontal (coronal) plane: Divides the body into anterior and posterior parts.

• Transverse (horizontal) plane: Divides the body into superior and inferior parts; produces a cross section.

• Oblique section: Cuts made at angles other than 90° to the vertical plane.

Body Cavities

• Dorsal body cavity: Protects the nervous system; includes cranial cavity (encases brain) and vertebral (spinal) cavity (encases spinal cord).

• Ventral body cavity: Houses internal organs (viscera); includes thoracic cavity (pleural cavities, mediastinum, pericardial cavity) and abdominopelvic cavity (abdominal and pelvic cavities).

Membranes in the Ventral Body Cavity

• Serosa (serous membrane): Thin, double-layered membranes covering surfaces in the ventral body cavity.

• Parietal serosa: Lines cavity walls.

• Visceral serosa: Covers organs.

• Serous fluid between layers reduces friction.

• Examples: Pleura (lungs), pericardium (heart), peritoneum (abdominopelvic organs).

Abdominopelvic Regions and Quadrants

• Four quadrants: Right upper (RUQ), left upper (LUQ), right lower (RLQ), left lower (LLQ).

• Nine regions: Umbilical, epigastric, pubic (hypogastric), right/left inguinal (iliac), right/left lateral (lumbar), right/left hypochondriac.

Other Body Cavities

• Open cavities: Oral, digestive, nasal, orbital, middle ear.

• Closed cavities: Synovial cavities (joints).

Clinical Relevance

• Accurate use of anatomical terminology is critical to avoid medical errors (e.g., wrong-site surgery).