The Human Body: An Orientation

Overview of Anatomy and Physiology

• Principle of Complementarity: Anatomy and physiology are inseparable. Function always reflects structure, meaning that what a structure can do depends on its specific form. Structure dictates function.

• Anatomy: The study of body structures, divided into several sub-disciplines:

  • Gross or Macroscopic: Study of large, visible structures.

  • Microscopic: Study of structures too small to be seen with the naked eye.

  • Developmental: Study of structural changes that occur throughout the lifespan.

• Physiology: The study of the function of body parts. Characteristics include:

  • The ability to focus at many levels of organization.

  • The study of basic physical principles.

  • The study of basic chemical principles.

  • Subdivisions traditionally based on organ systems.

  • Frequent focus on the cellular and molecular levels.

Core Principles of Physiology

1. Structure and Function are Complementary: Form follows function; the physical makeup of a part determines its performance capabilities.

2. Homeostasis: The process of maintaining internal stability to ensure survival.

3. Gradients: These drive physiological processes (e.g., movement of substances from high to low concentration).

4. Communication: Required throughout the body, typically mediated by nerve impulses or hormones.

Interdependence of Body Cells and Systems

• Multicellular Nature: Humans are multicellular organisms. To function, individual cells must be kept alive.

• System Cooperation: All body functions are spread among different organ systems that cooperate to maintain life. Cells depend on these organ systems to meet survival needs.

• Path of Nutrient/Waste Exchange: Via the blood, the cardiovascular system distributes $O_2$ and nutrients to all body cells and delivers wastes and $CO_2$ to disposal organs. Nutrients and wastes pass between the blood and cells via the interstitial fluid.

The 11 Organ Systems

• Integumentary System:

  • Forms the external body covering.

  • Provides separation from the external environment.

  • Protects deeper tissues from injury.

  • Synthesizes Vitamin D.

  • Houses cutaneous receptors, sweat glands, and oil glands.

  • Involved in thermoregulation.

• Skeletal System:

  • Protects and supports body organs.

  • Provides a framework for muscle attachment used for movement.

  • Site of blood cell formation.

  • Bones store essential minerals.

• Muscular System:

  • Allows for manipulation of the environment.

  • Maintains posture.

  • Produces heat.

• Nervous System:

  • Acts as the fast-acting control system.

  • Responds to internal and external changes.

  • Activates appropriate muscles and glands.

• Endocrine System:

  • Regulates body functions.

  • Uses hormones to regulate the functions of glands and tissues.

• Cardiovascular System:

  • Pumps deoxygenated blood to the lungs and oxygenated ($O_2$) blood to tissues.

  • Removes wastes from tissues.

  • Transports cells, nutrients, and other substances.

• Lymphatic System:

  • Returns excess tissue fluid to the cardiovascular system.

  • Contains white blood cells involved in immunity.

• Respiratory System:

  • Delivers oxygen to the blood.

  • Removes carbon dioxide ($CO_2$) from the body.

  • Maintains the acid-base balance of the blood.

• Digestive System:

  • Takes in nutrients and breaks them down.

  • Absorbs nutrients into the blood.

  • Removes food wastes and eliminates unabsorbed matter as feces.

  • Maintains fluid, electrolyte, and acid-base balance.

• Urinary System:

  • Removes nitrogenous metabolic wastes.

  • Eliminates excess ions.

  • Maintains fluid, electrolyte, and acid-base balance.

  • Stimulates blood cell production.

• Reproductive Systems:

  • Female: Produces and transports oocytes; serves as the site for fetal development, nourishment, childbirth, and lactation; secretes hormones; involved in sexual function.

  • Male: Produces and transports sperm; secretes hormones; involved in sexual function.

Characteristics of Living Organisms

• Cellular Composition: Composed of one or more cells.

• Metabolism: Includes both catabolism (breaking down substances) and anabolism (building up substances).

• Growth: An increase in the size of body parts or the organism as a whole.

• Movement: Of body parts or substances within the body.

• Responsiveness: The ability to sense and react to stimuli (e.g., the withdrawal reflex or control of breathing rate).

• Excretion: The removal of wastes resultant from metabolism and digestion.

• Reproduction: Occurs at the cellular level for growth or repair, and at the organismal level to produce offspring.

Survival Needs

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

• Oxygen ($O_2$): Essential for the release of energy from foods.

• Water: Provides the environment necessary for chemical reactions and serves as the fluid base for secretions and excretions.

• Normal Body Temperature: Necessary because it affects the rate of chemical reactions within the body.

• Appropriate Atmospheric Pressure: Required for adequate breathing and gas exchange in the lungs.

Homeostasis and Control Mechanisms

• Definition: The maintenance of stable internal conditions despite continuous changes in the environment. It is maintained by the contributions of all organ systems.

• Communication: Monitoring and regulation require communication via nerve impulses and hormones.

• Variable: The factor or event being regulated.

• Components of a Control Mechanism:

  1. Receptor (Sensor): Monitors the environment and responds to stimuli (changes in controlled variables).

  2. Control Center: Receives input from the receptor, determines the set point (the level at which the variable is maintained), and determines the appropriate response.

  3. Effector: Receives output from the control center and provides the means for the response.

• Negative Feedback:

  • The most common feedback mechanism in the body.

  • The response reduces or shuts off the original stimulus.

  • Examples: Regulation of body temperature, regulation of blood volume by Antidiuretic Hormone (ADH), and regulation of blood glucose.

• Positive Feedback:

  • The response enhances or exaggerates the original stimulus.

  • Usually controls infrequent events that do not require continuous adjustment.

  • Examples: Labor (childbirth) and platelet plug formation (blood clotting).

The Language of Anatomy

• Anatomical Position:

  • Body is erect.

  • Feet are slightly apart.

  • Palms face forward.

  • Directional terms always refer to the anatomical position.

  • "Right" and "left" refer to the sides of the body being viewed, not the observer's right and left.

• Regional Terms:

  • Axial: Includes the head, neck, and trunk.

  • Appendicular: Includes the limbs.

• Anatomical Variability:

  • 90% of all structures in the human body match textbook descriptions.

  • Variations exist, such as a nerve or blood vessel being out of place or a small muscle being missing.

  • Some extreme variations are inconsistent with life.

Directional Terms

1. Superior (cranial): Toward the head or upper part of a structure.

2. Inferior (caudal): Away from the head or toward the lower part of a structure.

3. Anterior (ventral): Toward the front of the body.

4. Posterior (dorsal): Toward the back of the body.

5. Medial: Toward the midline of the body.

6. Lateral: Away from the midline of the body.

7. Intermediate: Between a more medial and a more lateral structure.

8. Proximal: Closer to the origin of the body part or point of attachment of a limb.

9. Distal: Farther from the origin of a body part or point of attachment of a limb.

10. Superficial: Toward or at the body surface.

11. Deep: Away from the body surface; more internal.

Body Planes and Sections

• Section: A cut made along a body plane.

• Body Plane: A flat surface along which the body or a structure may be cut for study.

• Common Planes:

  • Median (Midsagittal) Plane: Vertically divides the body into right and left halves (e.g., through the vertebral column).

  • Frontal (Coronal) Plane: Vertically divides the body into anterior and posterior parts (e.g., showing the right lung, heart, left lung, and liver).

  • Transverse (Horizontal) Plane: Horizontally divides the body into superior and inferior parts (e.g., showing the aorta, pancreas, spleen, and liver).

  • Oblique Section: A cut made diagonally between the horizontal and vertical planes.

Body Cavities and Membranes

• Dorsal Body Cavity: Contains the Cranial cavity and Vertebral cavity.

• Ventral Body Cavity: Divided into:

  • Thoracic Cavity: Contains the Pleural cavities, Mediastinum, and Pericardial cavity.

  • Abdominopelvic Cavity: Contains digestive organs and urinary/reproductive structures.

• Serous Membranes (Serosa):

  • Double-layered membranes in the ventral cavity.

  • Parietal Serosa: Lines the internal body cavity walls.

  • Visceral Serosa: Covers the internal organs.

  • The layers are separated by a cavity filled with serous fluid.

  • Specific Serosae:

    • Pericardium: Associated with the heart.

    • Pleurae: Associated with the lungs.

    • Peritoneum: Associated with the abdominopelvic cavity.

• Other Cavities (Exposed to Environment): Oral and digestive cavities, Nasal cavity, Orbital cavities, and Middle ear cavities.

• Other Cavities (Not Exposed): Synovial cavities (found in joints).