Unit 1 – The Basics: Introduction to Anatomy and Physiology

Introduction to Anatomy and Physiology

Anatomy and physiology, commonly abbreviated as A&P, is the comprehensive study of human structure and function, essentially representing the biology of the human body. This field serves as a critical foundation for advanced study in various disciplines, including healthcare, pathophysiology, and other health-related fields. A fundamental principle of biology is that anatomy and physiology are always related because structure determines what functions can take place.

At the molecular level, shape determines function. For instance, fibrous proteins like collagen are structural proteins shaped like a rope. This specific structure provides strength to the skin and prevents it from tearing. In contrast, globular or round proteins such as hemoglobin are structured to transport oxygen within the blood. Other examples of molecular-level structure-function relationships include enzymes, which catalyze or speed up chemical reactions, and plasma membrane proteins, which transport substances, facilitate cell communication, or help identify cells to the rest of the body.

At the cellular level, the specific structure of skeletal muscle cells allows for contraction, which facilitates movement. A muscle fiber consists of many fibrils that give the cell its characteristic striated appearance. At the individual organ level, the heart's structure includes four hollow chambers—the atria and ventricles—composed of cardiac muscle cells, enabling the function of pumping blood throughout the body. Similarly, the bones of the skull are heavy and secure to protect brain function, while the thin air sacs of the lungs permit the movement of gases between the lungs and the blood. The urinary bladder features a thick, transitional epithelium that stretches readily to permit distension by contained urine, holding up to $800 - 11$ (presumably ml). The liver is structured to filter blood and produce bile.

Definitions and Methods of Anatomical Study

Anatomy is defined as the study of the structure and shape of the body, body parts, and their relationships to one another. The term originates from the Greek "ana-" meaning up and "tomia" meaning cutting (from "temnein" to cut). Physiology is the study of the normal functioning of a living organism and its component parts, including all physical and chemical processes. It comes from the Ancient Greek "physis" meaning nature or origin, and "-logia" meaning study.

Anatomical study can involve invasive and non-invasive techniques. Invasive techniques involve entering the body through the skin, mucosa, or natural orifices for assessment or treatment; these carry higher risks such as infection, bleeding, pain, and tissue injury but provide direct samples or visualization with higher specificity. Non-invasive techniques do not break the skin or enter the body, assessing or treating from the outside with lower risk and minimal discomfort. Methods of examination include inspection (visual examination), palpation (gently touching body surfaces with hands), auscultation (listening to body sounds with a stethoscope), and percussion (tapping on the body surface with fingertips to listen for echoes).

Cadaver dissection involves the cutting and separation of tissues to reveal their relationships and examine the structure of the human body. An autopsy, also known as a necropsy or postmortem examination, is the dissection and examination of a dead body, its organs, and structures to "see for oneself." Autopsies are typically performed within $24$ hours of death before organ deterioration or embalming occurs. The primary goals of an autopsy are to determine the cause of death, identify diseases not detected during life, determine the extent of injuries and their contribution to death, and identify hereditary conditions. Exploratory surgery is a surgical investigation used to diagnose the cause of a disorder or disease, sometimes leading to definitive treatment if a previously undiagnosed lesion is identified and rectified.

Medical Imaging Techniques

Medical imaging allows for the visualization of internal structures without surgery and is useful for confirming diagnoses. Conventional radiography, or X-ray, uses a single burst of X-rays to produce a $2-D$ image on film. While it has poor resolution for soft tissues, its major use is in osteology. Computed Tomography (CT Scan) uses a moving X-ray beam to produce a video image of a cross-section of the body, revealing more soft tissue detail such as kidney and gallstones; multiple scans can build $3D$ views.

Angiography or arteriography is used to visualize the inside, or lumen, of blood vessels and organs, specifically arteries, veins, and heart chambers. This can be performed using X-rays with catheters, CT, or Magnetic Resonance Imaging (MRI). MRI uses a magnetic field and radio waves to create detailed images of organs and tissues by temporarily realigning hydrogen atoms in the body, which then produce signals used to create cross-sectional images.

Ultrasound (US) utilizes high-frequency sound waves emitted by a handheld device to produce a safe, non-invasive, and painless image or sonogram. It is commonly used for fetal monitoring and examining pelvic and abdominal organs, the heart, and blood vessel flow. Positron Emission Tomography (PET) uses small amounts of radiolabeled biologically active compounds (tracers) introduced via injection or inhalation to produce images showing the distribution of the tracer. PET finds clinical applications in oncology, brain, and heart imaging.

Subspecialties of Anatomy and Physiology

Anatomy is divided into microscopic and macroscopic (gross) branches. Subspecialties include cytology (the study of cells, such as erythrocytes, neutrophils, and platelets), histology (the study of tissues, including epithelium, connective, muscle, and nervous tissues), and embryology/developmental anatomy (the study of the first $8$ weeks of development after fertilization). Other branches include surface anatomy (the study of surface markings to understand internal anatomy through visualization and palpation), systemic anatomy (the study of systems like the respiratory or nervous system), and regional anatomy (the study of specific body regions such as the head or chest).

Physiology subspecialties include neurophysiology (nerve cell function), endocrinology (hormones and body function control), cardiovascular physiology (heart and blood vessel functions), immunology (body defenses), respiratory physiology (air passageways and lungs), renal physiology (kidney function), and exercise physiology (changes due to muscular activity). Pathophysiology focus on the functional changes associated with disease and aging.

The Hierarchy of Complexity and Levels of Organization

The human body is organized into a hierarchy of increasing complexity. Atoms (protons, neutrons, electrons) combine to form molecules. Molecules, including the four biologically important organic molecules—proteins (made from $20$ amino acids), complex carbohydrates (from simple sugars), nucleic acids (from nucleotides), and lipids (from fatty acids and glycerol)—form organelles. Organelles are components of cells, which are the smallest structural and functional units of the body.

Tissues are groups of cells and surrounding materials working together for a particular function. Organs are composed of two or more tissues working together for specific functions, often with recognizable shapes. Organ systems consist of one or more organs providing a common function. The organism is the highest level, composed of all the integrated organ systems.

The Eleven Systems of the Human Body

Integumentary System: Components include skin, hair, fingernails, toenails, and glands. Functions involve protection, temperature regulation, waste elimination, vitamin D synthesis, and sensation detection.
Skeletal System: Components include bones, joints, and cartilages. Functions include support, protection, providing surface area for muscle attachment, aiding movement, housing blood-cell-producing cells, and storing minerals and lipids.
Muscular System: Components specifically include skeletal muscle tissue and tendons. It participates in body movements, maintains posture, and produces heat.
Nervous System: Components include the brain, spinal cord, nerves, and special sense organs. It generates action potentials to regulate body activities, detects changes in the environment, and responds via muscular contractions or glandular secretions.
Endocrine System: Components include hormone-producing glands such as the pineal, hypothalamus, pituitary, thymus, thyroid, parathyroid, adrenal, pancreas, ovaries, and testes. It regulates body activities by releasing chemical messengers into the blood.
Cardiovascular System: Components include blood, heart, and blood vessels. The heart pumps blood carrying oxygen and nutrients to cells and removing wastes; it also helps regulate temperature and acid-base balance.
Lymphatic System and Immunity: Components include lymphatic fluid/vessels, spleen, thymus, lymph nodes, and tonsils. Functions include returning proteins and fluid to the blood, carrying lipids, and housing B and T cells for immune protection.
Respiratory System: Components include the lungs, pharynx, larynx, trachea, and bronchial tubes. It transfers oxygen to the blood and removes carbon dioxide, helps regulate acid-base balance, and produces sound.
Digestive System: Components include the gastrointestinal tract (mouth, pharynx, esophagus, stomach, intestines, anus) and accessory organs (salivary glands, liver, gallbladder, pancreas). It achieves physical and chemical breakdown of food and absorbs nutrients.
Urinary System: Components include kidneys, ureters, urinary bladder, and urethra. It produces, stores, and eliminates urine, regulates blood volume and chemical composition, and maintains mineral and acid-base balance.
Reproductive Systems: Components include gonads (testes or ovaries) and associated organs (uterine tubes, uterus, vagina, mammary glands in females; epididymis, ductus deferens, seminal vesicles, prostate, penis in males). Functions include gamete production and hormone release.

Basic Life Processes

There are six basic life processes that distinguish living from non-living things:

Metabolism: The sum of all biochemical processes within cells, tissues, organs, and organ systems.
Responsiveness: The ability to detect and respond to changes in the internal and external environment.
Movement: Motion occurring at intracellular, cellular, and organ levels.
Growth: An increase in the number of cells, the size of cells, tissues, organs, and the overall body.
Differentiation: The process where unspecialized cells develop into specialized cells, such as hematopoietic stem cells developing into various blood cells like erythrocytes, neutrophils, or B cells.
Reproduction: The formation of new cells for growth, repair, or replacement, or the production of a new individual.

Homeostasis and Feedback Systems

Homeostasis is the maintenance of the body's internal environment within physiological limits. It is a dynamic condition of equilibrium produced by the interaction of organ systems and regulatory processes. Body fluids are organized into compartments: intracellular fluid (ICF) inside cells and extracellular fluid (ECF) outside cells. Interstitial fluid is the ECF that fills the space between cells and tissues and is considered the "internal environment." The composition of interstitial fluid changes as substances like glucose, oxygen, and ions move back and forth across capillary walls.

Homeostasis is continually disrupted by external stimuli (heat, cold, lack of oxygen) and internal stimuli (psychological stress, exercise). Regulation is managed by the nervous system (rapid nerve impulses) and the endocrine system (slower-acting hormones). A feedback system is a cycle of events where the body is monitored for variables known as controlled conditions, such as body temperature ( $37.0\,^\circ\text{C}$ / $98.6\,^\circ\text{F}$ ), blood glucose ( $~90\,mg/100\,ml$ ), and blood pressure.

A feedback system consists of three components. The Receptor (sensor) monitors changes and sends input via an afferent pathway. Receptors include mechanoreceptors (touch/pressure), thermoreceptors (temperature), photoreceptors (light), chemoreceptors (chemicals), and nociceptors (pain). The Control Center (integrating center) sets the range of values, evaluates input, and generates output commands (nerve impulses or hormones) via an efferent pathway. The Effector receives output and produces a response that changes the controlled condition.

Negative feedback systems reverse a change in a controlled condition. For example, if blood pressure rises, baroreceptors send impulses to the brain, which then sends impulses to the heart to decrease the heart rate, returning blood pressure to normal. Similarly, when blood glucose is high, the pancreas releases insulin to promote glucose uptake; when low, it releases glucagon to break down glycogen. Positive feedback systems strengthen or reinforce a change, such as during childbirth (oxytocin increases uterine contractions) or blood clotting (platelets release chemicals to attract more platelets until the break is sealed).

Homeostatic Imbalances

Disruption of homeostasis can lead to disorder, disease, aging, or death. A disorder is a general term for any abnormality of function, while a disease is a more specific illness characterized by recognizable signs and symptoms. A local disease affects one part of the body, whereas a systemic disease affects the entire body or several parts. Signs are objective changes that can be observed and measured (e.g., fever, rash), whereas symptoms are subjective changes not apparent to an observer (e.g., headache, nausea). Diagnosis is the art of distinguishing one disease from another through medical history and physical examination.