Introduction to General Physiology and Homeostasis

Definitions of Anatomy and Physiology

• Anatomy: The branch of biology dealing with the identification, study, and understanding of the various anatomical structures making up the human body. This includes macroscopic structures such as bones, muscles, and organs like the heart, pancreas, and kidneys, as well as the microscopic tissues and cells that compose them.
• Physiology: The study of underlying cellular mechanisms and how cells communicate with each other. By understanding how individual cells function, one can understand the broader functions of organs and organ systems within the human body.

Levels of Biological Organization

• Atoms and Elements: The smallest unit of organization, also known as elements on the periodic table (e.g., Carbon, Calcium, Hydrogen, Oxygen, and Nitrogen). Atoms occupy space and possess mass.
  • Atomic Structure: Atoms consist of a nucleus containing positively charged protons and neutral neutrons (no charge). Negatively charged electrons reside in orbits surrounding the nucleus.
• Molecules and Macromolecules: Small molecules, such as water ($H_2O$) and glucose ($C_6H_{12}O_6$), are formed when atoms combine. These can further form macromolecules, which serve as structural components of cells.
• Cells: The smallest fundamental unit of life. A single cell constitutes a living organism.
• Tissues: Groups of similar cells with specialized functions.
• Organs: Forged from multiple different tissue types. For example, the stomach is an organ composed of:
  • An outer fibrous connective protective layer.
  • Smooth muscle for movement and digestion-related contractions.
  • Connective tissue for supporting and binding.
  • An innermost mucosa lining made of columnar epithelial tissue that secretes mucus.
• Organ Systems: Groups of organs working together. For example, the digestive system includes the stomach, intestines, pancreas, liver, and gallbladder, working to digest nutrients and absorb them into the bloodstream.
• Organism: Many organ systems working in coordination characterize the organism.

The Cellular and Chemical Levels of Life

• Prokaryotic Cells: Meaning "before nucleus," these cells lack a membrane-bound nucleus. Bacterial cells are the primary example of prokaryotes.
• Eukaryotic Cells: Cells containing a "true nucleus," which is a membrane-bound organelle housing the DNA or genetic material. These cells contain complicated organelles with specialized functions.
• Macromolecules at the Chemical Level:
  • Nucleic Acids: Includes DNA and RNA.
  • Proteins: Essential for various cellular functions.
  • Carbohydrates: Sugars used for energy.
  • Lipids: Fats, including cholesterol-related fats.

Primary Tissue Types

• Epithelial Tissue: Involved in protection and secretion. It covers body surfaces and lines organs.
  • Squamous: Flat-shaped cells.
  • Columnar: Long, tall cells; often involved in mucus secretion.
  • Cuboidal: Cube-shaped cells.
• Connective Tissue: A vast category providing protection and support.
  • Bone: Physical framework and protection.
  • Blood: A liquid connective tissue.
  • Adipose: Fat storage for energy and immunity.
  • Elastic Tissue and Cartilage: Flexible support structures.
• Muscle Tissue: Specialized for movement. It is categorized as:
  • Voluntary (Skeletal): Attached to the bones of the skeletal framework.
  • Involuntary (Cardiac): Found specifically in the heart.
  • Involuntary (Smooth): Found in organs such as the bladder and uterus, as well as within blood vessels.
• Nervous Tissue: Located in the brain, spinal cord, and nerves. It facilitates communication between the nervous systems, organs, and muscles.

The Concept of Homeostasis

• Definition: The process by which the body self-regulates to maintain an optimal, stable internal environment. "Stasis" refers to stability. This mechanism involves the fine-tuning of cell functions to keep the organism alive.
• Equilibrium: The body aims for a state of balance. Every system in the body works to maintain this equilibrium.
• Optimal Physiological Sets:
  • Body Temperature: Optimal temperature is approximately $37\,^\circ\text{C}$ or $98.4\,^\circ\text{F}$.
  • Blood pH: The optimal range is between $7.35$ and $7.45$, maintained primarily by the respiratory and urinary (kidney) systems.

Homeostatic Control Mechanisms: Intrinsic and Extrinsic

• Intrinsic Controls: These are inherent to an organ or tissue. For example, during exercise, blood vessels naturally dilate (increase in diameter) to deliver more oxygen and nutrients to tissues.
• Extrinsic Controls: These involve the nervous system and hormones.
  • Example: Epinephrine is a neurotransmitter/hormone that increases heart rate above the average resting rate of $60$ to $100\,\text{bpm}$ to meet the demands of physical activity.

Components of Homeostatic Feedback Systems

1. Variable: The factor that must stay balanced.
2. Stimulus: Something that changes the variable and knocks homeostasis off balance.
3. Receptor (Sensor): Specialized nerve cells that receive signals and detect changes.
4. Control Center (Integrating/Coordinating Center): Usually a part of the brain that receives, interprets, and processes sensory information, then sends motor instructions.
5. Effector: A gland, organ, muscle, or blood vessel that carries out a response.
6. Response: The action taken to restore balance and return to the stable state.

Case Studies in Homeostasis: Temperature and Glucose

• Thermoregulation (Body Temperature):
  • High Temperature (Hyperthermia): Thermoreceptors in the skin and blood vessels detect the rise. The hypothalamus (the body's thermostat) receives the signal. Effectors include sweat glands (secreting moisture to cool) and blood vessels (dilating to radiate heat).
  • Low Temperature (Hypothermia): Thermoreceptors send signals to the hypothalamus. Effectors include skeletal muscles (shivering/contracting to generate heat) and blood vessels (constricting to prevent heat loss from the skin surface).
• Blood Glucose Regulation:
  • Normal Fasting Range: $70$ to $100\,\text{mg/mil}$.
  • Rising Blood Glucose: Eating sugar (a cupcake) acts as a stimulus. Beta cells in the pancreas detect the change and release the hormone insulin. Insulin moves glucose into cells or into the liver to be converted to glycogen (reserves), lowering blood sugar.
  • Falling Blood Glucose: Skipping a meal causes blood sugar to drop. Alpha cells in the pancreas detect this and release the hormone glucagon. Glucagon breaks down glycogen reserves into glucose to raise blood sugar levels.

Negative and Positive Feedback Loops

• Negative Feedback Loop: The primary regulatory mechanism in physiology. The body's response is opposite to the initial stimulus, reducing the effect of the stimulus to restore balance. Most body systems (e.g., thermoregulation and glucose control) operate on negative feedback.
• Positive Feedback Loop: A mechanism that exaggerates or increases the response until a specific goal is achieved.
  • Example: Childbirth:
    1. Stimulus: The baby's head pushes against the cervix.
    2. Detection: Stretch receptors in the cervix send signals to the brain.
    3. Hormonal Response: The brain releases oxytocin.
    4. Effector Action: Oxytocin reaches the myometrium (smooth uterine muscle), causing it to contract.
    5. Loop Continuation: Contractions push the baby further against the cervix, triggering more oxytocin and even stronger contractions.
    6. Resolution: The loop continues until the expulsion of the baby and placenta, at which point homeostasis is regained.