Human Biology & Physiology Cheat Sheet

A comprehensive overview of the fundamental levels of organization, chemical principles, cellular processes, and organ system functions within the human body, essential for understanding health and disease.

Core Principles

• Levels of Organization: Matter progresses from atoms to molecules, cells, tissues, organs, organ systems, and finally, the complete organism.
• Chemical Bonding: Atoms form molecules through ionic (transfer), covalent (sharing), and hydrogen (weak attraction) bonds, crucial for biological structures.
• Cellular Basis of Life: Cells are the fundamental units, with specialized organelles performing vital functions, and their cycle is tightly regulated.
• Homeostasis: The body maintains a stable internal environment through feedback mechanisms, primarily negative feedback, to ensure optimal function.
• System Integration: Organ systems (nervous, circulatory, respiratory, etc.) work cooperatively to maintain life and respond to stimuli.
• Metabolism and Energy: Organisms utilize catabolic and anabolic processes to break down substances for energy and build complex molecules.

Formulas

• Atomic Number = Protons
• Atomic Mass = Protons + Neutrons
• $Na^+ + Cl^- \rightarrow NaCl$ (Ionic Bond Example)
• Glucose + O₂ → CO₂ + H₂O + ATP (Carbohydrate Catabolism)
• Resting Membrane Potential: Inside Negative (High K⁺ inside, High Na⁺ outside)
• Action Potential: Depolarization (Na⁺ in) → Repolarization (K⁺ out)

Pro Tips

• Reactivity is essential for life, driven by unfilled electron shells.
• Carbon's ability to form diverse structures makes it the backbone of biological molecules.
• Water's polarity makes it an excellent solvent for biological processes.
• Negative feedback is the primary mechanism for maintaining homeostasis.
• The sodium-potassium pump is crucial for maintaining cell membrane potential.
• Myelin dramatically increases nerve impulse speed via saltatory conduction.
• The heart's electrical conduction system ensures coordinated pumping.
• Alveoli provide a massive surface area for efficient gas exchange.
• Nephrons are the functional units of the kidney, filtering blood and producing urine.
• Bone remodels constantly in response to mechanical stress.
• Muscle contraction relies on the sliding of actin and myosin filaments.
• The small intestine's villi and microvilli vastly increase nutrient absorption.
• Hormones act slowly but have prolonged effects by binding to specific receptors.
• Meiosis creates genetic diversity through crossing-over and independent assortment.
• The immune system has both rapid, non-specific innate defenses and slower, specific adaptive responses.
• Pharmacokinetics (ADME) determines how a drug affects the body over time.

Pitfalls to Avoid

• Confusing atomic number (protons) with atomic mass (protons + neutrons).
• Assuming all atoms with full shells are inert; some can still participate in weak interactions.
• Overlooking the importance of hydrogen bonds in biological molecules like water and DNA.
• Forgetting that active transport requires ATP energy.
• Misunderstanding the difference between depolarization and repolarization in action potentials.
• Confusing the brain's grey/white matter arrangement with the spinal cord's.
• Assuming arteries and veins have identical structures and functions.
• Not recognizing the role of accessory organs in digestion.
• Confusing the functions of the anterior and posterior pituitary.
• Mistaking meiosis (gamete production) for mitosis (somatic cell division).
• Underestimating the importance of MHC molecules in T cell activation.
• Confusing communicable with non-communicable diseases.
• Ignoring drug interactions, which can significantly alter effects.
• Assuming resistance only applies to antibiotics; it affects antivirals and antiparasitics too.

Myth vs Reality

• All ions are harmful.: Ions are charged atoms essential for nerve impulses, muscle contraction, and maintaining fluid balance (e.g., Na⁺, K⁺, Ca²⁺).
• The body only uses negative feedback.: While negative feedback is dominant for homeostasis, positive feedback (e.g., childbirth, blood clotting) amplifies responses.
• All cells divide continuously.: Cells exist in different states: G₀ (non-dividing), G₁, S, G₂ (interphase), and M (mitosis). Some, like neurons, rarely divide.

Statistics

• Alveoli per lung: ~150 million
• Small intestine absorption efficiency: ~90%
• Blood filtered per day (kidneys): ~180 Liters
• Cardiac output percentage to kidneys: 20-25%
• Normal body temperature: ~38°C
• Blood pH range: 7.35–7.45
• Blood volume (male): 4–6 Liters
• Blood volume increase in pregnancy: Up to 50%
• Lymph drained daily: ~3 Liters

Timeline

• Early Embryonic Development: Cleavage, morula formation, blastocyst development, implantation (approx. 7-10 days post-fertilization).
• 1st Trimester (0-13 weeks): Gastrulation (formation of germ layers), early organogenesis, placenta formation.
• 2nd Trimester (14-26 weeks): Rapid fetal growth, organ maturation, development of fetal viability.
• 3rd Trimester (27-40 weeks): Final organ function development, significant growth, fat deposition.
• Birth: Labor and delivery, often involving positive feedback loops (oxytocin).

People

• Robert Hooke: Coined the term 'cell' after observing cork under a microscope.
• Louis Pasteur: Developed pasteurization and contributed to germ theory, impacting understanding of disease transmission.
• Gregor Mendel: Father of genetics, studied inheritance patterns in pea plants, laying groundwork for understanding heredity.

Quiz

• Which level of organization is formed by groups of similar cells working together?: Tissue level
• What is the primary function of mitochondria?: ATP production
• Which hormone lowers blood glucose levels?: Insulin