Human Body Systems, Homeostasis, Nutrition & Disease

Life Science accounts for 40% of the GED Science exam, and Assessment Target L.a focuses on how the human body maintains internal stability, processes nutrients, and defends against disease. You are never required to memorize obscure anatomical Latin; instead, questions give you a physiological scenario, feedback diagram, or clinical trial table and ask how two organ systems work together or how a feedback loop restores balance. For example, when you sprint up a flight of stairs, your muscular system demands more ATP energy, your respiratory system increases breathing rate to bring in O₂ and expel CO₂, and your circulatory system raises heart rate to transport oxygen and glucose from the digestive tract to muscle cells—all coordinated by signals from the nervous and endocrine systems.

1. Major Body Systems & Their Interactions (L.a.1): • Circulatory (heart, arteries, veins, capillaries): Transports oxygen, nutrients, hormones, and immune cells. • Respiratory (lungs, trachea, alveoli, diaphragm): Exchanges O₂ and CO₂ between air and blood. • Digestive (stomach, small & large intestines, liver, pancreas): Breaks food down mechanically and chemically into absorbable glucose, amino acids, and fatty acids (absorbed through villi in the small intestine into capillaries). • Nervous (brain, spinal cord, neurons): Sends rapid electrical and neurotransmitter signals to detect stimuli and coordinate responses. • Immune (white blood cells/leukocytes, antibodies, lymph nodes, spleen): Identifies and destroys invading pathogens. 2. Homeostasis and Negative Feedback Loops (L.a.2): Homeostasis is the maintenance of a stable internal environment (core body temperature ~37°C, blood glucose ~70–100 mg/dL, blood pH ~7.4) despite external changes. • Negative Feedback (Reverses a deviation back toward the set point): - Thermoregulation: Too hot → hypothalamus triggers sweating (evaporative cooling) and vasodilation (widening skin blood vessels). Too cold → shivering and vasoconstriction. - Blood Glucose Regulation: After a meal (high blood glucose) → pancreas secretes INSULIN → liver and muscle cells take up glucose and store it as glycogen → blood glucose falls. During fasting (low blood glucose) → pancreas secretes GLUCAGON → liver breaks glycogen back down into glucose. 3. Nutrients & Dietary Function (L.a.3): • Carbohydrates (sugars, starches, fiber): Primary rapid source of cellular energy (4 kcal/g; glucose). • Lipids / Fats (oils, phospholipids): Long-term energy storage (9 kcal/g), cell membranes, thermal insulation, and steroid hormones. Unsaturated plant fats are healthier for cardiovascular health than saturated or trans fats. • Proteins (built from amino acids): Enzymes, muscle fibers, antibodies, and structural tissue repair. • Vitamins & Minerals: Micronutrients needed in small amounts as enzyme cofactors (e.g., Iron in hemoglobin for O₂ transport; Calcium and Vitamin D for bone density; Vitamin C for collagen and immunity). 4. Pathogens, Antibiotics vs Vaccines & Prevention (L.a.4): • Bacteria: Single-celled living prokaryotes (e.g., strep throat, tuberculosis, Salmonella). Treated with ANTIBIOTICS, which target bacterial cell walls or ribosomes. • Viruses: Non-living genetic material (DNA or RNA) inside a protein capsid that must hijack a host cell to replicate (e.g., influenza, measles, HIV, COVID-19). Antibiotics DO NOT work on viruses! Overusing antibiotics breeds antibiotic-resistant bacteria via natural selection. • Vaccines: Expose the immune system to a harmless weakened/inactivated pathogen or antigen so memory B and T cells produce rapid antibodies upon future exposure (preventative active immunity).