ASVAB General Science — Biology and the Human Body

Urinary/renal system: kidneys (2), ureters (2), bladder, urethra. Functions: (1) Filter blood to remove metabolic wastes (urea, creatinine, excess salts and water); (2) Regulate fluid and electrolyte balance; (3) Maintain blood pH; (4) Regulate blood pressure (renin); (5) Produce erythropoietin (stimulates RBC production); (6) Activate vitamin D. Kidney function: each kidney contains ~1 million nephrons (functional units). Nephron stages: (1) Glomerulus — filters blood plasma; (2) Proximal tubule — reabsorbs water, glucose, ions; (3) Loop of Henle — concentrates urine; (4) Distal tubule — fine-tunes electrolytes; (5) Collecting duct — final water reabsorption based on ADH. Major body systems: (1) Skeletal — bones, joints; support, movement, blood cell production; (2) Muscular — skeletal/smooth/cardiac muscle; movement; (3) Cardiovascular — heart, blood, vessels; circulation; (4) Respiratory — lungs, airways; gas exchange; (5) Digestive — GI tract, accessory organs; nutrient absorption; (6) Urinary — kidneys, bladder; filtration; (7) Nervous — brain, nerves; control, sensation; (8) Endocrine — glands; hormone regulation; (9) Immune/Lymphatic — defense, fluid balance; (10) Integumentary — skin; barrier; (11) Reproductive — gonads; reproduction.

Skin is the largest organ, with surface area ~1.5-2 square meters and accounting for ~15% of body weight. Skin (integumentary system) functions: (1) Barrier — protects against infection, dehydration, UV radiation, mechanical injury; (2) Thermoregulation — sweating cools body; vasodilation/constriction adjusts heat loss; (3) Sensation — touch, pressure, temperature, pain receptors; (4) Vitamin D synthesis — UV light converts cholesterol precursor to vitamin D; (5) Excretion — small amounts of waste through sweat. Skin layers: (1) Epidermis — outermost, no blood vessels, contains keratinocytes (produce keratin), melanocytes (produce melanin/pigment), Langerhans cells (immune); (2) Dermis — beneath epidermis; contains blood vessels, nerves, sweat glands, sebaceous (oil) glands, hair follicles; (3) Hypodermis (subcutaneous) — fat, connective tissue, larger vessels. Largest internal organ: liver (~1.5 kg, ~3% body weight). Other big organs: lungs, brain (1.4 kg), heart, kidneys. The brain is highly metabolically active despite only 2% of body weight — uses 20% of body's energy.

The small intestine is where most chemical digestion and virtually all nutrient absorption occur. About 6-7 meters long in adults; three sections: duodenum (first 25 cm; receives bile from gallbladder and pancreatic enzymes), jejunum (next ~2.5 m; most absorption), ileum (final ~3.5 m; absorbs vitamin B12, bile salts, remaining nutrients). The inner surface is highly folded and lined with villi (finger-like projections) and microvilli (tiny projections on villi cells), creating enormous surface area (~250 m²) for absorption. Digestive process by organ: (1) Mouth — mechanical breakdown by teeth; saliva contains amylase (starts carbohydrate digestion); (2) Esophagus — peristalsis moves food to stomach; (3) Stomach — mechanical churning; HCl creates acidic environment; pepsin begins protein digestion; food becomes chyme; (4) Small intestine — bile (emulsifies fats), pancreatic enzymes (lipase for fats, amylase for carbs, proteases for proteins) complete digestion; absorption into blood (sugars, amino acids) and lymph (fats); (5) Large intestine (colon) — absorbs water and electrolytes; houses beneficial bacteria; forms feces; (6) Rectum/anus — stores and eliminates waste. Accessory organs: liver (produces bile, processes nutrients), gallbladder (stores bile), pancreas (digestive enzymes and insulin).

DNA (deoxyribonucleic acid) is the genetic material in all known living organisms. Structure: (1) Double helix — two strands wound around each other; (2) Nucleotides — building blocks; each contains a sugar (deoxyribose), phosphate group, and one of four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G); (3) Base pairing — A always pairs with T (2 hydrogen bonds); C always pairs with G (3 hydrogen bonds); this complementary pairing allows accurate replication; (4) Sugar-phosphate backbone — sides of the 'ladder'; bases are the 'rungs'. DNA structure discovered by Watson, Crick, Franklin, Wilkins (1953). DNA → genes → chromosomes: a gene is a DNA sequence coding for a protein; genes are organized into chromosomes; humans have 46 chromosomes (23 pairs). The Central Dogma: DNA → RNA → protein. (1) Transcription: DNA is copied into messenger RNA (mRNA) in the nucleus; (2) Translation: mRNA is read by ribosomes; transfer RNAs (tRNAs) bring amino acids; amino acids link to form proteins. Codons: three-letter mRNA sequences each coding for one amino acid (e.g., AUG = methionine/start). DNA replication: occurs before cell division; semi-conservative (each new molecule has one old and one new strand); enzymes include helicase (unwinds), DNA polymerase (adds nucleotides). Mutations: changes in DNA sequence (point mutations, insertions, deletions); can be harmful, neutral, or beneficial.

Mitosis is cell division producing two genetically identical diploid daughter cells. Phases: (1) Prophase — chromosomes condense, nuclear envelope breaks down, spindle fibers form; (2) Metaphase — chromosomes line up at cell equator; (3) Anaphase — sister chromatids separate, pulled to opposite poles; (4) Telophase — nuclear envelopes reform around each set; (5) Cytokinesis — cytoplasm divides, two cells separate. Mnemonic: PMAT (Prophase, Metaphase, Anaphase, Telophase). Uses of mitosis: growth (single fertilized egg → trillions of cells); tissue repair (healing wounds, replacing dead cells); asexual reproduction (in some organisms). Versus MEIOSIS: produces four genetically different haploid gametes (sperm, eggs); occurs in reproductive cells only; involves two divisions and genetic recombination through crossing over. Meiosis introduces genetic variation; mitosis preserves it. Cell cycle: G1 (growth) → S (DNA replication) → G2 (preparation) → M (mitosis). G0 — non-dividing state for cells like neurons. Cell cycle regulation involves checkpoints; failure can lead to cancer (uncontrolled cell division). Tumor suppressor genes (p53) and oncogenes regulate the cycle; mutations disrupt control.

Red blood cells (RBCs/erythrocytes) — most abundant blood cells (~4-6 million per microliter). Primary function: gas transport. Hemoglobin: iron-containing protein; each hemoglobin molecule binds 4 oxygen molecules; gives RBCs and blood their red color. RBC characteristics: biconcave disc shape (increases surface area, allows flexibility); no nucleus in mature cells (more room for hemoglobin); ~120 day lifespan; produced in bone marrow (stimulated by erythropoietin from kidneys); destroyed in spleen and liver. Blood composition: (1) Plasma (~55%) — water, proteins, nutrients, waste, hormones; (2) Cells (~45%): RBCs (most), white blood cells (immune), platelets (clotting). White blood cells (leukocytes): fight infection and disease; ~5,000-10,000/μL; types include neutrophils (most common, fight bacteria), lymphocytes (T and B cells; antibodies, immune memory), monocytes (become macrophages), eosinophils (parasites, allergies), basophils (allergies). Platelets (thrombocytes): cell fragments; ~150,000-400,000/μL; activate to form clots, stopping bleeding. Anemia: low RBC count or hemoglobin; causes fatigue, weakness, pallor; types include iron deficiency, B12/folate deficiency, blood loss, hemolytic, aplastic. Polycythemia: too many RBCs.

Mitochondria produce ATP (cellular energy) through aerobic respiration — the powerhouse of the cell.

O negative is the universal donor for red blood cells — it lacks A, B, and Rh antigens, making it compatible with all blood types.

DNA (Deoxyribonucleic Acid) is the molecule that contains the GENETIC INSTRUCTIONS for every living organism. Located primarily in the NUCLEUS of cells. STRUCTURE: Double helix — two strands wound around each other; each strand made of nucleotides with four bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C). BASE PAIRING: A pairs with T; G pairs with C. FUNCTION: Stores instructions for making proteins (via transcription → RNA → translation → protein); copied during cell division so each daughter cell receives a complete copy; mutations in DNA can cause cancer or heritable diseases. GENOME: Complete DNA sequence of an organism — human genome has ~3 billion base pairs.

WHITE BLOOD CELLS (leukocytes) are primarily responsible for FIGHTING INFECTION as part of the immune system. ASVAB General Science covers the circulatory/immune system. Blood components: RED BLOOD CELLS (carry oxygen via hemoglobin); WHITE BLOOD CELLS (fight infection/immunity); PLATELETS (help blood clot); PLASMA (the liquid that carries cells and nutrients). Knowing the function of each blood component — especially white blood cells fighting infection — is commonly tested.