Human Body Systems: Practice Questions & Explanations

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).

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.

The circulatory (cardiovascular) system — the heart, blood, and blood vessels — transports oxygen, nutrients, hormones, and waste products throughout the body. The respiratory system (lungs) brings oxygen into the blood and removes carbon dioxide, but the circulatory system does the actual delivery to cells. The digestive system breaks down food into nutrients, and the nervous system handles signaling. These systems work together, but transport of oxygen and nutrients to cells is the circulatory system's main job, carried by the blood the heart pumps.

The pancreas produces insulin, a hormone that regulates the level of glucose (sugar) in the blood by helping cells absorb it. When insulin production or function is impaired, the result can be diabetes. The liver stores and releases glucose and performs many other functions but does not make insulin; the kidneys filter blood to make urine; and the stomach digests food. Knowing key glands and the hormones they produce — such as the pancreas and insulin — is standard human-body content on the General Science subtest.