The heart is the correct answer! The heart has four chambers: right atrium, right ventricle, left atrium, left ventricle. The right side pumps deoxygenated blood to the lungs (pulmonary circulation). The left side pumps oxygenated blood to the body (systemic circulation). Blood vessels: arteries carry blood away from the heart (thick, muscular walls); veins carry blood toward the heart (thinner walls, valves); capillaries are microscopic vessels where exchange of gases and nutrients occurs (thin walls, one cell thick). The circulatory system also includes the lymphatic system (returns excess fluid to the blood). The adult human has about 60,000 miles (100,000 km) of blood vessels – enough to circle the Earth more than twice! The circulatory system is essential for life; heart disease is the leading cause of death worldwide.

The left ventricle pumps oxygenated blood to the body! The left ventricle has the thickest muscle because it must generate high pressure to send blood throughout the body. The right ventricle pumps deoxygenated blood to the lungs (shorter distance, less pressure). The heart sounds ("lub-dub") are caused by valves closing: "lub" = tricuspid and mitral valves closing; "dub" = aortic and pulmonary valves closing. The heart's electrical system (sinoatrial node, atrioventricular node, Purkinje fibers) controls the heartbeat. The SA node (natural pacemaker) generates about 60-100 beats per minute at rest. Heart rate increases with exercise, stress, or illness. The average resting heart rate is about 60-100 bpm (adults). Babies have higher heart rates (100-160 bpm). Athletes may have lower resting heart rates (40-60 bpm) due to more efficient hearts. The heart beats about 2.5 billion times in an average lifetime.

Arteries carry blood away from the heart! The word "artery" comes from Greek "arteria" (air pipe), because ancient anatomists thought arteries carried air (they were empty in cadavers). The pulmonary artery is an exception: it carries deoxygenated blood from the right ventricle to the lungs. The pulmonary vein carries oxygenated blood from the lungs to the left atrium. Veins have valves (like one-way doors) that prevent blood from flowing backward. This is especially important in the legs, where blood must flow upward against gravity. When you contract your leg muscles (walking), they squeeze veins, pushing blood upward (the skeletal muscle pump). Prolonged standing can cause blood to pool in leg veins, leading to varicose veins or fainting. Capillaries are so narrow that red blood cells must pass through single file. The total length of capillaries in the human body is about 25,000 miles (40,000 km).

Red blood cells carry oxygen! There are about 25 trillion red blood cells in the adult human body (about 5 million per microliter of blood). Each red blood cell contains about 270 million hemoglobin molecules; each hemoglobin can carry 4 oxygen molecules. Red blood cells live about 120 days and are destroyed in the spleen. About 2 million red blood cells are produced per second in bone marrow. White blood cells are larger and have a nucleus. There are several types: neutrophils (most common, fight bacteria), lymphocytes (B cells and T cells, part of adaptive immunity), monocytes (become macrophages), eosinophils (fight parasites), basophils (involved in allergic reactions). White blood cells increase during infection (leukocytosis). Platelets are cell fragments (no nucleus) that clump together to form clots. Without platelets, a small cut could cause fatal bleeding. Hemophilia is a genetic disorder where blood does not clot properly. Blood plasma is 92% water, with dissolved proteins (albumin, globulins, fibrinogen), electrolytes, glucose, hormones, and waste products. Blood makes up about 7-8% of body weight (about 5-6 liters in an adult).

The pulmonary artery carries deoxygenated blood to the lungs! In the lungs, carbon dioxide is removed and oxygen is added. Oxygenated blood returns to the left atrium via the pulmonary vein (the only vein that carries oxygenated blood). The aorta is the largest artery; it carries oxygenated blood from the left ventricle to the body. The superior vena cava and inferior vena cava bring deoxygenated blood from the body to the right atrium. The heart also has its own blood supply (coronary circulation). Coronary arteries branch off the aorta and supply oxygen to the heart muscle. Blockage of a coronary artery causes a heart attack (myocardial infarction). The flow of blood is one-way due to valves (tricuspid, pulmonary, mitral, aortic). Backflow of blood (regurgitation) can occur if valves are damaged. The heart sounds ("lub-dub") are caused by valve closures. Heart murmurs are abnormal sounds due to turbulent blood flow (sometimes harmless, sometimes indicating valve problems).

Systolic is the top number! Normal adult blood pressure is around 120/80 (systolic/diastolic). Prehypertension: 120-139/80-89. Hypertension stage 1: 140-159/90-99. Hypertension stage 2: 160+/100+. Low blood pressure (hypotension) can cause dizziness or fainting. Blood pressure varies with age, activity, stress, and health. A sphygmomanometer (blood pressure cuff) is used to measure blood pressure. The cuff inflates to block blood flow; as the cuff deflates, the technician listens for Korotkoff sounds (thumping) with a stethoscope. The first sound is the systolic pressure; the last sound is the diastolic pressure. Pulse rate is the number of heartbeats per minute. The average resting heart rate for adults is 60-100 bpm. Athletes may have rates as low as 40 bpm. Tachycardia is a fast heart rate (>100 bpm); bradycardia is a slow heart rate (<60 bpm). The pulse can be felt at several points: radial (wrist), carotid (neck), brachial (inner elbow), femoral (groin), popliteal (behind knee), dorsalis pedis (top of foot).

Lymph nodes filter lymph! There are about 500-600 lymph nodes in the human body, concentrated in the neck, armpits, groin, chest, and abdomen. They range in size from a pinhead to a small bean. When you have a sore throat, you may feel swollen lymph nodes in your neck (cervical nodes). The spleen is the largest lymphatic organ; it filters blood, removes old red blood cells, and stores white blood cells. The thymus is where T cells (a type of white blood cell) mature. The tonsils and adenoids are lymphatic tissues in the throat that trap pathogens. The appendix (once thought vestigial) may serve as a reservoir for beneficial gut bacteria. Lymph capillaries in the small intestine (lacteals) absorb dietary fats (which gives lymph a milky appearance after a fatty meal). The lymphatic system lacks a pump (like the heart); lymph moves through muscle contractions and breathing (pressure changes). Lymphedema is swelling caused by blockage of lymph vessels (e.g., after cancer surgery). The lymphatic system is part of the immune system and is essential for survival.

Cardiovascular disease is the leading cause of death! Modifiable risk factors (smoking, diet, exercise, blood pressure, cholesterol) account for most heart disease. High cholesterol can be treated with statins. High blood pressure is often called the "silent killer" because it has no symptoms. A heart attack (myocardial infarction) occurs when blood flow to part of the heart is blocked. Symptoms include chest pain (angina), shortness of breath, nausea, pain radiating to left arm or jaw. Immediate treatment (aspirin, clot-busting drugs, angioplasty) can save lives. A stroke (cerebrovascular accident) occurs when blood flow to part of the brain is blocked (ischemic stroke) or when a blood vessel in the brain bursts (hemorrhagic stroke). FAST is a mnemonic for stroke symptoms: Face drooping, Arm weakness, Speech difficulty, Time to call emergency services. Prevention: healthy diet (low in saturated fat, trans fat, salt, added sugar), regular exercise, maintaining healthy weight, not smoking, limiting alcohol, managing stress. Heart attacks kill more women than breast cancer, but women often have different symptoms (nausea, back pain, fatigue) and are more likely to be misdiagnosed.

The SA node is the natural pacemaker! If it fails, the AV node can take over (slower rate, about 40-60 bpm). If both fail, Purkinje fibers can fire (very slow, about 20-40 bpm). Artificial pacemakers are implanted devices that deliver electrical impulses to regulate the heartbeat. An ECG records the P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization). Heart rate variability (HRV) is the variation in time between heartbeats; high HRV is generally a sign of good health (low HRV is linked to stress and cardiovascular risk). The heart's electrical system is sensitive to electrolytes (potassium, calcium, sodium), drugs, and nerve signals (sympathetic nervous system speeds up heart rate, parasympathetic slows it down). Exercise training can increase HRV and lower resting heart rate. Meditation and deep breathing can also increase HRV. The SA node was discovered by Martin Flack (1907). Artificial pacemakers were first implanted in the 1950s (Rune Elmqvist, Åke Senning). Modern pacemakers can sense the heart's natural rhythm and deliver impulses only when needed (demand pacemakers).

Pulmonary circuit is the correct answer! The word "pulmonary" means "related to the lungs." The systemic circuit supplies oxygen and nutrients to all body tissues and removes waste. The circulatory system also transports hormones (from endocrine glands), immune cells, and heat (to regulate body temperature). When you exercise, blood vessels in muscles dilate (widen) to increase blood flow; blood vessels in less active areas constrict. This is controlled by the sympathetic nervous system. Blood pressure is regulated by the kidneys (renin-angiotensin system) and the baroreceptor reflex. The circulatory system works closely with the respiratory system (gas exchange), digestive system (nutrient absorption), urinary system (waste removal), and endocrine system (hormone transport). You have learned about the heart's chambers, blood vessels, blood components, blood flow, blood pressure, lymphatic system, cardiovascular disease, and the heart's electrical system. Great job! You are now a circulatory system expert.