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Field notes · chapter 48

The Circulatory System

pp. 1064–1084 · Raven Part VII

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  1. Two system types. Open circulation (most mollusks, arthropods): the fluid is hemolymph, and there is no difference between circulating fluid and the extracellular fluid bathing tissues. Closed circulation (cephalopods, annelids, all vertebrates): blood stays inside vessels the whole time. Watch Open and closed circulatory systems | Body fluids and circulation | Biology | Khan Academy on YouTube, from Khan Academy India - English
  2. Mammalian blood by volume: plasma 55%, red blood cells 45% (the hematocrit), buffy coat of platelets + leukocytes <1%. Fish blood is very different: plasma 69%, RBCs 30%, buffy coat 1%. Watch Blood, Part 1 - True Blood: Crash Course Anatomy & Physiology #29 on YouTube, from CrashCourse
  3. Plasma is about 92% water (91.5% in the detailed breakdown), 7% plasma proteins, 1.5% other solutes. Plasma proteins: albumin 54%, globulins 38%, fibrinogen 7%, others 1%. The liver makes most of them. Watch Blood, Part 1 - True Blood: Crash Course Anatomy & Physiology #29 on YouTube, from CrashCourse
  4. Plasma minus fibrinogen = serum. Main plasma ions: Na⁺, Cl⁻, HCO₃⁻.
  5. Counts to memorize: red blood cells 4–6 million/mm³ (about 5 million per microliter); platelets 150,000–300,000/mm³; leukocytes are less than 1% of blood cells — only 1 or 2 leukocytes per 1000 erythrocytes. Watch Blood, Part 2 - There Will Be Blood: Crash Course Anatomy & Physiology #30 on YouTube, from CrashCourse
  6. Leukocyte ranking, most to least: neutrophils (60–70%) > lymphocytes (20–25%) > monocytes (3–8%) > eosinophils (2–4%) > basophils (0.5–1%). Memory hook: "Never Let Monkeys Eat Bananas." Watch Blood, Part 2 - There Will Be Blood: Crash Course Anatomy & Physiology #30 on YouTube, from CrashCourse
  7. Mature mammalian erythrocytes have no nucleus. Every other vertebrate keeps nucleated red cells. Mammalian RBCs are biconcave discs with a central depression that does not go all the way through. Watch Blood, Part 1 - True Blood: Crash Course Anatomy & Physiology #29 on YouTube, from CrashCourse
  8. Clotting cascade in order: vessel constricts → platelets stick and plug → cascade triggered → prothrombin activator + prothrombin + Ca²⁺ → thrombin; thrombin + fibrinogen + Ca²⁺ → fibrin. Fibrin threads trap erythrocytes and tighten the plug. Activating thrombin is the last step of the cascade. Watch Coagulation Cascade and Fibrinolysis - clotting factors, regulation and control mechanism on YouTube, from Armando Hasudungan
  9. Soluble fibrinogen becomes insoluble fibrin. A clot that breaks loose and travels can block a brain vessel (stroke) or a heart vessel (heart attack). Watch Coagulation Cascade and Fibrinolysis - clotting factors, regulation and control mechanism on YouTube, from Armando Hasudungan
  10. Low blood oxygen → the kidney converts a plasma protein into erythropoietin → myeloid stem cells make more red cells (erythropoiesis). Platelets are cytoplasm fragments pinched off megakaryocytes, about 3 µm across. Watch Blood, Part 2 - There Will Be Blood: Crash Course Anatomy & Physiology #30 on YouTube, from CrashCourse
  11. Heart evolution: lancelet tubular heart → fish 2-chambered → amphibians and most reptiles 3-chambered (2 atria, 1 ventricle) → crocodilians, birds, mammals 4-chambered. Bird and mammal four-chambered hearts are convergent evolution, not shared ancestry. Watch How many chambers do animals' hearts have? | Body fluids and circulation | Biology | Khan Academy on YouTube, from Khan Academy India - English
  12. Fish heart order: sinus venosus → atrium → ventricle → conus arteriosus. This firing order is conserved in all vertebrates even as the chambers change. Fish have single-circuit circulation, and blood loses a lot of pressure crossing the gills. Watch How many chambers do animals' hearts have? | Body fluids and circulation | Biology | Khan Academy on YouTube, from Khan Academy India - English
  13. Blood through the human heart: superior + inferior venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary trunk → pulmonary arteries → lungs → pulmonary veins → left atrium → bicuspid (mitral) valve → left ventricle → aortic valve → aorta → body → venae cavae. Watch Circulatory System and Pathway of Blood Through the Heart on YouTube, from Amoeba Sisters
  14. Only pulmonary arteries carry deoxygenated blood and only pulmonary veins carry oxygenated blood. "Artery = oxygenated" is false. Watch Circulatory System and Pathway of Blood Through the Heart on YouTube, from Amoeba Sisters
  15. Systole = contraction, diastole = relaxation. One cardiac cycle = atrial systole, then ventricular systole, then a shared diastole. Average human heart rate is about 70 beats/min. Watch Cardiovascular | Cardiac Cycle: Digital Version on YouTube, from Ninja Nerd
  16. Heart sounds come from valves closing, not muscle contracting. Lub = softer, longer, AV valves (tricuspid + mitral) closing at the start of ventricular systole. Dub = louder, shorter, semilunar valves (pulmonary + aortic) closing at the start of diastole. Watch Cardiovascular | Cardiac Cycle: Digital Version on YouTube, from Ninja Nerd
  17. Conduction path: SA node → both atria → AV node (delay ~0.1 sec) → AV bundle (bundle of His) → left and right bundle branches down the septum → Purkinje fibers → ventricular muscle. A connective tissue ring blocks every other route from atria to ventricles. Watch Cardiovascular | Electrophysiology | Intrinsic Cardiac Conduction System on YouTube, from Ninja Nerd
  18. The AV node delay of ~0.1 second is what lets the atria finish emptying before the ventricles squeeze. Watch Cardiovascular | Electrophysiology | Intrinsic Cardiac Conduction System on YouTube, from Ninja Nerd
  19. ECG: P wave = atrial depolarization (just before atrial systole). QRS complex = ventricular depolarization (just before ventricular systole) — bigger because the ventricles are thicker, and atrial repolarization is hidden inside it. T wave = ventricular repolarization, with all four chambers in diastole. Watch ECG | Body fluids and circulation | Biology | Khan Academy on YouTube, from Khan Academy India - English
  20. Cardiac output = stroke volume × heart rate. Resting example: 70 mL/beat × 70 beats/min = 4900 mL/min = 4.9 L/min — roughly your entire blood volume every minute. Watch Cardiovascular | Cardiac Output on YouTube, from Ninja Nerd
  21. Exercise redistribution (Table 48.2): muscle flow jumps 1,000 → 12,500 mL/min (20% → 71.4% of CO). Brain flow barely moves (700 → 750 mL/min) but its share falls 14.0% → 4.3%. Coronary flow rises 200 → 750 mL/min at about the same share. Watch Cardiovascular | Cardiac Output on YouTube, from Ninja Nerd
  22. Blood pressure is written systolic/diastolic, measured with a sphygmomanometer on the brachial artery. Healthy young adult ≈ 120/80 mm Hg. Hypertension = systolic above 140 or diastolic above 90. Watch Blood Vessels, Part 2: Crash Course Anatomy & Physiology #28 on YouTube, from CrashCourse
  23. BP = CO × R. Raise cardiac output or raise resistance and pressure rises. Pressure falls steadily from the aorta to the vena cava, and the biggest drop happens at the arterioles. Watch Blood Vessels, Part 2: Crash Course Anatomy & Physiology #28 on YouTube, from CrashCourse
  24. Resistance scales with 1/radius⁴. Halve the radius and resistance goes up 16×. That is why arterioles, not capillaries, are the main resistance vessels. Watch Blood Vessels, Part 1 - Form and Function: Crash Course Anatomy & Physiology #27 on YouTube, from CrashCourse
  25. Vessel walls, outside in: connective tissue → smooth muscle → elastic fibers → endothelium. Arteries and veins have all four; arteries have much thicker smooth muscle and two elastic layers. Capillaries are endothelium only. Watch Blood Vessels, Part 1 - Form and Function: Crash Course Anatomy & Physiology #27 on YouTube, from CrashCourse
  26. Venous pressure is only about one-tenth of arterial pressure, so veins need help: the venous pump (skeletal muscle squeezing) plus one-way venous valves. Veins hold most of the body’s blood volume. Overstretched veins with leaky valves = varicose veins. Watch Blood Vessels, Part 1 - Form and Function: Crash Course Anatomy & Physiology #27 on YouTube, from CrashCourse
  27. Lymph path: lymphatic capillaries (blind-ended) → lymphatic vessels with one-way valves → two trunks → left and right subclavian veins. Lymph moves by skeletal muscle squeezing; some vertebrates have lymph hearts. Watch Lymphatic System: Crash Course Anatomy & Physiology #44 on YouTube, from CrashCourse
  28. Blood pressure hormones: renin → angiotensin → vasoconstriction + aldosterone → Na⁺ and water retention → BP up. ANP from the stretched right atrium does the opposite: more Na⁺ excreted, BP down. Nitric oxide from endothelium relaxes smooth muscle and dilates vessels (this is how nitroglycerin treats angina). Watch Blood Vessels, Part 2: Crash Course Anatomy & Physiology #28 on YouTube, from CrashCourse
  29. Baroreceptors in the aortic arch and carotid arteries sense pressure. Falling BP → fewer signals to the cardiac center → more sympathetic, less parasympathetic output → heart rate and vasoconstriction rise.
  30. Cardiovascular disease is the #1 killer in the US: 80+ million affected, 600,000+ deaths/year. Atherosclerosis = fat, cholesterol, smooth muscle, fibrin and debris narrowing the lumen. LDL delivers cholesterol to cells (the "bad" one); HDL hauls it to the liver for disposal (the "good" one). Watch Atherosclerosis - Pathogenesis, risk factors and complications on YouTube, from Dr Matt & Dr Mike