Formed elements in blood (Table 48.1)
7 rows| Component | Class | Share | Function |
|---|---|---|---|
| Red blood cells (erythrocytes) | — | 4–6 million/mm³ | O₂ and CO₂ transport on hemoglobin |
| Neutrophils | Granular | 60–70% of WBCs | Phagocytosis |
| Eosinophils | Granular | 2–4% of WBCs | Allergic responses and parasitic infections |
| Basophils | Granular | 0.5–1% of WBCs | Release histamine for vasodilation in inflammation |
| Lymphocytes | Agranular | 20–25% of WBCs | Produce antibodies; destroy foreign invaders |
| Monocytes | Agranular | 3–8% of WBCs | Become large macrophages that settle in tissues |
| Platelets | — | 150,000–300,000/mm³ | Instrumental in clotting |
Evolution of the vertebrate heart
6 rows| Group | Heart | Circuits | Mixing of blood |
|---|---|---|---|
| Lancelets (ancestral chordates) | Simple tubular heart, peristaltic | Single | Not applicable |
| Fishes | 2 chambers (sinus venosus + atrium; ventricle + conus arteriosus) | Single circuit | None, but large pressure loss at the gills |
| Amphibians | 3 chambers — 2 atria, 1 ventricle | Pulmonary + systemic (pulmocutaneous) | Some mixing, limited by ventricular ridges |
| Most reptiles | 3 chambers with a partial ventricular septum | Pulmonary + systemic | Reduced mixing |
| Crocodilians | 4 chambers — complete septum | Pulmonary + systemic | None |
| Birds and mammals | 4 chambers, nearly identical by convergent evolution | Fully separate double circulation | None |
Blood vessels compared
6 rows| Vessel | Wall layers | Pressure | Job |
|---|---|---|---|
| Artery | All 4; thick smooth muscle, two elastic layers | Highest; ~120/80 in the aorta | Carry blood away from the heart; recoil between beats |
| Arteriole | All 4; thick muscle relative to size | Steep drop happens here | Main resistance vessel — controls flow by constricting or dilating |
| Capillary | Endothelium only | Low | All exchange with tissues; slowest flow, largest total cross-section |
| Venule | All 4, thin | Low | Collect blood from capillaries |
| Vein | All 4; thin muscle layer, plus one-way valves | ~1/10 of arterial | Return blood to the heart; hold most of the blood volume |
| Lymphatic vessel | Built like a vein, with valves; capillaries are blind-ended | Very low | One-way drainage of excess interstitial fluid |
Cardiac output redistribution: rest vs moderate exercise (Table 48.2)
5 rows| Organ | Rest (mL/min) | Exercise (mL/min) | % of CO at rest → exercise |
|---|---|---|---|
| Muscle | 1,000 | 12,500 | 20.0% → 71.4% |
| Brain | 700 | 750 | 14.0% → 4.3% (flow nearly unchanged) |
| Heart (coronary) | 200 | 750 | About the same share |
| Skin (cutaneous) | Low | Raised | Rises — dumps exercise heat |
| Kidneys and digestive tract | High | Reduced | Both flow and share drop |
ECG waves and what is happening
3 rows| Wave | Electrical event | Mechanical event that follows |
|---|---|---|
| P wave | Atrial depolarization | Atrial systole — atria push blood into the ventricles |
| QRS complex | Ventricular depolarization (atrial repolarization is hidden inside it) | Ventricular systole — AV valves close ("lub"), then ejection |
| T wave | Ventricular repolarization | Diastole in all four chambers; semilunar valves close ("dub"), ventricles refill |
Exam traps
11 pairsQuestions get built out of near-misses. If you can state each difference in one sentence, you will not lose those points.
ArteriesvsVeins
Direction, not oxygen, is the definition: arteries carry blood AWAY from the heart, veins carry it BACK. The pulmonary artery carries deoxygenated blood and the pulmonary veins carry oxygenated blood, so "arteries are oxygenated" is false. Structurally: arteries have thick smooth muscle and two elastic layers; veins are thin-walled and have one-way valves.
SystolevsDiastole
Systole = contraction, diastole = relaxation. Always say which chamber: during ventricular systole the atria are relaxing. The higher number in a BP reading is systolic.
SA nodevsAV node
The SA node is the pacemaker — it fires first, spontaneously, and sets the rate. The AV node is the only gateway from atria to ventricles and its job is to delay the signal ~0.1 sec so the atria finish emptying first.
P wave and QRS complex (DE-polarization)vsT wave (RE-polarization)
P = atria depolarizing; QRS = ventricles depolarizing, bigger because ventricular muscle is thicker. Both come just BEFORE a contraction. The T wave is ventricular RE-polarization, when all four chambers are in diastole. Atrial repolarization has no wave of its own — it is buried inside the QRS.
"Lub" (first sound)vs"Dub" (second sound)
Lub is softer and longer: AV valves (tricuspid + mitral) closing at the START of ventricular systole. Dub is louder and shorter: semilunar valves (pulmonary + aortic) closing at the START of diastole. Both sounds are valves closing — never muscle contracting.
Tricuspid valvevsBicuspid (mitral) valve
Tricuspid = RIGHT side, between right atrium and right ventricle. Bicuspid/mitral = LEFT side. "Try before you buy": tri on the right, bi on the left.
Open circulatory systemvsClosed circulatory system
Open: fluid (hemolymph) leaves vessels and bathes tissues directly, then drains back — one-way path, no separation from extracellular fluid. Closed: blood never leaves the vessels — a true loop. Vertebrates are closed, but their LYMPHATIC system is open-ended.
FibrinogenvsFibrin
Fibrinogen is SOLUBLE and always dissolved in plasma. Thrombin cuts it into insoluble FIBRIN threads, which is what actually forms the clot mesh. Plasma minus fibrinogen = serum.
ProthrombinvsThrombin
Prothrombin is the inactive liver-made precursor. Thrombin is the active enzyme, and activating it is the LAST step of the cascade — after which fibrinogen becomes fibrin.
AtherosclerosisvsArteriosclerosis
Athero- = fatty plaque (cholesterol, smooth muscle, debris) narrowing the lumen. Arterio- = calcium deposits HARDENING the wall so it cannot expand. They usually occur together, but the buildup material differs.
Arterioles are the resistance vesselsvsCapillaries are the exchange vessels
Resistance scales with 1/radius⁴, so arterioles cause the biggest pressure drop. Capillaries are narrower individually but so numerous that their total cross-sectional area is the largest — which is why blood moves SLOWEST there, giving time for exchange.