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Field notes · chapter 48

The Circulatory System

pp. 1064–1084 · Raven Part VII

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48.1

Invertebrate Circulatory Systems

pp. 1064–1065
  • The kind of circulatory system an invertebrate has tracks its size, complexity, and lifestyle. Small and thin animals need less machinery.
  • Sponges and most cnidarians use environmental water as their circulatory fluid. Sponges draw water through many incurrent pores and expel it through one osculum. Hydra moves water through a gastrovascular cavity that serves digestion and circulation at once.
  • This works in Hydra because the body wall is only two cell layers thick, so every cell touches either the outside water or the gastrovascular cavity.
  • Pseudocoelomate invertebrates (roundworms, rotifers) circulate using body cavity fluid, stirred by body movements. Again, they are small or long and thin.
  • Larger animals have tissues several cell layers thick. Interior cells are too far from any surface to exchange directly, so an internal fluid moved by a circulatory system is required.
  • In an open circulatory system (most mollusks and arthropods) there is no distinction between circulating fluid and extracellular fluid. That fluid is called hemolymph. Watch Open and closed circulatory systems | Body fluids and circulation | Biology | Khan Academy on YouTube, from Khan Academy India - English
  • In an insect, a muscular tube — the heart — pumps hemolymph through channels and cavities in the body; the fluid then drains back into the central body cavity and is recirculated. Flow is a one-way path, not a loop.
  • In a closed circulatory system, the fluid — blood — is always inside vessels that carry it away from and back to the heart. Found in cephalopod mollusks, annelids, and all vertebrates. Watch Open and closed circulatory systems | Body fluids and circulation | Biology | Khan Academy on YouTube, from Khan Academy India - English
  • Earthworm example: a dorsal vessel contracts rhythmically as the pump, pushing blood through five small connecting arteries (which also pump) into a ventral vessel that carries blood posteriorly before it reenters the dorsal vessel. Smaller branches serve the tissues.
48.2

The Components of Vertebrate Blood

pp. 1065–1068
48.3

Vertebrate Circulatory Systems

pp. 1068–1071
  • Closed systems and larger hearts put a premium on efficiency. Linking circulation tightly to respiration is what allowed vertebrates to diversify on land and in water and to get large.
  • Ancestral chordates likely had a simple tubular heart, like a lancelet — barely more than a heavily muscled zone of the ventral artery, contracting in peristaltic waves.
  • Gills demanded a better pump, so fish evolved a true chamber-pump heart: four structures in series forming two pumping chambers. Sinus venosus + atrium = first chamber; ventricle + conus arteriosus = second.
  • The fish contraction order — sinus venosus, atrium, ventricle, conus arteriosus — is conserved in all vertebrates even though chamber names and positions changed.
  • Fish circulation is a single circuit: heart → gills → body → heart. Its weakness is that blood loses substantial pressure crossing the gill capillaries, so it reaches the tissues slowly and at low pressure. Watch How many chambers do animals' hearts have? | Body fluids and circulation | Biology | Khan Academy on YouTube, from Khan Academy India - English
  • Lungs required a second pump. Blood from the lungs returns to the heart first via pulmonary veins, then gets pumped out again to the body. That two-loop design is double circulation: the pulmonary circuit (heart ↔ lungs) plus the systemic circuit (heart ↔ body).
  • Amphibians have three chambers — two atria, one ventricle — so some mixing of oxygenated and deoxygenated blood is unavoidable. Two features limit it: the divided atrium (right gets body blood, left gets lung blood) and internal ridges and recesses in the single ventricle that keep the streams somewhat separate. A partial wall in the conus arteriosus steers deoxygenated blood toward the pulmocutaneous artery and oxygenated blood toward the aorta. Watch How many chambers do animals' hearts have? | Body fluids and circulation | Biology | Khan Academy on YouTube, from Khan Academy India - English
  • Amphibians also use cutaneous respiration, so the pulmocutaneous circuit sends blood to both lungs and skin. Turtles do cutaneous respiration too.
  • Reptiles reduce mixing further with a septum that partially divides the ventricle; in crocodilians the septum is complete, giving two separate ventricles. The conus arteriosus has become absorbed into the trunks of the great arteries. Watch How many chambers do animals' hearts have? | Body fluids and circulation | Biology | Khan Academy on YouTube, from Khan Academy India - English
  • Mammals, birds, and crocodilians have four-chambered hearts — two atria and two ventricles — keeping pulmonary and systemic blood completely separate. Bird and mammal hearts are so alike that one diagram serves both, which is a striking case of convergent evolution. Watch How many chambers do animals' hearts have? | Body fluids and circulation | Biology | Khan Academy on YouTube, from Khan Academy India - English
  • The four-chambered heart is a double pump with valves preventing backflow, producing one-way continuous flow.
  • Full path, starting at the right atrium: superior and inferior venae cavae deliver oxygen-poor blood → right atrium → tricuspid (right AV) valve → right ventricle → pulmonary valve → pulmonary trunk → right and left pulmonary arteries → lungs, where CO₂ is unloaded and O₂ binds hemoglobin. Watch Circulatory System and Pathway of Blood Through the Heart on YouTube, from Amoeba Sisters
  • Continuing: pulmonary veins → left atrium → bicuspid (left AV) valve, known medically as the mitral valve → left ventricle → aortic valve → aorta → systemic arteries → tissues, where O₂ is unloaded and CO₂ picked up → systemic veins → venae cavae → back to the right atrium. Watch Circulatory System and Pathway of Blood Through the Heart on YouTube, from Amoeba Sisters
  • The efficiency of double circulation is thought to have been important in the evolution of endothermy, which requires a high metabolic rate to hold body temperature at a set point.
  • Throughout vertebrate history the sinus venosus has been the pacemaker. It is a major chamber in fish, reduced in amphibians, smaller still in reptiles, and gone as a separate chamber in birds and mammals — surviving as the SA node in the right atrial wall.
48.4

Cardiac Cycle, Electrical Conduction, ECG, and Cardiac Output

pp. 1071–1075
48.5

Blood Pressure and Blood Vessels

pp. 1075–1080
  • Blood pressure is the force blood exerts against vessel walls. Clinically it means pressure in the aorta and large arteries near the heart, which rises and falls as the left ventricle contracts and relaxes. You feel that swing as a pulse because elastic artery walls expand and recoil with each beat. Watch Blood Vessels, Part 2: Crash Course Anatomy & Physiology #28 on YouTube, from CrashCourse
  • Vessel path from the heart outward: arteries → arterioles → capillaries → venules → veins → back to the heart. Watch Blood Vessels, Part 1 - Form and Function: Crash Course Anatomy & Physiology #27 on YouTube, from CrashCourse
  • Auscultatory method: a sphygmomanometer cuff on the upper arm compresses the brachial artery, which is used because it sits at about heart level. Cuff pressure is raised above systolic (e.g., 150 mm Hg), squeezing the artery shut so there is no flow and no sound. Watch Blood Vessels, Part 2: Crash Course Anatomy & Physiology #28 on YouTube, from CrashCourse
  • As the cuff deflates to systolic pressure (e.g., 120), the artery pops open only at each pressure peak, producing turbulent spurts and a tapping sound. The pressure at the first sound is the systolic reading. Watch Blood Vessels, Part 2: Crash Course Anatomy & Physiology #28 on YouTube, from CrashCourse
  • Deflating further to diastolic pressure (e.g., 75) leaves the artery open through the whole cycle, so smooth laminar flow returns and the sound vanishes. The pressure where sound disappears is the diastolic reading. Watch Blood Vessels, Part 2: Crash Course Anatomy & Physiology #28 on YouTube, from CrashCourse
  • BP is reported systolic/diastolic. Healthy young adult ≈ 120/80 mm Hg; normal ranges are roughly 110–140 systolic and 70–80 diastolic. Hypertension = systolic above 140 or diastolic above 90. Watch Blood Vessels, Part 2: Crash Course Anatomy & Physiology #28 on YouTube, from CrashCourse
  • Pressure falls along the circuit: large oscillating pulses in the aorta and large arteries, smoothing and dropping through small arteries, with the steepest drop at the arterioles, then capillaries, venules, veins, ending near 0 mm Hg at the vena cava.
  • BP = CO × R (cardiac output times total resistance). Pressure rises when heart rate or stroke volume rises, or when vessels constrict. It falls when the heart slows or blood volume drops from dehydration or hemorrhage. Watch Blood Vessels, Part 2: Crash Course Anatomy & Physiology #28 on YouTube, from CrashCourse
  • Baroreceptors — stretch receptors in the aortic arch and carotid arteries — detect pressure changes. Falling BP means fewer signals to the brain’s cardiac center, which responds with more sympathetic and less parasympathetic output to the heart and vessels.
  • Kidney pathway: reduced renal blood flow → kidney cells release renin → renin activates angiotensin → angiotensin causes body-wide vasoconstriction and triggers the adrenal cortex to release aldosterone → aldosterone makes the kidneys retain Na⁺ and water → blood volume and pressure rise. Excess Na⁺ has the opposite effect: less aldosterone, more Na⁺ lost in urine.
  • Atrial natriuretic peptide (ANP) is released by the right atrial wall when stretched by high blood volume. It promotes Na⁺ excretion by the kidneys, lowering blood volume and pressure — a negative feedback loop opposing renin-angiotensin-aldosterone.
  • Nitric oxide (NO) is a gas made by endothelial cells; it diffuses into the smooth muscle layer and relaxes it, dilating the vessel. Nitroglycerin, long used for angina, works by releasing NO.
  • Vessel walls have up to four layers: endothelium (innermost, a single epithelial cell layer, present in every vessel), an elastic fiber layer (lets large arteries recoil after each beat), a smooth muscle layer (thick in arteries and arterioles, drives vasoconstriction and dilation), and an outer connective tissue layer. Watch Blood Vessels, Part 1 - Form and Function: Crash Course Anatomy & Physiology #27 on YouTube, from CrashCourse
  • Arteries and veins have the same four layers, but arteries have a much thicker smooth muscle layer and two elastic layers. Veins are thin-walled because venous pressure is only about one-tenth of arterial pressure. Watch Blood Vessels, Part 1 - Form and Function: Crash Course Anatomy & Physiology #27 on YouTube, from CrashCourse
  • Capillaries are endothelium only — no muscle, elastic, or connective layers. That single-cell wall is why exchange happens there, by diffusion, filtration through pores, and transport across cells. Subtypes: plain, fenestrated (pored), and sinusoid (large gaps, in liver, spleen, bone marrow). Watch Blood Vessels, Part 1 - Form and Function: Crash Course Anatomy & Physiology #27 on YouTube, from CrashCourse
  • Resistance rises steeply as radius shrinks: resistance scales with 1/radius⁴, so halving the radius multiplies resistance 16×. Small arteries and arterioles therefore create most of the resistance in the arterial tree.
  • Vasoconstriction (arteriole smooth muscle contracts) raises resistance and cuts flow to an organ; vasodilation lowers resistance and increases flow. Chronic arteriole vasoconstriction can produce hypertension.
  • Skin vessels use the same mechanism for heat regulation in both ectotherms and endotherms: more skin blood flow means more heat exchange with the environment, less flow conserves heat in the cold. Precapillary sphincters gate individual capillary beds.
  • Capillary network stats: every body cell sits within about 100 µm of a capillary; an average capillary is about 1 mm long and 8 µm wide — barely wider than a 5–7 µm red blood cell, which flexes to squeeze through.
  • Capillaries collectively have the largest total cross-sectional area of any vessel type, so blood slows down most there — maximizing exchange time. On the venous side, vessels merge, cross-sectional area shrinks, and blood speeds up again. Watch Blood Vessels, Part 1 - Form and Function: Crash Course Anatomy & Physiology #27 on YouTube, from CrashCourse
  • Venous return needs help. The venous pump — skeletal muscle contractions squeezing veins — plus one-way venous valves keep blood moving toward the heart. Veins hold most of the body’s blood volume and can stretch to store extra, which is why foot veins bulge after long standing. Watch Blood Vessels, Part 1 - Form and Function: Crash Course Anatomy & Physiology #27 on YouTube, from CrashCourse
  • Varicose veins: veins stretched too far by pooled blood leave the one-way valves unable to seal, so blood pools further instead of returning.
48.6

The Lymphatic System

p. 1080
48.7

Cardiovascular Disease

p. 1081