Source: Lectures 30–32
Tags: circulatory system, open vs closed circulation, hemolymph, blood vessels, blood flow, cardiac anatomy, cardiac cycle, EKG, blood pressure, Poiseuille, hemodynamics
Difficulty: Intermediate Prerequisites: Basic cell biology, diffusion, membrane transport. Familiarity with smooth muscle and the autonomic nervous system will help.
This block of material covers how animals move fluids through their bodies, from organisms that rely on simple diffusion to those with complex four-chambered hearts. You will need to understand the physics of fluid flow (pressure gradients, resistance, Poiseuille's equation) and apply those principles to real cardiovascular anatomy. The sequence builds from vessel structure and flow physics (Lecture 30) through comparative cardiac anatomy and the cardiac cycle (Lecture 31) to electrical conduction, autonomic regulation, and blood pressure homeostasis (Lecture 32). If you are behind on diffusion or the autonomic nervous system, revisit those topics first.
Circulatory systems exist on a spectrum from open (hemolymph bathing tissues directly) to closed (blood confined in vessels under high pressure). Blood flow obeys Poiseuille's equation, where small changes in vessel radius have an outsized effect on flow rate. The mammalian heart generates pressure through a coordinated cardiac cycle controlled by intrinsic pacemaker cells and modulated by the sympathetic and parasympathetic nervous systems, with blood pressure maintained via baroreceptor-mediated negative feedback.
Open circulatory system
A circulatory system in which hemolymph is not confined to vessels but flows through body cavities (the hemocoel), bathing tissues directly before returning to the heart through ostia. In simple terms: the "blood" sloshes freely around the organs instead of staying inside tubes.
Closed circulatory system
A circulatory system in which blood is confined within vessels and pumped by the heart through a continuous loop, allowing controlled, directed, high-pressure flow. Think of it as plumbing with sealed pipes, as opposed to flooding a room.
Hemolymph
The circulating fluid in open circulatory systems (insect "blood") that moves through the hemocoel and enters the heart via ostia.
Ostia
Tiny openings in the heart of organisms with open circulatory systems. They open during diastole (relaxation) to let hemolymph in and close during systole (contraction) to push it forward.
Bulk flow
Movement of blood driven by pressure differences, where fluid flows from regions of higher pressure to regions of lower pressure. In simple terms: blood moves because it is pushed by a pressure gradient, the same reason water flows downhill.
Tunica externa
Outermost layer of blood vessels, providing structural support.
Tunica media
Middle layer of blood vessels, composed of smooth muscle that controls vessel diameter.
Tunica intima
Innermost layer of blood vessels, consisting of endothelium that lines the vessel and allows smooth blood flow.
Elastic artery
Large artery with thick walls (all three tunica layers) designed to handle high pressure directly from the heart. The aorta is the primary example.
Muscular artery
Medium-sized artery with a prominent tunica media (smooth muscle), used to distribute blood to specific regions.
Arteriole
Small artery with a strong tunica media and endothelium that regulates blood flow into capillary beds. The primary site of resistance in the circulatory system.
Capillary
The smallest blood vessel, made only of tunica intima (endothelium). The site where exchange of gases, nutrients, and wastes occurs between blood and tissues.
Continuous capillary
Capillary with tightly connected endothelial cells and tight junctions, allowing limited and controlled exchange. Found in brain, muscle, and skin.
Fenestrated capillary
Capillary with pores (fenestrations) in its endothelial cells, allowing more rapid exchange. Found in kidney, endocrine organs, and intestines.
Sinusoidal capillary
Capillary with large gaps and irregular structure, allowing passage of large molecules and even whole cells. Found in liver and bone marrow.
Venule
Small vein with thin walls, primarily consisting of tunica externa and tunica intima.
Vein
Vessel that carries blood back toward the heart. Thinner walls than arteries, all three tunica layers present, and often equipped with one-way valves.
Diastole
Phase of the cardiac cycle where the heart relaxes, creating negative pressure so blood flows into the chambers.
Systole
Phase of the cardiac cycle where the heart contracts, creating positive pressure so blood is pushed out.
SA node (sinoatrial node)
Specialised muscle cells in the right atrium that act as the primary pacemaker, initiating the electrical signal that drives each heartbeat.
AV node (atrioventricular node)
Receives the electrical signal from the SA node and relays it to the ventricles after a brief delay, ensuring the atria finish contracting before the ventricles begin.
Baroreceptors
Pressure sensors (especially in the carotid sinus) that monitor blood pressure and send signals to the cardiovascular control centre in the medulla, forming the basis of the MAP negative feedback loop.
Mean arterial pressure (MAP)
The average blood pressure in the arteries across the cardiac cycle. Calculated as MAP = CO x TPR (cardiac output times total peripheral resistance).
Some animals are small or thin enough that diffusion alone handles transport. Key examples:
Sponges (Porifera): water moves through pores (porocytes) into the spongocoel and out the osculum; choanocytes distribute materials
Cnidarians: use a single-opening gastrovascular cavity (mouth) to circulate water and nutrients
Platyhelminthes (flatworms): have a branched gastrovascular cavity (mouth and pharynx) that distributes nutrients throughout the body
Diffusion time scales with the square of distance (t is proportional to x squared). Double the distance, quadruple the time. Triple it, nine times the time. This is why diffusion works only over very short distances and why larger, thicker organisms need a circulatory system.
Open systems (most insects, many invertebrates):
Hemolymph is pumped by the heart into arteries, flows into the hemocoel, bathes tissues directly, and returns to the heart through ostia
Ostia open during diastole (heart relaxation) to fill the heart and close during systole (heart contraction) to prevent backflow
Lower pressure, less precise control of flow distribution
Closed systems (vertebrates, annelids, cephalopods):
Blood stays within vessels at all times: heart, arteries, capillaries, veins, back to heart
Maintains relatively high pressure, enabling faster and more efficient transport
Supports higher metabolic demands because flow can be directed to specific tissues
Pressure is maintained by enclosed vessels, strong ventricular contractions, and elastic, muscular vessel walls
All circulatory systems share three general characteristics: one or more pumps to drive flow, a system of tubes for the fluid to travel through, and a circulating fluid.
Blood vessels are built from up to three layers: tunica externa (outer, structural support), tunica media (middle, smooth muscle controlling diameter), and tunica intima (inner, endothelium for smooth flow).
From largest to smallest on the arterial side: elastic artery, muscular artery, arteriole, capillary. On the venous side: venule, vein, large vein.
Capillaries come in three types:
Continuous: tight junctions, small clefts, limited exchange (brain, muscle, skin)
Fenestrated: pores in endothelial cells, faster exchange (kidney, endocrine organs, intestines)
Sinusoidal: large gaps, irregular structure, allows passage of proteins and even cells (liver, bone marrow)
Blood flow is governed by bulk flow: fluid moves down a pressure gradient from high to low pressure. This flow is opposed by resistance (friction).
Law of bulk flow: Q = delta-P / R, where Q is flow (volume per unit time), delta-P is the pressure gradient, and R is resistance.
This is directly analogous to Ohm's law for electrical circuits (I = V / R).
Resistance formula: R = 8 eta L / r to the fourth power, where eta is fluid viscosity, L is tube length, and r is tube radius. Because radius is raised to the fourth power, small changes in radius produce enormous changes in resistance.
Poiseuille's equation combines these: Q = delta-P times r to the fourth / (8 eta L). Flow increases strongly with radius and pressure gradient.
Limitation: Poiseuille's equation assumes unbranched, rigid tubes with a simple, uniform fluid and steady flow. Real blood vessels are branched, flexible, and carry a complex fluid (blood), so the equation is an approximation.
Resistors in series vs parallel:
In series (sequential vessels): total R = R1 + R2 + R3, so resistance adds up
In parallel (capillary beds): 1/R total = 1/R1 + 1/R2 + 1/R3, so total resistance decreases
Velocity of flow: velocity = Q / A, where A is the total cross-sectional area. As total cross-sectional area increases (as in capillary beds), velocity decreases. Flow (Q) is constant throughout a closed circuit, but velocity varies inversely with cross-sectional area. Blood velocity is lowest in capillaries because they have the highest total cross-sectional area in the system.
Fish: 1 circuit, 2-chambered heart (one atrium, one ventricle). Blood flows heart, gills, body, heart.
Frog: 2 circuits, 3-chambered heart (two atria, one ventricle). Some mixing of oxygenated and deoxygenated blood occurs in the single ventricle.
Crocodiles, birds, mammals: 2 circuits, 4-chambered heart. Complete separation of oxygenated and deoxygenated blood.
The evolutionary trend is toward complete separation of pulmonary and systemic circuits, which prevents mixing and supports higher metabolic rates.
Two circuits:
Pulmonary circuit: right heart to lungs to left heart (deoxygenated blood picks up O2)
Systemic circuit: left heart to body tissues to right heart (oxygenated blood delivers O2)
Right side of the heart:
Superior and inferior vena cava bring deoxygenated blood into the right atrium
Right AV valve (tricuspid) between right atrium and right ventricle
Right ventricle pumps blood to the lungs via the pulmonary artery
Pulmonary semilunar valve prevents backflow into the right ventricle
Left side of the heart:
Pulmonary veins bring oxygenated blood into the left atrium
Left AV valve (mitral/bicuspid) between left atrium and left ventricle
Left ventricle pumps blood to the body via the aorta
Aortic semilunar valve prevents backflow into the left ventricle
Septum: wall separating the left and right sides of the heart (interatrial septum between atria, interventricular septum between ventricles).
Chordae tendineae: tendinous cords that anchor AV valves to papillary muscles, preventing valve prolapse during ventricular contraction.
Heart wall layers (outside to inside):
Pericardium: outer covering that encloses and protects the heart; parietal pericardium lines the pericardial cavity, pericardial fluid reduces friction
Epicardium (visceral pericardium): connective tissue and nerves lying directly on the myocardium
Myocardium: muscular middle layer responsible for contraction; comes in two types:
Compact myocardium: dense, tightly packed, highly vascularised, supplied by coronary arteries
Spongy myocardium: loosely organised with trabeculae (spaces), not highly vascularised, relies on diffusion from chamber blood
Endocardium: inner layer of endothelium and connective tissue lining the chambers
The cardiac cycle is the sequence of pressure and volume changes that produce one heartbeat.
Ventricular diastole (passive filling): atrial pressure exceeds ventricular pressure, AV valves open, ventricles fill passively with blood
Atrial systole: atria contract, pushing remaining blood into the ventricles
Isovolumetric contraction: ventricles contract, AV valves close (first heart sound), pressure rises but no blood leaves because semilunar valves are still closed. Volume stays constant.
Ventricular ejection: ventricular pressure exceeds arterial pressure, semilunar valves open, blood is ejected into the pulmonary artery (right) and aorta (left)
Isovolumetric relaxation: ventricles relax, semilunar valves close (second heart sound) as arterial pressure exceeds ventricular pressure. Volume stays constant, pressure falls.
AV valves (tricuspid on right, mitral/bicuspid on left) are passive, opening with pressure from atria. They are supported by chordae tendineae and papillary muscles to prevent prolapse.
Semilunar valves (pulmonary and aortic) open with ventricular contraction and are strong enough to withstand return pressure.
The left ventricle generates much higher pressure than the right ventricle because it pumps blood through the entire systemic circuit, not just the lungs.
The heart's electrical system drives the mechanical events of the cardiac cycle.
Conduction pathway: SA node, internodal pathways (spreads across atria), AV node (delays signal), AV bundle (bundle of His), left and right bundle branches, Purkinje fibres (spread signal rapidly through ventricles from apex upward).
Pacemaker cells (SA node):
Have an unstable resting membrane potential (pacemaker potential)
"Funny" channels (I_f) open at hyperpolarised voltages, allowing Na+ influx that slowly depolarises the cell
At threshold, L-type Ca2+ channels open, generating the action potential
After approximately 200 ms, Ca2+ channels close and K+ channels open, causing repolarisation
The cycle repeats continuously, making the SA node self-sustaining
Cardiac action potential (cardiomyocytes), distinct from the pacemaker potential:
Phase 0 (depolarisation): voltage-gated Na+ channels open, rapid depolarisation
Phase 1: Na+ channels inactivate, K+ channels open briefly, slight repolarisation
Phase 2 (plateau): inward rectifier K+ channels close, L-type Ca2+ channels open, Ca2+ influx maintains the plateau
Phase 3 (repolarisation): Ca2+ channels close, K+ channels open, K+ efflux repolarises the cell
Phase 4: return to resting membrane potential
The electrocardiogram detects the integrated electrical activity of the heart over time.
P wave: depolarisation of the atria (leads to atrial systole)
QRS complex: depolarisation of the ventricles and repolarisation of the atria (leads to isovolumetric contraction and ventricular systole)
T wave: repolarisation of the ventricles (leads to isovolumetric relaxation and ventricular diastole)
Clinical conditions:
Supraventricular tachycardia (SVT): rapid heart rhythm (150 to 250 bpm) originating above the ventricles, usually from a re-entrant loop at the atria or AV node. ECG shows a narrow QRS (<120 ms). Consequences include palpitations, dizziness, chest pain, shortness of breath, and risk of syncope.
Left bundle branch block (LBBB): electrical impulses are delayed or blocked in the left bundle branch, so the right ventricle depolarises first. Results in a wide QRS (>120 ms), ventricular dyssynchrony, and reduced cardiac efficiency.
The parasympathetic and sympathetic branches of the autonomic nervous system reciprocally control heart rate by adjusting how quickly the SA node pacemaker potential reaches threshold.
Sympathetic stimulation (increases heart rate and force):
Cardiovascular control centre (medulla) activates sympathetic neurons
Release norepinephrine (and adrenal medulla releases epinephrine)
Bind beta-adrenergic receptors on autorhythmic cells
Activate Gs, adenylyl cyclase converts ATP to cAMP, activates protein kinase
Increases activity of funny Na+ channels and T-type Ca2+ channels
Increased Na+ and Ca2+ influx produces a steeper pacemaker potential
Threshold reached sooner, heart rate increases
Sympathetic modulation of contraction force:
Norepinephrine or epinephrine binds beta-1 receptors on cardiomyocytes
Gs, adenylate cyclase, cAMP, protein kinase A (PKA) cascade
PKA phosphorylates L-type Ca2+ channels (more Ca2+ entry) and Ca2+ channels on the sarcoplasmic reticulum (more Ca2+ release)
Increased cytoplasmic Ca2+ drives stronger actin-myosin contraction
PKA also phosphorylates myosin (enhanced contraction) and SERCA (faster Ca2+ removal, shorter relaxation time)
Net effect: increased force of contraction, increased stroke volume
Parasympathetic stimulation (decreases heart rate):
Cardiovascular control centre activates parasympathetic neurons via the vagus nerve
Release acetylcholine (ACh)
Binds muscarinic receptors on autorhythmic cells
Activates Gi, opens K+ channels (increased K+ efflux) and reduces Ca2+ channel activity (decreased Ca2+ influx)
Cell hyperpolarises, pacemaker potential slope decreases
Longer time to reach threshold, heart rate decreases
Pressure across the system:
Blood pressure is highest and pulsatile in the left ventricle and aorta (peaking at approximately 120 mmHg). It progressively decreases through arteries, arterioles, capillaries, venules, and veins. The largest pressure drop occurs across the arterioles, because they are the primary site of resistance. Capillaries and veins have low, steady pressure approaching 0 mmHg.
Systolic pressure: the peak arterial pressure during ventricular contraction. Diastolic pressure: the lowest arterial pressure during ventricular relaxation.
During ventricular contraction, blood is ejected into the aorta, stretching elastic arterial walls. During relaxation, elastic recoil of those walls helps maintain pressure and push blood forward.
Venous return:
Veins have low, non-pulsatile pressure and rely on one-way valves and external forces to return blood to the heart. The skeletal muscle pump is the key mechanism: when skeletal muscle contracts, it compresses the vein, opening the proximal (toward-heart) valve and closing the distal valve, squeezing blood toward the heart. When the muscle relaxes, the distal valve opens to refill the segment.
Mean arterial pressure (MAP):
MAP = CO x TPR (cardiac output times total peripheral resistance). Based on Q = delta-P / R, where CO = MAP / TPR.
Baroreceptor reflex (MAP regulation):
When MAP rises: baroreceptor firing increases, afferent neurons signal the cardiovascular control centre (medulla), sympathetic output decreases, norepinephrine release drops
Result: arteriolar smooth muscle relaxes (vasodilation, decreased peripheral resistance), ventricular myocardium decreases force, SA node slows heart rate, cardiac output falls, MAP decreases back to normal
When MAP falls: the opposite occurs (increased sympathetic output, vasoconstriction, increased HR and contractility, MAP rises)
This is a negative feedback loop
Velocity, pressure, and cross-sectional area relationships:
Velocity: highest in arteries, lowest in capillaries (large total cross-sectional area), increases again in veins
Total cross-sectional area: small in arteries, peaks in capillaries, decreases toward veins
Velocity is inversely related to total cross-sectional area
Local flow control:
Local blood flow is controlled primarily by changing arteriole diameter, which has an outsized effect due to the r-to-the-fourth relationship in Poiseuille's equation. Arterioles regulate entry into capillary beds via precapillary sphincters, metarteriole pathways, and arteriovenous anastomoses.
Vasodilators (increase arteriole diameter, increase flow): metabolic signals (decreased O2, increased CO2, increased H+, increased K+, prostaglandins, adenosine, nitric oxide), decreased sympathetic tone, atrial natriuretic peptide.
Vasoconstrictors (decrease arteriole diameter, decrease flow): myogenic stretch, endothelins, increased sympathetic tone, angiotensin II, ADH, epinephrine, norepinephrine.
Intrinsic mechanisms (autoregulation): local negative feedback via metabolic and myogenic controls; matches blood flow to tissue needs.
Extrinsic mechanisms: neural and hormonal control at the system level; maintains MAP and redistributes blood during exercise or thermoregulation.
Formula | Equation | Variables |
|---|---|---|
Law of bulk flow | Q = delta-P / R | Q = flow, delta-P = pressure gradient, R = resistance |
Resistance | R = 8 eta L / r^4 | eta = viscosity, L = length, r = radius |
Poiseuille's equation | Q = delta-P x r^4 / (8 eta L) | Combines flow, pressure, and resistance |
Ohm's law (analogy) | I = V / R | I = current, V = voltage, R = resistance |
Blood velocity | v = Q / A | v = velocity, Q = flow, A = total cross-sectional area |
MAP | MAP = CO x TPR | CO = cardiac output, TPR = total peripheral resistance |
Resistors in series | R_total = R1 + R2 + R3 | Resistance adds |
Resistors in parallel | 1/R_total = 1/R1 + 1/R2 + 1/R3 | Resistance decreases |
Poiseuille's equation explains why atherosclerosis (narrowing of arteries) is so dangerous: a 50% reduction in radius reduces flow to 1/16th of normal. This is also the principle behind vasodilator drugs used to treat hypertension.
The baroreceptor reflex is why standing up quickly can cause dizziness: blood pools in the legs, MAP drops briefly, and the reflex takes a moment to compensate.
Students often think "open circulatory system" means the animal has no heart. It does not. Open systems have hearts; the fluid simply leaves the vessels.
Students confuse flow and velocity. Flow (Q) is constant throughout a closed circuit. Velocity varies inversely with total cross-sectional area. Blood slows down in capillaries because there are so many of them, not because flow decreases.
Students assume arteries always carry oxygenated blood. The pulmonary arteries carry deoxygenated blood to the lungs.
Students mix up the two types of semilunar valves. Remember: pulmonary semilunar valve is on the right side (blood to lungs), aortic semilunar valve is on the left side (blood to body).
The r^4 relationship in Poiseuille's equation is a very common exam question. Know that halving the radius increases resistance 16-fold.
Be able to trace blood flow through the entire mammalian heart, naming every chamber, valve, and vessel in order.
Understand the cardiac cycle sequence and which valves are open or closed at each stage.
Know the difference between the pacemaker potential (SA node, funny channels, Ca2+ channels) and the cardiomyocyte action potential (5 phases, Na+ and Ca2+ channels).
Be able to match EKG waves to mechanical events: P wave to atrial depolarisation/systole, QRS to ventricular depolarisation/systole, T wave to ventricular repolarisation/diastole.
Understand the baroreceptor reflex as a negative feedback loop and be able to trace it in both directions (MAP too high or too low).
Know the sympathetic pathway (norepinephrine, beta receptors, cAMP, increased rate and force) and the parasympathetic pathway (ACh, muscarinic receptors, K+ efflux, decreased rate).
True or false: In a closed circulatory system, blood velocity is constant throughout the circuit.
False. Flow (Q) is constant; velocity varies inversely with cross-sectional area.
Fill in the blank: The phase of the cardiac cycle where ventricular pressure rises but no blood leaves the heart is called ___.
Isovolumetric contraction.
True or false: The parasympathetic nervous system increases heart rate via norepinephrine.
False. The parasympathetic system decreases heart rate via acetylcholine and the vagus nerve.
Fill in the blank: If vessel radius is halved, resistance increases by a factor of ___.
16 (because resistance is proportional to 1/r^4).
True or false: Capillaries have the highest total cross-sectional area of any vessel type.
True.
Q: An insect's heart pumps hemolymph into the hemocoel. Explain why this animal does not require a closed circulatory system, and identify the structures that allow hemolymph to re-enter the heart.
A: Insects are generally small with high surface-area-to-volume ratios, so diffusion distances are short and a closed, high-pressure system is unnecessary. Hemolymph re-enters the heart through ostia, tiny openings that open during diastole and close during systole to prevent backflow.
Q: Using Poiseuille's equation, explain why arterioles are the primary site of resistance in the circulatory system and why small changes in arteriole diameter have a large effect on blood flow.
A: Poiseuille's equation states Q = delta-P x r^4 / (8 eta L). Because flow depends on the fourth power of the radius, a small decrease in arteriole radius produces a large increase in resistance and a large decrease in flow. Arterioles have small radii and thick smooth muscle in their tunica media, so they are ideally positioned to regulate resistance.
Q: Trace the pathway of a single electrical impulse through the heart, from initiation to ventricular contraction. For each structure, state what it does.
A: SA node (initiates the impulse) spreads the signal across the atria via internodal pathways, causing atrial contraction. The AV node receives and delays the signal (allowing atria to finish contracting). The signal then travels through the AV bundle (bundle of His) in the interventricular septum, splits into left and right bundle branches, and is distributed rapidly through Purkinje fibres to the ventricular myocardium, causing coordinated ventricular contraction from the apex upward.
Q: Explain how the baroreceptor reflex responds to a sudden drop in blood pressure. Include the role of the medulla, sympathetic nervous system, and at least two effector targets.
A: A drop in MAP decreases baroreceptor firing. Afferent neurons signal the cardiovascular control centre in the medulla, which increases sympathetic output. Increased norepinephrine release causes: (1) vasoconstriction of arteriolar smooth muscle, increasing peripheral resistance; (2) increased heart rate via the SA node; and (3) increased force of ventricular contraction, raising cardiac output. The net effect is an increase in MAP back toward normal (negative feedback).
This material connects directly to gas exchange (Lectures 33 to 34), because the circulatory system is the delivery network that moves O2 from respiratory surfaces to tissues and returns CO2 for elimination. Concepts of diffusion distance and surface area appear in both units.
The autonomic nervous system regulation covered here also ties to the broader nervous system material from earlier in the course (sympathetic vs parasympathetic pathways, neurotransmitter signalling).
The physics of fluid flow (Poiseuille's equation) will appear again when studying ventilation mechanics and airflow in the lungs.
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