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Learning Outline

Cardiovascular System

A&P 2


Blood “connects” the various regions of the internal fluid environment by providing a flowing, circulating stream of liquid that transports, protects, and regulates

Blood stays within a closed system of tubes (= vessels) and chambers (of the heart)

  • EXCEPTmyocardial infarction
    • WBCs travel in and out of the blood stream
    • Water and solutes diffuse in and out of blood stream

Cardiovascular disease is a major health concern slide

Recall that William Harvey is credited with first demonstrating the principle of a closed circulatory system in humans in The Anatomical Exercises: De Motu Cordis and De Circulatione Sanguinis (1649)

This figure shows one of many experiments that Harvey documents in his thorough deductive proof that the heart is a double pump and how blood actually circulates.

Harvey diagram of circulation

Anatomy of the Cardiovascular System

Anatomy of the Heart

Functional anatomy of the heart

  • General structure
    • Four chambers: two upper atria (sing. atrium) and two lower ventricles
      • Septum separates the “left heart” from the “right heart” skull icon
    • heart diagramLocated in mediastinum of thorax; about 2/3 of heart is left of median skull icon skull icon
      • Apex is lower tip of heart; points somewhat to the left slide
      • Base is broad top of heart skull icon
  • Heart wall
    • Endocardium — endothelial lining that is continuous with the endothelium (simple-squamous-like membrane) that lines the blood vessels
      • Has valves (see below) as in veins
    • Myocardium — cardiac muscle layer
      • heart diagramCardiac muscle tissue
        • Branched fibers interconnected by gap junctions at intercalated discs
        • Trabeculae carneae — thick branches of cardiac muscle seen inside the heart wall (covered with endocardium)
      • Very thick (thickest in chambers that pump harder [ventricles, esp. L ventricle])
      • Cardiac muscle of upper and lower heart each act as a syncytium
        • Because fibers are branched and connected by gap junctions, they all contract together as if they were one giant cell skull icon
        • This means wall of upper chambers contract more or less at once; likewise for lower chambers
      • Cardiac muscle is autorhythmic
      • Cardiac muscle does not fatigue
      • Cardiac muscle does not have tetanus
        • Instead, cardiac muscle has single prolonged contractions
        • Peak of contraction in cardiac muscle is several times longer than in the twitch of a skeletal muscle

Pericardium — multilayer sac around heart

  • Fibrous pericardium is outermost covering
  • Serous pericardium activity
    • Visceral layer of serous pericardium is also called “epicardium”
    • Parietal layer of serous pericardium lines the inside of the fibrous pericardium
    • Lubricating pericardial fluid between the two layers of the serous pericardium reduces friction

Chambers of the heart skull icon skull icon

  • Two atria (sing. “atrium”) literally, “lobby”
    • Heart’s upper chambers (L and R atria divided by a septum)
      • The auricle is the ear-like lateral out-pouching of each atrium
    • Receiving chambers —receive blood from veins
    • Pressurize blood, moving it to the lower chambers
  • Two ventricles —literally, “little chambers”
    • Heart’s lower chambers (L and R ventricles divided by a septum)
    • Pumping chambers —pressurize blood so it moves out of heart, maintaining a large enough pressure gradient to move blood most of the way around the circulatory loop
  • Although ventricles (esp. L ventricle) have thicker walls than atria, they all contain about the same volume of blood

Valves of the heart

  • Atrioventricular (AV) valves skull icon
    • Between each atrium and ventricle (R and L AV valves)
    • Have two (L) or three (R) flaps (leaflets) held by stringy chordae tendinae (lit. “tendon cords”) which are in turn attached to fingerlike projections inside the ventricles called papillary muscles
      • R AV valve also called tricuspid valve
      • L AV valve also called bicuspid valve
        • The L AV valve, because it only has two leaflets, reminded early anatomists (who were apparently affected by fumes of formaldehyde and alcohol, or perhaps just alcohol) of a bishop’s two-flapped, angled hat called a “miter” —for that reason the L AV valve is usually called the mitral valve
    • When the ventricle contracts, blood pushes the leaflets into the atrium
    • The leaflets stop at the point where they completely block blood flow back into the atrium because the chordae tendinae, held tightly by the papillary muscles, become taut
Mitral valve. In the cartoon is the mitral valve, turned upside down so you can see its similarity to a bishop’s hat with two angled flaps. mitral valve
Bishop’s miter. In the photo is the 11th Episcopal Bishop of Chicago, William Dailey Persell, sporting a white miter
click image to enlarge it
bishop persell
  • Semilunar (SL) valves skull icon skull icon
    • Three “pockets” in the endothelium, all facing each other
      • When blood goes “backward” pockets fill and press into each other, forming a barrier to further backflow
      • When blood goes “forward” pockets are pushed flat against vessel wall and blood flows easily past them
    • SL valves ensure one-way flow of blood (see Harvey’s diagram above)
    • Aortic SL valve is on left, preventing backflow from aorta into left ventricle
    • Pulmonary SL valve is on right, preventing backflow from trunk of pulmonary artery into right ventricle

lion trackSL valve sketch. Click on the the icon picture icon and print out the sketch from Leonardo da Vinci’s notebook showing SL valves. I have added labels to clarify each view.

lion trackWatch the valves in action! Click on the the icon picture icon to see a video of a transesophageal echocardiogram (TEE) that shows the valves working. A TEE is a sonogram of the heart taken from inside the esophagus, which runs right behind the heart. picture icon

Skeleton of the heart

  • Fibrous rings around the openings of the heart valves
  • Give a more rigid support to the valve openings (like a door jamb)
  • Provide electrical insulation between the atrial myocardium and the ventricular myocardium

“Wheresoever you go, go with all your heart.”

— Confucius

Pathway of blood through heart and “circulatory routes”

Circulatory routes (AKA “circulatory loops” or “circuits”)

  • Pulmonary route
    • From heart (right side) to gas-exchange tissues of lungs and back to heart (left side)
    • Blood loses CO2 and gains O2
  • Systemic route
    • From heart (left side) to “systemic” (non-pulmonary) tissues of body and back to heart (right side)
    • Blood loses O2 and gains CO2

Systemic route is longer and more extensive than pulmonary route

Blood goes through pulmonary route, then systemic, then pulmonary, and so on

Route through heart and circulatory routes: tv icon tv icon blog icon

  • R atrium
  • R AV valve
  • R ventricle
  • Pulmonary SL valve
  • Pulmonary arteries skull icon
  • Pulmonary capillaries
  • Pulmonary veins
  • L atrium
  • L AV valve (mitral valve)
  • L ventricle
  • Aortic semilunar valve
  • Aorta skull icon skull icon
  • Systemic arteries
  • Systemic capillaries
  • Systemic veins
  • Superior and inferior venae cavae (S vena cava & I vena cava)
  • R atrium
Note: You can start anywhere in this plan and return to the same spot. You must be able to put these structures in the order in which blood flows through them, starting from any point. Of course, you should be prepared for alternate names and other options.
lion trackThis animation may help you: Map of the Human Heart

Coronary circulation skull icon

  • Coronary is from “corona” meaning “crown” —referring to vessels encircling heart as a crown encircles the head
  • Supply blood to the myocardium
  • Anastomoses are critical when blockage occurs (singular, anatomosis)

A myocardial infarction (MI) may occur when a blood clot occurs in a narrow (perhaps partly blocked) area of a coronary artery and causes damage and death to the portion of the myocardium supplied by the blocked vessel. Acute MIs are often called “heart attacks.”

click image to enlarge it

heart diagram

Fetal circulation skull icon

  • Special circulatory route prior to birth
    • heart diagramUmbilical cord
      • Umbilical arteries (2)
      • Umbilical vein
    • Placenta slide
    • Ductus venosus
    • Foramen ovale
    • Ductus arteriosus
  • Changes to normal adult pattern at time of birth

Portal circulatory routes slide

  • Occurs when blood leaves a capillary bed and then moves through a “portal vein” to a second capillary network before returning to heart (“portal” means “gateway”)
  • Examples
    • Hypophyseal portal circulation
      • heart right arrow hypothalamus right arrow anterior pituitary right arrow heart
    • Hepatic portal circulation skull icon
      • heart right arrow digestive organs right arrow liver right arrow heart

Anatomy of blood vessels

heart diagramAlmost 100,000 km (more than 60,000 mi) of blood vessels in human body

Types slide

  • Arteries — large vessels that conduct blood away from heart, toward
    • Arterioles — small arteries that conduct blood way from larger arteries, toward
  • Capillaries — microscopic, thin-walled vessels where water and solutes move in and out of blood (“exchange vessels”)
    • Sinusoids are like capillaries, except they are not tubular (instead, they are irregular, microscopic spaces)
  • Veins — large vessels that conduct blood from venules toward the heart
    • Sinuses are especially large veins where blood “pools” as a method of storing excess blood volume in the case of a sudden loss of blood (making veins and sinuses “reservoir vessels”)
    • Venules — small veins that collect blood from capillaries (and sinusoids), conducting blood toward

lion trackNote: Vessels are named “artery” or “vein” based on the direction in which they conduct blood —NOT whether they contain oxygenated or deoxygenated blood

Wall of blood vessels

  • Tunica intima (“intimate garment” = “underwear”)
    • Endothelium (similar to simple squamous epithelium)
      • Smooth (for easy flow) and thin (for easy exchange)
    • In veins (only), this layer has semilunar (“half-moon”) valves (SL valves)
      • Three “pockets” in the “underwear,” all facing each other
        • When blood goes “backward” pockets fill and press into each other, forming a barrier to further backflow
        • When blood goes “forward” pockets are pushed flat against vessel wall and blood flows easily past them
    • SL valves ensure one-way flow of blood
  • Tunica media (“middle garment”)
    • Smooth muscle picture icon
      • Supports wall (prevents rupture) by resisting pressure of blood
      • Allows change in blood flow slide
        • Vasoconstriction — muscle contracts, reducing diameter
        • Vasodilation — muscle relaxes, increasing diameter
      • Regulated by nerves, hormones, and local regulators (e.g. prostaglandins)
    • Elastic tissue picture icon
      • Supports, but also allows recoil of an expanded vessel
    • Absent in capillaries
  • Tunica externa (“external garment;” also called tunica adventitia)
    • Fibrous tissue, adding to flexible strength of wall
    • Absent in capillaries
  • Overall, arteries are thicker-walled than veins, with more muscle

Functional principles

  • Arteries — “resistance vessels” can use their muscles to change resistance to blood flow, regulating where blood goes slide
  • Veins — “capacitance vessels” can increase their capacity (volume) by stretching and can thus act as blood reservoirs
  • Capillaries — the primary “exchange vessels” can move substances easily into and out of the blood tissue tv icon
    • Microcirculation refers to blood flow through capillary networks
    • A metarteriole is small connecting vessel from an arteriole that extends through a capillary bed (network)
      • Precapillary sphincters are valve-like muscles of the metarteriole that regulate blood flow into capillary networks, just controlling exactly where in a tissue blood flows
      • Metarterioles can also allow blood to bypass a capillary bed, thus acting as a “thoroughfare channel”



Physiology of the Cardiovascular System


Hemodynamics — study/analysis of blood flow

Heart acts as a pump

Cardiac cycle

  • Two-step cycle of contraction and relaxation
    • Contraction — systole (SIS-toh-lee)
    • Relaxation — diastole (dy-ASS-toh-lee)
  • Cardiac cycle is alternate systole/diastole of atria, ventricles, atria, ventricles, and so on
  • Five-step version of cardiac pumping cycle
    • Atrial systole
    • Isovolumetric ventricular contraction
    • Ejection
    • Isovolumetric ventricular relaxation
    • Passive ventricular filling
  • Heart sounds
    • 1st heart sound — lubb — AV valves closing (and ventricular contraction noise)
    • 2nd heart sound — dupp — SL valves closing

Electrical conducting system of the heart skull icon

  • Three kinds of myocardial fibers
    • Myocardial fibers (ordinary fibers)
    • Conducting fibers (conduct action potentials more rapidly than ordinary fibers)
    • Pacemaker fibers (“stronger” rhythm than ordinary fibers)
  • Action potentials spread along entire atrial myocardium (triggering atrial systole) then entire ventricular myocardium (triggering ventricular systole)
    • Must be sped up, to reach whole myocardium at about same time
    • Must be coordinated/linked, so atria and ventricles stay “in time” with each other
    • All cells within syncytium (atrial or ventricular myocardium) must follow the same pace (remember: each fiber has its own rhythm, which may be faster or slower than its neighbor)
  • Sinoatrial (SA) node is primary “pacemaker” of heart, setting rhythm for atrial myocardium and for AV node
    • See diagram for location of this and remaining structures
  • Atrioventricular (AV) node is pacemaker for ventricular myocardium
    • Follows signal from SA node, even though it COULD set its own pace
  • Atrioventricular (AV) bundle (bundle of His) rapidly conducts depolarization (action potential) through septum between ventricles to apex (bottom point) of heart
  • Purkinje fibers (subendocardial fibers) rapidly conduct depolarization through lateral walls of ventricles
  • Ectopic pacemakers are areas other than the SA node that “take over” (perhaps because the SA node is damaged)
    • “Ectopic” means “off place” or “out of place”
    • Ectopic pacemakers are usually not as efficient as the SA node, so this creates a problem
    • Can be treated by using artificial pacemakers

Electrocardiography (ECG or EKG)

  • Baseline of the ECG wave shows no change (no depolarization, no repolarization)
  • heart diagramDeviations (waves) show depolarization OR repolarization
    • P wave — depolarization of atrial myocardium
    • QRS complex — repolarization of atrial myocardium AND depolarization of ventricular myocardium
    • T wave — repolarization of ventricular myocardium
    • U wave — “hump” on the T wave (not usually present) — repolarization of papillary muscles


Normal ECG

  • Oddities in the ECG show damage to myocardium or other problems
    • Changes in intervals may mean a change in conduction velocity, which may mean a “heart block” or impairment of conduction

EKG intervals
ECG intervals

    • Fibrillation — asynchronous , uncoordinated contractions — no effective pumping



Regulation of electrical activity

  • Nervous regulation
    • Sympathetic fibers (via cardiac nerve) increase heart rate
    • Parasympathetic fibers (via vagus nerve) decrease heart rate
    • Reflexes
      • Baroreflexes (pressoreflexes) respond to changes in blood pressure
      • Chemoreflexes respond to changes in CO2 or pH or O2
      • Carotid bodies and aortic bodies contain receptors for these reflexes
  • Endocrine regulation (e.g. epinephrine, thyroid hormone)
  • Misc. factors: blood temp, pain, ions (Ca++, Na+, K+), exercise
  • Resting HR is about 65-80 beats/min

EinthovenThe electrocardiograph was invented at the beginning of the 20th century by the Dutch scientist Willem Einthoven, pictured here with his original apparatus. His electrocardiograph filled two rooms and required five operators. Electrical contact with the body was made by immersing limbs in buckets of salt water wired to the machine.

Einthoven’s machine was simply a recording voltmeter that recorded fluctuations in the polarization (potential) of the myocardium during the cardiac cycle, producing wavelike squiggles —an electrocardiogram (ECG or EKG). His invention was later used to visualize electrical activity in the nervous system—as we saw in A&P 1.

Einthoven also developed a way to send ECG signals to remote locations using telephone wires—a strategy called telemetry. Today, many cardiac care units in hospitals use telemetric ECG monitoring on a routine basis.

Click here to see more historic photos of Einthoven’s lab. slide

In 1984, Kevin Patton developed a method to monitor electrical activity of the heart in birds by way of RF telemetry (sending information by radio waves) using a tiny ECG device coupled to a radio transmitter in a small backpack. He and others used the method to detect stress events in captive wild birds.

The graph shows the chaotic nature of a hawk’s heart rate (beats/min), which is similar to the pattern in human heart activity. Notice that the average resting HR of this bird is over twice that of a human.

Click either image to enlarge it.

From Telemetry of Heart Rate in Large Raptors: a Method of Transmitter and Electrode Placement in Patton, Crawford, & Sawyer (1984) Raptor Research 18:2 (p. 59-61)

Patton's HR backback
Patton's HR graph

Measuring blood pressure

heart diagramUse of a sphygmomanometer, which is simply a pressure gauge calibrated to mm Hg (how much a column of mercury [liquid metal] will rise in a tube as a result of a certain amount of pressure)

Measures maximum and minimum pressures as the pulse wave passes by in an artery

  • Maximum pressure — systolic pressure (usually less than 120 mm Hg)
  • Minimum pressure — diastolic pressure (usually less than 80 mm Hg)

Reported as max/min or sys/dias, for example 120/80 or “one-twenty over eighty”

Hypertension (HTN) is “high blood pressure”

Circulatory shock results from extremely low blood pressure

General principles of blood vessel function and blood flow

Primary principle of circulation — blood flows down a pressure gradient

  • That is, blood flows from high pressure regions to lower pressure regions
  • Highest pressure in heart (during contraction), then arteries, then capillaries, then veins, then heart (during relaxation)
  • Perfusion pressure is the local pressure gradient needed to maintain blood flow through a tissue
    • Perfusion means “flow through”

Anything that affects blood pressure affects blood flow

Cardiac output (CO)

  • CO is the volume pumped out of the heart per unit of time
  • Expressed in units of ml/min or L/min (usually around 5 L/min for 70 kg adult male)
  • CO = SV x HR (SV — stroke volume)

Stroke volume (ml/beat)

  • Starling’s law of the heart slide
    • Also called the Frank-Starling mechanism
    • Cardiac muscle contraction increases in strength the more it is stretched (length-tension relationship)
    • If end-diastolic volume (EDV; blood volume in ventricle just before it contracts) is large, this will stretch the myocardium
      and increase the strength of that heart stroke (beat)
    • Thus, the heart pumps what it receives
    • The higher the end-diastolic volume, the higher the stroke volume
      • Affected by venous return of blood to the heart
        • Venous return is affected by total blood volume and operation of venous pumps —see below
      • Ejection fraction (EF) — ratio of SV to EDV
        • Usually 55% or higher in health adults
        • Heart failure — decline in EF
  • Changes in contractility (strength of contraction)
    • Epinephrine, norepinephrine (NE) increase contractility
    • Exercise

Heart rate (beats per minute; contractions per minute)

  • Affected by; autonomic nerves (chemoreflexes, pressure reflexes, stress response), hormones, drugs, sex, violence, online tests, etc. (see above)

Total blood volume

  • Affected by: dehydration, overhydration, hormones, kidney function, blood loss (hemorrhage, lit. “blood flow”), osmotic pressure (sufficient plasma proteins)

Peripheral resistance (PR; resistance to flow outside the heart)

  • Vessel length (length directly proportional to resistance)
    • Except during growth, doesn’t change much
  • Branching (number of branches directly proportional to resistance)
    • May change over time with functional or structural changes in tissues
  • Viscosity of blood
    • Viscosity is “thickness” of a fluid
      • The higher the viscosity, the more resistance to flow
    • Normally doesn’t change much
      • Could change when hematocrit (RBC%) changes; could also change with plasma protein fluctuations
    • Applies to ketchup, too . . . the thicker the ketchup, the more resistance to flow TV icon
  • Diameter of vessels
    • Resistance inversely proportional diameter of vessel (by an exponent of 4)
    • Vasomotor mechanism — smooth muscle in wall of vessel changes diameter of vessel slide
      • This is the usual way to regulate blood flow in local areas
    • Microcirculation (blood flow in capillaries)
      • RBCs must be deformable (bendable) or they won’t fit through some capillaries
      • Precapillary sphincters control flow locally
        • These muscle “valves” are on arterioles NOT on capillaries
  • Peripheral resistance can affect blood flow in local regions or the whole circulatory loop (total peripheral resistance; TPR)

Operation of venous pumps

  • Orthostatic effect (“standing up” effect)
    • Gravity causes blood to shift to the lowest venous reservoirs (legs)
  • Skeletal muscle pump
    • Skeletal muscles and SL valves of vein operate a “pump” that keeps pressure gradients up so that blood flows back to heart
  • Respiratory pump
    • Breathing movements and SL valves do the same thing as skeletal muscle pump

Pulse waves conserve energy and keep blood flowing continuously

  • Pulse waves are created in arteries every time the heart contracts and pushes blood into the arteries slide
  • Stress-relaxation effect
    • The arterial wall expands under the high pressure, absorbing some of the energy
    • During heart relaxation, the arterial wall recoils and thus releases some of the energy it had absorbed
      • Recoil of arterial walls thus keeps the pressure up, and thus keeps blood flowing, between heart contractions
      • If vessel walls were instead stiff, blood would only flow in spurts —not continuously
  • Pulse waves reflect heart rate and strength
  • heart diagramPulse waves are best palpated (felt with your hand) at pulse points
    • Areas where there is a large artery close to the skin and over a bone or other solid structure (so you can push it up against the bone and feel the pulse wave through the skin) slide

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