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    Circulation

    Circulation is the pressure-driven bulk flow of fluids that rapidly transports various products through the animal body.

    In many species, it also provides a source of hydraulic pressure that assists organ function.

    Plants have a vascular system, not a circulatory system.
    Fluids in plants move

    • in one direction, from roots to stomates (xylem)
    • in two directions between various tissues (phloem)

      ...but they do not circulate.

    We will focus only on the animal vascular/circulatory system.
    Because life is not fair.

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    The animal circulatory system mediates transport of
    • oxygen
    • carbon dioxide
    • nutrients
    • nitrogenous waste
    • hormones
    • agents of the immune system
    • other commodities

    In various species, the circulatory system also may be involved in

    • heat exchange
    • pH regulation
    • fluid volume in the body
    • formation of urine (by providing blood and fluid pressure to the kidneys)
    • proper function of genitalia (blood pressure)

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      William Harvey, 1628 was the first to provide a complete, detailed description of the "round trip" made by blood in the circulatory system.

      He elucidated how blood was pumped to the brain and body by the heart, and that valves in the veins kept blood from moving in only one direction.

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    Open vs. Closed Circulatory Systems

    Circulatory systems may be open or closed.

    Open Circulatory System:

    • Heart pumps haemolymph (blood) into a haemocoel body cavity.
    • Oxygenated, nutrient-laden haemolymph bathes the organs directly.
    • There is no distinction between haemolymph and interstitial fluid.
    • Muscle movement causes haemolymph to move within the haemocoel.
    • When the heart relaxes, blood flows back into the dorsal aorta through ostia.
    • Found in all Arthropods and all Molluscs except Cephalopods.

    Closed Circulatory System:

    • Heart pumps blood through a circuit of blood vessels.
    • Gas exchange and nutrient transfer takes place in the smallest diameter blood vessels.
    • May form one or two blood circuits.
    • Found in all Annelids, Cephalopods and Vertebrates.

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By BerserkerBen at English Wikipedia


    Blood and Blood Pigments

    Blood is a connective tissue composed of
    • an aqueous medium (plasma)
    • various cells (hemocytes)
    • cell fragments
    • proteins
      ...each of which has a specific function.

    Blood is the medium of oxygen, nutrient, and waste transport.


    Blood pigments vary among species.
    All types of blood contain cells carrying a metalloprotein respiratory pigment molecule
    whose function is to bind to and carry O2 and CO2.

    All blood pigments contain a central metal atom, instrumental in respiratory gas binding.

    • hemoglobin
      • central metal atom is iron
      • bound inside blood cells (in vertebrates, the erythrocytes)
      • can bind up to four oxygen molecules at a time
      • binding of one O2 molecule increases affinity for O2 (positive cooperative binding)
      • bright red when oxygenated; dark red when not oxygenated (not blue)
      • Question: Where does the "unoxygenated blood is blue" myth originate?
      • the most primitive animal respiratory pigment, it is also the most common
      • found in vertebrates and many invertebrates, but with different forms and functions

    • haemerythrin
      • central metal atom is iron
      • located inside blood cells
      • appears to have evolved from hemoglobin ancestral molecule
      • violet-pink when oxygenated, colorless when deoxygenated
      • also involved in immune response and tissue regeneration
      • found in sipunculids, priapulids, brachiopods, and the annelid genus Magelona

    • hemocyanin
      • central metal atom is copper
      • free-floating in blood plasma
      • blue when oxygenated
      • appears to have evolved independently, and is not related to hemoglobin
      • better than hemoglobin at binding O2 in cold, oxygen-poor medium
      • second most common respiratory pigment in animals
      • found in most arthropods, molluscs

    • chlorocruorin
      • central metal atom is iron
      • free-floating in the hemolymph
      • red when oxygenated in concentrated solution; green in dilute solution
      • found in many annelids (especially marine polychaetes)

    • erythrocruorin

    More information here.


    (click on pic for source)

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    Vertebrate Blood Cells

    Blood cells comprise the cellular component of blood.
    They are also known as
    • hematopoietic cells
    • hemocytes
    • hematocytes
    Blood cell production is known as hematopoiesis.

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Questions:
A high white blood cell count is often a sign of systemic infection.
Paradoxically, a LOW white count (leukopenia) can sometimes result from an infection.
How could this happen?
Other possible explanations for a low white blood cell count?

    Vertebrate Hematopoiesis

    Myeloblasts give rise to
    • neutrophils (nucleate leukocytes)
      • target bacteria, fungi
    • eosinophils (nucleate leukocytes)
      • target larger parasites (e.g., nematodes)
    • basophils (nucleate leukocytes)
      • release histamines for inflammatory response

    Lymphoblasts give rise to

    • lymphocyte (nucleate leukocytes)
      • immune cells reside primarily in the lymphatic system

    Monoblasts give rise to
    • monocytes (nucleate leukocytes)
      • phagocytes that reside primarily in body tissues

    Proerythroblasts give rise to
    • erythrocytes (anucleate red blood cells)
      • oxygen transport
      • Erythropoietin, a glycoprotein hormone
        produced in the kidneys,
        controls erythrocyte hematopoiesis.

    Megakaryoblasts give rise to

    • platelets (anucleate; = thrombocytes)
      • function in clotting

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    Mammalian Blood Cells

    Mammal erythrocytes are derived in that they lack a nucleus.
    Reptiles, including birds, have nucleated red blood cells.

    Kwestion Korner

      1. Many diapsid species do not exhibit obvious (to us) sexual dimorphism.
      However, with the proper tools and reagents, one can sex a bird or lizard from a blood sample.
        a. How is this possible?
        b. What would a vet need to observe to make the sex determination?

      2. One symptom of renal failure is anemia. Why?

      3. What hormone might be administered to treat anemia due to renal failure?

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    Human Blood Types

    Antigens in the RBC plasma membrane determine blood types.

    These are inherited in Mendelian fashion.

    There are actually 35 different human blood types,
    but the antigens most likely to trigger an immune response are:

    • ABO antigens
    • Rh-D (Rhesus factor D) antigens

    Which blood type is the "universal donor"?
    Which type is the "universal recipient"?
    Which type of blood can each type above safely receive?

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Hearts

A heart is a discrete, localized pumping structure that drives blood through the circulatory system.
Many animals have accessory or auxiliary hearts to assist the principal heart in pumping blood.

The muscular tissue of the heart is known as the myocardium.

Depending on the taxon, a heart may be

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The Vertebrate Heart

Question: Are the four-chambered hearts of mammals and birds/crocodilians homologous or convergent?

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    Modern vertebrate hearts may have
    two, three, or four chambers.

    Two-chambered hearts (fish) pump blood through
    one circuit of blood vessels:

    • heart --> gills --> body --> heart

    Three- and four-chambered hearts pump blood through
    two separate circuits:

      1. systemic circuit (heart --> body --> heart)
      2. pulmonary circuit (heart --> lungs --> heart)
      xxxOR
      xxxpulmocutaneous circuit (heart --> lungs/skin --> heart)

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    The Circulatory Plan: Blood Vessels

    Systemic and pulmonary circuits are connected in series.

    Arteries

    • are thick-walled with copious smooth muscle
    • travel away from the heart
    • branch into arterioles as they move away from the heart

    Arterioles branch into capillaries that form the microcirculatory bed.

    Capillaries converge onto venules, which, in turn, converge onto Veins.

    Veins

    • are relatively thin-walled
    • contain valves to prevent backflow
    • travel towards the heart

    The smooth muscle of arteries and arterioles are responsible for
    vasomotor control of blood distribution and pressure.

    • Vasoconstriction is a decrease in luminal radius.
    • Vasodilation is an increase in luminal radius.

    Blood flow rate is sensitive to lumen diameter.
    Arterial muscle action affects blood flow rate:

    • Environmental cold usually triggers vasoconstriction
    • Environmental warmth usually triggers vasodilation

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Blood flow from the cardiac blood vessels.
(click on image for a larger view)

    Systemic and Pulmonary Circuits

    The mammal heart will serve as our model.

    The left side of the heart receives freshly oxygenated blood
    from the lungs and pumps it to the systemic circuit.

    The flow of oxygenated blood:

      1. Lungs --> pulmonary veins --> left atrium
      2. Left atrioventricular valve --> muscular left ventricle.
      3. Left ventricle pumps blood --> aortic valve --> aorta.
      4. Aorta --> entire systemic circuit.

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    The right side of the heart receives deoxygenated blood
    from the body and pumps it to the pulmonary circuit.

    The flow of deoxygenated blood:

      5. Systemic vessels --> inferior and superior venae cavae
      6. venae cavae --> right atrium --> muscular right ventricle
      7. Right ventricle pumps blood --> pulmonary trunk.
      8. Pulmonary trunk branches into right and left pulmonary arteries.
      8. Pulmonary arteries --> lungs.

    Re-oxygenated blood flows from the lungs --> left atrium,
    and the cycle repeats.

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    Providing Blood to the Heart

    Circulation also must deliver oxygen to the myocardium itself.

      Compact Myocardium

      The homeotherm myocardium's oxygen need is second only to that of the brain.
      Birds and mammals have a compact myocardium:
      • Myocardium cells are very tightly packed
      • Thus, blood passing through the ventricles cannot oxygenate them.
      • Instead, coronary arteries
        • branch from the aorta
        • carry oxygenated blood to myocardium capillary beds
      • Deoxygenated exits via coronary veins.
      • Why is coronary artery disease so dangerous?

      Spongy Myocardium

      Primitive vertebrate hearts have a spongy myocardium.
      • Spongy myocardium is permeated by a branching network of open spaces.
      • These fill with blood from the lumen of the heart, providing oxygenation.
      • There are no coronary arteries.
      • Teleost fish, amphibians, and non-avian reptiles have spongy myocardia.
      • The single ventricle mixes oxygenated and unoxygenated blood
        to some degree.
      • Thus, this system is not as efficient as the compact myocardium.

      Mixed Structure Myocardium

      A heart with an outer compact layer and an inner spongy layer
      combines the oxygenation mechanisms of the previous two types.

      Such a heart provides greater oxygenation efficiency than a fully spongy myocardium.

    • Spongy myocardium is permeated by a branching network of open spaces.
    • These fill with oxygenated blood from the heart's lumen.
    • External coronary arteries oxygenate the outer compact layer.
    • Found in some species of very active fish (salmon, tuna, sharks)
      and in some reptiles and amphibians.


      Octopus: A Unique Mixed Structure Myocardium

      • Oxygenated luminal blood enters a capillary system
      • Capillaries weave through both the spongy and compact myocardium
      • De-oxygenation of blood takes place at the capillary/myocardium interface.
      • There are no coronary arteries.
      • Deoxygenated blood flows into coronary veins on the surface.
      • These return blood to the gills.

      Luminal blood of this system is richly oxygenated, providing very efficient myocardium oxygenation.

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    Pacemakers: The Rhythm of the Beat

    Hearts contract rhythmically. This rhythmic contraction reflects the synchronous depolarization of cardiac muscle cells.
    The rhythm of contraction is set by the heart's cardiac conduction system, a cell or set of cells that spontaneously initiates the rhythm of heart depolarization.

    • The period of contraction is systole (SIS'-tuh-lee).
    • The period of relaxation is diastole (di-AAH'-stuh-lee)

    • Stroke volume is the volume of blood pumped per heartbeat cycle.
    • Cardiac output is the volume of blood pumped by the heart per unit time (e.g., mL/min).
    • Cardiac output is the product of heart rate (beats/min) x stroke volume (mL/beat).

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    Neurogenic Hearts

    In a neurogenic heart, the impulse that causes heart contraction
    originates in neurons.

    A neurogenic heart cannot generate a beat without input from the nervous system.

    In most crustaceans, the cardiac ganglion--a cluster of neurons on the heart wall--produces rhythmic motor output that generates the heartbeat.

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Disturbing Video of the Day:

    Myogenic Hearts

    In a myogenic heart, the impulse that causes heart contraction
    originates in muscle cells.

    A myogenic heart can generate a beat without nervous system input.

    • Adjacent muscle cells connect via intercalated discs (gap junctions).
    • Depolarization of one cell depolarizes neighboring cells.
    • Each of these induces its neighbors to depolarize, and so on.
    • Result: Large-scale depolarization.

    Tunicates, most mollusks, and vertebrates have myogenic hearts.

    The vertebrate pacemaker is a group of specialized muscle cells
    that set the rhythm of contraction.

    The bird and mammal pacemaker is located in the right atrium wall.
    It is known as the sinoatrial node (SA) or sinus node.

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    Depolarization of the Mammal Heart

    The atria and ventricles are separated by a fibrous interventricular septum through which electrical signals cannot pass.
    • The atrioventricular (AV) node is an "electrical window" in the septum.
    • The atrioventricular bundle (a.k.a. bundle of His)
      • is composed of heart muscle cells specialized for electrical conduction
      • branches to enter the interventricular septum

    • His fibers connect with Purkinje fibers
      • large muscle cells
      • branch along the inner ventricular walls, just beneath the endocardium.

    • Depolarization begins in the SA node and spreads through atrial muscle.
    • Atrium contracts.

    • AV node depolarization spreads rapidly into the ventricles.
    • (Atrial muscle cells begin to repolarize.)

    • Nearly simultaneous depolarization of cells throughout ventricular myocardium leads to forceful ventricular contraction.

    Rapid delivery of the depolarizing wave ensures that
    all parts of the ventricular myocardium contract together.

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    Electrocardiogram

    Cardiac electrical conduction produces the heart's signature electrocardiogram,
    a recording of the electrical impulses driving heart contraction.

    • P wave - atrial depolarization
    • QRS waves - ventricular depolarization
        (Ventricular contraction begins at the peak of R.)
    • T wave - ventricular repolarization

    Anomalies in the timing, progression, and/or amplitude of these waves
    can signal cardiac pathology.

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Control of Heartbeat

Heart action is subject to hormonal, nervous, and intrinsic controls.

Hormonal Control

Nervous Control

Intrinsic Control

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