Download circulatory pathways and vessels

CHAPTER 9 SUMMARY
•Circulatory systems have evolved in most animal groups because diffusion is too slow
for distribution of O2, nutrients, wastes, and hormones. These systems can also transmit
pressure that can move flexible body parts, and provide the driving force for filtration in
renal organs.
•Circulation consists of up to three components: fluid, pump(s), and vessels.
•Open circulation (in most molluscs, arthropods, etc.) has a fluid called hemolymph that
moves freely among organs, and which may be pumped by cilia, body movements or
heart. There may be vessels, but they open up into body cavities.
•Closed circulation (in cephalopods, vertebrates, etc.) has a fluid called blood that is
pumped by a heart and stays entirely within blood vessels, with tissue exchange occurring
in tiny vessels called capillaries.
CIRCULATORY FLUIDS
•Fluids consist of liquid plasma and cellular elements. The percentage of whole-blood
volume occupied by blood cells is known as the hematocrit.
•Plasma is a complex liquid that serves as a transport medium. All plasma constituents
are freely diffusible across the capillary walls (in closed systems) except the plasma
proteins, which remain in the plasma and perform a variety of functions: creating
colloidal osmotic pressure, buffering pH changes, binding and transporting various
molecules such as cholesterol, iron and oxygen, participating in clotting and immune
processes.
•Cellular elements in vertebrates are erythrocytes, leukocytes, and clotting cells.
•Erythrocytes (red blood cells) are specialized for O2 transport in the blood. In
vertebrates, they are full of hemoglobin, an iron-containing protein that loosely,
reversibly binds with O2. This is useful because O2 is poorly soluble in blood.
Erythrocytes usually have a short life span. Undifferentiated pluripotent stem cells in
the red bone marrow give rise to all cellular elements of the blood. Erythrocyte
maturation is stimulated by erythropoietin, a hormone secreted by the kidneys in
response to reduced O2 delivery.
•Leukocytes (white blood cells) are the defense corps of the body. Leukocytes are
present in the blood only while in transit from their site of production and storage in
the bone marrow (and also in the lymphoid tissues in the case of the lymphocytes) to
their site of action in the tissues. All leukocytes have a limited life span and must be
replenished by ongoing differentiation and proliferation of stem cells.
•Clotting cells are thrombocytes in most vertebrates, and platelets in mammals; the
latter are actually cell fragments derived from large megakaryocytes in the bone
marrow. Clotting cells play an important role in hemostasis, the arrest of bleeding
from an injured vessel. Hemostasis steps are (1) vascular spasm, (2) platelet plugging,
and (3) clot formation. Spasm reduces blood flow through an injured vessel, whereas
aggregation of platelets at the site of vessel injury quickly plugs the defect. Platelets
start to aggregate upon contact with ex-posed collagen in the damaged vessel wall.
•Clot formation reinforces the platelet plug and converts blood near a vessel injury
into a gel. Most factors necessary for clotting are always present in the plasma in
inactive forms. When a vessel is damaged, exposed collagen initiates a cascade of
reactions involving successive activation of these clotting factors, ultimately
converting fibrinogen into fibrin. Fibrin, an insoluble threadlike molecule, is laid
down as the meshwork of the clot; the meshwork in turn entangles blood cells to
complete clot formation. When no longer needed, clots are dissolved by plasmin, a
factor also activated by exposed collagen.
•Insects contain similar clotting factors, though detailed reactions are not known.
CIRCULATORY PUMPS
Anatomical and Evolutionary Considerations
•Pumps come in a variety of forms: (1) flagella, (2) extrinsic muscle or skeletal pumps (in
which musculoskeletal movements push the circulatory fluid); (3) peristaltic pumps (in
which vessel walls contract in a wave); and (4) chamber muscle pumps (muscular
chambers that squeeze fluid through one-way valves).
•Many animals have primary or systemic hearts aided by auxiliary pumps, which usually
move fluid from distant parts of the body.
•Arthropods hearts draw in hemolymph through valved ostia and pump out into arteries
tha empty into a hemocoel.
•Vertebrate hearts began with two chambers: an atrium to collect returning blood, and a
ventricle to pump fluid to the body. This evolved in mammals into a four-chambered
(two atria, two ventricles), dual pump that moves fluid through two separate vessel
systems, the pulmonary (to the lungs) and systemic (to the rest of the body). One-way
valves direct the blood in the proper direction and prevent it from flowing in the reverse.
•Contraction in a mammalian heart of the spirally arranged cardiac muscle fibers
produces a wringing effect for efficient pumping. Also important for efficient pumping is
the fact that the vertebrate muscle fibers in each chamber act as a functional syncytium,
contracting as a coordinated unit.
Electrical Activity of Hearts
•Myogenic hearts are capable of initiating electrical signals on their own, usually starting
as special pacemaker cells. In most vertebrates, the cardiac impulse originates at the SA
node, the pacemaker of the heart, which has the fastest rate of spontaneous depolarization
to threshold.
•Once initiated, the action potential spreads in a mammalian heart throughout the right
and left atria, only by cell-to-cell spread of the impulse through gap junctions. The
impulse passes from the atria into the ventricles through the AV node, the only point of
electrical contact between these chambers. The action potential is delayed briefly at the
AV node, ensuring that atrial contraction precedes ventricular contraction to allow
complete ventricular filling. The impulse then travels rapidly down the interventricular
septum via the bundle of His and is rapidly dispersed through the myocardium via
Purkinje fibers and gap junctions.
•The action potentials of contractile cardiac muscle fibers exhibit a prolonged positive
phase, or plateau, accompanied by a prolonged period of contraction, which ensures
adequate ejection time. This plateau is primarily due to activation of slow Ca++ channels.
Because a long refractory period occurs in conjunction with this prolonged plateau phase,
summation and tetanus of cardiac muscle are impossible, thereby ensuring the alternate
periods of contraction and relaxation essential for pumping of blood.
•The spread of electrical activity throughout the heart can be recorded from the surface of
the body. This record, the ECG, can provide useful information about the status of the
heart.
Mechanical Events of the Cardiac Cycle
•The cardiac cycle consists of three important events: (1) The generation of electrical
activity as the heart autorhythmically depolarizes and repolarizes. (2) Mechanical activity
consisting of alternate periods of systole (contraction and emptying) and diastole
(relaxation and filling), which are initiated by the rhythmical electrical cycle. (3)
Directional flow of blood through the heart chambers, guided by valve opening and
closing induced by pressure changes that are generated by mechanical activity.
•The atrial pressure curve remains low throughout the entire cardiac cycle, with only
minor fluctuations. The aortic pressure curve remains high the entire time, with moderate
fluctuations (in humans, typically varying between a systolic pressure of 120 mm Hg and
a diastolic pressure of 80 mm Hg). The ventricular pressure curve fluctuates dramatically
because ventricular pressure must be below the low atrial pressure during diastole to
allow the AV valve to open so filling can take place, and it must be above the high aortic
pressure during systole to force the aortic valve open to allow emptying to occur.
Cardiac Output and Its Control
•Cardiac output, the volume of blood ejected by a ventricle per minute, is determined by
the heart rate times the stroke volume. This is the crucial physiological factor.
•Heart rate in vertebrates is varied by altering the balance of parasympathetic and
sympathetic influence on the SA node. Parasympathetic stimulation slows the heart rate,
and sympathetic stimulation speeds it up.
•Stroke volume depends on (1) the extent of ventricular filling, with an increased enddiastolic volume resulting in a larger stroke volume by means of the length-tension
relationship (intrinsic control), and (2) the extent of sympathetic stimulation, with
increased sympathetic stimulation resulting in increased contractility of the heart, that is,
increased strength of contraction and increased stroke volume at a given end-diastolic
volume (extrinsic control).
Nourishing the Heart Muscle
•Cardiac muscle is supplied with oxygen and nutrients by blood delivered to it by the
coronary circulation, not by blood within the heart chambers. Most coronary blood flow
occurs during diastole, because the coronary vessels are compressed by the contracting
heart muscle during systole. Coronary blood flow is normally varied to keep pace with
cardiac oxygen needs.
CIRCULATORY PATHWAYS AND VESSELS
Anatomical and Evolutionary Considerations
•Hemolymph and blood transport materials and transmit force as they are pumped under
pressure.
•Hemolymph may travel in arteries initially, but then emerges to bathe tissues directly.
•Blood flows in a closed loop between the heart and the tissues. Closed vascular
systems provide more precise control of fluid delivery to organs according to their
needs.
•Pressure can be used as a force to extend limbs (e.g., in arachnids), filter blood, and
create erections.
Flow Regulation and Hemodynamics
•Blood flow can be selectively directed to different organs as needed. The flow rate (F) of
blood through a vessel is directly proportional to the pressure gradient (P) and inversely
proportional to the resistance (R): F = P/R, or C.O. =∆P/R for a cardiovascular system.
•The higher pressure at the beginning of a vessel is established by the pressure imparted
to the blood by cardiac contraction. The lower pressure at the end is due to frictional
losses as flowing blood rubs against the vessel wall.
•Resistance, the hindrance to blood flow through a vessel, is influenced most by the
vessel’s radius. Resistance is inversely proportional to the fourth power of the radius,
so small changes in radius profoundly influence flow.
•Gravity creates a backpressure against which the heart must work, an important factor
in tall animals.
Open Circulation (Arthropods)
•In large crustacea, arteries may branch into fine capillary-like vessels in many organs,
and hemolymph flows through gills before returning to the heart. Insect circulation has
fewer arteries and no capillary-like vessels.
Closed Circulation (Vertebrates)
•The vascular system evolved from one circuit to two separate circuits in vertebrates:
•The primitive vertebrate flow pattern is: Heartgillsother organsheart. This is
the pattern in water-breathing fish, though agnathans have auxiliary hearts in some
veins.
•In lungfish, a separate circuit through lungs evolved, necessitating partial separation of
flow output from the heart. In many adult amphibians, flow goes first to lung and skin,
returns to the heart, then proceeds to the rest of the body.
•In most reptiles, the single ventricle has 3 subchambers; flow normally goes from the
heart to the lungs to the heart and then to the body, but flow can bypass the lungs
completely during diving. Crocodiles have similar basic flow, but with two separate
ventricles; and the foramen of Panizza allows blood to flow from the initial lung
circulation to the body, again bypassing the lung during diving.
•Birds and mammals have four-chambered hearts with completely separate flows to the
lungs (pulmonary) and body (systemic) vessels.
Vertebrate circulatory vessels have separate distinct functions:
•Arteries are large-radius, low-resistance passageways from the heart to the tissues; they
also serve as a pressure reservoir. Because of their elasticity, arteries expand to
accommodate the extra volume of blood pumped into them by cardiac contraction and
then recoil to continue driving the blood forward when the heart is relaxing. Systolic
pressure is the peak pressure exerted by the ejected blood against the vessel walls during
cardiac systole. Diastolic pressure is the minimum pressure in the arteries when blood is
draining off into the vessels downstream during cardiac diastole.
•Arterioles are the major resistance vessels, and control flow to individual organs. Their
high resistance produces a large drop in mean pressure between the arteries and
capillaries. Tone, a baseline of contractile activity, is maintained in arterioles at all times.
Arteriolar vasodilation, an expansion of arteriolar caliber above tonic level, decreases
resistance and increases blood flow through the vessel, whereas vasoconstriction, a
narrowing of the vessel, increases resistance and decreases flow. Arteriolar caliber is
subject to two types of controls:
•Local (intrinsic) controls involve local chemical changes associated with changes in
the level of metabolic activity; these act directly on the arteriolar smooth muscle to
induce changes in the caliber of the local arterioles. This local control mechanism
adjusts blood flow to the tissue to match momentary metabolic needs.
•Extrinsic control is accomplished primarily by sympathetic nerve influence and to a
lesser extent by hormones. Extrinsic controls are important in maintaining mean
arterial blood pressure. Arterioles are richly supplied with sympathetic nerve fibers,
whose increased activity produces generalized vasoconstriction and a subsequent
increase in mean arterial pressure. Decreased sympathetic activity produces
generalized arteriolar vasodilation, which lowers mean arterial pressure. These
extrinsically controlled adjustments of arteriolar caliber help maintain the appropriate
pressure head for driving blood forward to the tissues.
•Capillaries are thin-walled, small-radius, extensively branched vessels ideally suited to
serve as sites of exchange between the blood and surrounding tissues.
•The surface area for exchange is maximized and diffusion distance is minimized.
•Because of their large total cross-sectional area, the velocity of blood flow through
capillaries is relatively slow, providing adequate time for exchanges to take place.
•Two types of passive exchanges—diffusion and bulk flow—take place across
capillary walls. Individual solutes are exchanged primarily by diffusion down
concentration gradients. Lipid-soluble substances pass directly through the single layer
of endothelial cells lining a capillary, whereas water-soluble substances pass through
water-filled pores between the endothelial cells. Plasma proteins generally do not
escape.
•Imbalances in physical pressures acting across capillary walls are responsible for bulk
flow of fluid through the pores back and forth between the plasma and interstitial fluid.
Fluid is forced out of the first portion of the capillary (ultrafiltration), where outward
pressures (mainly capillary blood pressure) exceed inward pressures (mainly plasma-
colloid osmotic pressure). Fluid is returned to the capillary along its last half, when
outward pressures fall below inward pressures. The reason for the shift in balance
down the length of the capillary is the continuous decline in capillary blood pressure
while the plasmacolloid osmotic pressure remains constant. Bulk flow is responsible
for the distribution of extracellular fluid between the plasma and the interstitial fluid.
•Lymph vessels pick up leftover fluids. Normally, slightly more fluid is filtered than is
reabsorbed. The extra fluid, any leaked proteins, and tissue contaminants such as bacteria
are picked up by the lymphatic system. In amphibians and reptiles, lymph hearts aid flow
back to the blood circulation
•Veins are large-radius, low-resistance passageways for return of blood from the tissues
to the heart. Additionally, they can accommodate variable volumes of blood and therefore
act as a blood reservoir.
•The capacity of veins to hold blood can change markedly with little change in venous
pressure. Veins are thin-walled, highly distensible vessels that can passively stretch to
store a larger volume of blood.
•The primary force responsible for venous flow is the pressure gradient between the
veins and atrium (that is, what remains of the driving pressure from the heart).
•Venous return is enhanced by sympathetically induced venous vasoconstriction, by
external compression of the veins resulting from contraction of surrounding skeletal
muscles, and by other auxiliary pumps. One-way venous valves ensure that blood is
driven toward the heart and prevented from flowing back toward the tissues. Venous
return is also enhanced by respiratory activity, which produces a less-than-atmospheric
pressure in the chest cavity that encourages flow from the lower veins that are exposed
to atmospheric pressure to the chest veins, and by slightly negative pressures created
within the atria during ventricular systole and within the ventricles during ventricular
diastole exert a suctioning effect on venous return.
INTEGRATED CARDIOVASCULAR FUNCTION
•The cardiovascular system has two main goals: regulating gas transport as needed by the
body, and maintaining blood pressure in a viable range.
•Regulation of mean arterial pressure depends on control of its two main determinants,
cardiac output and total peripheral resistance.
•Control of cardiac output in turn depends on regulation of heart rate and stroke
volume, while total peripheral resistance is determined primarily by arteriolar
vasoconstriction.
•Short-term regulation of blood pressure is accomplished primarily by the baroreceptor
reflex. Carotid sinus and aortic arch baroreceptors continuously monitor mean arterial
pressure. When they detect a deviation from normal, they signal the medullary
cardiovascular center, which responds by adjusting autonomic output to the heart and
blood vessels to restore the blood pressure to normal.
•Long-term control of blood pressure involves maintenance of proper plasma volume
through the kidneys’ control of salt and water balance.