Biology 102
Fall 2000
R. Brundage
Lecture 4: Part III
Circulation
I.Circulatory Systems–An Overview
A.General Characteristics
1.A circulatory system is an internal transport system with three
components:
a.Blood is a fluid tissue composed of water, solutes, and formed
elements.
b.Blood vessels are tubes of various diameters through which the
blood is transported.
c.The heart is a muscular pump that generates pressure to keep the
blood flowing.
2.Arthropods and most mollusks have an open system:
a.Blood is pumped from a heart into large tissue spaces where
organs are "bathed."
b.Blood is returned to the heart at a leisurely rate.
3.Vertebrates have a closed system.
a.All the vessels and the heart are connected so that blood remains
enclosed.
b.Blood volume is constant and is equal to the heart’s output at any
time.
c.Flow rate slows as blood moves through the fine capillaries of
the capillary beds
B.Evolution of Vertebrate Circulatory Systems
1.Fishes have a one circuit plan which includes flow to the gills.
2.Amphibians two partially separated circuits with flow to lungs.
3.Birds and mammals have a two separate circuits of blood flow.
a.The right half of the heart receives deoxygenated blood and
pumps it to the lungs for oxygenation—pulmonary circuit.
b.The left half receives from the lungs and pumps the oxygen-rich
blood to all the body—systemic circuit.
C.Links With the Lymphatic System
1.The lymphatic system picks up excess fluids, solutes, and disease agents
from the interstitial fluid.
2.This lymph is cleansed by exposure to the infection-fighting cells before
being returned to the general circulation.
II.Characteristics of Blood
A.Functions of Blood
1.It carries oxygen and nutrients to cells, and it carries secretions and
wastes away from them.
2.It contains phagocytic cells that fight infection.
3.It helps stabilize internal pH.
4.It equalizes body temperatures in birds and mammals.
B.Blood Volume and Composition
1.An average-sized adult has a blood volume of 4-5 quarts.
2.Plasma
a.This fluid portion of the blood is mostly water.
b.Some plasma proteins (alpha and beta globulins) transport lipids
and vitamins; others function in immune responses (gamma
globulins) and in blood clotting (fibrinogen).
c.Plasma also contains ions, glucose, lipids, amino acids,
vitamins, hormones, and dissolved gases.
3.Red Blood Cells (Erythrocytes)
a.In mammals, red blood cells are biconcave disks that transport
oxygen.
b.Red blood cells contain hemoglobin, an iron-containing protein
that binds with oxygen.
c.They form in the red bone marrow from stem cells.
d.When mature they have no nuclei; they live about 120 days.
1.Phagocytic cells remove the oldest cells from the
bloodstream.
2.Cell count remains at 5.4 million/microliter for males and
4.8 for females.
4.White Blood Cells (Leukocytes)
a.Leukocytes remove dead or worn-out cells and protect us against
invading microbes and foreign agents.
b.Leukocytes are derived from stem cells in the bone marrow.
c.There are five types of white blood cells.
1.Lymphocytes, the "B" and "T" cells, are involved in the
immune responses.
2.Monocytes and neutrophils are the "search-and-destroy"
cells; monocytes differentiate into the macrophages.
3.Eosinophils and basophils have lesser roles in the
system.
5.Platelets
a.These are fragments of megakaryocytes produced by bone
marrow stem cells; they have no nucleus.
b.Substances released from platelets initiate blood clotting.
III.Blood Transfusion and Typing
A.Concerning Agglutination
1.In a blood transfusion it is important that there is no possibility of an
interaction between "self" markers and markers from the donor, which
would be recognized as "foreign."
2.If bloods from incompatible donors are mixed, the antibodies cause
agglutination (clumping).
B.ABO Blood Typing
1.ABO blood typing is based on surface markers on red blood cells.
2.Type A has A markers; type B has B markers; type AB has both markers;
type O has neither marker.
3.Blood types of donors and recipients must be carefully determined to
prevent agglutination.
C.Rh Blood Typing<
1.An Rh— person (lacks this marker) transfused with Rh+ blood (has this
marker) will produce antibodies to the Rh marker.
2.There are risks in pregnancy to a second Rh+ child if an Rh— woman
bore a previous child who was also Rh+ and thus left behind some
antibodies that can now seep into this second child and cause clumping.
3.In erythroblastosis fetalis, too many cells may be destroyed and the fetus
dies.
4.Medical treatment (RhoGam) given to the mother after the birth of the first
Rh+ baby can inactivate the Rh antibodies.
IV.Human Cardiovascular System
A."Cardiovascular" comes from the Greek words meaning "heart" "vessel."
1.Blood travel follows this route: heart ––> arteries ––> arterioles ––>
capillaries ––> venules ––> veins ––> heart
2.The human heart is divided into right and left halves.
3.Blood is transported from the right side of the heart to the lungs in the
pulmonary circuit; the blood is returned to the left side of the heart,
from which it is pumped to the rest of the body in the systemic circuit.
B.Usually a given volume of blood in either circuit passes through only one capillary
bed; an exception is blood from the digestive tract, which passes through the liver
before entering the general circulation.
V.The Heart is a Lonely Pumper
A.Heart Structure
1.The heart is a durable pump made mostly of cardiac muscle and enclosed
in a tough, fibrous sac; its chambers are lined with connective tissue and
endothelium.
2.Heart muscle cells are serviced by the coronary circulation, two arteries
that branch directly off the aorta.
3.Each half consists of an atrium (receiving) chamber and a ventricle
(pumping) chamber separated by an atrioventricular (AV) valve.
4.Blood exits each ventricle through a semilunar valve.
B.Cardiac Cycle
1.The cardiac cycle consists of a sequence of contraction (systole) and
relaxation (diastole).
2.As the atria fill, the ventricles are relaxed.
3.Pressure of the blood in the atria forces the atrioventricular valves to
open; the ventricles continue to fill as the atria contract.
4.The ventricles contract, the atrioventricular valves close, and blood flows
out through the semilunar valves.
5.The heart sound "lub" is made by the closing of the AV valves; the "dup"
sound is the closure of the semilunar valves.
C.Mechanisms of Contraction
1.Because of the close junction of cardiac muscle cells, they contract in
unison.
2.Excitation for a heartbeat is initiated in the sinoatrial (SA) node (also
known as the cardiac pacemaker) then passes to the atrioventricular (AV)
node for ventricular contraction; this is the cardiac conduction system.
3.The nervous system adjusts rate and strength.
VI.Blood Pressure in the Cardiovascular System
A.Blood is distributed by means of arteries, arterioles, capillaries venules and veins.
1.Two key factors influence the rate of flow through each type of blood
vessel:
a.The flow rate is directly proportional to the pressure gradient
between the start and end of the vessel.
b.The flow rate is inversely proportional to the vessel’s resistance
to flow.
2.Blood pressure drops along the way due to energy loss from resistance.
B.Arterial Blood Pressure
1.Arteries are large diameter vessels that present low resistance to flow as
they conduct blood away from the heart.
2.Because of their elastic walls, arteries tend to "smooth out" the pulsations
associated with the discontinuous pumping cycle of the heart.
3.Normal systolic pressure is 120 mm Hg; normal diastolic pressure is 80
mm Hg; the measuring device is a sphygmomanometer.
C.Resistance to Flow at Arterioles
1.Arteries branch into smaller arterioles which offer greater resistance to
flow and thus the greatest drop in blood pressure.
2.Arterioles serve as control points where adjustments can be made in blood
volume distribution.
3.Neural (sympathetic nerves) and endocrine (epinephrine and angiotensin)
signals cause changes in arteriole diameter by stimulating the muscle cells
in the walls.
a.If the blood pressure increases, the arterioles are instructed to
relax (vasodilation).
b.If the pressure decreases, the diameter of the arterioles decreases
(vasoconstriction).
D.Controlling Mean Arterial Blood Pressure
1.The nervous system and endocrine system also control the allocation of
more or less blood to different body regions at different times.
2.A baroreceptor (such as in the carotid arteries) is the main short-term
control over arterial pressure.
a.The medulla oblongata of the brain monitors signals from various
arteries to determine the rate of heartbeat and any changes needed
in vessel diameters.
b.Sympathetic and parasympathetic nerves make adjustments in
heart rate and vessel diameter to meet tissue needs.
3.Long-term control of blood pressure is exerted by the kidneys, which
adjust the volume and composition of the blood.
VII.From Capillary Beds Back to the Heart
A.Capillary Function
1.Capillaries are diffusion zones for exchanges between blood and
interstitial fluid.
a.A capillary is the smallest tube (red blood cells travel single file)
in the path of circulation.
b.Its wall consists of a single layer of endothelial cells, which
facilitates diffusion to and from the interstitial fluid.
2.Movement across the capillary is by several modes: diffusion (oxygen
and carbon dioxide), endo- and exocytosis (proteins), between the cells
(ions), and bulk flow (water).
a.At the beginning of a capillary bed, there is a movement of
plasma out into the interstitial fluid in a process known as
ultrafiltration.
b.Further on, some tissue fluid moves into the capillary through
clefts between its endothelial walls in a process known as
reabsorption.
B.Venous Pressure
1.Capillaries merge into venules then into veins.
2.Blood pressure and resistance to flow are both low; valves prevent
backflow.
3.Veins are blood volume reservoirs (50-60 percent of blood volume)
because their walls can distend or contract.
4.The movement of skeletal muscles squeezes the veins and pushes the
blood along against the forces of gravity.
VIII.Hemostasis
A.Hemostasis is the process of repairing the damage when a blood vessel ruptures.
B.There are several sequential steps:
1.Spasm of the smooth muscle in the damaged blood vessel stops blood
flow for a few minutes.
2.Platelets clump to plug the rupture.
3.The blood coagulates and forms a clot; the clot then retracts into a
compact mass.
IX.Lymphatic System
A.Lymph Vascular System
1.The lymph vascular system includes lymph capillaries and lymph vessels.
a.It returns excess fluid (mostly water and called lymph) and
proteins that have leaked out of the blood capillaries.
b.It transports fats that are absorbed from the intestine.
c.It brings pathogens, foreign cells, and cellular debris to the
lymph nodes for disposal.
2.Lymph capillaries begin blindly in the tissues of the body; they lead to
lymph vessels, which in turn lead to ducts that return the fluid to the
bloodstream.
B.Lymphoid Organs and Tissues
1.The lymph nodes (containing lymphocytes, plasma cells, and
macrophages) are located along the lymph vessels; their resident cells help
remove bacteria and cellular debris from the lymph.
2.The spleen removes spent RBCs and holds macrophages and reserve
RBCs; it produces red blood cells in human embryos.
3.The thymus secretes hormones that regulate the activity of lymphocytes
and is a site where they multiply and mature.