Chapter 39 Circulation

I. Circulatory System: An Overview
A. A circulatory system is an internal transport system with three components:
1 Blood is a fluid tissue composed of water, solutes, and formed elements.
2 The heart is a muscular pump that generates pressure to keep the blood flowing.
3 Blood vessels are tubes of various diameters through which the blood is transported. B. There are two basic types of circulatory systems:
1 Arthropods and most molluscs 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.
2 Vertebrates have a closed system.
a. All the vessels and the heart are connected so that blood remains enclosed.
b. Blood volume is constant; rate slows as blood moves through the fine tubes of the
capillary beds.
C. Functional Links With the Lymphatic System
1 The lymphatic system is a network of vessels and organs that house vast numbers of
infection-fighting cells.
2 The lymphatic system picks up excess fluids, solutes, and disease agents from the
interstitial fluid.
3 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 about 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.
3) When extra red blood cells are needed, the kidneys secrete an enzyme that converts a
plasma protein to the hormone erythropoietin, which stimulates the bone marrow to
increase production.
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. They function in blood clotting.

III. Cardiovascular System of Vertebrates
A. Blood Circulation Routes in Humans
1 The human heart is divided into right and left halves.
2 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.
3 Blood travel follows this route: heart Æ arteries Æ arterioles Æ capillaries Æ venules Æ veins
Æ heart
4 Usually a given volume of blood in either circuit passes through only one capillary bed; exception
is blood from the digestive tract, which passes through the liver before entering the general
circulation.
B. The Human Heart
1 Heart Structure
a. The heart is a durable pump made mostly of cardiac muscle and enclosed in a tough, fibrous
sac (pericardium); its chambers are lined with connective tissue and endothelium.
b. Each half consists of an atrium (receiving) chamber and a ventricle (pumping) chamber
separated by an atrioventricular valve.
c. Blood exits each ventricle through a semilunar valve.
d. Heart muscle cells are serviced by the coronary circulation, two arteries that branch
directly off the aorta.
2 Cardiac Cycle
a. The cardiac cycle consists of a sequence of contraction (systole) and relaxation (diastole).
b. As the atria fill, the ventricles are relaxed.
c. Pressure of the blood in the atria forces the atrioventricular valves to open; the ventricles
continue to fill as the atria contract.
d. The ventricles contract, the atrioventricular valves close, and blood flows out through the
semilunar valves.
e. The heart sound “lub” is made by the closing of the AV valves; the “dup” sound is the
closure of the semilunar valves.
3 Mechanisms of Contraction
a. Because of the close junction of cardiac muscle cells, they contract in unison.
b. Excitation for a heartbeat is initiated in the sinoatrial (SA) node, then passes to the
atrioventricular (AV) node for ventricular contraction; this is the cardiac conduction
system.
c. The nervous system adjusts rate and strength.
C. Blood Pressure in the Vascular System
1 Blood pressure drops along the way due to energy loss from resistance.
2 Arterial Blood Pressure
a. Arteries conduct blood away from the heart.
b. Because of their elastic walls, arteries tend to “smooth out” the pressure changes
associated with the discontinuous pumping cycle of the heart.
c. Normal systolic pressure is 120mm Hg; normal diastolic pressure is 80mm Hg; the measuring
device is a sphygmomanometer.
3 Resistance at Arterioles
a. Arteries branch into smaller arterioles which offer greater resistance to flow and thus a
drop in blood pressure.
b. Neural and endocrine signals cause changes in arteriole diameter by stimulating the muscle
cells in the walls.
c. Arterioles serve as control points where adjustments can be made in blood volume
distribution.
4 Capillary Function
a. Capillaries are diffusion zones for exchanges between blood and interstitial fluid.
b. A capillary is the smallest tube (red blood cells travel single file) in the path of circulation; it
consists of a single layer of endothelial cells.
c. 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).
5 Venous Pressure
a. Capillaries merge into venules then into veins.
b. Blood pressure and resistance to flow are both low; valves prevent backflow.
c. Veins are blood volume reservoirs (60 percent of blood volume) because their walls can
distend or contract.
D. Controls Over Blood Flow
1 Maintaining Blood 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. If the blood pressure increases, the arterioles are instructed to relax (vasodilation).
c. If the pressure decreases, the diameter of the arterioles decreases (vasoconstriction).
d. Hormones such as epinephrine and angiotensin also assist.
2 Control of Blood Distribution
a. Sympathetic and parasympathetic nerves make adjustments in heart rate and vessel diameter
to meet tissue needs.
b. Local conditions, such as need for more oxygen and nutrients in active skeletal muscle, cause
changes in the rate of flow near those tissues.
E. Hemostasis
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 contracts.
a. In the intrinsic clotting mechanism, internal damage exposes collagen in the vessel walls; this
in turn activates a plasma protein that triggers the formation of thrombin, which acts on
fibrinogen to form insoluble threads that will entrap blood cells
b. In the extrinsic clotting mechanism, blood clotting is triggered by the release of substances
outside the blood itself due to damage there.
F. Blood Typing
1 All cells of the human body have surface proteins and other molecules that serve as “self”
markers.
2 The human body also has antibodies that recognize markers on foreign cells.
3 ABO Blood Typing
a. ABO blood typing is based on surface markers on red blood cells.
b. Type A has A markers; type B has B markers; type AB has both markers; type O has neither
marker.
c. If bloods of certain donors and recipients are mixed, agglutination (clumping) will occur.
4 Rh Blood Typing
a. An Rh– person (lacks this marker) transfused with Rh+ blood (has this marker) will produce
antibodies to the Rh marker.
b. 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.
c. In erythroblastosis fetalis, too many cells may be destroyed and the fetus dies.
d. Medical treatment given to the mother after the birth of the first Rh+ baby can inactivate the
Rh antibodies.

IV. Lymphatic System
A. The lymphatic system returns excess tissue fluid to the bloodstream via transport tubes. B. It also contains lymphoid organs, which take part in defense responses.
C. Lymph Vascular System
1 The lymph vascular system includes lymph capillaries, lymph vessels, and ducts.
2 It returns excess fluid (called lymph) and proteins, transports fats, and brings foreign materials
to the lymph nodes for disposal.
3 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.
D. Lymphoid Organs
1 They contain lymphocytes, which originate from stem cells in the bone marrow.
2 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.
3 The spleen removes spent RBCs and holds macrophages and reserve RBCs; it produces red blood
cells in human embryos.
4 The thymus secretes hormones that regulate the activity of lymphocytes and is a site where they
multiply and mature.