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23 CIRCULATION
CHAPTER OUTLINE
Circulation (p. 508)
23.1
23.2
Open and Closed Circulatory Systems (p. 508; Fig. 23.1)
A. Circulatory systems may be open or closed.
B. In open circulatory systems, there is no distinction between the circulating fluid (blood, or
hemolymph) and the extracellular fluid of the body tissues.
C. In closed circulatory systems, the circulating fluid (blood) is enclosed within blood vessels
that transport blood to and from a muscular heart.
D. Arteries carry blood away from the heart, exchange of gases and nutrients occurs through
thin-walled tiny capillaries, and veins return blood to the heart.
E. As blood plasma passes through capillaries, the pressure forces some of its fluid out of the
capillary walls; lymph vessels collect and return this fluid to the venous blood.
F. The Functions of Vertebrate Circulatory Systems
1. The circulatory system transports gases, nutrients, and hormones to body cells, and
removes wastes from cells.
2. The cardiovascular system also functions in temperature regulation.
3. The circulatory system protects against injury and foreign microbes or toxins introduced
into the body.
Architecture of the Vertebrate Circulatory System (p. 510; Figs. 23.2-23.6)
A. The vertebrate circulatory system is made up of a muscular heart that pushes the blood
through the body, a network of blood vessels to carry the blood, and the blood itself.
B. The circulatory system is often referred to as the cardiovascular system.
C. Blood flows through the body in a cycle, from the heart, through a system of vessels: blood
leaves the heart in arteries; from the arteries, blood passes into smaller arterioles; tiny vessels
called capillaries connect arterioles to venules, or small veins; venules and then veins carry
blood back to the heart.
D. Through the walls of capillaries, the exchange of nutrients, gases, and wastes between the
blood and the tissues occurs.
E. Capillary beds can be opened or closed based on the physiological needs of the tissues;
precapillary sphincters can contract or relax and affect whether blood flows into a capillary
bed for exchange of gases and metabolites.
F. Although each capillary is very narrow, there are so many of them that the capillaries have
the greatest total cross-sectional area of any other type of blood vessel.
G. Arteries: Highways from the Heart
1. Arterial walls have several layers: the innermost layer is smooth; next is a layer of elastic
fibers surrounded by a layer of smooth muscle; and the outermost layer is connective
tissue.
2. Arteries are both flexible and strong in order to carry high-pressure blood to all portions
of the body.
3. Arterioles are much smaller than arteries, and their muscular layer can be influenced by
hormones or nerves: arterioles can constrict or expand to affect blood flow during periods
of stress or body activity.
H. Capillaries: Where Exchange Takes Place
1. Capillaries are where O2 and food molecules are transferred from the blood to the body’s
cells and waste CO2 is picked up.
2. Capillaries are one cell thick with very narrow diameters, just wide enough to allow red
blood cells to pass through; this design ensures that the gases and metabolites transported
in the blood are carried close to the tissue cells that need them.
I.
23.3
23.4
Veins: Returning Blood to the Heart
1. Veins return blood to the heart.
2. They have the same layers as arteries, but the elastic and muscle layers are thinner.
3. Veins have valves inside that prevent the backflow of low-pressure blood as it returns to
the heart.
The Lymphatic System: Recovering Lost Fluid (p. 513; Figs. 23.7, 23.8)
A. The cardiovascular system is very leaky; from capillary exchange, the body loses about
4 liters of fluid each day.
B. To collect and recycle this fluid, the body uses a second circulatory system called the
lymphatic system.
C. The lymphatic system is a network of vessels filled with a fluid called lymph.
D. Lymphatic capillaries gather excess fluid from the tissues; the fluid, now lymph, moves
through the lymph vessels and eventually reenters the bloodstream through veins in the neck.
E. The lymphatic system has three important functions: it returns proteins to the bloodstream, it
transport fats absorbed from the intestines, and it aids in the body’s defenses.
F. Swellings along lymph vessels called lymph nodes and a lymph organ called the spleen are
where bacteria and dead blood cells are destroyed, and the thymus produces white blood
cells.
Blood (p. 514; Figs. 23.9, 23.10)
A. Blood Plasma: The Blood’s Fluid
1. Blood flows through the vessels of the circulatory system, carrying with it oxygen,
nutrients, and wastes.
2. The fluid portion of the blood is called the plasma, and it carries metabolites and waste
products, salts and ions, and plasma proteins.
3. Some plasma proteins, such as serum albumin, help to maintain the osmotic pressure of
the blood so it does not lose too much water to the tissues.
4. Starvation and protein deficiency can result in reduced levels of protein in the blood,
leading to edema, or swelling of the tissues.
5. Other plasma proteins include antibodies, globulins, and fibrinogen; fibrinogen (which
converts into fibrin) is required for blood clotting.
B. Blood Cells: Cells That Circulate Through the Body
1. Three types of cells make up almost half the volume of blood.
2. Erythrocytes, also known as red blood cells, carry oxygen to the cells of the body.
3. A protein called hemoglobin takes up most of the space inside a red blood cell.
4. Hemoglobin binds oxygen in areas where oxygen is plentiful, such as in the lungs, and
gives it up in tissues where oxygen is needed.
5. White blood cells, or leukocytes, are essentially colorless; there are several different
kinds, all of which help defend the body against invading microorganisms and other
foreign substances.
6. Some types of leukocytes are able to migrate out into tissues to attack pathogens.
7. The third type of cell exists as cell fragments; these platelets play a role in blood
clotting.
Evolution of Vertebrate Circulatory Systems (p. 516)
23.5
23.6
Fish Circulation (p. 516; Fig. 23.11)
A. A fish heart is essentially a tube consisting of a series of four chambers.
B. The first two chambers, the sinus venosus and atrium, are the collecting chambers.
C. The second two chambers, the ventricle and the conus arteriosus, are pumping chambers.
D. The sinus venosus and conus arteriosus chambers are greatly reduced in higher vertebrates.
E. Blood enters the heart at the sinus venosus, where the wavelike contraction of the heart
begins.
Amphibian and Reptile Circulation (p. 517; Fig. 23.12)
A. Amphibians and reptiles overcame the problems of living on land in part with the evolution
of the lung, and a simultaneous change in circulation.
B.
23.7
Amphibians and reptiles thus have two circulations: a pulmonary circulation that goes
between the heart and the lungs, and a systemic circulation that goes between the body and
the heart.
C. The amphibian heart has structural features to prevent the mixing of deoxygenated from the
body with oxygenated blood from the lungs.
Mammalian and Bird Circulation (p. 518; Figs. 23.13, 23.14, 23.15)
A. Mammals, birds, and crocodiles have a four-chambered heart that is two separate pumps
working together.
B. The increased efficiency of this double circulatory system allowed for endothermy and the
higher metabolic rate required to support endothermy.
C. Circulation Through the Heart
1. In the mammalian heart, one side of the heart pumps blood to the lungs to pick up
oxygen, while the other side distributes oxygenated blood to the rest of the body.
2. Oxygen-rich blood returns from the lungs through pulmonary veins to the left atrium of
the heart and flows mostly passively through the mitral valve into the left ventricle.
3. The thick-walled left ventricle contracts, sending oxygenated blood through a large
artery called the aorta and out to the body.
4. Backflow of blood from the aorta is prevented by an aortic valve.
5. Blood travels through the body's arteries and arterioles to capillaries.
6. Blood is returned to the heart through venules and veins that merge to form the superior
vena cava and inferior vena cava, which drain into the right atrium.
7. Blood flows from the right atrium through the tricuspid valve to the right ventricle.
8. The right ventricle contracts, pushing blood through the pulmonary valve into
pulmonary arteries that lead to the lungs.
D. Monitoring the Heart’s Performance
1. The performance of the heart can be monitored by several methods.
2. Listening to its sounds using a stethoscope can determine the presence of an
incompletely closing valve, a condition called a murmur.
3. Arterial blood pressure can be measured using a sphygmomanometer.
4. The greatest pressure, systolic pressure, occurs when the ventricles contract.
5. When the atria are filling, pressure is at its lowest in the arteries, and is called diastolic
pressure.
E. How the Heart Contracts
1. The contraction of heart muscle is carefully controlled.
2. First the atria contract together, then the ventricles contract together.
3. Contraction is initiated by a cluster of self-exciting cells called the sinoatrial (SA) node
(which evolved from the sinus venosus) located in the upper wall of the right atrium.
4. A wave of depolarization is initiated there and rapidly spreads throughout the atria,
followed by their contraction.
5. The depolarization wave travels to the ventricles through the atrioventricular (AV) node
and rapidly spreads from there to the bundle of His and Purkinje fibers, and then to the
ventricles, which contract.
6. The electrical activity of the heart can be measured by a recording called an
electrocardiogram (ECG or EKG).
KEY TERMS
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open circulatory system (p. 508) In an open circulatory system, there is no distinction between the
blood and the extracellular fluid of the body tissues.
closed circulatory system (p. 508) In a closed circulatory system, the blood is enclosed within vessels
that transport it to and from a heart.
gastrovascular cavity (p. 508)
hemolymph (p. 508)
interstitial fluid (p. 509)
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countercurrent heat exchange (p. 509)
cardiovascular system (p. 510)
arteries (p. 510) Arteries carry high-pressure blood away from the heart.
capillaries (p. 510) It is through these tiniest of the blood vessels that gas and nutrient exchange with
tissue cells occurs.
veins (p. 510) Veins return blood to the heart.
lymphatic system (p. 513) The lymphatic system consists of a system of vessels, similar in structure to
veins, that collects and recycles fluid. Portions of the lymphatic system house cells of immunity.
lymph (p. 513)
lymph nodes (p. 513)
plasma (p. 514) The fluid portion of blood.
erythrocytes (p. 514) Red blood cells; carry oxygen in association with hemoglobin.
leukocytes (p. 515) White blood cells; fight against infection.
platelets (p. 515) Fragments of cells that function in blood clotting.
atrium (p. 516) Collecting chamber of the heart.
ventricle (p. 516) Pumping chamber of the heart.
pulmonary circulation (p. 517) The system of blood vessels leading from the heart to the lungs and
back.
systemic circulation (p. 517) The system of blood vessels leading from the heart to the body and back.
cutaneous respiration (p. 517)
heart (p. 518)
heart murmur (p. 519)
diastolic (p. 519)
systolic (p. 519)
sinoatrial (SA) node (p. 520) The pacemaker of the heart.
electrocardiogram (p. 520)
atrioventricular (AV) node (p. 520)