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1 Testis
2 Interstitial (Leydig) cell
3 Basement membrane
4 Sertoli cell
5 Spermatogonium
6 Primary spermatocyte
7 Secondary spermatocyte
8 Spermatid
9 Seminiferous tubule
10 Tunica albuginea
11 Epididymis
12 Spermatic cord
Sagittal section of left testis
Medial view
Photomicrograph of testis
40x
The testes are oval-shaped organs about 2 inches (5 cm) long and 1 inch (2.5 cm) wide
that occupy the scrotal sac of a male. They are covered by a tough fi brous tunic and
wrapped in a serous sac that separates them from the external tissues that surround
Testes
them. Internally, the testes consist of numerous small compartments created by connective tissue bands that
project inward from the outer fi brous tunic. Each testicular compartment is occupied by a thin, highly coiled
seminiferous tubule. This thin tube is the site of sperm production. Situated between the tubules are the interstitial cells (of Leydig). It is these large interstitial cells that secrete the steroidal hormones in the testis.
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In addition to the endocrine organs discussed
on the preceding pages, there are other endocrine tissues in the body. These include tisOther Endocrine Structures
Other organs with endocrine tissues
Stomach (upper left), kidney (upper right), heart (lower right),
placenta (lower left), and adipose tissue (center)
sues in the wall of the gastrointestinal tract that produce hormones such as gastrin and secretin, tissues in the kidney that
produce renin and erythropoietin, tissues in the atrium of the heart that produce atrial natriuretic peptide, tissues of the placenta that produce human chorionic gonadotropin, estrogens, and progesterone, and adipose tissue that produces leptin.
These hormones have a variety of functions, from stimulating the release of digestive enzymes, to raising blood pressure,
to decreasing blood pressure, to regulating reproductive cycles, and suppressing appetite.
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16 Cardiovascular System
If you have ever planted a
garden of signifi cant size, you have probably experienced the importance of an irrigation system. At its simplest, an irrigation system is
a network of channels or furrows that deliver needed water from
one main source to the roots of all the garden’s plants. Like an
irrigation system, the body’s blood vessels form an extensive
network of “irrigation channels” to deliver needed fl uid — in
this case the homeostatically maintained blood — to all the
body’s cells. In fact, this delivery system is probably the most
phenomenal irrigation network imaginable. Emanating
from a muscular pump, the heart, these vessels form an
extensive system of tubular roadways that carry nourishing
blood away from the heart and toward the tissues. Th ey
then make a “U-turn” through small permeable, exchange
vessels, the capillaries, which feed all the body’s cells. Here,
life-supporting molecules, such as water, oxygen, glucose,
and amino acids are delivered to the cells, and the by-products
of cellular metabolism are picked up from the surrounding
tissue fl uid. Th e blood then fl ows back to the heart through
a series of return vessels, the veins, that parallel the delivery
vessels. Th is circular pattern of fl ow to and from the heart
constitutes the vascular (blood vessel) component of the
cardiovascular (circulatory) system. Th is irrigation network
is so impressive, that if all the blood vessels of the body were
placed end-to-end they would extend 25,000 miles (96,500 km),
which is approximately two times the equatorial circumference
of the earth.
Th e irrigation network of blood vessels are of no value without
a pump. Th e heart is the dual, self-regulating pump that generates
the pressure to drive the blood through this impressive irrigation
network. It pumps the blood through two cycles — a pulmonary
cycle to pick up oxygen from the lungs and a systemic cycle to deliver
the oxygen to all the cells of the body. Soon aft er conception, and up
until death, the heart pumps blood. It averages approximately 70 beats
per minute, or about 3 billion contractions in an average lifetime.
Th e fi nal aspect of the cardiovascular system is the accessory drainage
network — the lymphatics. Th ese small vein-like vessels insure that the
cardiac return equals the cardiac output. Th is chapter will depict the anatomy
of this amazing muscular pump and the vascular and lymphatic roadways that
distribute the blood throughout the body.
Find more information
about the cardiovascular
system in
REAL ANATOMY
262
1 Erythrocyte (red blood cell)
2 Leukocyte - neutrophil (white blood cell)
3 Leukocyte - monocyte (white blood cell)
4 Thrombocyte (platelet)
5 Blood plasma
Blood smear
700x
In the histology chapter we learned that the fl uid material we call blood has been historically classifi ed as
a connective tissue. This classifi cation was a result of the fact that, like other connective tissues, blood
has more extracellular matrix than cells. More recently, however, blood has been placed in a tissue catBlood
egory of its own — the hematolymphoid complex. The extracellular portion of the blood is a water solution that gives rise to
its liquid nature. Blood is closely related to other aqueous fl uids within the body, in fact most of the other body fl uids, such
as interstitial fl uid, lymph, cerebrospinal fl uid, and aqueous humor, arise from the blood. These extracellular fl uids are the
water environment that nourish, protect, and exchange with every cell of the body. This water environment is derived from
the blood, renewed by the blood, and returned to the blood. Dispersed in the blood plasma are the three groups of blood
cells — erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). The blood smear below
depicts the three cell categories.
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Heart
Posterior view
Heart
Anterior view
From its origin in the embryo as a simple pumping tube, the heart develops into a strong fi bromuscular
organ. During its development the original tubular pump is folded and subdivided into a four chambered
organ that has a pyramidal or conical form. It is approximately the size of a closed fi st and weighs
Heart
approximately 300 grams in males and a little less than this in females. For its small size, comprising only one half of one
percent of the total body mass, it is an important and functionally amazing organ. The wall of the heart consists of three
structural layers that each play signifi cant roles in its function as an effi cient pump. While the tissue makeup of this wall is
similar at any location in the heart, the thickness can vary considerably. Internally a septum and series of valves divide the heart
into four chambers through which the blood moves in a unidirectional fl ow. The chambers differ in structure and function, which
is primarily refl ected in the anatomy of their walls. Embedded within the walls of heart is a special electrical conduction system
that helps regulate its coordinated pumping action.
1 Right atrium
2 Left atrium
3 Right ventricle
4 Left ventricle
5 Right auricle
6 Left auricle
7 Aorta
8 Brachiocephalic artery
9 Left common carotid artery
10 Left subclavian artery
11 Pulmonary trunk
12 Right pulmonary artery
13 Left pulmonary artery
14 Ligamentum arteriosum
15 Superior vena cava
16 Inferior vena cava
17 Coronary sinus
18 Right coronary artery
19 Conus arteriosus branch
20 Marginal branch
21 Anterior interventricular artery
22 Lateral branches
23 Circumflex branch
24 Posterior interventricular artery
25 Anterior cardiac vein
26 Great cardiac vein
27 Posterior vein of left ventricle
28 Middle cardiac vein
29 Small cardiac vein
30 Right superior pulmonary vein
31 Right inferior pulmonary vein
32 Left superior pulmonary vein
33 Left inferior pulmonary vein
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Dissection of heart and pericardial sac
Anterolateral view
Transverse section of heart comparing ventricle thickness
Inferior view, left ventricle at right
Dissected heart showing interior of chambers
Anterior view
Heart
1 Parietal pericardium
2 Fibrous pericardium
3 Visceral pericardium
4 Epicardium
5 Myocardium
6 Endocardium
7 Right atrium
8 Right auricle
9 Interatrial septum
10 Fossa ovalis
11 Crista terminalis
12 Valve of inferior vena cava
13 Pectinate muscle
14 Tricuspid valve
15 Chordae tendineae
16 Trabeculae carnae
17 Papillary muscle
18 Right ventricle
19 Pulmonary valve
20 Left atrium
21 Left auricle
22 Bicuspid valve
23 Left ventricle
24 Aortic valve
25 Apex
26 Aorta
27 Brachiocephalic artery
28 Left common carotid artery
29 Left subclavian artery
30 Pulmonary trunk
31 Left pulmonary artery
32 Ligamentum arteriosum
33 Anterior interventricular artery
34 Lateral branches of interventricular artery
35 Superior vena cava
36 Right coronary artery
37 Left coronary artery
38 Right pulmonary veins
39 Left pulmonary veins
40 Diaphragm
41 Lung
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Dissection of heart revealing tricuspid valve
Anterior view
Heart dissection with atria and arteries removed
Superior view, anterior at top
Aortic valve
Superior view
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Muscular artery
100x
Elastic lamellae of aorta
640x
Section of aorta — large elastic artery
100x
Like all tubes in the body, blood vessels have a basic pattern of design that involves
three structural tunics, or layers. The inner layer of the vessel is the tunica intima.
This consists of the luminal endothelium and a thin network of underlying elastic
Blood Vessels
connective tissue. The middle layer of the vessel is the tunica media, which consists of varying amounts of smooth muscle
and elastic connective tissue. Variations in the tunica media defi ne the different types of blood vessels. The outer layer, the
tunica externa, is a dense connective tissue outer coat. The designations — elastic arteries, muscular arteries, arterioles,
venules, and veins — are based on size differences and the differences in the vessels’ tunica media. Elastic arteries have
a thick elastic tunica media. Muscular arteries have a tunica media dominated by smooth muscle. Arterioles are tiny arteries
with a muscular tunica media. All the venous vessels have a thin, almost non-existent tunica media. The smallest blood
vessels, the capillaries, loose all the layers of their wall except the inner endothelium. These microscopic, thin walled tubes
become the exchange vessels of the system.
1 Endothelium of tunica intima
2 Internal elastic membrane of tunica intima
3 Elastic lamellae of tunica media
4 Smooth muscle cells of tunica media
5 Connective tissue of tunica externa
6 Red blood cells
7 White blood cells
8 Venous valves
9 Nerve
10 Striated skeletal muscle
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Arteriole
500x
Capillary
1000x
Neurovascular bundle — note thin-walled vein fi lled with red
blood cells (6) compared to thick-walled muscular arteries (4)
100x
Longitudinal section of vein showing valves
Anterior view
Transverse section of vein showing valves
Superior view
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1 Heart
2 Pulmonary trunk
3 Right pulmonary artery
4 Left pulmonary artery
5 Right superior pulmonary vein
6 Right inferior pulmonary vein
7 Left superior pulmonary vein
8 Left inferior pulmonary vein
9 Aorta
10 Right coronary artery
11 Left coronary artery
12 Right common carotid artery
13 Right subclavian artery
14 Left common carotid artery
15 Left subclavian artery
Dissections of pulmonary trunk, arteries, and veins
Anterosuperior view below, anterior view above
The vascular system consists of two long circular loops of continuous
branched tubing that each begin and end with the heart. Leaving the
right ventricle and returning to the left atrium is the smaller pulmonary
Pulmonary Circuit
circulation. This circular loop courses through the lung tissues where its smallest vessels form an extensive interface with
the small air sacs of the lungs. This important interface is the site of exchange of O2
and CO2
between the blood and air.
16 Superior vena cava
17 Inferior vena cava
18 Trachea
19 Right principal bronchus
20 Left principal bronchus
21 Esophagus
22 Thyroid gland
23 Vagus nerve
24 Pulmonary plexus
25 Posterior vagal trunk
26 Esophageal plexus
27 Anterior vagal trunk
28 Anterior scalene muscle
29 Cricothyroid muscle
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