Page 335 - Anatomy and Physiology of Farm Animals, 8th Edition

320 / Anatomy and Physiology of Farm Animals

Vessels irregular intervals throughout the venous
and lymphatic systems. A valve frequently
VetBooks.ir Blood Vessels is present where two or more veins unite to

form a larger vein. The valves ensure a uni-
Blood vessels resemble the branching of a directional flow of venous blood toward
tree in that the arteries start as large ves- the heart.
sels and divide into smaller and smaller
branches. The smallest arteries are arteri- Lymphatic Vessels
oles, which are continuous with the small-
est blood vessels, capillaries. Capillaries The walls of capillaries are thin enough to
again unite to form small venules that permit fluid as well as nutrients and gases
come together to form larger and larger to escape into spaces between tissue cells.
veins. The largest veins (cranial and caudal Some of this extracellular fluid (ECF) does
venae cavae) empty into the atria of the not re‐enter the vascular space directly but
heart (Fig. 17‐1). is instead recovered by thin‐walled lym-
Arteries and arterioles are tubular struc- phatic vessels. Lymphatic vessels resemble
tures that carry blood away from the heart. veins in that they contain numerous valves
Like all blood vessels, they are lined with permitting flow only toward the heart. The
endothelium. The walls of arteries tend to smallest lymphatic vessels are blind capil-
be thick and elastic, properties that are lary‐sized structures that begin in intercel-
important in maintaining blood pressure. lular spaces, where they accumulate
Smooth muscle in the walls of smaller extracellular fluid. Fluid within the lym-
arteries controls the diameter of these phatic vessels, called lymph, is transported
vessels. to larger and larger lymph vessels and
Capillaries are tiny tubes composed finally emptied into the cranial vena cava
almost entirely of endothelium, a continu- or one of its tributaries. The tracheal
ation of the simple squamous epithelium trunks, two large lymph vessels draining
that lines the heart and blood vessels. the head and neck, usually terminate in the
These thin‐walled vessels are only large jugular veins. Lymph from the caudal half
enough in diameter to accommodate a sin- of the body is delivered to the large tho-
gle file of erythrocytes. The wall acts as a racic duct (of which there may be one or
semipermeable membrane that permits two), which traverses the thoracic cavity
water, oxygen, and nutrients to leave the adjacent to the aorta to empty its lymph
blood for tissue cells and permits waste into the cranial vena cava.
products from tissue cells to enter the Movement of lymph is driven largely by
blood. Much of the fluid that passes out of gravity or changing pressures of adjacent
the capillaries into tissue spaces again structures. For example, contraction of a
returns to the blood by passing back muscle applies pressure to the adjacent
through the capillary walls. Some fluid lymphatic vessels and forces the lymph
remains in the tissues, and excess fluid farther toward the heart, since the valves
normally is removed by lymph vessels. prevent backflow. The lymph is filtered by
Capillaries unite to form venules, which nodular structures called lymph nodes (see
merge into larger and larger veins. Veins Chapter 16) scattered along the course of
are larger in diameter than the arteries most lymphatic vessels.
they parallel and have much thinner walls.
Venous blood pressure is typically quite
low. Contraction of skeletal muscles in the Pulmonary Circulation
limbs and trunk squeezes the thin‐walled
veins, assisting the flow of venous blood The pulmonary circulation is the part of the
back toward the heart. Valves, usually con- vascular system that circulates the blood
sisting of two cusps each, are scattered at through the lungs (Fig. 17‐1). Deoxygenated