Page 342 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy of the Cardiovascular System / 327

left azygos veins, but more usually have a branches there, finally ending in the sinu-
soids of the liver. Here the blood comes
single left azygos vein, which empties
VetBooks.ir directly into the right atrium with the cor- into direct contact with cells of the liver.
After being acted upon by the liver cells
onary sinus. The pig possesses a left azygos
vein (Fig. 17‐3), which empties into the (see Chapter 21), the blood passes from the
coronary sinus. sinusoids of the liver into the liver’s venous
system and eventually empties into the
Caudal Vena Cava caudal vena cava.

The caudal vena cava is formed in the Fetal Circulation
abdomen by the junction of the paired
internal and external iliac veins. These Throughout gestation, the fetus depends
bring venous blood from the gluteal and on the dam for the nutrients, water, and
perineal regions and the pelvic limbs, oxygen needed for growth and for the
respectively. The caudal vena cava also elimination of carbon dioxide and other
receives lumbar veins, testicular or ovar- waste products of fetal metabolism. During
ian veins, renal veins, and various others fetal development, the lungs are collapsed
from structures associated with the body and not ventilated, and the pulmonary
walls. Just caudal to the point at which the vascular beds have high resistance to blood
caudal vena cava passes through the caval flow. The fetal circulation therefore
foramen of the diaphragm, it receives a bypasses these pulmonary capillary beds.
number of short hepatic veins directly Immediately after birth, however, the new-
from the liver.
born needs to direct its blood through the
pulmonary vessels for oxygenation. The
Portal System heart and circulatory system are arranged
in such an elegant way that the cardiopul-
A portal system is one in which a vessel monary circulation just moments after
divides into a bed of capillaries, recom- birth is profoundly different from that
bines to form another vessel, and then exhibited just prior to the first breath
redivides into a second capillary bed. The (Fig. 17‐10).
hypothalamohypophysial portal system Because it is exchanging gas, supplying
was described in Chapter 13 in relation to nutrients, and removing metabolic waste
the pituitary gland. In birds and in some products, the placenta must necessarily
reptiles and amphibians, part of the venous receive a large proportion of the fetus’s
blood returning from the pelvic limbs circulating blood.
enters the kidneys to form a renal portal It does so via two large umbilical arter-
system (see Chapter 30). ies, extending from the caudal end of the
In the hepatic portal system, blood that abdominal aorta through the umbilical
has perfused the capillary beds of the cord to the placenta. After passing through
viscera is brought to the liver by a single the placental capillary bed, the oxygenated
large vein, the portal vein, and then is blood is returned to the fetus by a single
redistributed into a second capillary bed umbilical vein, which passes into the sub-
within the substance of the liver (Fig. 17‐5). stance of the liver. Most of the blood
Tributaries to the portal vein include returning from the placenta in the umbili-
the gastric vein from the stomach, the cal vein is delivered directly into the caudal
splenic vein from the spleen, the mesen- vena cava, bypassing the hepatic sinusoids
teric veins from the intestines, and the via a fetal diversion, the ductus venosus.
pancreatic veins from the pancreas. The Two features of the fetal heart allow
portal vein enters the liver and immedi- blood to bypass the pulmonary circulation.
ately breaks up into smaller and smaller During development of the heart, the wall