During the evolution of the synapsid reptiles and
the early mammals (a period of more than 150 million years), a number
of changes evolved in the cardiovascular and lymphatic systems. Red blood
cells lost their nuclei, the ancestral ventricle was divided in two, the
blood vessels were reorganized, new antibodies evolved, and lymph nodes
and tonsils were added to the lymphatic system.
THE CARDIOVASCULAR SYSTEM
In mammals, the red bone marrow became the site of red blood cell and
granulocyte synthesis while sites such as the thymus and spleen are important
for the maturation and proliferation of agranulocytes. Mammals were the
first to develop the blood cell known as platelets. In mammals, circulating
red blood cells lack nuclei (although a percentage of the red blood cells
in reptiles are also anucleate).
In the mammalian heart, an interventricular septum completely divided
the ancestral ventricle to creates right and left ventricles. The intrinsic
conduction system of the heart developed an AV node. Mammals evolved vagal
pathways from the nucleus ambiguous to regulate the heart.
Mammals developed complex coiled masses of blood vessels called rete mirable
which occur in many mammalian tails, perhaps as a heat-saving device;
humans still have the coccygeal glomus in their tail. Mammals lost their
lateral head veins which were replaced by internal jugular veins. Mammals
lost the ancestral renal portal system. Iliac veins now emptied into the
posterior vena cava (as opposed to the lateral abdominal veins or renal
portal veins). Mammals also lost the carotid duct connecting the carotid
artery to dorsal aorta. In mammals, the right branch of the fourth aortic
arch degenerated (its only remnant being the base of the right subclavian
artery) and the left branch of the fourth aortic arch composes the aorta.
THE LYMPHATIC SYSTEM
Valves (present in primitive lymph "hearts") developed in lymphatic
vessels. Mammals evolved true lymph nodes, increased the number of lymph
nodes sites over ancestral lymphatic tissue concentrations and evolved
palatine and lingual tonsils only in mammals. The thymus forms from the
ventral portions of pouches 3 and 4 instead of dorsal portions.
It seems that there was a duplication of the IgY gene in the ancestors
of mammals and the two forms were modified to form IgG and IgY. All mammals
use IgM, IgG, IgA, and IgE and lack the IgY used in reptiles. IgE only
exists in mammals but present in all 3 groups groups of mammals. Much
of the expansion of the expansion of the interleukin family has occurred
mammals. Interleukin-6 evolved in mammals.