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 Structure:
The heart is made of cardiac muscle, and the
arteries, the efferent vessels, contain smooth muscle,
but the veins, the afferent vessels with less muscle, have internal valves.
Capillaries are thin-walled tubes that form a large network connecting
arteries and veins. Blood circulates in this closed system, returning to
the heart. Lymphatic fluid, collected by a set of lymphatic
vessels with "one-way" flow from the spaces between cells,
enters into the subclavian veins through lymph ducts. Read about the comparative
anatomy and physiology of the circulatory and the respiratory systems
of vertebrates.
 Function:
The heart is the circulatory pump that maintains the flow of blood by maintaining
blood pressure in the arteries. The veins to the lungs
carry deoxygenated blood for absorbing oxygen,
and the pulmonary arteries return oxygenated blood to the left atrium of
the heart for entry into the left ventricle. This muscle pushes the blood
into the aorta for distribution through the arteries to the capillaries
in the head and other body regions. The pumping action of the heart and
the muscles of the arteries, keep a pressure in the circulation that causes
blood to flow forward. The capillary vessels, where the flow is slow, are
the site of gas and nutrient exchange with the tissues. Because there is
little blood pressure in the veins, backflow is prevented by valves. Because
the blood vessels leak fluid to the cells and the loose connective tissues
around them, the lymphatic system collects fluids to maintain the proper
balance of fluids, and nutrients
including digested proteins and
fats . Regulation of the circulatory system is by the autonomic
nervous system and hormones from the adrenal
medulla. The circulating blood is formed in bone marrow by a process
called hematopoeiesis.
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