Circulatory and Lymphatic Physiology - Anatomy

Veins always carry blood towards the heart. The blood in veins is mostly deoxygenated, however the pulmonary vein, which goes from the lungs to the left atrium, carries newly oxygenated blood back to the heart for it to be pumped to the rest of the body.

In contrast, arteries always travel away from the heart and usually carry oxygenated blood, with the exception of the pulmonary arteries. Arteries and arterioles have a thick layer of smooth muscle that helps to regulate blood pressure. Veins may have some smooth muscle, but are not nearly as significant in helping to regulate blood flow.

A common misconception is that all veins carry deoxygenated blood. In reality, all veins are responsible for bringing blood back to the heart. Generally, blood traveling toward the heart is deoxygentated. The pulmonary veins, however, bring blood that has just received oxygen from the lungs back to the heart. The pulmonary veins are the only veins in the body to carry oxygenated blood.

Similarly, the pulmonary arteries are the only arteries to carry deoxygentated blood away from the heart. All arteries carry blood away from the heart, but most contain oxygenated blood. The vena cavae are large veins that carry deoxygenated blood from the body back to the right atrium. This blood is then transferred to the right ventricle, and the then pulmonary arteries for transport to the lungs. The path of blood from the heart to the lungs and back is known as the pulmonary circuit.

During times of stress or physical activity, sympathetic nerves can stimulate blood vessels to constrict and dilate in order to redirect blood to the needed areas in the body. For example, sympathetic innervation can direct blood away from the skin and digestive tracts to facilitate muscle action.

Arterioles are typically surrounded by smooth muscle and can be constricted in order to redirect blood flow. Arteries also contain smooth muscle, but are generally too large to have specific, well-controlled effects on blood flow regulation. Venules have very little smooth muscle, and capillaries have none. Remember that the walls of capillaries consist of only a single layer of endothelium.

Arteries and veins differ in a few key ways. Arteries are much thicker than veins in order to compensate for the larger pressure exerted on them. Layers of smooth muscle in arteries is used to modulate this high pressure, allowing the vessel to expand or constrict. Veins have much larger lumens than arteries and have valves in order to prevent the backflow of blood. Veins have much lower pressure, but also must force blood to flow against gravity in order to carry it from the periphery back to the heart. This is accomplished by the venous valves.

Veins and arteries are the two primary vessels of the circulatory system. Blood is pumped from the heart into arteries, which branch into smaller arterioles. Arterioles terminate in capillary beds, which are specialized to allow for gas exchange. Blood leave the capillaries through venules, which expand into veins that carry the blood back to the heart. Veins always travel toward the heart and arteries always travel away from the heart.

Efferent arterioles are specialized arterioles in the kidneys that transport blood between two separate capillary beds, known as the renal portal system.

Blood flows through the circulatory system as a result of pressure generated by the heart. As blood moves throughout the circulatory system, pressure is lost due to friction generated between blood and blood vessel walls. Therefore, pressure falls continuously as blood moves farther from the heart. Since the body wants to maintain a forward, unidirectional flow of blood, the highest blood pressure is found in the aorta, and the lowest blood pressure is found in the venae cavae - just before emptying into the right atrium.

Angiogenesis is the development and growth of new blood vessels. The process is regulated by a class of peptides called angiogenic factors. Hematopoiesis is the process by which blood cells are made, in the bone marrow. Atherosclerosis refers to the thickening and the loss of the elasticity of the arteries due to deposition of plaques. Hypertension refers to high blood pressure. Vasoconstriction involves the contraction of smooth muscle surrounding vessels, decreasing blood flow and increasing blood pressure in those vessels.

The scalp bleeds profusely not only because the densely packed hair follicles demand a greater blood supply, but also because the superficial fascia (that binds the skin to the connective tissue of the occipitofrontalis muscle) prevents vascular dilation. Because of this, clotting is more difficult.

Blood flow in the veins is much slower because the blood is much farther from the left ventricle. Since the force of this powerful contraction is far "behind" venous flow, blood in veins is more likely to flow backward than in an artery. Because of this, veins have valves to prevent backflow much like the valves in the heart prevent back flow between chambers. The reason the blood flow in deep veins is generally faster than that of superficial veins is because muscle contractions help to move blood along in deep veins. A prime example is the lower legs. Contraction of the gastrocnemius (calf) muscle causes blood to move up through the lesser saphenous vein.

Arteries are strong elastic vessels that carry blood moving away from the heart. As arteries move away from the heart they become smaller (think of a tree and its branches, the trunk is larger than the branches and as each branch grows out it becomes smaller) and are referred to as arterioles, which connect to capillaries. Capillaries penetrate nearly all tissue; their walls are very thin and allow exchange of materials (oxygen, nutrients) between blood and tissues. Veins are thinner-walled and less muscular. The smallest ones are called venules and connect to capillaries.

Blood flow through veins is not very efficient. Slow and weak "pushing" by arteries does not contribute much to blood flow, as the hydrostatic pressure in veins is roughly zero. However, contraction of the diaphragm and skeletal muscles, along with the one way valves in veins (these prevent back flow) are important factors.

Note: Blood clots can occur if blood does not flow properly through veins. This can occur if a person doesn't move enough (for example a long international flight).

Venules are formed from merged capillaries. They progressively merge to form veins. Veins are thin-walled, are under low pressure, and contain the highest proportion of the blood in the cardiovascular system. The largest vein in the body is the vena cava, which returns blood to the heart.

Capillaries have the largest total cross-sectional and surface area. Arterioles are the site of highest resistance in the cardiovascular system. Arteries are thick-walled with extensive elastic tissue and smooth muscle.

The nasal cavity has a rich blood supply that derives from branches of both the internal and external carotid arteries. The maxillary artery, facial artery, and ophthalmic artery also contribute branches to provide blood supply to the nose. This being said, the posterior nose is supplied by the internal and external carotid arteries, the maxillary branch of the external carotid artery, and the sphenopalatine artery, which branches off the maxillary artery.

Veins are the most compliant entity within the circulatory system. The compliance of a systemic vein is 24 times that of its corresponding artery. Veins serve as the major blood reservoir within the human body, and are 8 times as distensible as arteries.

The cephalic vein is a superficial vein of the upper limb. It communicates with the basilic vein via the median cubital vein. It travels along the anterolateral surface of the biceps brachii muscle. The cephalic veins empties into the axillary vein (a continuation of the brachial and basilic veins). The cephalic vein terminates at the lateral margin of the first rib where it becomes the subclavian vein.

Fluid movement into and out of capillaries is mainly dependent on two forces: hydrostatic pressure and osmotic pressure. Hydrostatic pressure is determined by fluid volume and the pressure of the fluid against the capillary walls. Osmotic pressure is determined by osmotic gradient, pulling water in the direction of higher protein concentration. Osmotic pressure is largely directed into the capillary due to the high concentration of albumin in the plasma. The osmotic pressure outside of the capillary remains relatively constant, however, as the capillary moves from the arteriole end to the venule end the hydrostatic pressure drops significantly. Eventually, the osmotic pressure in the interstitium becomes greater than the hydrostatic pressure in the capillary, and fluid returns to the capillaries.

The walls of arteries, arterioles, veins, and venules are all much too thick to allow for the diffusion of substances. However, capillaries only have simple squamous endothelial cells lining their walls, without muscular and/or elastic tissue surrounding them. Their walls are thin enough to allow for diffusion of substances into the interstitium.

Pericytes, sometimes referred to as Rouget cells, are cells that wrap around the endothelial cells of capillaries. These cells play an important role in angiogenesis, the process of formation of new blood vessels. Monocytes are macrophages of the immune system, Purkinje fibers are found in the heart, and allow conductance of electricity to the cardiomyocytes, Schwann cells are glial cells in the peripheral nervous system that myelinate axons.

Capillaries have the largest total cross sectional and surface area. They are thin-walled and consist of a single layer of endothelial cells surrounded by basal lamina. Capillaries are the site of exchange of nutrients, water, and gases.

Arteries are thick-walled, with extensive elastic tissue and smooth muscle. Arteries are under high pressure.