Blood Vessels Health Article

Veins and venules

Veins are blood vessels that carry blood from the capillary beds to the heart. Capillaries give rise to venules (small veins that have walls composed of a thin layer of endothelial cells), which in turn converge to form veins. Blood from the head, neck, and arms is carried to the superior vena cava, while the inferior vena cava receives blood from the trunk and legs; these large veins empty into the right atrium of the heart. The veins carry blood that is oxygen-poor, with the exception of the pulmonary vein, which carries oxygenated blood from the lungs to the heart.

The walls of veins are thinner and the lumens larger than those of arteries. They can accommodate a large blood volume and may act as blood reservoirs, containing up to 70% of the body's total blood volume. Veins and venules are therefore called capacitance vessels. Most veins have a system of valves, paired folds of the tunica intima that prevent the backflow of blood.

Blood pressure

Blood pressure is defined as the force per unit area that flowing blood exerts on the wall of a vessel; it can be represented by the equation Blood pressure = flow resistance. Blood pressure is typically expressed in mm Hg (read as "millimeters of mercury"). It is usually recorded as two numbers: systolic pressure over diastolic pressure. Systole is the period of the cardiac cycle in which the aortic valve opens and blood flows into the aorta; systolic pressure is the maximal pressure during systole. Likewise, diastole is the period in which the left ventricle relaxes so it can refill with blood; diastolic pressure is therefore measured during diastole. It is generally assumed that a healthy young adult should have a blood pressure of 120/80 mm Hg (i.e. systolic pressure of 120 mm Hg and diastolic pressure of 80 mm Hg).

Blood pressure is proportional to blood flow (the amount of blood flowing through a vessel per unit time) and vascular resistance. Pressures vary throughout the cardiovascular system depending on the type and size of blood vessel. The highest systemic blood pressure is found in the aorta and diminishes progressively along the arterial system; it reaches its lowest point in the veins.

There are a number of factors that influence blood pressure. An individual's physical characteristics (i.e. sex, age, weight, race, or socioeconomic status) may positively or negatively affect blood pressure. Activities such as eating, drinking, sleeping, or smoking cause changes in pressure, as do mental activities or emotions such as anxiety or apprehension. Various disorders such as atherosclerosis, anemia, and diabetes mellitus have adverse affects on blood pressure.

Capillary dynamics

The capillary bed is the site at which gases, nutrients, and wastes are exchanged between the blood and surrounding tissues. It is surrounded by interstitial fluid, or lymph, which is produced by the lymphatic system. Substances are moved between blood and interstitial fluid across the capillary wall by means of diffusion (movement from a high to a low concentration). Oxygen and nutrients move from the blood to interstitial fluid, while carbon dioxide and wastes move in the opposite direction. Gases such as oxygen or carbon dioxide and lipid-soluble nutrients diffuse across the cell membranes of endothelial cells. Small openings in the capillary wall called slit pores or clefts exist where endothelial cells border each other; small water-soluble nutrients or wastes may diffuse through these clefts.

There are two types of pressure that are involved in capillary dynamics. Hydrostatic pressure is the force per unit area exerted by a fluid (blood) against a vessel wall. Colloid osmotic pressure is the pressure required to prevent osmosis of fluid across a semi-permeable membrane. Transcapillary filtration is determined not only by these pressures inside the blood vessels, but also by the same pressures outside the blood vessels. Osmotic pressure is an indirect measurement of the relative concentrations of water and solute in a solution; the higher the osmotic pressure of the solution, the lower the water concentration and therefore the higher the solute concentration of the solution. In a capillary, osmotic forces are exerted primarily by proteins, which are relatively impermeable to the capillary wall.