form a closed delivery system that begins and ends at the heart.
the innermost tunic, in intimate contact with the blood in the lumen, forming a slick surface that minimizes friction as blood moves through the lumen.
in vessels larger than 1 mm, consisting of a basement membrane and loose connective tissue, supports the endothelium.
the middle tunic, mostly circularly arranged smooth muscle cells and sheets of elastin, the bulkiest layer in arteries, responsible for maintaining blood pressure and continuous blood circulation.
reduction in lumen diameter as the smooth muscle contracts
increase in lumen diameter as the smooth muscle relaxes
the outermost layer is composed largely of loosely woven collagen fibers protect and reinforce the vessel, and anchor it to surrounding structures.
carry blood away from the heart, “branch,” “diverge,” or “fork” as they form smaller and smaller divisions.
carry blood toward the heart, “join,” “merge,” and “converge” into the successively larger vessels approaching the heart.
have intimate contact with tissue cells and directly serve cellular needs. Exchanges between the blood and tissue cells occur primarily through the gossamer-thin capillary walls.
conducting, thick-walled arteries near the heart—the aorta and its major branches.
distributing arteries, which deliver blood from elastic arteries to specific body organs
smallest of the arteries delivers blood from the muscular arteries to the capillaries
abundant in the skin and muscles, are most common. They are continuous in the sense that their endothelial cells provide an uninterrupted lining, adjacent cells being joined laterally by tight junctions.
are similar to the continuous variety except that some of the endothelial cells in fenestrated capillaries are riddled with oval pores, or fenestrations
are highly modified, leaky capillaries found only in the liver, bone marrow, spleen, and adrenal medulla.
interweaving networks of capillaries
The flow of blood from an arteriole to a venule—that is, through a capillary bed
are formed when capillaries unite.
the smallest venules, consist entirely of endothelium around which pericytes congregate. They are extremely porous and fluid and white blood cells move easily from the bloodstream through their walls.
feeds the capillary bed
a vessel structurally intermediate between an arteriole and a capillary
intermediate between a capillary and a venule.
number 10 to 100 per capillary bed, depending on the organ or tissues served. They usually branch off the metarteriole
surrounds the root of each true capillary at the metarteriole and acts as a valve to regulate blood flow into the capillary
Vein Tunica Externa
is the heaviest wall layer, Consisting of thick longitudinal bundles of collagen fibers and elastic networks
are formed from folds of the tunica intima, and resemble the semilunar valves of the heart in both structure and function
such as the coronary sinus of the heart and the dural venous sinuses of the brain, are highly specialized, flattened veins with extremely thin walls composed only of endothelium.
interconnections where vascular channels unite
one arterial branch meets another supplying the same area.
anastomoses provide alternate pathways
the metarteriole–thoroughfare channel shunts of capillary beds that connect arterioles and venules.
is the volume of blood flowing through a vessel, an organ, or the entire circulation in a given period (ml/min).
Blood pressure (BP)
the force per unit area exerted on a vessel wall by the contained blood, is expressed in millimeters of mercury (mm Hg).
is opposition to flow and is a measure of the amount of friction blood encounters as it passes through the vessels.
friction encountered in the peripheral (systemic) circulation, well away from the heart
the internal resistance to flow that exists in all fluids and is related to the thickness or “stickiness” of a fluid.
Total blood vessel length
the relationship between total blood vessel length and resistance is straightforward: the longer the vessel, the greater the resistance.
is the pressure peak, averages 120 mm Hg in healthy adults. Blood moves forward into the arterial bed because the pressure in the aorta is higher than the pressure in the more distal vessels
the aortic valve closes, preventing blood from flowing back into the heart, and the walls of the aorta (and other elastic arteries) recoil, maintaining sufficient pressure to keep the blood flowing forward into the smaller vessels. During this time, aortic pressure drops to its lowest level (approximately 70 to 80 mm Hg in healthy adults).
Mean arterial pressure (MAP)
the pressure that propels the blood to the tissues. Diastole usually lasts longer than systole, so the MAP is not simply the value halfway between systolic and diastolic pressures. Instead, it is roughly equal to the diastolic pressure plus one-third of the pulse pressure.
MAP = diastolic pressure + (pulse pressure)/3
During inhalation, abdominal pressure increases, squeezing the local veins and forcing blood toward the heart. At the same time, the pressure in the chest decreases, entry into the right atrium.
consisting of skeletal muscle activity. As the skeletal muscles surrounding the deep veins contract and relax, they “milk” blood toward the heart, and once blood passes each successive valve, it cannot flow back.
The neural center that oversees changes in the diameter of blood vessels, a cluster of neurons in the medulla.
postganglionic visceral efferent fibers innervating the smooth muscles of vessel walls.
state of moderate constriction the arterioles are almost always in
neural receptors located in the carotid sinuses (dilations in the internal carotid arteries, which provide the major blood supply to the brain).
Carotid Sinus Reflex
slowing of the heart beat on pressure on the carotid artery at the level of the cricoid cartilage.
help maintain adequate blood pressure in the systemic circuit as a whole.
in the aortic arch and large arteries of the neck transmit impulses to the cardioacceleratory center, which then increases cardiac output, and to the vasomotor center, which causes reflex vasoconstriction. The rise in blood pressure that follows speeds the return of blood to the heart and lungs.
Adrenal medulla hormones
during periods of stress, the adrenal gland releases norepinephrine (NE) and epinephrine to the blood, NE has a vasoconstrictive action. Epinephrine increases cardiac output and promotes generalized vasoconstriction.
when blood pressure or blood volumes are low, the kidneys release the hormone renin. Renin acts as an enzyme, ultimately generating angiotensin II that stimulates intense vasoconstriction, promoting a rapid rise in systemic blood pressure
Atrial Natriuretic Peptide (ANP)
Causes blood volume and blood pressure to decline.
stimulates the kidneys to conserve water. It is not usually important in short-term blood pressure regulation, but when blood pressure falls to dangerously low levels (as during severe hemorrhage), much more ADH is released and helps restore arterial pressure by causing intense vasoconstriction.
pulse, blood pressure, respiratory rate and body temperature
pressure wave that is transmitted through the arterial tree.
clinically important arterial pulse points because they are compressed to stop blood flow into distal tissues during hemorrhage.
or low blood pressure, is a systolic pressure below 100 mm Hg.
high blood pressure.
Blood flow through body tissues
the automatic adjustment of blood flow to each tissue in proportion to the tissue’s requirements at any instant.
Nitric oxide (NO)
is a powerful vasodilator which acts via a cyclic GMP second-messenger system.
are among the most potent vasoconstrictors known.
refers to the dramatically increased blood flow into a tissue that occurs after the blood supply to the area has been temporarily blocked.
When muscles become active, blood flow increases
is the force exerted by a fluid pressing against a wall.
Capillary hydrostatic pressure (HPc)
the pressure exerted by blood on the capillary walls.
interstitial fluid hydrostatic pressure
acting outside the capillaries and pushing fluid in
Colloid osmotic pressure
the force opposing hydrostatic pressure, is created by the presence in a fluid of large nondiffusible molecules, such as plasma proteins, that are unable to cross the capillary wall.
which results from large-scale loss of blood, as might follow acute hemorrhage, severe vomiting or diarrhea, or extensive burns. If blood volume drops rapidly, heart rate increases in an attempt to correct the problem. Thus, a weak, “thready” pulse is often the first sign of hypovolemic shock.
blood volume is normal, but circulation is poor as a result of an abnormal expansion of the vascular bed caused by extreme vasodilation.
or pump failure, occurs when the heart is so inefficient that it cannot sustain adequate circulation.
complex network of blood vessels
routes blood through a long loop to all parts of the body before returning it to the heart
is the short loop that runs from the heart to the lungs and back to the heart.
Three important differences between systemic arteries and veins
Arteries run deep while veins are both deep and superficial.
Venous pathways are more interconnected.
The brain and digestive systems have unique venous drainage systems.
The endothelial lining of blood vessels is formed by mesodermal cells, which collect in little masses
Right & Left
Right & Left
Superior & Inferior