Learning Objectives - Blood Vessels
Describe the three layers that typically form the wall of a blood vessel, and state the
function of each.
Tunica Intima - the innermost tunic, in intimate contact with the blood in the lumen. This tunic contains the endothelium, the simple squamous epithelium that lines the lumen of all vessels. The endothelium is continuous with the endocardial lining of the heart, and its flat cells fit closely together
In vessels larger than 1 mm in diameter, a subendothelial layer, consisting of a basement membrane and loose connective tissue, supports the endothelium.
forms a slick surface that minimizes friction as blood moves through the lumen.
Tunica Mediais - the middle tunic, mostly circularly arranged smooth muscle cells and sheets of elastin, the bulkiest layer in arteries
chief responsibility for maintaining blood pressure and continuous blood circulation. The activities of the tunica media are critical in regulating circulatory dynamics because small changes in vessel diameter greatly influence blood flow and blood pressure.
Tunica Externa the outermost layer is composed largely of loosely woven collagen fibers
protect and reinforce the vessel, and anchor it to surrounding structures
Define vasoconstriction and vasodilation.
•Vasoconstriction - reduction in lumen diameter as the smooth muscle contracts
•Vasodilation - increase in lumen diameter as the smooth muscle relaxes
Compare and contrast the structure and function of the three types of arteries.
•Elastic Arteries – called conducting, thick-walled arteries near the heart—the aorta and its major branches. These arteries are the largest in diameter, ranging from 2.5 cm to 1 cm, and the most elastic
Large lumens make them low-resistance pathways that conduct blood from the heart to medium-sized arteries. They are relatively inactive in vasoconstriction, they can be visualized as simple elastic tubes.
Elastic arteries are pressure reservoirs, expanding and recoiling as blood is ejected from the heart. Consequently, blood flows fairly continuously
•Muscular Arteries – distributing arteries, distally the elastic arteries give way to the muscular, or distributing, arteries, which deliver blood to specific body organs and account for most of the named arteries studied in the anatomy laboratory. Their internal diameter ranges from that of a little finger (1 cm) to that of a pencil lead (about 0.3 mm). Muscular arteries have the thickest tunica media of all vessels. Their tunica media contains relatively more smooth muscle and less elastic tissue than do elastic arteries
active in vasoconstriction and less distensible.
•Arterioles - smallest of the arteries, Larger arterioles have all three tunics, but their tunica media is chiefly smooth muscle with a few scattered elastic fibers. Smaller arterioles, which lead into the capillary beds, are little more than a single layer of smooth muscle cells spiraling around the endothelial lining.
Describe the structure and function of a capillary bed.
The microscopic capillaries are the smallest blood vessels. Their exceedingly thin walls consist of just a thin tunica intima. In some cases, one endothelial cell forms the entire circumference of the capillary wall.
Continuous 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
Fenestrated Capillaries are similar to the continuous variety except that some of the endothelial cells in fenestrated capillaries are riddled with oval pores, or fenestrations The fenestrations are usually covered by a delicate membrane, or diaphragm (probably condensed basal lamina material), but even so, fenestrated capillaries are much more permeable to fluids and small solutes than continuous capillaries are.
Sinusoids Capillaries - are highly modified, leaky capillaries found only in the liver, bone marrow, spleen, and adrenal medulla.
•Terminal Arteriole - feeds the capillary bed
•Metarteriole - a vessel structurally intermediate between an arteriole and a capillary
•True Capillaries number 10 to 100 per capillary bed, depending on the organ or tissues served. They usually branch off the metarteriole
•Precapillary Sphincter - surrounds the root of each true capillary at the metarteriole and acts as a valve to regulate blood flow into the capillary
•Thoroughfare Channel - intermediate between a capillary and a venule.
•Venules – are formed when capillaries unite.
Postcapillary Venules – 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.
The larger venules have one or two layers of smooth muscle cells (a scanty tunica media) and thin externa as well.
•exchange of materials (gases, nutrients, hormones, and so on) between the blood and the interstitial fluid
Describe the structure and function of veins, and explain how veins differ from arteries.
•Veins usually have three distinct tunics, but their walls are always thinner and their lumens larger than those of corresponding arteries
•There is relatively little smooth muscle or elastin in the tunica media, which is poorly developed and tends to be thin even in the largest veins. The tunica externa is the heaviest wall layer. Consisting of thick longitudinal bundles of collagen fibers and elastic networks, it is often several times thicker than the tunica media. In the largest veins—the venae cavae the tunica externa is further thickened by longitudinal bands of smooth muscle.
•With their large lumens and thin walls, veins can accommodate a fairly large blood volume. Veins are called capacitance vessels and blood reservoirs because up to 65% of the body’s blood supply is found in the veins at any time
•Venous valves - are formed from folds of the tunica intima, and resemble the semilunar valves of the heart in both structure and function
Define blood flow, blood pressure, and resistance, and explain the relationships between these factors.
Blood flow - 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).
Resistance - is opposition to flow and is a measure of the amount of friction blood encounters as it passes through the vessels.
Blood flow (F) is directly proportional to the difference in blood pressure (ΔP) between two points in the circulation, that is, the blood pressure, or hydrostatic pressure, gradient. Thus, when ΔP increases, blood flow speeds up, and when ΔP decreases, blood flow declines. Blood flow is inversely proportional to the peripheral resistance (R) in the systemic circulation; if R increases, blood flow decreases.
These relationships are expressed by the formula F=∆P/R
Describe how blood pressure differs in the arteries, capillaries, and veins.
•Any fluid driven by a pump through a circuit of closed channels operates under pressure, and the nearer the fluid is to the pump, the greater the pressure exerted on the fluid. The dynamics of blood flow in blood vessels is no exception, and blood flows through the blood vessels along a pressure gradient, always moving from higher- to lower-pressure areas.
Arteries must be able to endure high pressure of blood, from the aorta to the ends of the arterioles is about 60 mm Hg
Capillaries are fragile and high pressures would rupture them, and most capillaries are extremely permeable and thus even the low capillary pressure forces solute-containing fluids (filtrate) out of the bloodstream into the interstitial space.
Venous blood pressure is steady and changes very little during the cardiac cycle. The pressure gradient in the veins, from venules to the termini of the venae cavae, is only about 15 mm Hg
•Arteries receive blood from the heart, the blood they receive is under a lot of pressure. Can you picture how much pressure the arteries near the heart (like the pulmonary trunk and the aorta) have to withstand every time the ventricles squirt out a load of blood? At the same time, this pressure helps the blood move through the arteries-- even when the arteries are running in opposition to gravity
•The veins only receive the blood after it has travelled quite far from the heart. The blood pressure in the veins is thus much less; the blood is certainly much less likely to burst through walls of the veins than arteries. Also, because the blood pressure is small in the veins, it is not going to be enough to return all that blood to the heart; in fact, the blood could easily back up or collect in these vessels
List and explain the factors that influence blood pressure, and describe how blood
pressure is regulated.
•Arterial blood pressure reflects two factors:
How much the elastic arteries close to the heart can be stretched (their compliance or dispensability)
The volume of blood forced into them at any time. If the amounts of blood entering and leaving the elastic arteries in a given period were equal, arterial pressure would be constant. Blood pressure rises and falls in a regular fashion in the elastic arteries near the heart.
•Central among the homeostatic mechanisms that regulate cardiovascular dynamics are those that maintain blood pressure, principally cardiac output, peripheral resistance, and blood volume. Cardiac output (blood flow of the entire circulation) and peripheral resistance relate to blood pressure.
o F = ΔP/R
o CO = ΔP/R
o ΔP = CO X R
•blood pressure varies directly with CO and R.Additionally, blood pressure varies directly with blood volume because CO depends on blood volume (the heart can’t pump out what doesn’t enter its chambers). So in theory, a change (increase or decrease) in any of these variables would cause a corresponding change in blood pressure. However, what really happens in the body is that changes in one variable that threaten blood pressure homeostasis
Define hypertension. Describe its manifestations and consequences.
•Hypertension - high blood pressure, condition of sustained arterial pressure of 140/90 or higher, and the higher the pressure, the greater the risk for serious cardiovascular problems.
Heredity. Hypertension runs in families. Children of hypertensive parents are twice as likely to develop hypertension as are children of normotensive parents, and more blacks than whites are hypertensive. Many of the factors listed here require a genetic predisposition, and the course of the disease varies in different population groups.
Diet. Dietary factors that contribute to hypertension include high intakes of salt (NaCl), saturated fat, and cholesterol and deficiencies in certain metal ions
Age. Clinical signs of the disease usually appear after age 40.
Stress. Particularly at risk are “hot reactors,” people whose blood pressure zooms upward during every stressful event.
•strains the heart and damages the arteries.Prolonged hypertension is the major cause of heart failure, vascular disease, renal failure, and stroke. Because the heart is forced to pump against greater resistance, it must work harder, and in time the myocardium enlarges.When finally strained beyond its capacity to respond, the heart weakens and its walls become flabby. Hypertension also ravages the blood vessels, accelerating the progress of atherosclerosis the vessels become increasingly blocked, blood flow to the tissues becomes inadequate and vascular complications appear in the brain, heart, kidneys, and retinas of the eyes.
Explain how blood flow is regulated in the body in general and in specific organs.
•Autoregulation is the automatic adjustment of blood flow to each tissue in proportion to its needs, and is controlled intrinsically by modifying the diameter of local arterioles.
•Metabolic controls of autoregulation are most strongly stimulated by a shortage of oxygen at the tissues.
•Myogenic control involves the localized response of vascular smooth muscle to passive stretch.
•Long-term autoregulation develops over weeks or months, and involves an increase in the size of existing blood vessels and an increase in the number of vessels in a specific area, a process called angiogenesis.
Outline factors involved in capillary dynamics, and explain the significance of each.
•Vasomotion, the slow, intermittent flow of blood through the capillaries, reflects the action of the precapillary sphincters in response to local autoregulatory controls.
•Capillary exchange of nutrients, gases, and metabolic wastes occurs between the blood and interstitial space through diffusion.
•Hydrostatic pressure (HP) is the force of a fluid against a membrane.
•Colloid osmotic pressure (OP), the force opposing hydrostatic pressure, is created by the presence of large, nondiffusible molecules that are prevented from moving through the capillary membrane.
•Fluids will leave the capillaries if net HP exceeds net OP, but fluids will enter the capillaries if net OP exceeds net HP.
Define circulatory shock. List several possible causes.
Circulatory shock is any condition in which blood volume is inadequate and cannot circulate normally, resulting in blood flow that cannot meet the needs of a tissue
•Hypovolemic shock results from a large-scale loss of blood, and may be characterized by an elevated heart rate and intense vasoconstriction.
•Vascular shock is characterized by a normal blood volume, but extreme vasodilation, often related to a loss of vasomotor tone, resulting in poor circulation and a rapid drop in blood pressure.
•Transient vascular shock is due to prolonged exposure to heat, such as while sunbathing, resulting in vasodilation of cutaneous blood vessels.
•Cardiogenic shock occurs when the heart is too inefficient to sustain normal blood flow, and is usually related to myocardial damage, such as repeated myocardial infarcts
Trace the pathway of blood through the pulmonary circuit, and state the importance of this special circulation.
Iron poor blood returns from the systemic circuit to the right atrium, pumped through the tricuspid valve into the right ventricle. The right ventricle pumps the blood through the pulumonary valve into the pulmonary trunk to the lungs. Carbon dioxide is exchanged is released and oxygen is picked up in the lungs. The blood then moves through the pulmonary veins and into the left atrium to be pumped through the systemic system.
Describe the general functions of the systemic circuit.
The general function of the systemic system is to pump oxygenated blood to all the tissues of the body. Pick up cell waste in the tissue and return it to the heart.
Name and give the location of the major arteries and veins in the systemic circulation.
Aorta supplies the upper body, arches and descends past the diaphragm into the abdominal aorta supplying the lower body.
Common carotid Arteries supplies the head
Subclavian Arteries supplies the upper limb
Internal iliac arteries branches into the femoral arteries that supplies the lower limb
Femoral veins drains into the External Iliac veins to drain the lower limbs
Inferior Vena Cava drains the lower body
Cephalic veins and the basilic veins drain into the subclavian veins to drain the upper limb
Internal and External jugular veins drain the head
Superior Vena Cava drains the upper body into the heart
Describe the structure and special function of the hepatic portal system.
The hepatic portal system carries nutrient-rich blood (which may also contain toxins and microorganisms) from the digestive organs to the liver; where it can be “treated”before it reaches the rest of the body. As the blood percolates slowly through the liver sinusoidal capillaries, hepatocytes process nutrients and toxins, and phagocytic cells rid the blood of bacteria and other foreign matter
Provide examples of changes that often occur in blood vessels as a person ages
•Blood pressure changes with age: The arterial pressure of infants is about 90/55, but rises steadily during childhood to an average of 120/80, and finally increases to 150/90 in old age
•vascular disease increases with age, leading to varicose veins, tingling in fingers and toes, and muscle cramping
•Atherosclerosis begins in youth, but rarely causes problems until old age