Chapter 20 - Vessels and Circulation
"Anastomosis"
"A cross-connection between adjacent channels, tubes, fibers, or other parts of a network." -communication between blood vessels by means of collateral channels when usual routes are obstructed. -Blood vessels can change, adapt and grow, usually as a result of injury, blockage, or weight gain and are therefore ultimately unique to individuals
Describe hypertension, including causes, symptoms, and implications.
"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. -Causes: *Heredity* Hypertension runs in families. *Diet* Dietary factors that contribute to hypertension include high intakes of salt (NaCl), saturated fat, and cholesterol and deficiencies in certain metal ions *Obesity* This is obvious *Age* Clinical signs of the disease usually appear after age 40. *Diabetes mellitus* A group of diseases that result in too much sugar in the blood (high blood glucose). *Stress* Particularly at risk are "hot reactors," people whose blood pressure zooms upward during every stressful event. *Smoking/Nicotine* Yeah don't do this -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.
Differences between Arteries and Veins
(1) *Direction of Bloodflow:* Arteries carry blood from the heart to the rest of the body, whereas veins carry blood from the rest of the body back to the heart. (2) *Oxygen:* Almost all arteries carry oxygenated blood and almost all veins carry deoxygenated blood. The only exceptions are the pulmonary artery, which carries deoxygenated blood from the heart to the lungs, and the pulmonary vein, which carries oxygenated blood from the lungs to the heart. (3) *Thick Vs Thin:* Arteries have a thick elastic muscle layer, whereas the muscle layer for veins is much thinner. This is because the heart pumps blood into the arteries at high pressures, so the walls of the arteries must be able to cope with the changes in pressure during a heartbeat. Veins carry blood at much lower pressures so do not need such a thick wall. (4) *Diameter and Pressure:* Arteries have a much narrower lumen (the hole at the center that the blood flows through) than veins. This helps keep higher blood pressures in the arteries, which is needed to keep blood flowing quickly to body tissues. The endothelium of arteries also fold when they are constricted while this does not happen in veins. Veins have "Capacitance" the ability to distend even at low pressure (5) *Pressure and Backflow:* Veins have valves and arteries do not. In arteries, blood flows in the right direction because of the heart pumping it forwards at high pressures. The lower blood pressure in veins means that valves are needed to stop blood flowing backwards (for example, in veins in the legs, blood needs to flow upwards against the pull of gravity). The higher pressure of the arteries means that they are usually set deeper within the body and injuries or cuts to arteries are typically much more severe. (6) *Effects of Muscle Contraction* The muscle layer surrounding veins cause the otherwise flat veins to extend and become round and open when they are flexed. The opposite is true of arteries. Arteries are naturally rounded and open and their muscle layer causes vasoconstriction to narrow the diameter of the vessel.
From Artery to Capillary...
(1) *Elastic Arteries* These are the major large and tough arteries of the body. (Aorta, Pulmonary Trunk & Arteries, Carotid Arteries...) They're designed to deal with a high amount of blood pressure going through them (2) *Muscular Arteries* Slightly smaller arteries compared to Elastic Arteries. These have a thickened muscular layer designed to change the size of the arteries to fit the body's needs (3) *Arteriole* (Arteriole don't have a Tunica Externa) These are simple smaller abundant branches of arteries that spread into Continuous Capillaries. (4) *Capillary Bed* These are small vessels consisting of only endothelial cells. Some capillaries are so thin that only one red blood cell can flow through single file at a time. The way that nutrients and gases exit the capillaries is through the borders of these endothelial cells that make up the walls of the capillaries
From Capillary to Vein...
(1) *Fenestrated Capillary* (Meaning "Windowed Capillary) These have small holes within the endothelial cells which help increase the rate that water and solutes or little charged ions enter and exit. (2) *Venule* Essentially tiny veins with much thinner muscular layer. (3) *Medium-Sized Vein* Bigger vein with bigger muscle layer (4) *Large Vein* Even larger veins with larger muscle layers, although it is important to remember that while both feature a solid thick tunica externa layer on the outside all veins have a significantly smaller muscle layer compared to arteries.
Types of Blood Vessels
*Arteries* Takes blood away from the heart. These are thick, strong, and handle high pressure. *Arterioles* "Mini-Arteries" that branch off of arteries *Capillaries* The smallest blood vessels that bridge arterioles and venules. They physically take the blood (and the oxygen and nutrients inside of the blood) to the cells that need them. *Venules* Capillaries eventually all merge back together to form "Venules" These take deoxygenated blood back into the larger veins that take the blood back into the heart. *Veins* Also very large vessels although structurally are a bit different from arteries.
Regulation of Blood Vessels
*Baroreceptor Reflexes* -Carotid Sinuses -Aortic Sinuses (You have to ability to detect the amount of pressure going through these arteries and communicate with your brain to balance the pressure.) Some of the ways the brain can help regulate blood pressure in response are as follows: -*Antidiuretic Hormone* (AHD) Retains body fluids to increase blood volume if pressure were to be too low. -*Angiotensin II* This raises blood pressure by a number of actions; vasoconstriction, sympathetic nervous stimulation, increased aldosterone production, renal actions and more. -*Erythropoietin* (EPO) Hormone that stimulates the making of RBCs. This can increase the thickness of the blood which would make it more viscous and resistant if necessary.
Explain blood flow, blood pressure, and resistance, and relate each to the other both figuratively and mathematically.
*Blood flow* -Volume of blood flowing through a vessel, tissue, or organ, (ml/min). -Volume / time -Down a pressure gradient -Resistance -Blood pressure = hydrostatic --Systemic arterial --Usually measured on brachial artery *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* the opposition to flow and is a measure of the amount of friction blood encounters as it passes through the vessels. *Blood flow (BF)* 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 *Poiseuille's equation in a nutshell:* -Blood flow = Δ P/ Resistance -Resistance = Δ P/ Blood flow -Poiseuille's original equation related 3 variables: --viscosity --vessel length --radius. Resistance = 8ηλ / πr4 η = viscosity λ = vessel length, and r = vessel radius -Viscosity and vessel length change slowly -Radius values can be changed rapidly (150 lbs = 60,000 miles of vessels! Every 10 lbs extra adds 2 - 4K miles Slow change) -Even small changes in r will dramatically impact flow -Viscosity is directly proportional to resistance and indirectly proportional to flow. -Length is directly proportional to resistance due to increased surface area -Diameter is inversely proportional to resistance --Vascular tone is primary determinant --May change frequently throughout the day
Relate the structure and function of the three types of arteries.
*Elastic Arteries* -Conducting Arteries -Closest to the heart -High percentage of elastic fibers = Strong Recoil -Thick Walled -These arteries are the largest in diameter -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. They're pressure reservoirs, expanding and recoiling as blood is ejected from the heart. Consequently, blood flows fairly continuously *Muscular Arteries* -Distributing Arteries -Vasoconstrictions -Decreased elastic = low ability to expand -No "Line of demarcation" (Dividing line) -Distally the elastic arteries give way to the muscular arteries, which deliver blood to specific body organs and account for most of the named arteries studied in lab. 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 elastic arteries. *Arterioles* -Lead to capillaries -Very thin layers -Resistance vessels -Vascular tone -Primary site of pressure regulation -These are the 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 (shaped like rings surrounding the vessel).
Blood Vessel Conditions/Disorders
*Hyper/Hypotension* High blood pressure / Low pressure -Chronically elevated blood pressure (140/90 or greater) -"Silent Killer" -Can lead to heart attack, stroke, aneurism, peripheral arterial disease, kidney disease *Hypovolemia* -Low blood volume -Bleeding, dehydration, vomiting, severe burns, hypertension meds -10-20% blood loss *Hypervolemia* -Excessive fluid volume -Water, sodium retention -Heart failure, liver disease, kidney disease, hyperaldosteronism, steroid treatments *Arteriosclerosis* -Hardening and lessening of the diameter of a blood vessel. -Reduced compliance -Endothelial injury caused by --High glucose --Infection --Tobacco --Excessive lipids -Management: --Lifestyle changes --Balloon/Stent Angioplasty --Endarterectomy (Surgery to remove plaque) --Coronary bypass *Atherosclerosis* fatty deposits that can clog arteries. These buildups are called plaque. They're made of cholesterol, fatty substances, cellular waste products, calcium and fibrin (a clotting material in the blood). Sometimes compared to a plumbing problem however that's not a perfect comparison because buildups don't just form on artery walls but inside them. As plaque builds up, an artery wall gets thicker. This narrows the opening, reducing blood flow and the supply of oxygen to cells. This can ultimately cause a heart attack or death, but can be prevented by proper diet and exercise *Edema* (Symptom, not a disease) -Pooling of blood leads to increase in interstitial fluid due to pressure -Hypertension, heart failure, renal failure -Can be detected by pressing on the skin and seeing how deep and long the compression mark with stay visible. (0mm = no pitting edema, 4mm = severe pitting edema) *Aneurysm* A slight tear or break in the wall of the lumen into the other layers of the blood vessel. This causes a pooling bulge inside the walls of the vessel which gradually weakens it and can burst which causes severe damage. It doesn't come with a lot of symptoms even though it can be lethal if it forms on a critical vessel. *Hemorrhage* -Uncontrolled blood loss -Increases cardiac output, vasoconstriction -Endocrine response to try and restore volume -Internal bleeding (A burst aneurysm could also lead to a hemorrhage.) *Thrombus* Formation of a blood clot that doesn't disintegrate overtime blocking off the blood flow. If the thrombus gets dislodged it becomes whats called an "Embolism" which is even more dangerous and potentially lethal *Pulmonary Embolism* If that dislodged Embolism makes its way into part of the lungs that can kill you. *Varicose Veins* -Defective valves allow for blood accumulation -Veins distend and twist -Can be painful -Worsen over time This results in a positive feedback loop, only getting worse as time goes on unless medically treated. Compression socks or not standing in one place for long periods of time help with this. *Hemorrhoids* Essentially varicose veins located in the anus, caused by straining on those muscles. Drink more water, eat more fiber, and chilling out when you're taking a dump will all help prevent hemorrhoids. Surgery can fix this but its not ideal. *Angiogenesis* The forming of blood vessels when you have a tumor developing within an organ secreting certain chemicals that encourage angiogeneis which helps the tumor grow.
Name and describe the locations of all major arteries and veins in systemic circulation.
*Major Arteries* 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 *Major Veins* 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
Distribution of Blood
*Systemic Venous Sytem = 64%* - Venules = 25% - Veins = 39% *Systemic Arterial System = 13%* - Aorta, Arteries, Arterioles *Pulmonary Circuit = 9%* - All Arteries, Capillaries, and Veins associated with the Lungs *Heart = 7%* *Systemic Capillaries = 7%*
Compare and contrast the structures and functions of the vessel wall layers.
*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 Media* - 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 and describe vasoconstriction and vasodilation.
*Vasoconstriction* - reduction in lumen diameter as the smooth muscle contracts *Vasodilation* - increase in lumen diameter as the smooth muscle relaxes
Capillary Beds
-A vast network of capillaries between arterioles and venules. These tiny little vessels are what allow nutrients and gases to exit and the wastes to come in. -Precapillary sphincters help regulate flow: Little muscular doorways leading to different open and close to regulate flow. This can be used to prioritize certain body functions of others, such as skeletal muscles over your digestive organs to allow for fight or flight. *Venous System:* -Diameter increases from smaller to larger, but cross sectional area decreases -From venules to veins, pressure decreases but velocity increases -Any action increasing venous flow will increase venous return -Unlike arterial constriction, venous constriction increases flow --Smooth muscle contraction = more rounded lumen = decreased resistance --Venoconstriction = increased blood return to heart = increased preload / stretch *Capillary Exchange:* -Bulk Flow: mass movement of fluids through capillary beds and tissues -Filtration: movement from higher pressure in capillary to lower pressure in tissues -Reabsorption: movement from higher pressure in tissues to lower pressure in capillaries -Hydrostatic pressure: pressure of any fluid enclosed in a space; force exerted on vessel walls -Osmotic pressure: pressure driving reabsorption; draws fluid back into capillary; determined by osmotic gradients --Plasma proteins, not formed elements, determine osmotic gradient -Lymph capillaries pick up excess and return recycled blood plasma to circulation
Describe the factors that influence blood pressure, including ways it 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. -- F = ΔP/R -- CO = ΔP/R -- 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 Effects of Exercise on Vascular Homeostasis: *Heart* -Increases cardiac output, size, mass, and efficiency of the heart *Tissues* -Perfusion increases *Vessels* -Venous return increases, so preload rises and heart rate is lowered -Decreases plaque formation and lowers cholesterol
Describe some changes that 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
Describe how blood flow is regulated systemically as well as in specific organs.
Blood flow is continuously reallocated depending upon tissue needs. 3 mechanisms ensure adequate blood flow, pressure, distribution, and perfusion: -Neural -Endocrine -Autoregulatory *Autoregulation* 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.
Vessel Structure
Both arteries and veins have these layers, although the relative size of these layers are different in the veins and arteries. Shared Structures: -Lumens -Walls of living cells and connective tissue -Three distinctive layers (called tunics) *Tunica Intima* (Innermost Layer) -Epithelial and connective layers --Lined with endothelium --Endothelium continuous throughout vascular system --Endothelins can constrict vessels -Basal lamina --Binds endothelium to connective tissue --Strength, flexibility, permeability -Internal elastic membrane --Large arteries only --Tunica media boundary *Tunica Media* (Middle Layer) -Thick, smooth muscle and connective tissue --Collagen/Elastin --Circular elastic fibers --Vasocontstriction --Vasodilation --Nervi vasorum --Longitudinal muscle -External elastic membrane --Separates media from externa (Enables constriction and dialation to adjust blood flow) *Tunica Externa* (Outer layer) -Connective Tissue -Mostly collagen with some elastic --Thickest layer in veins and some arteries --Outer layer contiguous with connective tissue for stability (gives sturdy protection and anchors the vessel securely to the outside tissues)
Describe circulatory shock and discuss its significance.
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* 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* 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
Blood Pressure
Heart rate is measured in two numbers: Systolic/Diastolic: -Systolic: ventricular contraction -Diastolic: ventricular relaxation -Pulse Pressure: difference b/t systolic and diastolic --Should be 25% of systolic -Mean Arterial Pressure: "average" pressure of arterial blood (Blood pressure is a measurement of the pressure the ventricles producing at peak contraction // over the lowest the pressure gets when relaxed) -Blood pressure is equal to "Cardiac Output" multiplied by "Peripheral Resistance" --Cardiac Output = Stroke Volume x Heart Rate --Peripheral Resistance = The resistance determined by vessel diameter/length, blood viscosity, and potential drug/hormone influences. *Normal blood pressure* should be about 120/80 Which is a measure of the range of pressure in an artery. You do this by inflating the Sphygmomanometer (Cuff) around an artery to temporarily cut off the blood flow, then you slowly release the air out listening for the the first time you hear a heart beat, and what ever number the needle shows when you first hear this heart beat is the first (Systolic) number. This is the amount of pressure it took for the blood to overtake the cuffs pressure. You continue to listen as the pressure decreases even more inside the cuff and the last moment you hear a heartbeat is the second (Diastolic) number. The number between the Systolic and Diastolic numbers (Usually .about 40) tells doctors a lot about your blood pressure and whats going on inside your circular system These are referred to as the *Sounds of Korotoff:* turbulent blood flow; 1st sound = systolic 2nd sound = diastolic *Pulse:* (expansion and recoil from elastic fibers in arteries) -Diminishes over distance from heart -Measures HR -Strength indicative of ventricular contraction and cardiac output -Taken from temporal, facial, common carotid, brachial, radial, femoral, popliteal, posterior tibial, and dorsalis pedis artery. *Variables Affecting Blood Flow and Blood Pressure:* -Cardiac output -Volume of blood -Viscosity of blood -Blood vessel length and diameter -"Compliance" (ability to expand to accommodate increased content) -Veins are more compliant than arteries --Decreased compliance = decreased blood flow
Describe blood's pathway through the pulmonary circuit and discuss the implications of this circuit.
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.
Return of Blood Through the Veins
Since veins don't have a thick muscular layer, and have considerable less blood pressure compared to arteries, how do they effectively return blood back to the heart? *Valves* These prevent backflow inside the veins, which is especially important for veins that have to fight against the forces of gravity all the way up to the heart. *Skeletal Muscle Contractions* Veins tend to be located next to skeletal muscle, so when you contract/move your arms or legs around you squeeze and push the blood back into the heart. This is why staying active and exercising is so important. Blood can pool and not circulate properly if you remain stagnant which can result in soreness, clots, edema, varicose veins, or many other problems Venous pressure must exceed atrial pressure; 2 factors help maintain the pressure gradient (1) Atrial Pressure -Low during diastole (2) Physiologic "pumps" -Skeletal muscle --One-way valves -Respiratory --Increase in thoracic volume = lower pressure and increased flow into thoracic veins
The Cardiovascular System
Structure and Function of Vessels Artery - away from the heart Vein - towards heart Vascular transport supports life by distributing and facilitating the exchange of gases, nutrients, and other substances with body cells. Vascular impairment can result in tissue injury, impaired metabolism, and system failure. *Systemic Circuit* -Oxygen-rich blood to body tissues -Oxygen-poor blood from tissues to right atrium -64% of blood volume *Pulmonary Circuit* -Oxygen-poor blood from systemic circulation to lungs for gas exchange -Oxygen-rich blood to left atrium
Starling's Law
The Frank-Starling law of the heart states that: "The stroke volume of the heart increases in response to an increase in the volume of blood filling the heart (the end diastolic volume) when all other factors remain constant"
Describe unique aspects of fetal circulation.
The fetal circulation has several unique features. The fetus uses the placenta as lungs. The umbilical blood vessels carry blood between the placenta and the fetus. Three special structures are the ductus venosus, foramen ovale, and ductus arteriosus. *Hemangioblasts* differentiate into angioblasts, which give rise to formed elements and blood islands. *Angiogenesis* creation of new vessels from existing ones; follows nerve development
Discuss the 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.
Discuss 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
Describe the structure and function of a capillary bed and their surrounding structures.
The microscopic capillaries are the smallest blood vessels. Their very thin walls consist of just a thin tunica intima. In some cases, one endothelial cell forms the entire circumference of the capillary wall. The cite of exchange of materials (gases, nutrients, hormones, and so on) between the blood and the interstitial fluid Capillaries - Perfusion, Microcirculation *Continuous Capillaries* -Most vascular tissue -Complete endothelium -Tight junctions -Transport vesicles (Brain is exception) -Blood-brain barrier -Most common capillary -Abundant in the skin and muscles They are continuous in the sense that their endothelial cells provide an uninterrupted lining, adjacent cells being joined laterally by tight junctions *Fenestrated Capillaries* -Pores (Fenestrations) in addition to tight junctions -Permeable to larger molecules -Occurs in small intestine, kidneys, choroid plexuses are similar to continuous except that some of the endothelial cells are riddled with oval pores The fenestrations are usually covered by a delicate membrane, or diaphragm. These capillaries are much more permeable to fluids and small solutes than continuous capillaries are. *Sinusoids Capillaries* -Highly modified -Most permeable (leaky capillaries) -Located in liver, bone marrow, spleen, adrenal medulla. *Terminal Arteriole* -feeds the capillary bed *Metarteriole* -Similar to both arterioles and capillaries -Tunica media discontinuous; rings of smooth muscle sphincters -Lead to capillary beds --Diapedesis --Perfusion (a vessel structurally intermediate between an arteriole and a capillary) *True Capillaries* -Usually 10-100 per capillary bed. -Branch off the metarteriole *Precapillary Sphincter* -Surrounds the root of each true capillary at the metarteriole -Acts as a valve to regulate flow into the capillary *Thoroughfare Channel* -Intermediate -Between a capillary and a venule.
