Vasculature: General Anatomy of Blood Vessels - Montana State University PDF

Vasculature Outline General anatomy of blood vessels Arteries Capillaries Veins Physiology of circulation Blood pressure Regulation of blood pressure Capillary exchange Venous return Circulatory shock General anatomy of blood vessels Three categories of b...

Vasculature Outline General anatomy of blood vessels Arteries Capillaries Veins Physiology of circulation Blood pressure Regulation of blood pressure Capillary exchange Venous return Circulatory shock General anatomy of blood vessels Three categories of blood vessels Arteries Veins Capillaries What's the difference between them? Direction of blood flow The pressure they withstand Histological structure of their walls General anatomy of blood vessels: vessel wall Arteries and veins are both composed of the 3 tissue layers Tunica interna Tunica media Tunica Externa Capillaries only have 1 layer Tunica interna  General anatomy of blood vessels: vessel wall Tunica interna: inner lining of vessel Creates the borders of the lumen Direct contact with blood Consists of simple squamous epithelium called the endothelium Acts as a selectively permeable barrier to materials entering or leaving the bloodstream Also secretes chemicals to prevent blood vessels and platelets from adhering Remember back to when we talked about platelets – break in vessel exposed rough interior to adhere to Also secretes chemicals that dilate or constrict blood vessels Important for regulating blood flow General anatomy of blood vessels: vessel wall Tunica media: middle layer Usually the thickest Consists of smooth muscle, collagen, and elastic tissue Strengthen the vessels Prevents blood pressure from rupturing the vessel Smooth muscle of this layer carries out vasoconstriction and vasodilation General anatomy of blood vessels: vessel wall Tunica externa: outermost layer Consists of loose connective tissue Often blends into surrounding tissues and anchors the blood vessels to them Anchors the vessel to adjacent tissues Provides passage for small nerves, lymphatic vessels, and other smaller blood vessels  Reminders before we take a closer look All vessels that carry blood away from the heart = arteries All vessels that carry blood back to the heart = veins Microscopic vessels that connect the smallest arteries to the smallest veins = capillaries Arteries: Qualities Resilient structure Each heartbeat causes a surge in pressure as blood is ejected into them More muscular than veins Retain round shape when empty 3 types are categorized by size Conducting (large) Distributing (medium) Resistance (small) Large arteries branch into smaller and smaller arteries  Arteries: Conducting (elastic or large) Includes the largest arteries Aorta Pulmonary trunk Can be up to an inch in diameter Tunica media is dominated by layers of perforated elastic sheets of tissue Smooth muscle and collagen are also present but are less visible An abundance of elastic tissues allows these arteries to expand during systole and recoil during diastole Elasticity protects smaller vessels downstream by reducing pressure surge during ejection from the heart Elastic recoil prevents blood pressure from dropping too low when the heart relaxes and propels blood Arteries: Distributing (muscular or medium) Smaller than elastic Distribute blood to specific organs Think of conducting arteries like an interstate highway and distributing arteries as exit ramps/state highways that serve individual towns Tunica media is dominated by smooth muscle Less elastic tissue than conducting Examples: femoral, brachial, or renal artery Serving specific “towns” (leg, arm, and kidney) Arteries: Resistance (small) Too variable in number and location for many of them to have individual names Tunica media is primarily smooth muscle Very little elastic tissue The smallest of them are known as arterioles Point of control over how much blood an organ or tissue receives  Capillaries For the blood to serve any purpose, materials like wastes, hormones, and nutrients must pass through the vessel wall to reach the tissues This occurs primarily in the capillaries Often called exchange vessels There are estimated to be about a billion capillaries in the body Enough to ensure each cell is within 4-6 cell widths of the nearest capillary  Capillaries Composed of an endothelium and a basement membrane Their walls are as thin as 0.2 𝜇m to 0.4𝜇m Range in diameter about 5𝜇m to 9𝜇m For comparison, the diameter of a strand of hair is ~50𝜇m Red blood cells are about 7.5𝜇m Stretch and elongate to fit Capillaries 3 types of capillaries Continuous Fenestrated Sinusoids Capillaries: Continuous Most common type Tubes of endothelial cells rolled up like a burrito and held together by tight junctions Gaps between cells called intercellular clefts Allows water and small solutes to pass through the cells Sometimes have pericytes Can differentiate into new endothelial cells and smooth muscle cells and can help with the repair of the capillaries or other vessels Capillaries: Fenestrated Capillaries with large filtration pores For comparison Intercellular clefts are about 4nm wide Filtration pores are 20nm to 200nm wide Allows much larger molecules to pass through Found in organs like the kidneys where rapid filtration is needed  Capillaries: Sinusoids Irregularly shaped, conforms to the shape of the surrounding tissue Passages are 30𝜇m to 40𝜇m wide Endothelial cells are separated by wide gaps Cells also have large pores through them Proteins and blood cells can pass through these pores Location they are found explains these large gaps and pores – examples: Liver How albumin, clotting factors, and other proteins made by the liver enter circulation Bone marrow How newly formed cells enter circulation  Capillary beds Capillaries are organized in capillary beds Webs of 10 to 100 vessels per arteriole Each capillary has a precapillary sphincter at the beginning Dialates to let blood into a capillary Constricts to prevent blood flow Blood flow through the capillary beds is regulated to match the metabolic needs of the tissues There isn’t enough blood in the body to fill the entire vascular system at once So ¾ are shut down at any given time  Veins Regarded as capacitance vessels Relatively thin-walled, flaccid, and expand easily to accommodate increased volume In other words, have a greater capacity for blood than arteries At rest, ~64% of blood is in systemic veins Only 15% is in the arteries Why are they so flaccid and accommodating?  Veins Distant from the ventricles of the heart, so subjected to lower pressure Their blood flow is steady rather than pulsating with the heartbeat as it does in the arteries Thus, veins do not require the muscular or elastic walls to withstand pressure surges Veins Small veins merge to form larger and larger veins 3 types of veins based on size Venules Medium veins Large veins Veins: Venules Receive blood from capillaries Range up to 1mm in diameter The smallest venules have no tunica media and are porous This is how most white blood cells leave the bloodstream to migrate into tissues  Veins: medium veins Range up to 10mm in diameter Have a thicker tunica media and externa compared to venules Most veins with individual names are in this category Ex: brachial veins Many have venous valves Under less pressure, so much of blood movement is reliant on skeletal muscle Valves prevent backflow More to come on this! Veins: large veins Have a diameter greater than 10mm Includes veins that empty into the heart Two vena cava and four pulmonary veins  Variations in circulatory routes Most common route Oxygenated blood leaves the heart via arteries Passes through the capillary bed Deoxygenated blood returns to the heart from veins Variation to this pattern is the portal system Blood flows through two capillary beds in a row before returning to the heart Portal systems are found where a substance is to be picked up by one capillary bed and immediately given off to another Stay tuned for more on this when we get to the digestive system! Anastomoses are another variation Routes in which the blood bypasses capillaries and goes directly from an artery to a vein one vein to another One artery to another Outline General anatomy of blood vessels Arteries Capillaries Veins Physiology of circulation Blood pressure Regulation of blood pressure Capillary exchange Venous return Circulatory shock  Physiology of circulation Blood flow is the amount of blood passing any given point Perfusion is the flow relative to a given mass of tissue Milliliters of blood per minute per 100 grams of tissue Thus, a small organ can have greater perfusion but lower flow than a larger organ Ex) pituitary glad could have higher perfusion but lower flow than the thigh muscle Blood flow is governed by the fundamental principles that govern the flow of any fluid – pressure, and resistance Blood pressure Blood pressure is the force exerted by blood on a vessel wall Blood always flows down a gradient from a point of high pressure to a point of lower pressure The greater the pressure difference the greater the flow Think of a garden hose, when you open the tap a little, you only get a trickle of water – when you open it wider, water gushes out Flow increases because you have increased the pressure difference between the beginning and the end of the hose  Blood pressure Arterial blood pressure is expressed as a ratio of systolic pressure and diastolic pressure Systolic pressure: generated by contraction (systole) of the left ventricle Diastolic pressure: the minimum BP falls when ventricles relax (diastole) Measured with a sphygmomanometer Expressed in millimeters of mercury (mmHg) A typical healthy adult is 120/80  Blood pressure Persistent high blood pressure is associated with increased risk of cardiovascular disease Blood pressure often rises in middle age and older individuals as arteries become stiff and less resilient Persistent resting blood pressure of 130/80 is considered hypertension Does not include temporary elevations due to emotions or exercise Chronic low blood pressure is called hypotension Could result from blood loss, dehydration, or inability to regulate blood pressure fluctuation in old age No specific number criteria for hypotension Blood pressure Determined by 3 principles Cardiac output Stroke volume x heart rate Blood volume Regulated mainly by kidneys Resistance to flow What we’ll talk about next!  Peripheral resistance Resistance = measure of difficulty of blood flow through a vessel caused by friction between moving fluid and the vessel walls Peripheral resistance = opposition to flow in vessels away from the heart As opposed to resistance encountered in the heart itself When resistance increases, flow decreases unless the heart pumps harder to compensate Resistance is determined by 3 principal variables Viscosity Vessel length Vessel radius Peripheral resistance: viscosity The “thickness” of blood Higher viscosity increases resistance and impedes flow Think sucking honey through a straw vs water What determines blood viscosity? Concentration of erythrocytes Concentration of albumin Peripheral resistance: vessel length The farther a liquid travels through a vessel, the more cumulative resistance it encounters Pressure and flow both decline with distance  Peripheral resistance: vessel radius The most important variable in flow Vessel length and blood viscosity don’t change from one minute to the next The only way to control peripheral resistance from moment to moment is by adjusting the radius of the blood vessels Vasoconstriction: narrowing of the vessel Vasodilation: widening of the vessel Achieved by contraction or relaxation of the smooth muscle in the tunica media Regulation of blood pressure and flow Perfusion of each organ depends on an ever-changing demand for blood Mechanisms controlling perfusion Local Neural hormonal Regulation of blood pressure and flow: Local control A tissue with a high metabolic rate has an increased need for oxygen and nutrients but also produces more by-products such as carbon dioxide, lactic acid, and others Vessels around these tissues dilate and perfusion increases to meet demands Result of local chemicals, like histamine and nitric oxide, released by platelets, endothelial cells, and connective tissue cells Long-term high metabolic demand causes angiogenesis – new blood vessels grow into tissue creating a denser capillary network Long-term muscular conditioning Cancerous tumors  Regulation of blood pressure and flow: neural control Blood vessel diameter is also regulated by the autonomic nervous system Vessels can have sensory nerve endings called baroreceptors Monitor blood pressure and send signals to the medulla oblongata of the brainstem When BP rises above normal, the vasomotor center of the medulla oblongata sends a signal to blood vessels to dilate This decreases peripheral resistance – less resistance to overcome – less pressure needed - BP falls When BP falls below normal, the vasomotor center sends a signal for vessels to constrict This increases peripheral resistance – more resistance to overcome – more pressure needed – BP rises These reactions are called baroreflexes  Regulation of blood pressure and flow: neural control Blood pressure is constantly adjusted to respond to a variety of circumstances Example: standing up quickly Gravity draws blood downward BP in the head and neck fall Baroceptors detect fall in BP The medulla responds by sending signals to constrict blood vessels and raise HR Maintaining blood flow to the brain and preventing you from fainting  Regulation of blood pressure and flow: hormonal control Hormones can affect blood pressure by constricting or dilating blood vessels Epinephrine and Norepinephrine From the adrenal medulla and sympathetic nerves, respectively Dialiate some vessels and constricts others Ex) dilates arteries of skeletal muscle and constricts arteries of the digestive tract and skin During exercise, prioritizes blood to muscles Angiotensin II Produced through collaborative action of the liver, kidneys, and lungs Potent vasoconstrictor that raises BP Angiotensin-converting enzyme (ACE) is needed to produce this hormone Often treat hypertension with ACE inhibitors which block this enzyme thus lowering angiotensin II levels and BP  Regulation of blood pressure and flow: hormonal control Other hormonal mechanisms affect blood pressure by regulating urinary loss of water which in turn affects blood volume Natriuretic peptides Secreted by the heart when BP is too high Stimulates the kidneys to secrete more sodium Water follows by osmosis and is excreted by the body – lowering blood volume and BP Aldosterone Promotes sodium and water retention Raises blood volume and BP Antidiuretic hormone Promotes water retention and there for raises blood volume and BP Capillary fluid exchange Capillaries are the “business end” of the cardiovascular system Allows for the exchange of materials – the entire point of the system Capillaries are the site of most exchange Facilitated by: Diffusion Filtration osmosis  Capillary fluid exchange: diffusion If a substance is more concentrated in the blood than surrounding tissue (or vice versa) and the substance is capable of crossing the vessel wall – the substance will move via diffusion Diffuse directly through the plasma membrane Oxygen Carbon dioxide Steroid hormones Diffuse through clefts between cells Glucose electrolytes Capillary fluid exchange: filtration Blood pressure forces fluid through the capillary wall carrying solutes with it Think of pour over coffee  Capillary fluid exchange: osmosis High concentration of sodium, protein, and erythrocytes in the blood Water moves from areas of low solute concentration to areas of high Thus, capillaries tend to absorb water from surrounding tissues Some solutes may be dissolved in the water and follow along the water into the blood This is called solvent drag  Capillary fluid exchange Near the arterial end of the capillary Primarily gives off fluid to tissues and delivers nutrients and oxygen Near the venous end Primarily absorbs fluid and picks up materials such as metabolic wastes and excess water Capillary exchange How does it do both? Blood entering the capillary from the arteriole is under higher pressure – filtration overrides osmosis so more fluid leaves the capillary than enters By the time blood reaches the venous end, lower pressure, so osmosis overrides pressure and absorbs more fluid than it gives off  Venous return After blood has exchanged materials in the capillary beds, it’s time to return to the heart Flow back to the heart is called venous return Factors contributing to venous return Gravity Aids and opposes Anything above the heart – gravity helps anything below the heart – gravity opposes Blood pressure Much less pressure than in arteries but still important Not enough to move blood against gravity Skeletal muscle pump Thoracic pump  Venous return: skeletal muscle pump As muscles of limbs contract and relax, they squeeze blood vessels and promote the flow of blood Medium veins in limbs have one-way valves pointing upward As muscles contract, forces blood up As muscles relax, valves prevent the backflow of blood  Venous return: thoracic pump When you inhale, your chest expands Pressure in the thoracic cavity drops below pressure in the abdominal cavity The pressure difference squeezes the abdominal portion of the inferior vena cava, pushing blood blood up Think of squeezing the bottom of a ketchup bottle The thoracic pump and skeletal pump are part of why exercise and deep breathing are good for circulation  Circulatory shock If the heart fails to pump enough blood to meet the demands of the organs = circulatory shock This can be the result of major blood loss, tumors compressing vessels and blocking blood flow, venous pooling Venous pooling can result from Allergic reactions which cause widespread vasodilation Standing still too long – skeletal muscle pump not active This can lead to syncope (fainting) Most of the time, the body can recover from circulatory shock, but in severe cases, it can lead to death