Capillary Structure and Function

• Systemic arteries supply blood rich in oxygen to body tissues
• Systemic veins return blood with less oxygen to the heart
• Pulmonary arteries carry blood low in oxygen only to the lungs for gas exchange
• Pulmonary veins return oxygenated blood from the lungs to the heart to be pumped back into systemic circulation

Shared Structures

• Blood vessels create a closed system of tubes
• They carry blood from the heart and transport to body tissues
• Blood then returns to the heart, is pumped to oxygenate in the lungs, and is transported back to the tissues
• The hollow passageway for blood flow in vessels is the lumen
  • Arteries carry blood from the heart to tissues
  • Arterioles are small arteries that connect to capillaries
  • Capillaries exchange substances between blood and body tissues
  • Venules connect capillaries to larger veins
  • Veins convey blood from tissues back to the heart
• Arteries carry blood away from the heart
• Veins carry blood toward the heart
• Artery and vein walls have three coats/tunics
  • Tunica intima: Innermost layer of endothelium continuous throughout the cardiovascular system
  • It is smooth, with a basement membrane
  • Arteries have the internal elastic lamina (elasticity) while veins do not
  • Tunica media: The thickest layer is for contractility.
  • Smooth muscle contracts, constricting vessels and decreasing blood flow
  • Smooth muscle relaxes, dilating vessels and increasing blood flow
  • Arteries have an external elastic membrane; veins have less smooth muscle
  • Tunica externa: Elastic and a tough outer layer

Comparison of Tunics in Arteries and Veins

• Arteries have thick walls with small lumens, generally appearing rounded
• Veins have thin walls with large lumens, appearing flattened
• Artery endothelium appears wavy (due to smooth muscle constriction)
• Vein endothelium appears smooth
• Larger arteries contain an internal elastic membrane; it's absent in veins
• Arteries: Tunica media is the thickest layer; smooth muscle cells and elastic fibers predominate
• Veins: Smooth muscle cells and collagenous fibers predominate in tunica media
• Larger arteries have an external elastic membrane; it is absent in veins
• Arteries: Tunica externa is thinner than tunica media, has collagenous/elastic fibers, vasa vasorum
• Veins: Tunica externa is the thickest layer, has collagenous/smooth fibers, vasa vasorum

Walls of Vessels

• The walls of the larger vessels are too thick for nutrient diffusion
• Larger arteries and veins contain vasa vasorum (small blood vessels) within their walls for critical exchange

Arteries

• Arteries conduct blood away from the heart and exhibit elasticity
• Elasticity allows acceptance of blood under high pressure from ventricular contraction
• This pushes it throughout the system
• Elastic vessels store mechanical energy, functioning as a pressure reservoir
• Recoil releases stored energy as kinetic energy that propels blood
• Contractility (due to tunica media smooth muscle) allows artery size changes to limit bleeding from wounds
• Elastic arteries are large arteries
• They have more elastic fibers, less smooth muscle so receive blood under pressure
• They are also known as "conducting arteries" as they conduct blood from the heart to medium-sized muscular ones
• Muscular arteries have much smooth muscle, to adjust blood flow rate to tissues

Arterioles

• Arterioles ("resistance vessels") are small, microscopic arteries which deliver blood to capillaries
• Vasoconstriction decreases the lumen size of blood vessel, while vasodilatation increases it
• Arterioles regulate blood flow from arteries to capillaries
• They regulate resistance, altering arterial blood pressure
• Capillaries connect arterioles and venules
• Microcirculation is blood flow through capillaries
• Capillaries are near every cell in the body, but their distribution varies with the metabolic activity of the tissue
• The function of capillaries facilitate exchange between blood and tissue cells, through interstitial fluid.
• Capillary walls contain a single layer of endothelium cells and a basement membrane
• Capillaries form networks, increasing the surface area that allows a rapid material exchange
• Blood flow capillaries controlled by vessels; precapillary sphincters regulate blood flow
• 3 Capillary types with different porosity
  • Continuous capillaries' plasma membrane forms a complete tube interrupted by small intercellular gaps; the most common type
  • Fenestrated capillaries' plasma membrane has pores
  • Sinusoids may have large basement membrane gaps

Metarterioles and Capillary Beds

• Arterioles give rise to metarterioles
• Precapillary sphincters at the metarteriole-capillary junction can regulate blood flow
• Capillaries branch to form networks, increasing surface area and material exchange
• Arterial Anastomoses are the union of two or more artery branches that supply the same region
• They provide alternate routes for blood to reach an organ or tissue
• Collateral circulation: alternate blood flow through an anastomosis.
• Arteriovenous anastomoses can connect a vein (venules) and an artery (arterioles) directly
• Dilated blood bypasses the capillary bed and enters venous circulation, with the sympathetic nervous system controlling them

Venules and Veins

• Venules ("little veins") are small vessels formed from several capillaries joining
• Venules merge and drain blood into veins
• Veins have three tunics -- inner to outer- intima, media and externa, but they're thinner than arteries
• Veins have less elastic tissue/smooth muscle, so are thinner-walled than arteries
• Veins contain valves to prevent backflow of blood
• Vascular (venous) sinuses are veins with thin walls and lacks smooth to alter diameters
• Weak valves leads to varicose veins. Causes being congenital, standing, pregnancy or aging
• Resting blood is mostly in systemic veins and venules (64%), known as blood reservoirs
  • They store blood; vasoconstriction can move blood to other body parts if needed
  • Hemorrhage vasoconstriction of reservoirs makes up blood loss through veins (abdominal organs and skin principally)

Comparison of Arteries and Veins

• Arteries conduct blood away from the heart versus Veins conduct blood towards the heart
• Arteries rounded versus Veins irregular/often collapsed
• Arteries have high blood pressure versus Veins have low
• Arteries have thick walls versus Veins have thin walls
• Systemic arteries have high relative oxygen concentration, pulmonary arteries have low concentration
• Systemic veins have low oxygen concentration, pulmonary veins have high concentration
• Valves present in Veins mostly in limbs and body inferior

Blood Flow, Blood Pressure, and Resistance

• Blood flow is movement through a vessel, tissue, or organ, initiated by ventricular contraction
• Ventricular contraction ejects blood into major arteries
• Blood encounters smaller arteries/arterioles, then capillaries & finally venules and veins of the venous system
• Resistance impedes or slows blood flow
• Blood pressure: the force exerted by blood upon vessel walls/heart chambers

Pulse

• Pulse is the expansion/elastic recoil of an artery wall with each heartbeat
• Any artery near the surface/over hard tissue strongest at arteries nearest the heart or the radial artery
• 70-80 beats per minute is a normal resting pulse rate
  • Tachycardia: rapid resting heart/pulse rate (>100 beats/min)
  • Bradycardia: slow resting heart/pulse rate (<60 beats/min)

Measurement of Blood Pressure

• Blood pressure is pressure via blood on an artery wall when the left ventricle undergoes systole and diastole
• Measured using a sphygmomanometer, in one of the brachial arteries
  • Systolic blood pressure: force of blood recorded during ventricular contraction
  • Diastolic blood pressure: force of blood recorded during ventricular relaxation
  • Normal blood pressure for young adult: 120/80 mm Hg (females 8-10 mm Hg less)
• Pulse pressure is the difference between systolic and diastolic pressure as it gives info on artery condition measured at 40mm Hg
• Mean arterial pressure (MAP) is the "average" pressure in arteries that drives blood that serves tissues

What Affects Blood Flow

• Circulation happens when the heart can overcome peripheral resistance
• Peripheral resistance is resistance of the entire cardiovascular system
• Resistance opposes blood flow as a result of friction between blood and vessel walls like a river bank situation
  • Vascular resistance depends mostly on blood vessel diameter, blood viscosity, total vessel length
• Total peripheral or systemic vascular resistance given systemic vessels resistance
• Most resistance is from arterioles, capillaries, and venules due to their small diameters
• Blood flow is directly proportional to pressure (pressure and flow both rise) and inversely proportional to peripheral resistance
• Pressure gradient determines flow
• Blood travels towards capillaries, artery diameters decrease while vein diameters increase as blood returns to the heart
• Arterial pressure drops with blood pushed into smaller vessels and BP drops
• Blood moves more slowly versus flow in capillaries due to capillary exchange
• Because arterial pressure has high ventricular systole it falls during ventricular diastole and elastic arterial walls stretch/recoil

Disorders of the Blood Vessels

• Atherosclerosis occurs because smooth muscle cells proliferate and fatty substances accumulate in medium-sized versus large artery walls
• Endothelial damage is a contributor
• Diagnosing coronary artery disease (CAD) requires cardiac catherization and cardiac angiography
• Treatment consists of drugs or coronary artery bypass grafting.

Venous System

• Veins have a pressure greater than the heart's atriae for blood to flow back
• Two things maintain this pressure gradient between the veins and the heart
  • Skeletal muscular contractions, valves in veins of extremities, and pressure changes from breathing
  • Moves from venules to veins, average blood pressure drops, but blood velocity increases because more pressure drives blood back toward the heart

About Capillary Exchange

• The primary purpose of the cardiovascular system facilitates gases, nutrients, wastes, and other substances to and from body cells
• Substances leave/enter via diffusion, transcytosis, bulk flow (filtration and absorption)
• Diffusion: This is capillary exchange
  • O2, CO2, glucose, amino acids, hormones diffuse
  • Plasma solutes (except larger proteins) move across capillary walls
  • Water transfer
• Transcytosis is when insoluble lipid molecules move across due to cells entering endocytosis and exiting exocytosis

Bulk Flow (filtration & reabsorption)

• Diffusion handles solute exchange, but filtration determines the volume of liquid in the bloodstream/ interstitial fluid
• Capillary walls have different diffusion routes
  • Water, ions, small molecules go through pores versus ions through protein channels
  • Large water molecules unavailable due bloodstream
  • Lipids, oxygen, carbon dioxide diffuse via epithelial membrane but plasma proteins aren't
• Water and dissolved solute movement is based on capillary hydrostatic/blood pressure vs blood colloidal osmotic/plasma proteins
• Net filtration pressure out of vessel: (NFP) equals CHP minus BCOP Capillary BP declines arterial to venous the rates of change on length capillary,
• Net movement out beginning vs in at end
• Filtered fluid mostly reabsorbed
• An arteriolar end is when capillary net pressure is increased, net flow goes into tissue, venule end when blood/colloid bring fluid back
• Excess interstitial fluid aka edema is caused by hydrostatic/venous or low plasma vs permeability vs fluid retention Excess fluid is picked up by lymph capillaries

Homeostatic Regulation of the Vascular System

Homeostasis maintains cardiac function and redirects blood flow because tissues is more active

• Nerual Regulation (Cardiovascular centers in Medulla Oblongata & ANS)

About Central Regulation

• Cardiovascular center (CV): neurons in the part of medulla controls heart vessel.
• CV receives input from higher sensory (baroreceptor) brain regions
  • Sympathetic increases heart contractibility vs parasympathetic (Vagus/nerves) decreases
• Sympathetic division sends impulses to smooth muscle walls versus tone
• Autoregulation: This localized response vs changes temperature, oxygen and carbon dioxide for example

Baroreceptor Reflexes:

• Baroreceptors: pressure-sensitive neurons measure blood vessel versus aorta walls
  • Carotid sinus reflex ensures correct versus sinus (and brain) pressure
  • Aortic reflex is same as sinus but aorta
• Pressure via slows (if too high), or accelerates/vasocontricts or decreases if fall (vessel and/or heart)

Chemoreceptors

• Chemical sensors versus sinus and aorta monitor levels oxygen, CO2 and IonHydrogen.

Endocrine Regulation

• Epinephrine/norepinephrine increase versus vasoconstriction as sympathetic stimulation, and filling
• Antiduretic hormone increases water resorption/urine/blood vs concentration

Renin

• Renin is released when renal blood pressure falls. Renin activates Angiotensin1

• Angiotensin I is then converted to angiotensin II via angiotensin convert enzyme (ACE) which raises everything listed above

• Erythropoietin increases volume by stimulating RBC

• Excessive triggers peptide walls increasing salt loss or reducing the blood

Autoregulation of Perfusion

• A tissue needs perfusion demand for materials
• Chemical: WBC makes vessels expand and platelets contract when chemicals (NO and serotonin) release or tissue damage occurs
• Myogenic response to stretch contraction through changes flow

Effect of Exercise

• Exercise makes adjustments through all vascular homeostasis

Light vs Heavy Exercise

• Resting cardio stands (5.8), vasoldiation resistance flow (capilliarial), skeletal muscles squeeze return, cardio output increases by more intake

• Light is to Heavy: cardio increases vs blood falls as nonessentials or GI/kidney ones Only 30 minutes of exercise can lower hearth attacks

What to Look For

• Vascular functions or volumes affect vascular homeostasis and tone

High and Low Pressure

• Systolic ( > 140) and Diastolic ( > 90) versus high
• Primary hypertension is elevated without cause (95-90%) and or high pressure damages
• Damage leads to arteries, brain heart or kidney failure before symptoms surface
• To improve one must drink less or exercise more
• Hypotension happens if blood not controlled in hemostasis versus shock and strokes

Homeostatic Shock and Development

• Shock: heart fails to give right amount of nutrient
• Results include membrane distfunction + loss Types shock - volume related, cardiac and obstructive Shock includes angiotensin activation division, or secretions for example Symptoms - clammy to cool skin or more issues

Last part includes GermLayer making plasma/cell tissue

Explore capillary types, arteriovenous anastomoses, and precapillary sphincters. Understand alternate blood flow, fenestrated capillaries, and venous valve function. Differentiate arteries from veins based on vessel structure and characteristics.