Blood Vessels and Circulation

The Circulatory System: Blood Vessels and Circulation

• The circulatory system anatomy and physiology are based on the unity of form and function.

General Anatomy of Blood Vessels

• Key learning outcomes include describing blood vessel structure, types of arteries/capillaries/veins, tracing blood flow from and back to the heart, and identifying route variations.
• Arteries carry blood away from the heart.
• Veins return blood to the heart.
• Capillaries connect the smallest arteries to the smallest veins, forming a circuit.

The Vessel Wall

• Arteries and veins possess walls with three layers called tunics.
• The tunica interna lines the blood vessel and has contact to blood.
• The tunica interna consists of endothelium which is simple squamous epithelium.
  • It is a selectively permeable barrier.
  • Chemicals for dilation or constriction are secreted.
  • It normally repels blood cells and platelets to prevent clotting.
  • When tissue is inflamed, it produces cell-adhesion molecules.
  • Cell adhesion causes leukocytes to congregate where defensive actions are needed.
• The tunica media is the middle layer.
  • It consists of smooth muscle, collagen, and elastic tissue.
  • It strengthens vessels and prevents blood pressure from rupturing them.
  • Muscle contraction controls blood vessel diameter.
• The tunica externa is the outermost layer.
  • It has loose connective tissue which often merges with the tissue of nearby blood vessels, nerves, or organs.
  • It anchors the vessel and provides passage for small nerves and lymphatic vessels.
  • The vasa vasorum are small vessels supply blood to outer half of wall in the larger vessels.

Arteries

• Arteries divide into three classes based on size: conducting, distributing, and resistance arteries.
• Conducting arteries are the biggest.
  • Examples include the aorta, common carotid, subclavian, pulmonary trunk, and common iliac arteries.
  • They have an internal elastic lamina at the border between the tunica interna and media.
  • They have an external elastic lamina at the border between the media and externa.
  • They expand during systole and recoil during diastole.
  • Expansion reduces pressure on downstream vessels.
  • Recoil maintains pressure during relaxation to keep blood flowing.
• Distributing arteries distribute blood to specific organs.
  • Examples are brachial, femoral, renal, and splenic arteries.
  • Smooth muscle layers make up three-fourths of the thickness of its wall.
  • Internal and external elastic laminae are thick.
• Resistance Arteries are small arteries.
  • They have a thicker tunica media in proportion to their lumen, along with very little tunica externa.
  • Arterioles are these arteries, and they are the smallest.
    • They have a 200 mm diameter, with only one to three layers of smooth muscle.
    • They control amount of blood to various organs.
• Metarterioles (thoroughfare channels) are short vessels that link arterioles directly to venules in some places (e.g., mesenteries).
  • They provide shortcuts allowing blood to bypass capillary beds.

Aneurysm

• An aneurysm is a weak point in an artery or the heart wall.
• It forms a thin-walled, bulging sac that pulsates with each heartbeat and may rupture.
• In a dissecting aneurysm, blood accumulates between tunics of the artery, separating them, typically due to degeneration of the tunica media.
• Common locations include the abdominal aorta, renal arteries, and arterial circle at the base of the brain.
• Aneurysms can cause pain by putting pressure on surrounding structures, and can rupture, causing hemorrhage.
• They result from congenital blood vessel weakness, trauma, bacterial infections, or atherosclerosis and hypertension.

Arterial Sense Organs

• Sensory structures that monitor blood pressure and chemistry reside in the walls of major vessels.
• Information is transmitted to the brainstem to control heart rate, blood vessel diameter, and breathing.
• Carotid sinuses:
  • Baroreceptors in the walls of internal carotid artery.
  • They monitor blood pressure.
  • Transmit signals through the glossopharyngeal nerve.
  • Allow for baroreflex.
• Carotid bodies:
  • Oval bodies near the common carotids where they divide.
  • They are chemoreceptors, which monitor blood chemistry.
  • They transmit signals through the glossopharyngeal nerve to the brainstem's respiratory centers.
  • Adjust respiratory rate is adjusted to stabilize pH, CO2, and O2.
• Aortic bodies:
  • One to three chemoreceptors in the walls of the aortic arch.
  • They share same structure and function as carotid bodies, but innervation is by the vagus nerve.

Capillaries

• Capillaries are exchange vessels where gasses, nutrients, wastes, and hormones pass between the blood and tissue fluid.
• Capillaries are part of the microvasculature, which also includes arterioles and venules.
• Nearly every cell in the body is close to a capillary, except in tendons, ligaments, epithelia, cornea, and lens of the eye.
• Capillaries are composed of endothelium and basal lamina.
• There are three capillary types. They are distinguished by permeability: continuous, fenestrated, and sinusoidal.
• Continuous Capillaries are
  • Found in most tissues.
  • Endothelial cells held together by tight junctions; form a continuous tube with intercellular clefts separates the cells.
    • Small solutes can pass through intercellular clefts, but not things like plasma protein and other large molecules can not.
  • Have a basal lamina, which is a thin protein-carbohydrate layer surround endothelium.
  • Have pericytes, which wrap around the capillaries.
  • Pericytes contains the same contractile protein as muscle.
    • Pericytes contract and regulate blood flow and can differentiate for growth and repair.
• Fenestrated Capillaries:
  • Found in organs that require rapid absorption or filtration (e.g., kidneys, small intestine).
  • Endothelial cells contain filtration pores (fenestrations) with a 20 to 100 nm diameter.
    • These are spanned by very thin glycoprotein membrane, which is much thinner than cell’s plasma membrane.
    • Allow passage of only small molecules.
    • Proteins and larger particles generally stay in bloodstream.
• Sinusoids are
  • Found in the liver, bone marrow, and spleen
  • Irregular blood-filled spaces
  • Endothelial cells are separated by wide gaps have large fenestrations with no basal lamina
  • They allow proteins (albumin), clotting factors, and new blood cells to enter the circulation.
• Capillary beds are networks of 10-100 capillaries.
  • Usually supplied by a single arteriole or metarteriole.
  • Drain into a venule or the distal end of a metarteriole.
  • 75% of body's capillaries are shut down at any given time.
    • Control involves constriction of upstream arterioles.
  • precapillary sphincters control flow.
    • With relaxed sphincters, capillaries will have good blood flow.
    • When sphincters contract, they constrict the entry to the capillary, and blood typically bypasses the capillary.

Veins

• Veins are the capacitance vessels of the cardiovascular system.
• They are thin-walled and flaccid.
• They collapse when empty and expand easily.
• Veins contain more blood than arteries. And their blood is easier to contain too.
  • At rest, about 64% of blood is in veins while 13% if in the arteries.
• Blood flow is steady here, not pulsed as in arteries.
• They have a relatively low blood pressure.
  • It Averages 10 mm Hg with little fluctuation.
• There are various types of veins, listed from smallest to largest: post capillary venules, muscular venules, medium veins, large veins, and venous sinuses.
• Postcapillary venules:
  • Smallest veins with 10-20 um diameter.
  • They consists of tunica interna with only a few fibroblasts around it and it has no muscle.
  • More porous than capillaries, so they also exchange surrounding fluid.
  • Leukocytes leave bloodstream through venule walls.
• Muscular venules
  • Receive blood from postcapillary venules.
  • Up to 1 mm in diameter.
  • Consist of one or two layers of smooth muscle in the tunica media and thin tunica externa.
• Medium veins
  • Up to 10 mm in diameter.
  • Thin tunica media, thick tunica externa, and tunica interna form venous valves.
    • Varicose veins may result from failure of these valves.
  • Rely on skeletal muscle pump to propel venous blood back to heart
• Large veins
  • Diameter >10 mm.
  • Smooth muscle in all three tunics.
  • Relatively thin tunica media with a moderate amount of smooth muscle.
  • Tunica externa is the thickest layer with longitudinal bundles of smooth muscle.
  • Examples: the venae cavae, pulmonary veins, internal jugular veins, and renal veins.
• Venous sinuses
  • Occur in select locations.
  • Modified veins with specially thin walls, large lumens, and no smooth muscle.
    • They are not capable of vasoconstriction.
  • Examples: dural sinuses form in the brain and coronary sinus found in heart.

Varicose Veins

• Blood pools in the lower legs of people who are standing for long periods, stretching the veins.
• Cusps of valves pull apart in enlarged superficial veins, which further weakens vessels.
• Blood backflows, further distending the vessels their walls grow weak and develop into varicose veins with dilation.
• Hereditary weakness, obesity, and pregnancy promote problems.
• Hemorrhoids are varicose veins of the anal canal.

Circulation Routes

• Blood travels a common route:
  • Starting at the heart, it flows into the arteries, then arterioles, to the capillaries, and to the venules and then to the veins.
  • In this route, blood passes through only one network of capillaries from the time it leaves the heart until it returns.
• There may be alternate pathways that involve two capillary beds or no capillary beds
• The portal system uses
  • Portal system is a circulatory network where blood passes through two consecutive capillary networks before returning to the heart.
  • Examples are
    • In kidneys
    • Between hypothalamus and anterior pituitary
    • Between intestines and liver
• Anastomosis is convergence between two vessels.
  • Arteriovenous anastomosis is when artery flows directly into vein, bypassing capillaries
  • Venous anastomosis are common when one vein empties into another.
  • Arterial anastomosis occur when two arteries merge with to provide collateral routes of blood supply.

Blood Pressure, Resistance, and Flow

• Key learning outcomes involve explaining the relationship between blood pressure, resistance, and flow; describing how to measure and express blood pressure; calculating pulse pressure and mean arterial pressure; detailing factors that determine resistance to blood flow; explaining how vessel diameter affects blood pressure and flow; and knowing local, neural, and hormonal influences on blood flow.
• Understanding the blood supply to a tissue it is expressed in terms of flow and perfusion.
  • Flow is the amount of blood flowing through an organ, tissue, or blood vessel in a given time, measured in mL/min.
  • Perfusion is the flow per given volume or mass of tissue in a given time, measured in mL/100g/min.
  • Total flow at rest is constant and is to the cardiac output is about 5.25 L/min.
• Hemodynamics are physical principles of blood flow based on pressure and resistance.
  • The greater the pressure difference between two points, the greater the flow. The greater the resistance, the less the flow.

Blood Pressure

• Blood pressure (BP) is the force blood exerts against a vessel wall

• Blood pressure is measured at the brachial artery using a sphygmomanometer.

  • A close approximation of pressure at exit of left ventricle.

• Two pressures are recorded during blood pressure measurement:

• Systolic pressure is the peak arterial BP taken during ventricular contraction (ventricular systole).

• Diastolic pressure is the minimum arterial BP taken during ventricular relaxation (diastole) between heart beats

• A normal BP value for young adult is 120/75 mm Hg.

• Stress of blood vessels is measured by pulse pressure.

  • Pulse pressure is the difference between systolic and diastolic pressure.
    • This is an Important measure of driving force on circulation and of stress exerted on small arteries by pressure surges generated by the heart.

• Also measured is the mean arterial pressure:

  • This is Diastolic pressure + (one-third of pulse pressure).
  • Serves as average blood pressure that has the most influences risk level that affects one's kidney.

• Since pressure varies across the cardiac cycle, blood flow in arteries is pulsatile.

  • Speed surges from 40 cm/s to 120 cm/s.
  • Blood spurts intermittently from an open artery.

• In capillaries and veins, blood flows at a steady speed.

  • Bleeding from veins tends to be slow and steady.

• BP tends to rise with age.

  • Arteriosclerosis is the stiffening of arteries due to deterioration of elastic tissues of artery walls.
  • Atherosclerosis is the build up of lipid deposits that become plaques.

• Systemic blood pressure decreases farther from heart.

• Hypertension is

  • Chronic resting blood pressure higher than 130/80
  • Can weaken arteries, cause aneurysms, promote atherosclerosis.

• Hypotension is

  • Chronic low resting BP
  • Caused by blood loss, dehydration, anemia.
  • Includes no specific numerical criterion for hypotension.

• BP is determined by three variables:

  • Cardiac output
  • Blood volume
  • Resistance to flow

• Blood volume is regulated mainly by kidneys, and this gives the the largest singular organ that affects blood pressure unless considering the heart.

Peripheral Resistance

• Peripheral resistance opposes the flow of blood in the vessels away from the heart.
• Resistance depends on three variables in blood:
  • Blood viscosity from plasma protein and red blood cells.
    • Low viscosity in red blood cells or plasma protein speeds the flow.
    • dehydration declines of fluid in the body will hinder it.
  • Pressure declines with distance of the liquid when it travels through a tube.
  • Greatest control over blood flow is vessels radius.
    • Greatest control over blood flow
• Blood exhibits laminar flow, so dilating helps vessels.
• Vasomotion controls vasoconstriction and vasodilation.
• Also, more blood volume will decrease the resistance, for example, larger radii and more cross sections from capillaries which all lower the resistance. Volume is further compliant by being further away also.
• Arterioles are most significant point of control due to its positioning and they outnumber other types of artery. They area also muscular to maintain the flow.

Regulation of Blood Pressure and Flow

• Vasomotion is a quick, powerful way to alter blood pressure and flow.
• Three ways of controlling vasomotor activity:
  • Local control
  • Neural control
  • Hormonal control
• Local auto regulation:
  • Autoregulation enables tissues to regulate their blood supply.
    • If tissue is inadequately perfused(deoxygenated), then waste accumulates, stimulating perfusion.
• Neural control;
  • Remote control from vessel and neural system in center to automatic.
  • Vasomotor's center provides the sympathetic controls such as baro and chemo.
• Hormonal control;
  • Uses kidney processes such as aldosterone or angiotensin for increased absorption for higher blood volume.

Two Purposes of Vasomotion

• Serves to do the the overall.
• Selectively reroute region which required concentrated perfusion.