Book Notes

20.1 General Anatomy of the Blood Vessels

The Vessel Wall

• Tunica interna

• Tunica media

• Tunica externa

• network of smaller vessels serving the larger one called the vasa vasorum

Arteries

• considered resistance vessels

• relatively strong and resilient tissue

• Three Classes

  • Conducting (elastic or large) arteries

    • carotid and subclavian arteries conducting arteries expand as they receive blood blood during ventricular systole

  • Distributing (muscular or medium) arteries

    • smaller branches that distribute blood to specific organs

    • both the internal and external elastic lamilae thick and often conspicuous

  • Resistance arteries

    • too variable and location for many of them to have individual names

    • arterioles - smallest, major point of control over how much blood an organ or tissue receives

  • Aneursym

    • weak point in an artery or the heart wall

    • it forms a thin walled, buging sac that pulsates with each beat

  • Metarterioles

    • link arterioles directly to venules and provide shortcuts through which blood can bypass the capillaries

Arterial Sense Organs

• Carotid sinuses - baroceptors - monitor BP

  • common carotid - neck

  • internal carotid - brain

  • external carotid - face

• Carotid bodies

  • located near the branch of the common carotid arteries

  • chemorecptors

  • transmit signals to brainstem respiratory cneters, which adjust breathing to stabilize the blood [H and its CO2 and O2 levels

• Aortic bodies

  • chemoreceptors located in the aortic arch

  • similar to the carotid bodies and same fucntion

  • transmit their signals to the brainstem via the vagus nerves

Capillaries

• Only three places that nutrients, wastes, hormones, and leukocytes can pass into the tissues - the capillaries, some venules, and sinusoid

• exchange vessels of the CV system

• arterioles, capillaries, and venules are also called the microvasculature (microcirculation)

• Three Types

  • Continuous capillaries

    • occur in most tissues and organs

      • skeletal muscles, lungs, & brain

    • basal lamina & intercellular clefts

    • contain pericytes - contract & regulate blood flow, contribute to vessel growth and repair

  • Fenestrated capillaires

    • have endothelial with patches of filtration pores

    • allow rapid passage of small molecules, retain most proteins and larger particles in the bloodstream

    • important in kidneys, endocrine glands, small intestine, & choroid plexuses of the brain

  • Sinusoid

    • irregular blood filled spaces in the liver, bone marrow, spleen, and some other organs

    • no basal lamina

    • small molecules, proteins, and blood cells can pass through

    • conform to shape of surrounding tissues

Capillary Beds

• organized webs of capillaries

• supplied by a single arteriole or metarteriole

• at the end, capillaries transition to venules

• ¾ of the body’s capillaries are shut down bc there isnt enough blood to supply all of them at once

• Capillary flow = perfusion, is regulated by dilation or constriction\

• capillary beds w/ metaarterioles there is often a precapillary sphincter

  • relaxed = well perfused

  • regulates blood flow

Veins

• capacitance vessels bc they’re relatively thin-walled and flaccid, and expand easily to accommodate an increased volume of blood

• greater capacity for blood than arteries

• at rest can hold 64%

• averages 10 mm Hg

• smaller veins are tributaries

• Types of Veins

  • postcapillary venules

    • smallest of the veins

    • 10 to 20 micrometers

    • have tunica interna

    • receive blood from capillaries directly or by way of the distal ends of the metarterioles

    • more porous than capillaries

    • most leukocytes emigrate from the bloodstream

  • muscular venules

    • larger than postcapillary venules

    • receive blood from the postcapillary venules

    • tunica media and a tunica externa

  • Medium veins

    • larger than muscular venules

    • range up to 10mm

    • have individual names

    • have tunica interna, basement membrane, loose connective tissue, and sometimes elastic lamina, tunica media

    • can form venous valves - infoldings of the tunica interna that meet in the middle of the lumen

    • upwards flow of blood depends on the squeezing of skeletal muscle

      • skeletal muscle pump

      • varicose veins as a results of failure of valves

  • Large Veins

    • greater than 10 mm

    • thin tunica, tunica media, tunica externa

    • venae venae cavae, pulmonary veins, internal jugular veins, & renal veins

Circulatory Routes

• heart → arteries → capillaries → veins → heart

• There is three portal systems in the body

  • kidneys

  • connect hypothalamus to anterior pituitary gland

  • intestines to liver

• Anastomosis = point of convergence btw two blood vessels

• ateriovenous anastomosis = blood flows from an artery directly into a vein and bypasses capillaires

• venous anastomosis = one vein empties directly into another

• arterial anastomoses = two arteries merge

20.2 Blood Pressure, Resistance, and Flow

Overview

• Flow: the amount of blood passing through an organ, tissue, or blood vessel in a given time (mL/min)

• Perfusion: flow per given volume or mass of tissue

• In a resting individual, total flow is quite constant and equal to cardiac output

• Hemodynamics, the physical principles of blood flow, are based mainly on pressure and resistance

  • F α ∆P/R

Blood Pressure

• BP is the force exerted by blood on a vessel wall

• typically measured at the brachial artery of the arm with a sphygmomanometer

• Systolic is measured at the first sound of heart beat

• Diastolic is measured at the last beat

• Systolic pressure is the peak pressure generated by contraction of the left ventricle

• Diastolic pressure is the minimum to which the BP calls when the ventricle is in diastole

  • The difference in the two is called pulse pressure

    • E.g. 120-75 = 45 mm Hg

• Determined by: cardiac output, blood volume, and resistance to flow

  • blood volume is regulated by kidneys

• Mean arterial pressure = MAP

  • obtained by adding diastolic and one third the pulse pressure

  • it is the mean arterial pressure that most influences the risk of disorders such as atherosclerosis, kidney failure, edema, aneurysm, and syncope

• Blood flow in the arteries is pulsatile

• In capillaries and veins, the blood flows at a steady speed with little if any pulsation

• Arteriosclerosis: hardening of the arteries, caused by free radicals which gradually deteriorate the elastic and other tissues of the arterial walls

• Atherosclerosis: growth of lipid deposits in the arterial walls, the deposits can become calcified complicated plaques, giving the arteries a hard, crunchy likes consistency

• Hypertension: high BP, chronic, temporary based on emotion or exercise, can weaken arteries and cause aneurysms and promotes development of atherosclerosis

• Hypotension: chronic low BP, possible consequence of blood loss, dehydration, anemia, or other factors & is normal in those approaching death

Peripheral Resistance

• Opposition to flow in vessels

• away from the heart

• pressure and resistance aren’t independent variables, pressure is affected by resistance, flow is affected by both

• Resistance hinges on three variables: blood viscosity, vessel length, and vessel radius'

• Flow is fastest in the aorta

• from aorta to capillaries, velocity diminishes for three reasons

  • The blood has traveled a greater distance, so friction has slowed down

  • the arterioles and capillaires have smaller radii and therefore put up more resistance

  • even though the radii of the individual vessels become smaller as we progress farther from the hear, the number of vessels and their total cross sectional area become greater and greater

• From capillaries to vena cava, velocity rises again

• Arterioles are the most significant point of control over peripheral resistance and blood flow bc

  1. they’re on the proximal sides of the capillary beds, so they’re best positioned to regulate flow into the capillaries and thus regulate perfusion of the organs

  2. They greatly outnumber any other class of arteries and thus provide the most numerous control points

  3. they’re more muscular in proportion to their diameters than any other class of blood vessels and are highly capable of changing radius

• Viscosity

  • stems from plasma proteins (albumin) and erythrocytes

  • deficient in erythrocytes = anemia

  • deficient in albumin = hypoproteinemia, which reduces viscosity & speeds up blood flow

  • Polycythemia & dehydration = viscosity increase & flow decrease

• Vessel Length

  • farther a liquid travels through a tube, the more cumulative friction it encounters

• Vessel Radius

  • vasoconstriction - narrowing of a vessel

    • when smooth muscle of tunica media contracts

  • vasodilation - widening of a vessel

    • muscular passitivity - relaxation of smooth muscle

  • Vasoconstriction + vasodilation = vasomotion

    • controlled by a nucleus in the medulla oblongata - the vasomotor center

Regulation of Blood Pressure and Flow

• Three ways of controlling vasomotor activity: local, neural, and hormonal mechanisms

  • Local

    • Autoregulation is the ability of tissues to regulate their own blood supply

    • If waste products accumulate, they stimulate vasodilation and blood flow increases

    • If blood flow carries away those products, there is a vasoconstriction

    • Platelets, endothelial cells, and perivascular tissues can secrete vasoactive chemicals, including histamine, bradykinin, and prostaglandins

    • If blood flow is cut off for a while and then restored, there can be reactive hyperemia, and increase in the normal blood flow

    • Over a longer time, hypoxic tissue can initiate angiogenesis-growth of new blood vessels. This is seen in the regrowth of uterine lining after menstruation

    • Malignant tumors secrete growth factors that increases its blood supply and with-it growth of the tumor. One way to attack tumor growth is to inhibit this tumor angiogenesis

  • Neural

    • There is sympathetic control of blood vessels throughout the body, from the medulla

    • Sympathetic nerve fibers stimulate most vessels to constrict and allow for vasodilation by reducing the rate of firing of those nerve fibers

    • Precapillary sphincters have no innervation and react to local and hormonal stimuli

    • The vasomotor center is an integrating center for three autonomic reflexes-baroflexes, chemoreflexes and medullary ischemic reflex

    • The baroflex is a negative feedback response to an increase in blood pressure from the carotid sinuses-it inhibits sympathetic impulses causing vasodilation-it is a short term-regulation for BP