LESSON PLAN Sheet 1€¦ · Web viewIn addition, their endothelial cells may have large fenestrations, and intercellular clefts between adjacent endothelial cells are also large (Figure

Unit 4 Cardiovascular System

PART 3: BLOOD VESSELS

Introduction Blood vessels form a closed system of tubes that carry blood away from the heart, transport it to the

tissues of the body, and then return it back to the heart. In this part of the course we examine the histology of blood vessels and anatomy of the primary

routes of arterial and venous systems. Another main focus is hemodynamics, the means by which blood flow is altered and distributed and

by which blood pressure is regulated.

Structure and Function of Blood Vessels The blood vessel system is composed of arteries, arterioles, capillaries, venules, and veins.

o Arteries carry blood away from the heart to other organs.o Arterioles are small arteries that connect to capillaries.o Capillaries allow the exchange of substances between the blood and tissues.o Venules connect capillaries to veins.o Veins carry blood back to the heart.

Vessels that carry blood to and from the lungs form the pulmonary circulation. Vessels that carry blood to and from the rest of the body form the systemic circulation.

Basic Structure of a Blood Vessel There are three layers in a generalized blood vessel (Figure 21.1):

o The tunica interna, closest to the lumen, has direct contact with the blood and is continuous with the endocardium. It consists of an inner endothelium (simple squamous epithelium) and a basement membrane.

The endothelium forms a slick barrier that promotes blood flow.o The tunica media consists of smooth muscle and elastic fibers that provide for changes in

lumen diameter.o The tunica externa, consisting of elastic and collagen fibers, forms a protective outer layer.

The major structural and functional characteristics of blood vessels are describe in Table 21.1

ZOOL-1072/HEAL-2070 J. Taylor Red River College 1


Arteries Arteries carry blood away from the heart to the tissues. Arterial blood is under high pressure. The wall of an artery follows the basic structural plan described above, with the following

specializations (Figure 21.1a):o Tunica interna includes an internal elastic lamina, which provides the artery with some extra

strength and elasticity.o Tunica media contains plentiful elastic fibers, which give arteries compliance.

Compliance – ability to stretch or expand without tearing.o Tunica media contains thick layer of smooth muscle

Vasoconstriction – contraction of tunica media; causes decrease in lumen diameter. Vasodilation – relaxation of tunica media; causes increase in lumen diameter.

o Tunica media includes an external elastic lamina, which provides the artery with some extra strength and elasticity.

o Tunica externa is relatively thick and protective. The functional properties of arteries are:

o Elasticity, due to the elastic tissue in the tunica interna and media, allows arteries to accept blood under great pressure from the contraction of the ventricles and to send it on through the system.

o Contractility, due to the smooth muscle in the tunica media, allows arteries to increase or decrease lumen size (vasoconstriction/vasodilation) and to limit bleeding from wounds (vascular spasm).

Elastic arteries Large arteries with more elastic fibers and less smooth muscle are called elastic arteries and are able

to receive blood under pressure and propel it onward (Figure 21.2). They are also called conducting arteries because they conduct blood from the heart to medium sized

muscular arteries. They function as a pressure reservoir.

o Recoil of elastic arteries keeps blood flowing during ventricular relaxation (diastole).

Muscular Arteries Muscular arteries have a large amount of smooth muscle in their walls and distribute blood to

various parts of the body.o Vascular tone is important in maintaining blood pressure and blood flow.

Muscular arteries have less recoil than elastic arteries because they have a reduced amount of elastic fibers.

Anastomoses Anastomoses are the union of the branches of two or more arteries supplying the same region. They provide alternate routes for blood to reach a tissue or organ. The collateral circulation is the alternate flow of blood to a body part through an anastomosis. Arteries that do not anastomose are known as end arteries. Occlusion of an end artery interrupts

the blood supply to a whole segment of an organ, producing necrosis (death) of that segment.

Arterioles Arterioles are very small, almost microscopic, arteries that regulate the flow of blood to capillaries

(Figure 21.3).



The terminal end of an arteriole is called the metarteriole. A precapillary sphincter located between the metarteriole and capillary regulates blood flow into the capillary.

Through vasoconstriction (decrease in the size of the lumen of a blood vessel) and vasodilation (increase in the size of the lumen of a blood vessel), arterioles assume a key role in regulating blood flow from arteries into capillaries and in altering arterial blood pressure.

Capillaries Capillaries are microscopic vessels that usually connect arterioles and venules (Figure 21.3). The flow of blood through the capillaries is called the microcirculation. Capillaries are found near almost every cell in the body, but their distribution varies with the

metabolic activity of the tissue. The primary function of capillaries is to permit the exchange of nutrients and wastes between the

blood and tissue cells through interstitial fluid. Capillary walls are composed of only a single layer of cells (endothelium) and a basement membrane

(Figure 21.1). Capillaries branch to form an extensive capillary network throughout the tissue. This network

increases the surface area, allowing a rapid exchange of large quantities of materials. There are three types of capillaries (Figure 21.4):

o Continuous capillaries are composed of a single layer of endothelial cells that form a continuous tube. Diffusion of materials across the capillary occurs at intercellular clefts, which are gaps found between adjacent endothelial cells. Continuous capillaries are found in skeletal and smooth muscle, connective tissues, and the lungs.

o Fenestrated capillaries are composed of a layer of endothelial cells that form a continuous tube. They differ from continuous capillaries in that the endothelial cell membranes have many small holes through them called fenestrations (imagine a pasta strainer). Diffusion of materials across the capillary occurs through the fenestrations Sinusoids have a larger lumen than other capillaries. In addition, their endothelial cells may have large fenestrations, and intercellular clefts between adjacent endothelial cells are also large (Figure 21.4c). Compared to continuous capillaries, the presence of fenestrations increases the rate of diffusion. Fenestrated capillaries are found in tissues that must move large amounts of material across the capillary (e.g. kidneys, small intestine, choroid plexuses of the brain, ciliary processes of the eyes, and endocrine glands)

o Sinusoids are found in the liver, red bone marrow, and spleen. Due to the large openings between the sinusoid’s lumen and the interstitial fluid, the rate of diffusion is greatest in sinusoids.

Venules Venules are small vessels that are formed from the union of several capillaries; venules merge to

form veins (Figure 21.1) They drain blood from capillaries into veins.

Veins Veins consist of the same three tunics as arteries but have a thinner tunica interna and media and a

thicker tunica externa.o Veins have less elastic tissue and smooth muscle, no elastic laminae, and are therefore

thinner-walled than arteries. They have little compliance or vascular tone.

o Veins contain valves to prevent the backflow of blood (Figure 21.5).



Vascular (venous) sinuses are veins with very thin walls with no smooth muscle to alter their diameters. Examples are the brain’s superior sagittal sinus and the coronary sinus of the heart (Figure 20.3c).

Weak valves can lead to varicose veins. (Clinical Connection)

Blood Distribution At rest, the largest portion of the blood is in systemic veins and venules, collectively called blood

reservoirs (Figure 21.6).o They store blood and, through venous vasoconstriction, can move blood to other parts of

the body if the need arises.o In cases of hemorrhage, when blood pressure and volume decrease, vasoconstriction of

veins in venous reservoirs helps to compensate for the blood loss. The principal reservoirs are the veins of the abdominal organs (liver and spleen) and skin.

Capillary Exchange Substances enter and leave capillaries by diffusion, transcytosis, and bulk flow.

Diffusion The most important method of capillary exchange is simple diffusion.

o Substances such as O2, CO2, glucose, amino acids, hormones, and others diffuse down their concentration gradients.

o All plasma solutes, except larger proteins, pass freely across most capillary walls.o The prime exception of diffusion of water-soluble materials across capillary walls is in the

brain where the blood-brain barrier exists.

Transcytosis Some materials cross the capillary membrane by transcytosis, the enclosing of substances within

tiny vesicles that enter cells by endocytosis.

Bulk Flow: Filtration and Reabsorption



Whereas diffusion is more important for solute exchange between plasma and interstitial fluid, bulk flow is more important for regulation of the relative volumes of blood and interstitial fluid.

o Movement of fluid from capillaries into interstitial fluid is called filtration.o Movement of fluid from the interstitial fluid into capillaries is called reabsorption.

The movement of water and dissolved substances (except proteins) through capillaries is dependent upon hydrostatic and osmotic pressures (Figure 21.7):

o Blood hydrostatic pressure (BHP) - the pressure that water in blood plasma exerts against blood vessel walls. BHP “pushes” fluid out of capillaries and into interstitial spaces.

BHP is about 35 mmHg at the arterial end of the capillary. BHP is about 16 mmHg at the venous end of the capillary.

o Interstitial fluid hydrostatic pressure (IFHP) - the pressure that water in interstitial fluid exerts against blood vessel walls. IFHP “pushes” fluid out of interstitial spaces back into capillaries.

Assume that IFHP equals 0 mmHg along the length of the capillary.o Blood colloid osmotic pressure (BCOP) - the force caused by the colloidal suspension of

large proteins in blood plasma. BCOP “pulls” fluid from interstitial spaces into capillaries. BCOP averages 26 mmHg in most capillaries.

o Interstitial fluid osmotic pressure (IFOP) - the force caused by the presence of proteins in interstitial fluid. IFOP “pulls” fluid from capillaries into interstitial spaces.

Assume that IFHP equals 1 mmHg along the length of the capillary.



Whether fluid leaves or enters capillaries depends on the balance of the four pressures. The net filtration pressure (NFP), which indicates the direction of fluid movement, is calculated as follows:

NFP = (pressures that promote filtration) – (pressures that promote reabsorption)NFP = (BHP + IFOP) – (BCOP + IFHP)

In the arterial end of the capillary:NFP = (35 + 1) – (26 + 0) = 10 mmHg.

Thus there is a net outward pressure of 10 mmHg pushing fluid out of the capillary.

In the venous end of the capillary:NFP = (16 + 1) – (26+0) = -9 mmHg.

Thus there is a net inward pressure of 9 mmHg pushing fluid into the capillary.

Occasionally, the balance of filtration and reabsorption between interstitial fluid and plasma is disrupted, allowing an abnormal increase in interstitial fluid called edema. Edema may be caused by several factors including increased blood hydrostatic pressure in capillaries due to an increase in venous pressure and decreased concentration of plasma proteins that lower blood colloid osmotic pressure.

HEMODAYNAMICS: FACTORS AFFECTING BLOOD FLOW Blood flow is the volume of blood that flows through any tissue in a given time period (mL/min). Total blood flow is cardiac output (CO).

o Recall that cardiac output depends on heart rate (HR) and stroke volume(SV). CO=HR X SV

The distribution of cardiac output to various tissues depends on two factors:1. The pressure difference that drives the blood flow through a tissue.2. The resistance to blood flow in specific blood vessels.

Blood flows from regions of high pressure to regions of low pressure; the greater the pressure difference, the greater the blood flow.

But, the higher the resistance, the smaller the blood flow.

Blood Pressure Blood pressure (BP) is the pressure exerted by blood on the wall of an artery when the left ventricle

undergoes systole and then diastole (Figure 21.8). It is measured by the use of a sphygmomanometer, usually in one of the brachial arteries.

o Systolic blood pressure is the force of blood recorded during ventricular contraction (Figure 21.8).

o Diastolic blood pressure is the force of blood recorded during ventricular relaxation.o Pulse pressure is the difference between systolic and diastolic pressure.

As blood leaves the aorta and flows through systemic circulation, pulse pressure progressively falls to 0 mm Hg by the time it reaches the right atrium (Figure 21.8).

o Mean arterial pressure (MAP) is the average blood pressure in arteries (roughly 1/3 of the way between the diastolic and systolic pressures).



There is an important relationship between cardiac output and mean arterial pressure:

MAP = CO X RIf resistance (R) remains steady, increased CO results in increased blood pressure.

If resistance (R) remains steady, decreased CO results in decreased blood pressure.

Blood pressure also depends on the total volume of blood in the cardiovascular system.Increased blood volume (e.g. water retention) causes increased blood pressure.Decreased blood volume (e.g. hemorrhage) causes decreased blood pressure.

Vascular Resistance Vascular Resistance is the opposition to blood flow due to friction between blood and the walls of

blood vessels. Vascular resistance depends on:

1. Diameter of the blood vessel - Resistance is inversely proportional to the 4th power of the radius of the blood vessel (R 1/ r4). In normal person language: the smaller the radius of the blood vessel, the greater the resistance it offers to blood flow.

2. Blood viscosity - Resistance is directly proportional to viscosity; the greater the viscosity, the greater the resistance (e.g. polycythemia).

3. Total blood vessel length – Resistance is directly proportional to blood vessel length; the longer a blood vessel, the greater the resistance to blood that flows through it.

Systemic vascular resistance (also known as total peripheral resistance) refers to all of the vascular resistances offered by systemic blood vessels; most resistance is in arterioles, capillaries, and venules due to their small diameters.

o A major function of arterioles is to control SVR – and therefore blood pressure and blood flow (CO) – by changing their diameters.

Venous Return Venous return occurs because of the pressure gradient between the venules and the right atrium. Venous return is affected by two main factors:

1. Skeletal muscle pump and valves in veins (Figure 21.9)2. Respiratory pump



Summary of Factors Affecting Blood Pressure. Changes noted in green boxes increase cardiac output; changes noted within blue boxes increase system vascular resistance

Velocity of Blood Flow The volume that flows through any tissue in a given period of time is blood flow. The velocity of blood flow is inversely related to the cross-sectional area of blood vessels; blood

flows most slowly where cross-sectional area is greatest (Figure 21.11). Blood flow decreases from the aorta to arteries to capillaries and increases as it returns to the heart.



CONTROL OF BLOOD PRESSURE AND BLOOD FLOW

Role of the Cardiovascular Center The cardiovascular center (CV) is a group of neurons in the medulla that regulates heart rate,

contractility, and blood vessel diameter. The CV receives input from higher brain regions and sensory receptors (baroreceptors and

chemoreceptors) (Figure 21.12). Output from the CV flows along sympathetic and parasympathetic fibers.

o Sympathetic impulses along cardioaccelerator nerves increase heart rate and contractility.o Parasympathetic impulses along vagus nerves decrease heart rate.o The sympathetic division also continually sends impulses to smooth muscle in blood vessel

walls via vasomotor nerves. The result is a moderate state of tonic contraction or vasoconstriction, called vasomotor tone.

Neural Regulation of Blood Pressure

Baroreceptor Reflexes Baroreceptors are important pressure-sensitive sensory neurons that monitor stretching of the walls

of blood vessels and the atria.o The cardiac sinus reflex is concerned with maintaining normal blood pressure in the brain

and is initiated by baroreceptors in the wall of the carotid sinus (Figure 21.13).o The aortic reflex is concerned with general systemic blood pressure and is initiated by

baroreceptors in the wall of the arch of the aorta or attached to the arch.o If blood pressure falls, the baroreceptor reflexes accelerate heart rate, increase force of

contraction, and promote vasoconstriction (Figure 21.14). Receptors sensitive to chemicals are called chemoreceptors.

o These receptors are located close to the baroreceptors of the carotid sinus and arch of the aorta.

o They monitor blood levels of oxygen, carbon dioxide, and hydrogen ion concentration.



Hormonal Regulation Hormones such as angiotensin II, epinephrine, norepinephrine, antidiuretic hormone, and atrial

natriuretic peptide affect blood pressure and blood flow by altering cardiac output, changing systemic vascular resistance, or adjusting the total blood volume.

Table 21.2 summarizes the relationship between hormones and blood pressure regulation.



Autoregulation of Blood Pressure The ability of a tissue to automatically adjust its own blood flow to match its metabolic demand for

supply of O2 and nutrients and removal of wastes is called autoregulation. In most body tissues, oxygen is the principal, though not direct, stimulus for autoregulation. Researchers have identified two general types of stimuli that cause autoregulatory changes in blood

flow: o Physical changes

Warming promotes vasodilation, and cooling causes vasoconstriction. Smooth muscle in arteriole walls exhibits a myogenic response

o Vasodilating and vasoconstricting chemicals Vasodilators include , K+, H+, lactic acid and adenosine (from ATP), kinins, histamine,

and nitric oxide (NO). Vasoconstrictors include thromboxane A2, superoxide radicals, serotonin, and

endothelins

CIRCULATORY ROUTES The blood vessels are organized into routes that deliver blood throughout the body. Figure 21.17

shows the circulatory routes for blood flow. The largest circulatory route is the systemic circulation. Other routes include hepatic portal circulation (Figure 21.28), pulmonary circulation (Figure 21.29)

and fetal circulation (Figure 21.30).

Systemic Circulation The systemic circulation takes oxygenated blood from the left ventricle through the aorta to all parts

of the body, including some lung tissue (but does not supply the air sacs of the lungs) and returns the deoxygenated blood to the right atrium.

o The aorta is divided into the ascending aorta, arch of the aorta, and the descending aorta.o Each section gives off arteries that branch to supply the whole body.

Blood returns to the heart through the systemic veins. All the veins of the systemic circulation flow into the superior or inferior venae cavae or the coronary sinus, which in turn empty into the right atrium.

The principal arteries and veins of the systemic circulation are described and illustrated in Exhibits 21.1-21.12 and Figures 21.18-21.27. Blood vessels are organized in the exhibits according to regions of the body. Figure 21.18 shows the major arteries. Figure 21.23 shows the major veins.

Pulmonary Circulation The pulmonary circulation takes deoxygenated blood from the right ventricle to the air sacs of the

lungs and returns oxygenated blood from the lungs to the left atrium (Figure 21.29). The pulmonary and systemic circulations differ from each other in several more ways.

o Blood in the pulmonary circulation is not pumped so far as in the systemic circulation and the pulmonary arteries have a larger diameter, thinner walls, and less elastic tissue. As a result, resistance to blood flow is very low meaning that less pressure is needed to move blood through the lungs.

o Because resistance in the pulmonary circulation is low, normal pulmonary capillary hydrostatic pressure is lower than systemic capillary hydrostatic pressure which tends to prevent pulmonary edema.



Systemic Circulation –principal arteries



Systemic Circulation – principal veins



Hepatic Portal Circulation The hepatic portal circulation collects blood from the veins of the pancreas, spleen, stomach,

intestines, and gallbladder and directs it into the hepatic portal vein of the liver before it returns to the heart (Figure 21.28). A shopping list will be provided in lab.

A portal system carries blood between two capillary networks, in this case from capillaries of the gastrointestinal tract to sinusoids of the liver. This circulation enables nutrient utilization and blood detoxification by the liver.

Fetal Circulation The fetal circulation involves the exchange of materials between fetus and mother. The fetus derives its oxygen and nutrients and eliminates its carbon dioxide and wastes through the

maternal blood supply by means of a structure called the placenta. Blood passes from the fetus to the placenta via two umbilical arteries (Figure 21.30) and returns

from the placenta via a single umbilical vein. At birth, when pulmonary, digestive, and liver functions are established, the special structures of

fetal circulation are no longer needed.o The ductus arteriosus becomes the ligamentum arteriosum shortly after birth.o The foramen ovale becomes the fossa ovalis shortly after birth.o The ductus venosus becomes the ligamentum venosum shortly after birth.o The umbilical arteries become the medial umbilical ligaments.o The umbilical vein becomes the ligamentum teres (round ligament).



Fetal Circulation and Changes at Birth


Documents

LESSON PLAN Sheet 1€¦ · Web viewIn addition, their endothelial cells may have large fenestrations, and intercellular clefts between adjacent endothelial cells are also large (Figure