Chapter 20 BV guide

Chapter 20 The Circulatory System: Blood Vessels and Circulation

After these lectures, you should be able to:

• Know the basic structure of blood vessels

• Describe the types of arteries, capillaries and veins

• Trace the most common route taken by the blood to and from the heart (and some variations of this route)

• Explain the relationship between blood pressure, resistance, and flow

• Calculate blood pressure, mean arterial pressure, and pulse pressure

• Describe the factors that influence resistance

• Understand the forces behind capillary filtration & absorption

A. Topics

1) General Anatomy of Blood Vessels

2) Circulatory Routes

3) Blood Pressure, Resistance, and Flow

4) Capillary Exchange

5) Special Focus on Hypertension

6) Anatomy of arteries and veins (to help you prepare for lab exam on blood vessel anatomy)

B. Lecture Outline

1) General Anatomy of Blood Vessels (Figs. 20.2, 20.9)

The Vessel Wall (fig 20.2)

The walls of arteries and veins have 3 layers called tunics:

Tunica interna (intima) Tunica media Tunica externa

• Lines the blood vessel and is exposed to blood

• Site of material exchange

• Secretes chemicals that stimulate Vasomotion

• Brings leukocytes to inflamed tissue • Strengthens vessels and prevents blood pressure from rupturing them

• Responsible for Vasomotion • Loose connective tissue that often merges with other vessels

• Anchors the vessel

• Provides passage for small nerves, lymphatic vessels, and vasa vasorum

I. Arteries: sometimes called resistance vessels for their relatively strong, resilient tissue structure that resists changes in pressure with each heartbeat. Rather, the arteries absorb this pressure and use it to propel blood forward into the circulatory system.

a. Conducting (elastic or large) arteries –

• Largest; expand during systole, recoil during diastole; lessens fluctuations in BP.

• e.g. pulmonary, aorta, and common carotid arteries.

b. Distributing (muscular or medium) arteries.

• Distributes blood to specific organs

• e.g. femoral and splenic arteries (most of “named” arteries are in this category).

c. Resistance (small) arteries. Arterioles control amount of blood to various organs.

d. Metarterioles (Figure 20.3a) Short vessels that connect arterioles to capillaries.

• Contain smooth muscle cells that form precapillary sphincters – these contract to close, or relax to open, the entrance to the capillary.

• When the sphincters are closed, the metarterioles acts as “thoroughfare channels” that move blood into the venous side of circulation.

SCRIPTURE CONNECTION: Ezekiel 36:26,27 & the hard heart

II. Capillaries (figs. 20.5-20.7)

Precapillary sphincters of metarterioles control which beds are well perfused.

a. Continuous capillaries - occur in most tissues

• Endothelial cells have tight junctions with intercellular clefts

• Allow passage of small solutes.

b. Fenestrated - kidneys, small intestine (organs that require rapid absorption or filtration).

• Endothelial cells have filtration pores (fenestrations)

• Allow rapid passage of small molecules.

c. Sinusoids - liver, bone marrow, spleen

• Irregular blood-filled spaces; some have extra large fenestrations.

• Allow passage of larger molecules such as proteins and blood cells.

III. Veins: are considered capacitance vessels for their relatively thin walls that expand easily

• Veins have lower blood pressure: avg. = 10 mmHg with little fluctuation.

• Thinner walls, less muscular and elastic tissue.

• Venous valves aid skeletal muscles in upward blood flow.

Summary of 3 categories of Blood vessels:

Arteries Capillaries Veins

• Large (conducting, elastic)

• Medium (distributing or muscular)

• Small (resistance)

• Arteriole

• Metarteriole (thoroughfare channels) • Continuous

• Fenestrated

• Sinusoids • Post capillary venules

• Muscular venules

• Medium (used for skeletal muscle pump)

• Large or sinuses

2) Circulatory Routes (Figs. 20.2, 20.9)

a. Most common route = heart-->arteries-->arterioles-->capillaries-->venules-->veins-->back to heart.

b. Portal system = blood flows through two consecutive capillary networks before returning to heart.

Example: between intestines - liver.

c. Anastomoses = artery directly to vein.

Find in fingers, toes, ears; reduces heat loss, allows blood to bypass exposed areas during cold.

3) Blood Pressure, Resistance, and Flow (figs 20.13, 20.14, 20.4, 20.10)

• Blood flow: amount of blood flowing through a tissue in a given time (ml/min).

• Perfusion: rate of blood flow per given mass of tissue (ml/min/g).

• Important for delivery of nutrients and oxygen, and removal of metabolic wastes.

• Blood flow and perfusion determined by blood pressure and resistance.

a. Blood Pressure - determined by cardiac output, blood volume, and peripheral resistance.

• Arterial Blood Pressure is measured using a sphygmomanometer at the brachial artery. Two measurements are reported: Systolic Pressure/Diastolic Pressure

o Systolic pressure: BP during ventricular systole.

o Diastolic pressure: BP during ventricular diastole.

Know normal values for blood pressure

Hypotension Normal Hypertension

Chronic resting bp of 90/60

Chronic resting bp of 120-135/75 Chronic resting bp of 140/90

• Pulse Pressure = Systolic Pressure – Diastolic Pressure

o an important measure of the force driving circulation and of the stress exerted on small arteries

• Mean Arterial Pressure (MAP) = Diastolic Pressure + 1/3 Pulse Pressure

o This estimates the mean pressure you would calculate by measuring blood pressure at several intervals throughout cardiac cycle

o Varies with gravity (lower in head than in feet)

o An important risk factor in clinical disorders such as syncope, atherosclerosis, kidney failure, edema, aneurysm.

b. Peripheral Resistance - determined by blood viscosity, blood vessel length, and blood vessel radius.

Viscosity: éviscosity = êblood flow

RBC count and albumin concentration elevate viscosity the most

Blood vessel length: édistance = êblood flow

The farther blood has to travel, the more cumulative friction it encounters

Blood vessel radius:

êradius = êblood flow; éradius = éblood flow

Blood flows more slowly close to the wall vs. the center. Larger center means faster blood flow

• Mathematically, blood flow is proportional to the fourth power of the radius: F µ r4

o This makes blood vessel radius the most powerful influence on blood flow

• This is the most adjustable variable and controls resistance quickly

• Vasomotion: change in vessel radius = vasoconstriction and vasodilation.

• 3 ways to control vasomotion:

1. Local control

2. Neural control (Baroreflex, Chemoreflex, Medullary Ischemic Reflex)

3. Hormonal control

Summary of Neural Control Mechanisms:

Baroreflex Chemoreflex Medullary Ischemic Reflex

Sensory receptors Baroreceptors in carotid sinus and aortic arch Chemoreceptors in carotid body and aortic bodies Receptors in medulla oblongata

What do the sensory receptors respond to? Changes in blood pressure Changes in blood chemistry (O2, CO2, pH) Changes in blood perfusion in the medulla

Responses êblood pressure

• Vasomotor center: vasoconstriction to éblood pressure

éblood pressure (see Figure 20.13)

• Vasomotor center: vasodilation to êblood pressure

• Cardiac center: stimulates parasympathetic fibers in vagus nerve -- ê Heart Rate (which also êblood pressure) êO2 & pH or éCO2

• Vasomotor center: vasoconstriction to éblood pressure (This é perfusion and rate of gas exchange in the lungs. The chemoreceptors will also é breathing rate to keep up with é perfusion).

éO2 & pH or êCO2

• Vasomotor center: vasodilation to êblood pressure êblood perfusion

• Vasomotor center: vasoconstriction to éblood pressure

• Cardiac center: stimulates sympathetic fibers -- éHeart Rate and contraction force

• Both changes are meant to restore cerebral perfusion

c. Two Main Purposes of Vasoreflexes (figs 20.14, 20.15)

1. Increase or decrease general blood pressure & flow rate of blood

2. Method of rerouting blood from one region to another for perfusion of individual organs. Blood is “rerouted” based on organ demand by vessel dilation or construction.

4) Capillary Exchange (figs. 20.16, 20.17)

Only occurs across capillary walls between blood and cells of surrounding tissues.

Routes of capillary fluid exchange

1. intercellular clefts

2. fenestrations (filtration pores)

3. through cytoplasm (diffusion)

Mechanisms involved: Diffusion, transcytosis, filtration, and reabsorption.

Filtration & Reabsorption (Figure 20.17):

1. Blood (hydrostatic) pressure drives fluid out of capillary.

high on arterial end of capillary, low on venous end.

2. Colloid osmotic pressure (COP) draws fluid into capillary (same on both ends).

results from plasma proteins (primarily albumin) - abundant in blood.

Outcome of 2 pressures: filtration or reabsorption depending on which pressure dominates.

Factors that lead to EDEMA (fluid accumulation in the body)

• Increased hydrostatic pressure

• Decreased colloid osmotic pressure

• Blockage of drainage to lymph capillaries

You should know the following Clinical Concepts:

• Coronary Artery Disease (CAD)

• Aneurism

• Varicose Veins

• Hypertension

• Hypotension

• Edema

• Circulatory Shock

5) Special Focus: Hypertension (“The Silent Killer”)

a. Definition: Chronic resting blood pressure >140/90

b. How big of a problem is Hypertension?

• According to the American Heart Association (AHA), ≈74 million Americans over the age of 20 have hypertension.

• Hypertension is responsible for ≈60,000 deaths/yr

• Most common cardiovascular disease, affecting 30% of Americans over the age of 50

c. Dangers of Hypertension:

• Weakens small arteries

o Cause aneuryisms

o Promote atherosclerosis because the elevated blood flow tears the endothelium and creates lesions that become focal points for plaques (Atherosclerosis then worsens the hypertension to form insidious positive feedback cycle)

• Damages the Heart – pressure in the arteries just superior to the heart increases afterload, so the ventricles must work harder to eject blood. The myocardium enlarges, but over time becomes overstretched and inefficient.

o This stroke volume   cardiac output   heart rate

• Damages the Kidneys: renal arterioles thicken, which  renal blood pressure   release of hormones that retain salt in an effort to  blood pressure  makes overall hypertension even worse

Hypertension is therefore a major cause of stroke, heart failure and kidney failure.

d. What causes Primary Hypertension?

Risk factors/causes for Primary Hypertension (90% of cases) include:

Unavoidable risk factors Avoidable risk factors

• Heredity – runs in some families

• Race – incidence is 30% in blacks than whites (incidence of stroke is 2X as high

• Gender – more common in men aged 18-54; more common in women aged 65+

• Aging – blood vessels become damaged with the aging process:

o Arteriosclerosis: hardening of arteries due to free radical damage -> gradual degeneration of elastic tissue and other tissues in blood vessel wall (“old rubber bands”)

o Atherosclerosis: lipid deposits that become calcified, complicated plaques (“like bone”)

• Stress/aggressive personality

• Obesity: each lb of fat requires miles of blood vessels to serve it -> vessel length -> resistance to blood flow

• Sedentary lifestyle: Aerobic exercise reduces hypertension by controlling weight, reducing emotional tension, and stimulating vasodilation

• Diet:

o Salt intake: blood osmolarity and blood volume (although the kidneys secrete excess salt except in older adults or those with reduced renal function

o Cholesterol & fat: contributes to development of atherosclerotic plaques

o Potassium & Magnesium: these ions help to reduce blood pressure

o Smoking: Nicotine stimulates heart to beat faster and harder, but causes vasoconstriction of blood vessels -> afterload against which myocardium must work

e. Treatment for Primary Hypertension:

• Stress reduction

• Weight loss

• Exercise

• Dietary changes (salt & fat/cholesterol intake, magnesium & potassium)

• Stop smoking

• Drugs to:

o urination in order to blood volume (e.g. diuretics)

o Keep your blood vessels vasodilated, e.g. ACE inhibitors & Beta blockers block production of Angiotensin II, a powerful vasoconstrictor

o  your heart rate, e.g. calcium channel blockers (e.g. verapamil, nifedipine)

f. What causes Secondary Hypertension (10% of cases)?

• Secondary to (results from) other identifiable disorders, e.g.

o Kidney disease (with hypersecretion of renin, which ultimately causes release of Angiotensin II and Aldosterone)

o Atherosclerosis

o Hyperthyroidism

o Cushing Syndrome

o Polycythemia

g. Treatment for Secondary Hypertension:

• Treatment of underlying cause

6) Anatomy of Arteries & Veins (figs. 19.5, 19.8, 20.20, 20.21. 20.22, 20.23, 20.24, 20.25, 20.29, 20.30, 20.31, 20.32, 20.33, 20.34, 20.35, 20.36, 20.37, 20.38)

a. Coronary Arteries (figs 19.5, 19.8)

The heart itself receives 250 ml of blood per minute, which is about 5% of the circulating blood. The blood is delivered to the heart via the coronary arteries.

The Left coronary artery (LCA) travels through the coronary sulcus and divides into 2 branches:

1. The Anterior interventricular branch supplies both ventricles and anterior two-thirds of the interventricular septum

2. The Circumflex branch supplies the left atrium and posterior wall of left ventricle

The Right Coronary Artery (RCA) passes along the coronary sulcus under right auricle and divides into 2 branches:

1. Right marginal branch supplies right atrium and sinoatrial node (pacemaker)

2. Posterior interventricular branch supplies the posterior walls of the ventricles as well as the posterior portion of the interventricular septum.

b. Pulmonary Arteries (figs. 19.5, 20.20)

The purpose of the pulmonary circuit is to exchange CO2 for O2.

Blood flow: Right Ventricle èPulmonary trunk è left and right pulmonary arteries èlungs

In both lungs, these arteries ultimately lead to small basketlike capillary beds that surround the pulmonary alveoli. This is where the blood unloads the CO2 and picks up O2.

c. Systemic Arteries (fig. 20.21)

1. Aorta and major branches:

The ascending aorta (fig. 20.23) rises for about 5 cm above the left ventricle. Its only branches are the coronary arteries.

Aortic arch (fig. 20.23):

1. Brachiocephalic trunk: subdivides into right common carotid (supplies right side of head) and right subclavian (supplies right shoulder and upper limb)

2. Left common carotid artery (supplies left side of head)

3. Left subclavian (supplies left shoulder and upper limb)

The descending aorta (fig.20.23) is called the thoracic aorta above the diaphragm and the abdominal aorta below the diaphragm. Gives rise to arteries of the abdomen and pelvic regions (see #3 below).

2. Arteries of Head and Neck (fig. 20.24)

Common carotids (one on each side of neck) split to form the external carotid (supplies external head structures except the orbits) and the internal carotid (supplies the brain)

A parallel set of arteries also run up the side of the neck, called the vertebral arteries. They converge on the underside of the brain to form the basilar artery.

Cerebral Arterial Circle (Circle of Willis) on base of brain is formed from anastomosis of basilar and internal carotid arteries. Supplies the brain, inner ear, and orbital structures

3. Arteries of the Abdominal and Pelvic regions:

Celiac Trunk (fig. 20.29, 20.30) gives rise to 3 branches:

1. Left gastric artery: supplies stomach and lower esophagus

2. Splenic artery: supplies spleen (with branches to stomach and pancreas)

3. Common hepatic artery: supplies liver and stomach

Superior and inferior mesenteric arteries (fig. 20.29, 20.31): The superior mesenteric artery is the most significant contributor to intestinal blood supply, serving nearly all the small intestines and the proximal half of the large intestine. The inferior mesenteric artery supplies the distal end of the large intestine.

Gonadal arteries (fig.20.29) supply the gonads (testes in males, ovaries in females)

Renal arteries (fig.20.29) supply the kidneys and adrenal glands

The 2 common iliac arteries (fig.20.29) arise by branching of the aorta, descend for another 5 cm, then each divides into an external and internal iliac artery. The external iliac arteries supply the lower limbs. The Internal iliac arteries supply pelvic wall and viscera

4. Arteries of the Upper Limb (fig.20.34): The upper limb is supplied by a prominent artery that changes name along its course from subclavian to axillary to brachial. The brachial artery ends just distal to the elbow (most common site of blood pressure measurement). The radial artery descends alongside the radius, supplying the lateral forearm muscles (most common place to take pulse is radial artery just proximal to the thumb). The ulnar artery descends medially alongside the ulna, supplying the medial forearm muscles.

5. Arteries of Lower Limb (fig.20.36): Branches to the lower limb arise from external iliac branch of the common iliac artery. The external iliac artery becomes the femoral artery.

Femoral artery passes through femoral triangle of thigh and gives off branches before continuing to the knee. The largest branch is the deep femoral artery, which supplies the thigh muscles. The popliteal artery is a continuation of the femoral artery in the popliteal fossa at the rear of the knee. It splits into the anterior and posterior tibial arteries.

Anterior tibial arteries give rise to the dorsal pedal artery (traverse ankle and supplies top or medial surface of foot). Posterior tibial arteries supplies flexor muscles along tibia and goes into plantar region of foot.

d. Coronary Veins and Pulmonary Veins (figs 19.5, 19.8): The Coronary sinus is a large transverse vein that travels in the coronary sulcus on the posterior side of the heart. Collects blood from smaller veins and empties into the right atrium. After blood unloads C02 and picks up O2, it flows into pulmonary venules and veins, ultimately leading to right and left main pulmonary veins (2 each) that exit the lungs and empty into the left atrium.

e. Systemic Veins (fig.20.22)

1. Superior and Inferior Vena Cavae (fig.19.5): The blood returns to the heart via 2 large veins, the superior and inferior vena cavae. The superior vena cava drains the head, neck and upper limbs, while the inferior vena cava drains the lower limbs and abdomen.

The inferior vena cava (IVC ) is the body’s largest blood vessel, (diameter of 3.5 cm). It forms by the union of the left and right common iliac veins at the level of vertebra L5.

2. Veins of head and neck (fig.20.26):

a. Internal jugular vein drains blood from the brain

b. Branches of external jugular vein drain external parts of the head (salivary glands, face muscles, scalp, etc)

c. Both join the Subclavian vein

3. Veins of Upper Limb (fig.20.35):

a. Superficial:

i. Cephalic vein (Intravenous fluids often given through this vein.)

ii. Basilic vein

iii. Medial cubital (site of blood draws)

b. Deep:

i. Radial veins

ii. Ulnar veins

c. The Axillary vein is formed by the union of the brachial and basilic veins. It passes through axillary region, picking up the cephalic vein along the way. At the lateral margin of first rib, it changes name to subclavian vein.

d. The subclavian continues into the shoulder posterior to the clavicle and ends where it meets the internal jugular vein in the neck. There it becomes the brachiocephalic vein.

4. Veins of Abdominal and Pelvic regions:

Hepatic Portal System (fig.20.33): receives all blood draining from abdominal digestive tract, as well as pancreas, gallbladder, and spleen. The hepatic portal system gives the liver first claim to digestive nutrients before the blood is distributed to the rest of the body. It also allows the blood to be cleansed of bacteria and toxins picked up from the intestines, an important function of the liver.

a. The inferior mesenteric vein receives blood from the rectum and distal part of the colon.

b. The superior mesenteric vein receives blood from the entire small intestine, ascending colon, transverse colon and stomach.

c. The splenic vein drains the spleen and travels across the abdominal cavity toward the liver. Along the way, it picks up pancreatic veins from the pancreas, then the inferior mesenteric vein, then ends where it meets the superior mesenteric vein.

d. Left gastric vein drains the stomach

e. The hepatic portal vein is the continuation beyond the convergence of the splenic and superior mesenteric veins. It travels about 8 cm upward and to the right, receive the cystic vein from the gallbladder, then enters the inferior surface of the liver.

f. In the liver, the hepatic portal vein leads to innumerable hepatic sinusoids

g. Blood from the sinusoids empties into the hepatic vein

h. On the other side of the liver, the hepatic vein drains into the inferior vena cava.

Renal Veins (fig.20.32) drain the kidneys and adrenal glands

Gonadal veins (fig. 20.32) drain the gonads (testes in males, ovaries in females)

Internal Iliac veins (fig. 20.32) drain the gluteal muscles, the medial aspect of the thigh, the bladder, rectum, prostate, and ductus deferens in males, the uterus and vagina in females. They unite with external iliac veins, which drain the lower limb, to form the common iliac veins, which then converge to form the inferior vena cava.

5. Veins of the Lower Limb (fig. 20.38)

a. Superficial: Great Saphenous vein (longest vein in the body) travels all the way up the leg and thigh to the inguinal region. Empties into femoral vein slightly inferior to the inguinal ligament. Commonly used for long-term administration of IV fluids. Portions of this vein are also commonly used for graphs in coronary bypass surgery. Common site for varicose veins.

b. Deep:

c. Deep:

i. 2 posterior tibial veins pass up leg embedded deep in calf muscles. They converge like inverted Y in to a single vein about 2/3 way up tibia

ii. 2 fibular veins ascend the back of leg and similarly converge like a Y

iii. Popliteal vein begins near the knee by convergence of y-shaped vessels in calf. Passes through the popliteal fossa in back of the knee.

iv. 2 anterior tibial veins travel up anterior compartment of leg between tibia and fibula. They aris from medial side of dorsal veinous arch, converge just distal to knee, then flow into popliteal vein.

v. Femoral vein continuation of popliteal vein in the thigh. Drains blood from thigh and femur.

vi. External iliac is formed by union of femoral and great saphenous veins near the inguinal ligament.