V. Arterial System: Arterioles

a. Smallest arteries
b. Lead to capillary beds
c. Control flow into capillary beds via vasodilation and vasoconstriction

VI. Capillaries

a. Microscopic blood vessels
b. Walls of thin tunica intima
c. In smallest one cell forms entire circumference
d. Pericytes help stabilize their walls and control permeability
e. Diameter allows only single RBC to pass at a time
f. In all tissues except for cartilage, epithelia, cornea and lens of eye
g. Provide direct access to almost every cell
h. Functions
i. Exchange of gases, nutrients, wastes, hormones, etc., between blood and interstitial fluid

3 Types of Capillaries

i. Continuous Capillaries
1. Abundant in skin and muscles
2. Tight junctions connect endothelial cells
3. Intercellular clefts allow passage of fluids and small solutes
4. Continuous capillaries of brain unique
5. Tight junctions complete, forming blood brain barrier
ii. Fenestrated Capillaries
1. Some endothelial cells contain pores (fenestrations)
2. More permeable than continuous capillaries
3. Function in absorption or filtrate formation (small intestines, endocrine glands, and kidneys)
iii. Sinusoid Capillaries
1. Fewer tight junctions; usually fenestrated; larger intercellular clefts; large lumens
2. Blood flow sluggish – allows modification
3. Large molecules and blood cells pass between blood and surrounding tissues
4. Found only in the liver, bone marrow, spleen, adrenal medulla
5. Macrophages in lining to destroy bacteria

j. Capillary Beds - Microcirculation

i. Interwoven networks of capillaries between arterioles and venules
ii. Terminal arteriole → metarteriole
iii. Metarteriole continuous with thoroughfare channel (intermediate between capillary and venule)
iv. Thoroughfare channel → postcapillary venule that drains bed
1. True capillaries normally branch from metarteriole and return to thoroughfare channel
v. Precapillary sphincters regulate blood flow into true capillaries
vi. Regulated by local chemical conditions and vasomotor nerves

VII. Venous System: Venules

a. Formed when capillary beds unite
b. Very porous; allow fluids and WBCs into tissues
c. Consist of endothelium and a few pericytes
d. Larger venules have one or two layers of smooth muscle cells

VIII. Veins

a. Formed when venules converge
b. Have thinner walls, larger lumens compared with corresponding arteries
c. Blood pressure lower than in arteries
d. Thin tunica media; thick tunica externa of collagen fibers and elastic networks
e. Called capacitance vessels (blood reservoirs); contain up to 65% of blood supply
f. Adaptations ensure return of blood to heart despite low pressure
i. Large-diameter lumens offer little resistance
ii. Venous valves prevent backflow of blood
iii. Most abundant in veins of limbs
iv. Venous sinuses: flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain)

IX. Vascular Anastomoses

a. Interconnections of blood vessels
b. Arterial anastomoses provide alternate pathways (collateral channels) to given body region
c. Common at joints, in abdominal organs, brain, and heart; none in retina, kidneys, spleen

a. Blood flow

i. Volume of blood flowing through vessel, organ, or entire circulation in given period
ii. Measured as ml/min
iii. Equivalent to cardiac output (CO) for entire vascular system
iv. Relatively constant when at rest
v. Varies widely through individual organs, based on needs

b. Blood pressure (BP)

i. Force per unit area exerted on wall of blood vessel by blood
ii. Expressed in mm Hg
iii. Measured as systemic arterial BP in large arteries near heart
iv. Pressure gradient provides driving force that keeps blood moving from higher to lower pressure areas

c. Resistance (peripheral resistance)

i. Opposition to flow
ii. Measure of amount of friction blood encounters with vessel walls, generally in peripheral (systemic) circulation
iii. Three important sources of resistance
1. Blood viscosity
a. The "stickiness" of blood due to formed elements and plasma proteins
b. Increased viscosity = increased resistance
2. Blood vessel length
a. Longer vessel = greater resistance encountered
3. Blood vessel diameter
a. Greatest influence on resistance
b. Frequent changes alter peripheral resistance
c. Varies inversely with fourth power of vessel radius
d. E.g., if radius is doubled, the resistance is 1/16 as much
e. E.g., Vasoconstriction → increased resistance
f. Abrupt changes in diameter or fatty plaques from atherosclerosis dramatically increase resistance
g. Irregular fluid motion → increased resistance

d. Relationship Between Blood Flow, Blood Pressure, and Resistance

i. Blood flow (F) directly proportional to blood pressure gradient (ΔP)
1. If ΔP increases, blood flow speeds up
ii. Blood flow inversely proportional to peripheral resistance (R)
1. If R increases, blood flow decreases: F = ΔP/R
a. R more important in influencing local blood flow because easily changed by altering blood vessel diameter

XI. Systemic Blood Pressure

a. Pumping action of heart generates blood flow
b. Pressure results when flow is opposed by resistance
c. Systemic pressure is highest in aorta
i. Declines throughout pathway
d. Arterial Blood Pressure
i. Reflects two factors of arteries close to heart
1. Elasticity (compliance or distensibility)
2. Volume of blood forced into them at any time
ii. Systolic pressure: pressure exerted in aorta during ventricular contraction
1. Averages 120 mm Hg in normal adult
iii. Diastolic pressure: lowest level of aortic pressure
iv. Pulse pressure = difference between systolic and diastolic pressure

XII. Capillary Blood Pressure

a. Ranges from 17 to 35 mm Hg
b. Low capillary pressure is desirable
c. High BP would rupture fragile, thin-walled capillaries

XIII. Venous Blood Pressure

a. Small pressure gradient; about 15 mm Hg
b. Low pressure due to cumulative effects of peripheral resistance

XIV. Factors Aiding Venous Return

a. Muscular pump: contraction of skeletal muscles "milks" blood toward heart; valves prevent backflow
b. Respiratory pump: pressure changes during breathing move blood toward heart by squeezing abdominal veins as thoracic veins expand
c. Venoconstriction under sympathetic control pushes blood toward heart

XV. Maintaining Blood Pressure

a. Requires
i. Cooperation of heart, blood vessels, and kidneys
ii. Supervision by brain
b. Main factors influencing blood pressure
i. Cardiac output (CO)
ii. Peripheral resistance (PR)
iii. Blood volume
c. F = ΔP/R; CO = ΔP/R; ΔP = CO × R
i. Blood pressure = CO × PR (and CO depends on blood volume)
ii. Blood pressure varies directly with CO, PR, and blood volume
iii. Cardiac Output (CO)
1. CO = SV × HR; normal = 5.0-5.5 L/min
2. Determined by venous return, and neural and hormonal controls
3. Resting heart rate maintained by cardioinhibitory center via parasympathetic vagus nerves
4. Stroke volume controlled by venous return (EDV)

XVI. Control of Blood Pressure (Short Term)

i. Counteract fluctuations in blood pressure by altering peripheral resistance and CO
ii. Short-term Mechanisms: Neural Controls
1. Neural controls of peripheral resistance
2. Maintain MAP by altering blood vessel diameter
3. If low blood volume all vessels constricted except those to heart and brain
4. Neural controls operate via reflex arcs that involve
a. The Cardiovascular Center
i. Clusters of sympathetic neurons in medulla oversee changes in CO and blood vessel diameter
ii. Consists of cardiac centers and vasomotor center
iii. Vasomotor center sends steady impulses via sympathetic efferents to blood vessels → moderate constriction called vasomotor tone
iv. Receives inputs from baroreceptors, chemoreceptors, and higher brain centers
b. Baroreceptors
i. Located in : Carotid sinuses, Aortic arch, Walls of large arteries of neck and thorax
ii. Baroreceptor Reflexes
1. Inhibits vasomotor and cardioacceleratory centers, causing arteriolar dilation and venodilation
2. Stimulates cardioinhibitory center → decreased blood pressure
iii. → Reflex vasoconstriction → increased CO → increased blood pressure
1. Ex. Upon standing baroreceptors of carotid sinus reflex protect blood to brain; in systemic circuit as whole aortic reflex maintains blood pressure
c. Chemoreceptor Reflexes
i. Chemoreceptors in aortic arch and large arteries of neck detect increase in CO2, or drop in pH or O2
ii. Cause increased blood pressure by
iii. Signaling cardioacceleratory center → increase CO
iv. Signaling vasomotor center → increase vasoconstriction
d. Higher Brain Centers
i. Reflexes in medulla
ii. Hypothalamus and cerebral cortex can modify arterial pressure via relays to medulla
iii. Hypothalamus increases blood pressure during stress
iv. Hypothalamus mediates redistribution of blood flow during exercise and changes in body temperature
e. Hormonal Controls
i. Cause increased blood pressure
1. Epinephrine and norepinephrine from adrenal gland → increased CO and vasoconstriction
2. Angiotensin II stimulates vasoconstriction
3. High ADH levels cause vasoconstriction
ii. Cause lowered blood pressure
1. Atrial natriuretic peptide causes decreased blood volume by antagonizing aldosterone

XVI. Control of Blood Pressure (Long Term)

iii. Long-term Mechanisms: Renal Regulation
1. Long-term mechanisms control BP by altering blood volume via kidneys
2. Direct Renal Mechanism
a. Alters blood volume independently of hormones
b. Increased BP or blood volume causes elimination of more urine, thus reducing BP
c. Decreased BP or blood volume causes kidneys to conserve water, and BP rises
3. Indirect Mechanism
a. The renin-angiotensin-aldosterone mechanism
b. ↓ Arterial blood pressure → release of renin
c. Renin catalyzes conversion of angiotensinogen from liver to angiotensin I
d. Angiotensin converting enzyme, especially from lungs, converts angiotensin I to angiotensin II
i. Functions of Angiotensin II
1. Increases blood volume
2. Stimulates aldosterone secretion
3. Causes ADH release
4. Triggers hypothalamic thirst center

XVII. Monitoring Circulatory Efficiency

a. Vital signs: pulse and blood pressure, along with respiratory rate and body temperature
b. Pulse: pressure wave caused by expansion and recoil of arteries
i. Radial pulse (taken at the wrist) routinely used
ii. Pressure points where arteries close to body surface
iii. Can be compressed to stop blood flow

XVIII. Measuring Blood Pressure

a. Systemic arterial BP
b. Measured indirectly by auscultatory method using a sphygmomanometer
c. Pressure increased in cuff until it exceeds systolic pressure in brachial artery
d. Pressure released slowly and examiner listens for sounds of Korotkoff with a stethoscope
e. Systolic pressure, normally less than 120 mm Hg, is pressure when sounds first occur as blood starts to spurt through artery
f. Diastolic pressure, normally less than 80 mm Hg, is pressure when sounds disappear because artery no longer constricted; blood flowing freely

XIX. Variations in Blood Pressure

a. Transient elevations occur during changes in posture, physical exertion, emotional upset, fever. Age, sex, weight, race, mood, and posture may cause BP to vary
b. Hypertension: high blood pressure
i. Sustained elevated arterial pressure of 140/90 or higher
ii. Prolonged hypertension major cause of heart failure, vascular disease, renal failure, and stroke
iii. Heart must work harder → myocardium enlarges, weakens, becomes flabby
iv. Risk factors include heredity, diet, obesity, age, diabetes mellitus, stress, and smoking
v. No cure but can be controlled
1. Restrict salt, fat, cholesterol intake
2. Increase exercise, lose weight, stop smoking
3. Antihypertensive drugs
vi. Secondary hypertension less common
1. Due to identifiable disorders including obstructed renal arteries, kidney disease, and endocrine disorders such as hyperthyroidism and Cushing's syndrome
c. Hypotension: low blood pressure
i. Blood pressure below 90/60 mm Hg
ii. Usually not a concern
iii. Only if leads to inadequate blood flow to tissues
iv. Often associated with long life and lack of cardiovascular illness
v. Orthostatic hypotension: temporary low BP and dizziness when suddenly rising from sitting or reclining position

XX. Blood Flow Through Body Tissues

a. Tissue perfusion involved in
i. Delivery of O2 and nutrients to, and removal of wastes from, tissue cells
ii. Gas exchange (lungs)
iii. Absorption of nutrients (digestive tract)
iv. Urine formation (kidneys)
v. Rate of flow is precisely right amount to provide proper function

XXI. Velocity of Blood Flow

a. Changes as travels through systemic circulation
b. Inversely related to total cross-sectional area
i. Fastest in aorta; slowest in capillaries; increases in veins
ii. Slow capillary flow allows adequate time for exchange between blood and tissues

XXII. Autoregulation

a. Automatic adjustment of blood flow to each tissue relative to its varying requirements
b. Controlled intrinsically by modifying diameter of local arterioles feeding capillaries
c. Two types of autoregulation
i. Metabolic Controls
1. Vasodilation of arterioles and relaxation of precapillary sphincters occur in response to
2. Declining tissue O2
3. Substances from metabolically active tissues (H+, K+, adenosine, and prostaglandins) and inflammatory chemicals
4. Effects
a. Relaxation of vascular smooth muscle
b. Release of NO (powerful vasodilator) by endothelial cells
c. Endothelins released from endothelium are potent vasoconstrictors
d. NO and endothelins balanced unless blood flow inadequate, then NO wins
e. Inflammatory chemicals also cause vasodilation
ii. Myogenic Controls
1. Myogenic responses keep tissue perfusion constant despite most fluctuations in systemic pressure
2. Vascular smooth muscle responds to stretch
3. Passive stretch (increased intravascular pressure) promotes increased tone and vasoconstriction
4. Reduced stretch promotes vasodilation and increases blood flow to the tissue

XXIII. Long-term Autoregulation

a. Occurs when short-term autoregulation cannot meet tissue nutrient requirements
b. Angiogenesis - Number of vessels to region increases and existing vessels enlarge
i. Common in heart when coronary vessel occluded, or throughout body in people in high-altitude areas

XXIV. Blood Flow: Skeletal Muscles

a. Varies with fiber type and activity
b. Active or exercise hyperemia - blood flow increases in direct proportion to metabolic activity

XXV. Blood Flow: Brain

a. Metabolic controls - Decreased pH of increased carbon dioxide cause marked vasodilation
b. Myogenic controls - Decreased MAP causes cerebral vessels to dilate and Increased MAP causes cerebral vessels to constrict
c. Brain vulnerable under extreme systemic pressure changes
i. MAP below 60 mm Hg can cause syncope (fainting)
ii. MAP above 160 can result in cerebral edema

XXVI. Blood Flow: Skin

a. Supplies nutrients to cells (autoregulation in response to O2 need)
b. Helps regulate body temperature (neurally controlled) – primary function
i. As temperature rises (e.g., heat exposure, fever, vigorous exercise)
ii. Hypothalamic signals reduce vasomotor stimulation of skin vessels →Warm blood flushes into capillary beds →Heat radiates from skin
iii.
c. Provides a blood reservoir (neurally controlled)

XXVII. Blood Flow: Lungs

a. Pulmonary circuit unusual
i. Pathway short
ii. Arteries/arterioles more like veins/venules (thin walled, with large lumens)
b. Autoregulatory mechanism opposite that in most tissues
i. Low O2 levels cause vasoconstriction; high levels promote vasodilation
ii. Allows blood flow to O2-rich areas of lung

XXVIII. Blood Flow: Heart

a. During ventricular systole
i. Coronary vessels are compressed
ii. Myocardial blood flow ceases
iii. Stored myoglobin supplies sufficient oxygen
b. During diastole high aortic pressure forces blood through coronary circulation
c. During strenuous exercise
i. Coronary vessels dilate in response to local accumulation of vasodilators
ii. Blood flow may increase three to four times

XXIX. Blood Flow Through Capillaries

a. Capillary Exchange of Respiratory Gases and Nutrients
i. Diffusion down concentration gradients
ii. O2 and nutrients from blood to tissues
iii. CO2 and metabolic wastes from tissues to blood
iv. Lipid-soluble molecules diffuse directly through endothelial membranes
v. Water-soluble solutes pass through clefts and fenestrations
vi. Larger molecules, such as proteins, are actively transported in pinocytotic vesicles or caveolae
b. Fluid Movements: Bulk Flow
i. Fluid leaves capillaries at arterial end; most returns to blood at venous end
ii. Extremely important in determining relative fluid volumes in blood and interstitial space
iii. Direction and amount of fluid flow depend on two opposing forces: hydrostatic and colloid osmotic pressures
1. Hydrostatic Pressures - Tends to force fluids through capillary walls
2. Colloid Osmotic Pressures - Created by nondiffusible plasma proteins, which draw water toward themselves
iv. Hydrostatic-osmotic Pressure Interactions: Net Filtration Pressure (NFP)
1. NFP—comprises all forces acting on capillary bed
2. NFP = (HPc + OPif)¬ – (HPif + OPc)
3. Net fluid flow out at arterial end
4. Net fluid flow in at venous end
5. More leaves than is returned
6. Excess fluid returned to blood via lymphatic system

Circulatory Shock

Any condition in which Blood vessels inadequately filled, Blood cannot circulate normally
a. Results in inadequate blood flow to meet tissue needs
b. Hypovolemic shock: results from large-scale blood loss
c. Vascular shock: results from extreme vasodilation and decreased peripheral resistance
d. Cardiogenic shock results when an inefficient heart cannot sustain adequate circulation