Blood Vessels and Circulation Chapter 19

Blood Distribution at Rest

% of blood volume: 30 - 35% in pulmonary circuit, heart, systemic arteries, and capillaries 65 - 70% in systemic veins approximately 5 - 6 liters of blood within the closed circulatory system. Veins serve as a blood reservoir

Formula for Resistance:

(vessel length * viscosity of blood) / (vessel radius)^4

In the systemic circulation, mean arterial pressure range is:

100 mm Hg at the aorta - 35mm Hg at the arterial end of a capillary bed

Venous pressure in systemic circulation:

18 mm Hg to almost 0 mmHg at the distal end of the vena cava.

Venous pressure gradient in systemic circulation:

18mmHg in smallest veins - almost 0 mmHg at entrance of right atrium

Arteriovenous Anastomosis

A direct connection between an arteriole and venule that is not part of a capillary bed. It allows blood to bypass the capillary bed

Thoroughfare Channel

A direct connection between arterioles and venules within the capillary bed

Mean Arterial Blood Pressure

Although arterial pressure fluctuates, a single value is used to represent conditions within the body. MAP = diastolic pressure + 1/3 (pulse pressure) Ex. 120 (systolic)/80(diastolic) 80 mmHg + [1/3(120mmHg - 80 mmHg)] 80mmHg + [1/3 (40 mmHg)] 80 mmHg + 13.3 mmHg 93.3 mmHg

Resistance

Any force that opposes motion. The resistance of the cardiovascular system opposes movement of blood flow

Autoregulation and Changing Metabolic Activity

Autoregulation is the ability of individual vascular beds to control local blood flow in response to changing metabolic needs. Blood flow in most tissues increases in proportion to the metabolic demand of the tissue. An example is during strenuous exercise, blood flow may increase up to twenty times over resting levels. Most is mediated by the changes in the chemical composition of the interstitial fluid immediately surrounding the metabolically active tissue. Autoregulation causes immediate and localized homeostatic adjustments.

Muscular arteries or medium-sized arteries

Average diameter of 4mm Distributes blood to the body's skeletal muscle and organs (external carotid arteries of neck, brachial arteries of arm and femoral arteries of thigh) Characterized by a thick tunica media containing more smooth muscle than that of elastic arteries

Antidiuretic Hormone (ADH) Targets

Blood Vessels: peripheral vasoconstriction Kidneys: conservation of water (less urine production) Brain Centers: stimulates thirst to increase water consumption

Vasomotion

Blood flow into a capillary bed is generally constant, although blood flow into individual capillaries within the capillary bed varies due to opening and closing of pre-capillary sphincters. The rhythmic changes in capillary diameter are vasomotion

Neural Regulation of Blood Pressure

Blood pressure must be adequate to overcome resistance and provide perfusion of all tissues - but not be too high as to cause damage to the blood vessels. It is important that cardiovascular regulation (1) occurs in an appropriate time, (2) In an appropriate location and (3) without drastically changing blood pressure and blood flow to vital organs - brain, heart, liver, skeletal muscle

Capillaries

Blood vessels that allow for the exchange of nutrients and gases to the interstitial fluids due to tissue anatomy and the slow rate of blood flow through them Connect the arteries to the veins Millions of capillaries are found in the body

Renin-Angiotensin system targets:

Blood vessels: peripheral vasoconstriction resulting in increased resistance Kidney: decrease urine production Brain centers: stimulates thirst to increase water consumption

REMEMBER

CO α ∆P/R Or ∆P α (CO) (R)

The cross-sectional area of arteries increases as they approach the:

Capillaries

The diameter of arteries decrease as they approach the:

Capillaries

Continuous Capillary

Capillary lining is complete Materials can move in and out of the blood through endothelial cells (diffusion, pinocytosis, selective transport), which prevent the movement of large substances from the plasma Most common type of capillary. Can be found throughout the body except in cartilage and epithelia

Total Blood Flow =

Cardiac Output measured in liters/minute

Arteries

Carry blood away from the heart (Efferent)

Vasodilation and Vasoconstriction is the mechanism for what:

Changing resistance in any given time within the vascular system. Most peripheral resistance occurs in the arterioles - the resistance vessels.

Venules

Collect blood from capillary beds May resemble a capillary

Large Veins

Collect blood from medium-sized veins. All tunics are present Ex. superior and inferior vena cava

Medium-sized veins

Collect blood from the venules They have a thin tunica media with few smooth muscles; tunica externa thickest layer

Tunica Media

Contains concentric sheets of smooth muscle tissue in a framework of loose connective tissue When smooth muscle is stimulated (or contracts), arteries asoconstrict and the diameter of the lumen decrease. Relaxation of smooth muscle or vasodilation casues an increase in the diameter of the limen

Result of Atrial Natriuretic Peptide (ANP)

Decrease in blood volume and decrease in vascular resistance. Therefore, there is a decrease in blood pressure to restore normal blood pressure.

If cardiac output decreases, then total blood flow:

Decreases and less blood is available to body tissues

How much pressure must the heart generate to overcome this resistance of the blood vessels?

Enough pressure must be generated to overcome the resistance offered between the aorta as blood flows from the left ventricle to the vena cava as blood enters into the right atrium of the heart. Therefore, this is the pressure difference between the aorta (around 93 mmHg) and the vena cava (near 0 mmHg) - or about 93 mmHg of pressure. Approximately 30 mmHg pressure is generated by the right ventricle as blood enters into the pulmonary circulation, yet the blood flows to the heart at the same rate as the systemic circulation. Therefore, it is known that the resistance to blood flow must be about three times greater in the systemic circulation than the pulmonary circulation.

Light exercise

Extensive vasodilation occurs: skeletal muscle oxygen consumption increases. Peripheral resistance drops, blood flow through the capillaries increases and blood enters the venous system faster. b. The venous return increases: (1) Skeletal pump: venous return increases as skeletal muscle contractions squeeze blood along the peripheral veins. (2) Respiratory pump: Each inhalation creates a negative pressure in the thoracic cavity that pulls blood into the vena cava toward the heart. c. Cardiac output rises: due to increase in venous return (Starling response of the heart). CO keeps up with demand of the tissue for oxygen and blood pressure is maintained.

F (L/min) ~ r4

F = flow, ~ means proportional to and r = radius. For example, if a blood vessel increases its diameter by a factor of 2 (it is twice its original size), then flow would increase by a factor of 16 or 2^4

Arteries have a small cross-sectional area - resulting in:

Fast Velocity

Veins have a small cross-sectional area - resulting in:

Fast Velocity

Valves Structure

Folds of the tunica intima

Causes of Vascular Resistance:

Friction Viscosity Turbulence

The cross-sectional area of veins decreases as they approach the:

Heart

The diameter of veins increases as they approach the:

Heart

Turbulence

High flow rates, blood movement and changes in blood movement due to irregular surfaces and sudden changes in vessel diameter. Increasing turbulence will increase resistance.

Tunica Intima

Includes the endothelial lining and an underlying layer of connective tissue In arteries, the outer margin of the tunica intima contains a thick layer of elastic fibers called the internal elastic membrane

Result of Antidiuretic Hormone (ADH)

Increase in resistance and blood volume and therefore an increase in blood pressure in order to restore normal blood pressure.

Result of Renin-Angiotensin system

Increase in resistance and blood volume, and therefore an increase in blood pressure in order to restore normal blood pressure

Stimulating the vasomotor center and the sympathetic nervous system cause the following:

Increased peripheral resistance Larger circulating volume Redistribution of blood flow

If cardiac output increases, then total blood flow:

Increases, and more blood is available to body tissues

Vessel Length

Increasing the length of a blood vessel cumulates friction. Vessel length remains constant and this component of vascular resistance remains constant Ex. Blowing water out of a drinking straw vs. a garden hose

Vaso Vasorum

Is a network of small arteries and veins located in the wall of large blood vessels. These vessels supply blood to the smooth muscles and cells in the vessel wall

Aldosterone Targets

Kidney: conserve sodium

Atrial Natriuretic Peptide (ANP) Targets

Kidney: stimulates the release sodium into urine and therefore promotes the loss of water and increase in urine output. Peripheral blood vessels: stimulate vasodilation

Factors involved in the regulation of cardiovascular function:

Local Neural Endocrine

Capillary Exchange

Most tissues are no more than 3-4 cells away from a capillary. The anatomy of a capillary facilitates the rapid diffusion or transport of materials into or out of the circulation.

Diffusion occurs in capillaries by:

Moving through cell membranes Passing between adjacent cells Moving through cell membrane channels Coursing through pores of fenestrated capillaries Moving through openings in sinusoids

Cardiovascular Physiology

Must maintain blood flow through the peripheral tissues and organs

Diffusion

Net movement of molecules from areas of high concentration to areas of low concentration. Diffusion occurs most readily in areas where the concentration gradient is large, the distances involved are small and the substance, ion or molecule is small.

Imagine that you are outside; watering your front lawn with a garden hose, and the hose is not quite long enough to reach the corner of the yard. How can you make the water from the hose squirt the extra distance?

One way is to go back to the house and open up the faucet, so that more water is running through the hose (increase hydrostatic pressure by increasing the amount of water forcefully being pushed out of the hose - in other words, increase output or flow). Suppose you didn't feel like walking back to the house? You could use your finger to close off part of the opening at the end of the hose. This action decreases the diameter of the opening and increases resistance. Increasing resistance increases water pressure and therefore causes the water to squirt further

Tunica Externa

Outermost layer: forms a connective tissue covering around the vessel The connective tissue of the tunica externa tends to blend with neighboring tissues. This stabilizes and anchors the vessel

Valves Function

Prevent backflow of blood into the capillary network; keeps blood flowing in one direction. In veins, blood pressure is below the force of gravity. Also important are the skeletal muscle contractions that constrict veins in cycles so that blood will be pushed toward the heart

Atrial Natriuretic Peptide (ANP)

Produced by cardiac muscle cells in the wall of the right atrium ANP responds to excessive stretching of the right atrium during diastole. This occurs because of increased blood volume and increased venous return.

Aldosterone

Produced by the adrenal gland Responds to low sodium ion (Na+) levels in the blood

Antidiuretic Hormone (ADH)

Produced by the pituitary gland Responds to low blood pressures caused by a decrease in blood volume

Velocity

Rate of blood flow transported per unit time = 1 / total cross section area

Bulk Flow

Refers to movement of fluids and their solutes in one direction down a pressure gradient.

Renin-Angiotensin system

Renin is an enzyme produced by the kidney. Renin initiates a cascade chemical reaction that converts inactive angiotensinogen (circulating in blood plasma) to undergo a series of reactions to ultimately form the active hormone angiotensin II. Renin is released in response to low blood pressure and volume in the kidney.

What happens if the length of a vessel doubles?

Resistance increases (doubles), therefore blood flow decreases

What happens of the viscosity doubles?

Resistance increases (doubles), therefore blood flow decreases.

Peripheral Resistance

Resistance of the arterial system as a whole

Total Peripheral Resistance

Resistance of the entire circulatory system

Viscosity

Resistance to blood flow caused by interactions among molecules ad suspended materials in a liquid. An increase in formed elements or large proteins will increase viscosity and therefore increase resistance in blood. Whole blood has a viscosity about 5 times that of water.

Local Vasodilators

Response to inadequate perfusion due to increased tissue activity. Local vasodilators are produced at the tissue level and accelerate blood flow. Vasodilation results in increased blood flow, which tends to increase oxygen concentrations while decreasing metabolite concentrations in the interstitial fluid back toward resting values. Decrease in oxygen levels Decreased in nutrient levels Increased in CO2 levels Decrease in pH Elevated local temperature

Veins

Return blood from capillaries to the heart (Afferent)

Capillaries have a large cross-sectional area - resulting in:

Slow Velocity

Result of Aldosterone

Sodium retention results in water retention (less urine production) and therefore an increase in blood pressure in order to restore normal blood pressure.

Factors that regulate total blood flow affect:

The activity of the heart, tone of blood vessels, and volume of blood

What happens if the radius of a blood vessel doubles?

The blood vessel will be 1/16 as resistant or 1 / (2)4. Small changes in diameter cause relatively large changes in resistance; arterioles are able to dynamically adjust their diameters; they contain relatively more smooth muscle in their walls than other arteries. The way that the smooth muscle is arranged (circularly) allows for the arteriole to contract and the lumen of the arteriole to decrease in diameter. High resistance and variable diameter provide a mechanism by which arterioles can adjust blood flow through different tissues. Because resistance to blood flow through a tissue is determined almost exclusively by the resistance of the arterioles, parallel circuits can be adjusted by diameter of arterioles, and individual tissue beds can regulate the local flow of blood

If mean arterial blood pressure (MAP) increases:

The body responds by decreasing blood pressure to return this value back to normal Response by cardiac centers: There will be a decrease in cardiac output due to parasympathetic stimulation and inhibition of sympathetic activity. (decrease in heart rate) Response by vasomotor centers: There will be a decrease in resistance due to widespread vasodilation, due to inhibition of sympathetic activity (resulting in vasodilation of non-essential blood vessels)

Laminar Flow

The difference in flow rate within a conduit or tube. Fluid moves faster in the center of a tube and slows down its movement at the edge or near the wall. If the diameter of a blood vessel decreases, more blood is near the edge and movement of blood slows down. If the diameter of a blood vessel increases, less blood is near the edge of the vessel and movement of blood increases.

Blood Pressure Gradient

The driving force that propels blood through the vessels

Friction

The most important factor in vascular resistance is friction between moving blood and the vessel walls

Vascular Resistance

The opposition of blood flow in blood vessels

Hydrostatic Pressure

The physical force exerted by a fluid on a structure. Hydrostatic pressure forces water from a high to low pressure. Capillary hydrostatic pressure (CHP) is the force exerted per unit area by the blood as it presses against the capillary wall. CHP causes filtration from the capillary.

Vessel Diameter

The smaller the diameter vessel, the greater the resistance Ex. Drinking through a small vs. large straw

Vascular resistance due to friction depends on two factors:

Vessel Length Vessel Diameter

Sinusoids or Sinusoidal Capillaries

Wide-open, flattened specialized fenestrated capillaries May have an incomplete basement membrane Allows for the free exchange of water, solutes, large proteins and formed elements Blood moves through sinusoids slowly Can be found in the liver, spleen, and bone marrow

Elastic Recoil

a part of the pressure generated during ventricular systole is stored in the stretched walls of the arteries, and is then slowly released during ventricular diastole as blood flows out of the arterial system. The pressure oscillation decreases as branching into smaller arteries occurs until there is no elastic recoil within the smallest arteries; the elastic properties of the arteries help convert the pulsatile flow of blood from the heart into a more continuous flow through the rest of the circulation.

Vasomotor Center

autonomic regulation of blood vessels tone (and resistance) Only the sympathetic portion of the autonomic nervous system controls activity of the blood vessels. Blood vessel response is dependent on the type of receptor associated with the smooth muscle within the blood vessel.

Cardiac Center

autonomic regulation of heart activity (and cardiac output) Cardioinhibitory center decreases cardiac output through parasympathetic stimulation and sympathetic inhibition. The heart rate and strength of contraction will decrease.

Local Factors

autoregulation; immediate and local homeostatic adjustments

Capillary Beds

capillaries exist within an interconnected network of many vessels

Fenestrated Capillary

capillaries with small pores within endothelial cells. (endothelium is complete) The pores allow for rapid exchange of water and solutes (even small proteins) Located in areas in need of rapid exchange. Can be found in endocrine glands (release and rapid diffusion of hormones) and within kidney (for rapid filtration of blood)

Heavy exercise

cardiac output cannot keep up with oxygen demand of skeletal muscles. Therefore, there is a massive sympathetic stimulation -with increased heart rate and constriction of blood flow to non-essential organs. Blood is literally racing from the skeletal muscles to the lungs and heart, and then returns back to the muscles demanding oxygen. Although blood flow to most tissues is diminished, body temperature rises and skin perfusion increases to promote heat loss. Only the brain maintains its normal blood flow). Cardiac output can increase from the resting average of 5 - 6 liters / minute to 20-25 liters/minute.

Blood Flow =

cardiac output. Blood flow is the volume of blood moving through a given area in a given time expressed in L/minute. Factors affecting blood flow are (1) blood pressure gradients - ∆P and (2) resistance. F ~ ∆P/R Where F is blood flow; ∆P is the pressure gradient (the difference between the high pressure and low pressure); R = resistance Blood flow has a direct relationship to the pressure gradient and an indirect relationship to resistance.

Neural Mechanisms

central control that hinges on regulation of mean arterial blood pressure in order to maintain blood flow to essential organs. Immediate response.

Local regulation is a response to:

changes in metabolic activity of tissues. Homeostatic. The stimulus for local regulation is changing concentrations of chemicals around capillary beds

The circulatory system is a:

closed system

Blood is a fluid; and fluids cannot be:

compressed there must be flow in arteries, capillaries and veins if there is an increase in volume in one area of the circulatory system, then there must be a decrease in another area (if volume remains constant) Blood flow. the heart must generate enough pressure to force blood into the blood vessels and overcome vascular resistance to allow blood flow back to the heart

Vasoconstrictors

constrict precapillary sphincters causing a decrease in blood flow into a capillary bed.

Vasodilators

dilate precapillary sphincters causing an increase in blood flow into a capillary bed

Pressures are dissipated by:

flow

Hormonal Regulation of Blood Pressure

hormones provide for short and long-term regulation of cardiovascular performance. They typically regulate blood pressure by changing resistance or blood volume.

Cardioacceleratory Center

increases cardiac output through sympathetic stimulation and parasympathetic inhibition. The heart rate and strength of contraction will increase

Arterioles

internal diameter of 30 um or less the tunica media consists of six or less layers of smooth muscle The smallest arterioles do not have a tunica externa

Cardiovascular response to hemorrhaging - short term

involves adjusting to the loss of blood. a. Neural response: baroreceptor reflex in response to low blood pressure due to loss of blood volume b. Endocrine response: release of ADH, epinephrine, and angiotensin II c. The body is stimulated to (1) increase cardiac output and increase resistance (increases the heart rate and heart contractility; vasoconstriction of non-essential blood vessels) d. Result is the increase in MAP (mean arterial blood pressure)

Cardiovascular response to hemorrhaging - long term

involves restoring blood volume. a. Endocrine response: Release of ADH, aldosterone, and angiotensin II for water and sodium conservation. Release of erythropoietin: replace red blood cells. b. Result is the increase in MAP (mean arterial blood pressure)

Net filtration pressure (NFP)

is the difference between the net hydrostatic pressure and the net colloid osmotic pressure.

Blood Flow

is the volume of blood moving through a given area in a given time factors affecting blood flow are 1. resistance and 2. pressure

Elastic Arteries

large vessels with a diameter up to 1.5 cm (pulmonary trunk and aorta) transport large volumes of blood from the heart During ventricular systole, blood is ejected into elastic arteries and they expand. During ventricular diastole, elastic arteries recoil and return to their original size. Systole (expansion) - cushions rise in blood pressure Diastole (recoil) - slows drop in blood pressure Continuous blood flow even though the heart is discontinuous

Cardiovascular center

located in medulla oblongata. This complex integrates information and nerve responses for the regulation of blood pressure.

Venous return

movement of blood from the capillaries back to the heart via the veins

Pre-Capillary Sphincters

muscular sphincters that can constrict and thus restrict blood flow to the capillary guard. the entrance to each capillary

Structure of the Capillary

one layer thick of endothelial cells lying on a basement membrane average diameter is 8 um (close to the size of a RBC)

Capillary blood pressure in the systemic circulation:

pressure within capillary beds (range of 35 Hg mm on arterial end - 18mm Hg on venous end)

Venous pressure is not:

pulsatile

Arterial pressure is:

pulsatile The elasticity of the arterial walls allows for expansion and accommodation of the blood provided by ventricular systole. They then recoil during ventricular diastole - which pushes blood toward the capillaries

Colloid Osmotic Pressure

refers to the pull of water back into a tissue by the tissue's concentration of proteins (colloid). Osmotic pressure is high in plasma due to high concentrations of plasma proteins - such as albumin. The osmotic pressure of the blood is called blood colloid osmotic pressure (BCOP) because the suspended proteins in plasma cannot cross the cell membrane.

Baroreceptor Reflexes

respond to changes in blood pressure; they are specialized nerve receptors that monitor the degree of stretch in the walls of blood vessels. They consist of nerve endings within blood vessels located in the aortic sinus (expanded region within the ascending aorta - important in regulating systemic blood pressure) and the carotid sinus (expanded region near the base of the internal carotid arteries-important in monitoring blood pressure changes in the head and neck). Baroreceptors respond to stretch of the artery wall, not directly to pressure. Information from the baroreceptors is relayed to the cardiovascular center in the medulla oblongata.

Most of the decline in pressure occurs at the:

small arteries and arterioles

α (Alpha) Receptors

smooth muscle with α receptors contracts in response to sympathetic stimulation resulting vasoconstriction. Most blood vessels in the body have α receptors.

β (Beta) Receptors

smooth muscle with β receptors relaxes in response to epinephrine resulting in vasodilation. Smooth muscle in blood vessels with β receptors is located in skeletal muscle and the heart.

Endocrine Mechanisms

substances produced by tissues of the endocrine system to cause a response that is central, delayed and with short and long-term effects.

Capillary Blood Pressure

sufficient for exchange of materials between blood and surrounding tissues, but low enough to prevent capillary damage

Cardiovascular Regulation: Perfusion

the blood flow through tissues. Capillary blood flow adjusts to meet the demands of the tissues - both oxygen demands and nutrient demands. When a group of cells or tissue becomes active, blood flow must increase in order to deliver oxygen and nutrients needed for metabolism and to carry away waste product and carbon dioxide they generate. Cardiac output or blood flow depends on vascular resistance and blood pressure. Therefore, these three factors (CO, BP, PR) are the variable controls

If blood pressure falls below normal:

the body responds by raising mean blood pressure (MAP) to return this value back to normal Response by cardiac centers: There will be an increase in cardiac output due to stimulation of sympathetic innervation and inhibition of parasympathetic activity to the heart. (increase in heart rate and strength of heart contraction) Response by vasomotor centers: There will be an increase in resistance due to widespread peripheral vasoconstriction due to stimulation of sympathetic activity (resulting in vasoconstriction of non-essential blood vessels)

Pulse Pressure

the difference between systolic pressure and diastolic pressure; indicates the force of contraction by the ventricle. The size of the pulse pressure is a measure of the elasticity and recoil of arteries. Healthy arteries expand and recoil easily. If the arteries were not elastic or lost recoil, then it would be harder for the heart to pump blood. The pulse is the rhythmic throbbing sensation associated with pulse pressure

Diastolic Pressure

the minimum blood pressure measured when the artery is recoiled during ventricular diastole.

Reabsorption

the movement by bulk flow from the interstitial fluid through the capillary wall and into the bloodstream

Filtration

the movement by bulk flow through the openings in the capillary (for example: between adjacent cells or pores of fenestrated capillaries) from the bloodstream into the interstitial fluid.

Systolic Pressure

the peak blood pressure measured when the artery is maximally stretched during ventricular systole

Blood pressure within the systemic circulation:

the pressure difference between the beginning of the aorta to the entrance into the right atrium. Blood pressure is the force per unit area exerted on the wall of a blood vessel by its contained blood. (units are millimeters of mercury - mmHg).

Structure of wall of a vein

usually flattened or collapsed in sectional view relatively thin wall and large lumen thinner-walled to its companion artery thickest wall is the tunica externa lacks internal elastic membrane may contain valves

Structure of Arterial Wall

usually round and retain their shape in sectional view relatively thick wall and small lumen compared to its companion vein thickest tunic is the tunica media

Venous pressure determines the:

venous return

Backflow of blood is prevented by:

venous valves located in the veins below the level of the diaphragm

While you are standing, the venous blood must return from:

your body inferior to your heart - must overcome gravity.