Chapter 18: Blood Vessels
What are the characteristics of veins?
(1) veins function as blood reservoirs, (2) blood can be diverted from the veins to other parts of the body because veins typically have much thinner walls, fewer elastic fibers, less smooth muscle, and larger lumens than arteries.
When blood pressure decreases:
(1) ADH secretion triggers thirst and increases the amount of water retained by the kidneys, raising blood volume and blood pressure. (2) Renin secretion from the kidneys, triggered when the blood pressure drops, activates angiotensin-II, which induces thirst, causes sodium ion retention, and as a result increases blood volume. (3) Angiotensin-II also triggers the secretion of another hormone, aldosterone, which causes retention of sodium ions and water from the kidneys, increasing blood volume.
The heart has both contraction and relaxation periods. Pressure rises during ventricular systole and declines during ventricular diastole. This leads to two separate pressures in the arteries:
(1) Systolic pressure averages about 120 mm Hg, and (2) Diastolic pressure averages about 80 mm Hg when the person is at rest. The difference between the systolic and diastolic pressures—about 40 mm Hg, is known as the pulse pressure.
List some common causes of edema:
(1) an increase in the capillary hydrostatic pressure gradient due to hypertension or (2) a decrease in the colloid osmotic pressure due to liver disease, (3) cancer or (4) starvation
What are the three main factors that influence blood pressure?
(1) resistance, (2) cardiac output, and (3) blood volume
collateral vessels
Vascular Anastomoses are locations where vessels connect via pathways called __ ___
What drives most of the processes in the body?
Gradients drive most of the processes in the body whether concentration gradients, pressure gradients, or electrical gradients
What is the hydrostatic pressure in a capillary?
Hydrostatic pressure in a capillary changes from its arteriolar end, where it measures about 35 mm Hg, to its venular end, where it measures about 15 mm Hg. The hydrostatic pressure of the interstitial fluid is so low that it is functionally 0 mm Hg. Thus, there are steep hydrostatic pressure gradients on both ends of the capillary that drive water out of the capillary and into the interstitial fluid.
What are the most compliant vessels?
Veins
What are two notable differences between a typical artery wall and the wall of a typical vein?
a. Most arteries have thicker tunicae mediae than do veins, which reflects the role of the arteries in controlling blood pressure and blood flow to organs b. The internal and external elastic laminae are much more extensive in arteries than in veins, which reflects the fact that arteries are under much higher pressure than are veins
List and describe three ways in which the rate of venous return is increased:
a. Venous valves prevent backward flow in some veins while smooth muscle in vein walls can contract under sympathetic nervous system stimulation to increase the rate of venous return. b. Skeletal muscle pumps: skeletal muscles surrounding the deeper veins of the upper and lower limbs squeeze the blood in the veins and propel it upward (toward the heart) as they contract and relax. c. The respiratory pump helps propel blood through thoracic and abdominal cavity veins, driven by the rhythmic changes in pressure in the thoracic and abdominopelvic cavities that occur with ventilation.
The pulmonary and systemic circuit are composed of three kinds of blood vessels, what are they?
arteries, capillaries, and veins
How are veins classified?
by their size
Introduction to capillaries and tissue perfusion:
capillaries generally are found in clusters called capillary beds that wind their way through the cells of most tissues. The blood flow to a tissue through a capillary bed is known as tissue perfusion
Small increases in blood volume are offset by the ability of vessels to stretch, a property known as?
compliance
What is Edema?
is a condition characterized by an excessive amount of water in the interstitial fluid
tunics
the lumen are surrounded by several tissue layers, or ___.
metarterioles
the smallest arterioles
venules
the smallest veins
As the arterial system branches into more, progressively smaller vessels, what happens?
the total cross-sectional area increases
tunica intima
this is the innermost layer, is composed of endothelium, which is continuous with the inner lining of the heart, the endocardium
tunica media
this is the middle layer, composed of two layers: smooth muscle cells and elastic fibers
The total volume of blood is directly linked to what?
to the amount of water in the blood.
At the capillary's venular end, the NFP is
-7 mm Hg. NFP is a negative number, which means that water flows into the capillary. Colloid osmotic pressure is greater and water is absorbed into the capillary by osmosis.
At the capillary's arteriolar end, the NFP is?
13 mm Hg. This force drives water out of the capillary by filtration because hydrostatic pressure is greater at this end.
Pressure change is caused by what?
A pressure change (ΔP) is caused by altering cardiac output (CO) and/or peripheral resistance (PR). When cardiac output increases, blood pressure increases and vice versa
lumen
All blood vessels are tubular organs that contain a central space—the ___.
Peripheral Resistance:
Any factor that hinders blood flow through the vasculature contributes to the overall resistance of that circuit. As peripheral resistance increases, blood pressure increases
Systemic Capillary Pressure:
As pressure continues to decline throughout the remainder of the systemic circuit: pressure at the arteriolar end of a capillary bed is about 35 mm Hg. However, at the venular end of the capillary bed, the pressure has decreased to about 15 mm Hg
What is one factor that determines blood flow?
Blood flow generally matches cardiac output and averages 5-6 liters/min. Blood flow is directly proportional to the pressure gradient, meaning that blood flow increases when the pressure gradient increases and vice versa
What creates hydrostatic pressure?
Blood is the fluid in a vessel that is creating hydrostatic pressure
Where is blood pressure the highest?
Blood pressure is highest in the aorta and elastic arteries and declines slightly as it spreads throughout the muscular arteries
Disorders of Blood Pressure, Hypertension and Hypotension:
Blood pressure must be maintained within a certain normal range. If the blood pressure rises too high (a condition known as hypertension) or falls too low (a condition known as hypotension), severe disturbances in homeostasis may result
What is the difference in blood pressure in the systemic circuit versus the pulmonary?
Blood pressure remains fairly low throughout the pulmonary circuit but changes significantly as blood travels through the systemic circuit. Blood pressures averages about 15 mm Hg in the pulmonary circuit and about 95 mm Hg in the systemic circuit.
What is the function of the blood vessels?
Blood vessels transport blood to the tissues, where gases, nutrients, and wastes are exchanged, and then transport it back to the heart. They also regulate blood flow to tissues, control blood pressure, and secrete a variety of chemicals.
Capillary Structure and Function:
Capillaries are extremely thin vessels, with walls that are only about 0.2 μm in thickness. Each capillary consists of only an endothelium rolled into a tube and a small amount of basal lamina secreted by the endothelial cells
Types of Capillaries:
Capillaries in different parts of the body have slightly different functions and slightly different structures. The three types of capillaries and their characteristics are as follows: continuous capillaries, fenestrated capillaries, and sinusoidal capillaries
What are the two factors that determine the pressure gradient driving circulation?
Cardiac output and peripheral resistance- The relationship is expressed by this mathematical equation: ΔP = CO × PR
Capillary Net Filtration Pressure:
Colloid osmotic pressure and the hydrostatic pressure gradient drive water in opposite directions. Hydrostatic pressure pushes water out of the capillary and colloid osmotic pressure pulls water into the capillary. The difference between these gradients is the net filtration pressure (NFP).
What do most veins contain?
Many veins contain venous valves that prevent blood from flowing backward in the venous circuit
What are other characteristics of veins?
Most veins have a thin tunica media with few smooth muscle cells, and their diameter changes only slightly with vasodilation and vasoconstriction
Capillary Exchange:
Nutrients, gases, ions, and wastes must be able to cross the wall and travel between the blood in the capillary and the tissue cells. This movement of materials is known as capillary exchange
Osmotic Pressure:
Osmosis involves the movement of water from a solution with a lower solute concentration to one with a higher solute concentration
Systemic Venous Pressure and Mechanisms of Venous Return:
Pressure declines even further in venules and veins, dropping to about 4 mm Hg in the inferior vena cava and to 0 mm Hg in the right atrium
Short-Term Maintenance of Blood Pressure:
Short-term control of blood pressure is primarily accomplished by the nervous system and certain hormones of the endocrine system and achieved by the adjustment of resistance and cardiac output
What are smooth muscle cells in the tunica media innervated by?
Smooth muscle cells are innervated by the sympathetic nervous system called vasomotor nerves
What creates osmotic pressure?
Solute particles in a solution exert a force, or "pull," on water molecules called osmotic pressure (OP). Osmotic pressure is determined almost exclusively by the number of particles, not their size
The Baroreceptor Reflex:
The baroreceptor reflex arc is a negative feedback loop that responds to increases or decreases in blood pressure
Effects of Central Chemoreceptor Stimulation:
The central chemoreceptors respond to decreases in the pH of the interstitial fluid in the brain, which triggers another feedback loop that indirectly increases the activity of sympathetic neurons, resulting in vasoconstriction and a rise in blood pressure.
Blood Flow through Capillary Beds:
The flow of blood that takes place within the body's capillary beds is collectively called the microcirculation
Long-Term Maintenance of Blood Pressure by the Endocrine and Urinary Systems:
The long-term maintenance of blood pressure falls to the urinary system and certain hormones of the endocrine system that affect the kidneys. These systems control blood pressure by increasing or decreasing the amount of body water lost as urine, which affects blood volume.
List and describe the three variables that mainly contribute to resistance: Blood vessel length
The longer the blood vessel, the greater the resistance. More pressure is needed to propel blood through a long vessel than a short one
What accounts for the low pressure?
The low pressure is largely due to the high compliance of veins and the declining resistance as vessels merge and become larger
blood flow
The magnitude of the blood pressure gradient is one main factor that determines the __ __, or the volume of blood that flows per minute
Summary of Blood Pressure Maintenance:
The maintenance of blood pressure by the nervous, endocrine, and urinary systems
Capillary Pressures and Water Movement:
The movement of water across a capillary is driven by a process called filtration, which is the movement of a fluid by a force such as pressure or gravity
What is the osmotic pressure in capillary blood?
The osmotic pressure of capillary blood is about 25 mm Hg. This osmotic pressure is created almost exclusively by large proteins in the blood, especially the protein albumin. These proteins are too large to leave the capillary, so osmotic pressure remains consistent throughout the capillary's length while this pressure is very low in the interstitial fluid.
tunica extrena
The outermost layer, composed of dense irregular CT that supports the blood vessel and prevents it from overstretching
Effects of Peripheral Chemoreceptor Stimulation:
The peripheral chemoreceptors respond mostly to the level of oxygen in the blood. Although they primarily play a role in the regulation of breathing, they also affect blood pressure.
Describe the two circuits that carry blood through the body:
The pulmonary circuit, which transports blood between the heart and lungs, and the systemic circuit, which transports blood between the heart and body.
What is the function of the smooth muscle cells in the tunica media?
The smooth muscle cells control the diameter of the blood vessel and so the amount of blood that flows to organs
Discuss vasoconstriction and vasodilation as they relate to vessel diameter: vasoconstriction
These nerves stimulate the smooth muscle cells of the tunica media to contract, an action known as ____ that narrows the diameter of the vessel
What does the increase in area cause?
This increase in area causes the velocity of blood flow to decrease. For this reason, the velocity of blood flow is fastest in the aorta and slowest in the capillaries.
What accounts for the decrease in pressure?
This pressure decrease is largely due to the reduction in blood volume that takes place in capillaries.
Discuss vasoconstriction and vasodilation as they relate to vessel diameter: vasodilation
When sympathetic stimulation of the smooth muscle cells decreases, these cells relax and the vessel's diameter increases, a change called ____.
What does the difference in osmotic pressure create?
an osmotic pressure gradient known as the colloid osmotic pressure (COP), or the oncotic pressure. Colloid osmotic pressure draws water into the capillary by osmosis, a process known as absorption.
Define resistance:
any impedance to blood flow
What are the arteries?
are the distribution system of the vasculature. As they travel away from the heart, they branch into vessels of progressively smaller diameter that supply most tissues in the body with blood.
What are the capillaries?
are the exchange system of the vasculature. They are very small-diameter vessels that form branching networks called capillary beds. Gases, nutrients, wastes, and other molecules are exchanged between tissue cells and the blood through capillary walls.
Blood flow is inversely proportional to resistance:
as resistance increases, blood flow decreases
When blood pressure increases:
atrial cells secrete ANP, which causes the kidneys to excrete more water and sodium ions to decrease blood volume, and so blood pressure
Parasympathetic Effects on Blood Pressure:
axons of the parasympathetic system, via the vagus nerve, release acetylcholine primarily onto certain cardiac pacemaker cells and atrial cardiac muscle cells. This slows the heart rate, which decreases cardiac output and blood pressure
What happens when blood contains more water?
blood volume increases: as blood volume increases, blood pressure increases and vice versa
Pressure receptors, or baroreceptors, and chemoreceptors
detect blood oxygen, carbon dioxide, and hydrogen ion concentrations - are found in the aorta and the common carotid artery
What is the function of the metarterioles?
directly feed capillary beds in most tissues
What is the function of the venules?
drain blood from capillary beds
List and describe the three variables that mainly contribute to resistance: Blood vessel radius
dramatically affects resistance. Resistance varies inversely with the vessel's radius. As a vessel's radius increases (dilates) the resistance to blood flow decreases, and vice versa
Hormones that control resistance:
epinephrine and norepinephrine cause vasoconstriction and increase peripheral resistance, elevating blood pressure. Angiotensin-II is a vasoconstrictor and atrial natriuretic peptide (ANP) causes a mild decrease in peripheral resistance.
Hormones that control cardiac output include:
epinephrine, norepinephrine, and thyroid hormone
Fluid flows through from an area of higher hydrostatic pressure to the area of lower hydrostatic pressure through the capillaries until the gradient is extinguished. This fluid flow is a passive process known as
filtration
What are the veins?
function as the collection system of the vasculature. Veins drain blood from capillary beds and return it to the heart; small veins merge with other veins to become progressively larger vessels as they travel toward the heart.
muscular (distributing) arteries
have an intermediate diameter
Elastic (conducting) arteries
have the largest diameter
Pressures at Work in a Capillary: What are the two basic pressures that drive water movement at work within a capillary?
hydrostatic pressure and osmotic pressure. These forces promote movement in opposite directions—hydrostatic pressure drives water out of the capillary, whereas osmotic pressure generally draws fluid into the capillary.
Hydrostatic Pressure
is the force that a fluid exerts on the wall of its container. Blood pressure is equal to hydrostatic pressure
List and describe the three variables that mainly contribute to resistance: Blood viscosity
is the inherent resistance that all liquids have to flow. Blood has a high viscosity due to the proteins and cells that it contains
What is blood pressure?
is the outward force exerted by blood on the walls of blood vessels. Blood pressure is highest in the large systemic arteries and lowest in the large systemic veins
Cardiac Output (CO)
is the product of stroke volume (the amount of blood pumped with each beat) times heart rate (the number of beats per minute)
The other factor that determines blood flow is
resistance
arterioles
smallest diameter
Sympathetic Effects on Blood Pressure:
sympathetic axons release norepinephrine and epinephrine onto cardiac muscle cells and the smooth muscle cells of blood vessels, to produce two immediate changes: (1) an increase in heart rate and contractility, which increases cardiac output, and (2) vasoconstriction of all types of vessels, but especially arterioles, which increases peripheral resistance. Both changes increase blood pressure.
The thickness of the arterial wall depends on what two factors?
the artery's size and function
What determines the velocity with which blood flows?
the cross-sectional area of the blood vessel
Endocrine System Maintenance of Blood Pressure:
the endocrine system works with the nervous system to maintain nearly all aspects of homeostasis, including blood pressure although the hormonal responses are much slower
