from the right ventricle, blood is pumped at a low pressure to the lungs and then back to the left atria
from the left ventricle, blood is pumped at a high pressure to the rest of the body and then back to the right atria
There are 3 main types of vessels that carry blood around the body
Arteries and arterioles (small arteries)
carry blood away from the heart
Capillaries
allow for exchange of materials between the blood and the cells of the body
Veins and venules (small veins)
carry blood back to the heart
Arteries and arterioles are characterized by a divergent pattern of blood flow
Arteries and arterioles are characterized by a divergent pattern of blood flow
blood leaves each ventricle via a single artery but split into numerous and smaller diameter vessels
Arterioles branch into capillaries
capillaries are the most numerous blood vessel with the smallest diameter
Venules and veins are characterized by a convergent pattern of blood flow
blood flows out of many capillaries into a single venule with a larger diameter
from the venules, blood flows into veins that are larger in diameter which merge into a single vessel to deliver blood to the atria
~60% of the blood volume at rest is in the veins
All blood vessels are lined with a thin layer of endothelium, a type of epithelium which is supported by a basement membrane
All blood vessels are lined with a thin layer of endothelium, a type of epithelium which is supported by a basement membrane
called the tunica intima (or tunica interna)
only layer of capillary walls
The walls of most arteries and veins have layers of smooth muscle and/or elastic connective tissue called the tunica media and fibrous connective tissue called the tunica externa, surrounding the endothelium
the thickness of the tunica media and externa vary in different vessels depending on their function or the amount of internal (blood) pressure that they encounter
Most blood vessels contain vascular smooth muscle arranged in circular layers which is partially contracted at all times creating a condition known as muscle tone
Most blood vessels contain vascular smooth muscle arranged in circular layers which is partially contracted at all times creating a condition known as muscle tone
Additional contraction of the smooth muscle results in vasoconstriction which narrows the diameter of the vessel lumen
Relaxation of the smooth muscle results in vasodilation which widens the diameter of the vessel lumen
Neurotransmitters, hormones and paracrine signals influence vascular smooth muscle tone which in turn will affect blood pressure and blood flow throughout the cardiovascular system
Total blood flow through any level of the circulation is equal to the cardiac output
Total blood flow through any level of the circulation is equal to the cardiac output
if cardiac output is 5 L/min, the blood flow through all systemic capillaries is also 5 L/min
blood flow through the pulmonary side is equal to blood flow through the systemic circulation
prevents blood from accumulating in either the systemic or pulmonary loop
The distribution of systemic blood varies according to the metabolic needs of individual organs and is governed by homeostatic reflexes
The distribution of systemic blood varies according to the metabolic needs of individual organs and is governed by homeostatic reflexes
skeletal muscles at rest receive 21% of cardiac output, but during exercise when they use more O2 and nutrients and produce more CO2 and wastes receive as much as 85% of cardiac output
accomplished through the vasoconstriction and vasodilation of arterioles supplying blood to various regions, organs or tissues of the body
The ability to selectively alter blood flow to organs is an important aspect of cardiovascular regulation
Blood flow (F) through the vascular system is directly proportional to the pressure gradient (ΔP) between to points within the system: F ΔP
Blood flow (F) through the vascular system is directly proportional to the pressure gradient (ΔP) between to points within the system: F ΔP
if the pressure gradient increases, flow increases
if the pressure gradient decreases, flow decreases
blood pressure is the amount of force blood exerts outwardly on the wall of a vessel
The tendency of the vascular system to oppose blood flow is called its resistance (R) and is inversely proportional to flow: F 1/R
if the resistance increases, flow decreases
if the resistance decreases, flow increases
Combining the equations above results in: F ΔP/R
Aortic pressure reaches an average high of 120 mmHg during ventricular systole (systolic pressure) and falls steadily to a low of 80 mmHg during ventricular diastole (diastolic pressure)
Aortic pressure reaches an average high of 120 mmHg during ventricular systole (systolic pressure) and falls steadily to a low of 80 mmHg during ventricular diastole (diastolic pressure)
systolic pressure > 120 is called hypertension
systolic pressure < 100 is called hypotension
The highly elastic walls of the arteries allows them to capture and store the energy of ventricular ejection
note that the pressure in the aorta drops only to 80 mmHg (not to 0mmHg as observed in the ventricle) which keeps blood constantly moving (never stops)
energy stored by the arteries can be felt as a pulse
Blood pressure decreases as it flows downstream
A similar blood pressure profile (albeit lower) is observed on the pulmonary side of circulation
Arterial blood pressure is directly proportional to the amount of blood found in an artery
Arterial blood pressure is directly proportional to the amount of blood found in an artery
more blood in an artery = higher pressure
less blood in an artery = lower pressure
Since arterial pressure is pulsatile, the mean arterial pressure (MAP) is used to represent the driving pressure of blood through the vascular system
MAP = diastolic + 1/3 (systolic – diastolic)
MAP = 80 + 1/3 (120 – 80) = 93 mmHg in the aorta
Mean arterial pressure is a balance between blood flow into the arteries and blood flow out of the arteries
Mean arterial pressure is a balance between blood flow into the arteries and blood flow out of the arteries
if flow in exceeds flow out, pressure increases
if flow out exceeds flow in, pressure decreases
Blood flow in is equal to the cardiac output
Blood flow out is influenced primarily by the vascular resistance offered by the arterioles determined mainly by their diameter
MAPCO X Resistancearterioles
The central nervous system coordinates the reflex control of blood pressure
The central nervous system coordinates the reflex control of blood pressure
The main integrating center is a cluster of neurons in the medulla oblongata called the cardiovascular control center
Sensory input to the integrating center comes from a variety of peripheral sensory receptors stretch sensitive mechanoreceptors known as baroreceptors in the walls of the aorta and carotid arteries travel to the cardiovascular center via sensory neurons
Responses by the cardiovascular center is carried via both sympathetic and parasympathetic neurons and include changes in cardiac output and peripheral resistance which occur within 2 heartbeats of the stimulus
The baroreceptors are tonically active stretch receptors that fire action potentials continuously at normal blood pressures
The baroreceptors are tonically active stretch receptors that fire action potentials continuously at normal blood pressures
When blood pressure increases in the arteries stretches the baroreceptor cell membrane, the firing rate of the receptor increases
in response, the cardiovascular center increases parasympathetic activity and decrease sympathetic activity to slow down the heart
decreased sympathetic outflow to arterioles causes dilation allowing more blood to flow out of the arteries
When blood pressure decreases in the arteries, the cardiovascular center increases sympathetic activity and decreases parasympathetic activity creating opposite responses in the effectors to increase blood pressure
Although the volume of blood is usually relatively constant, changes in blood volume can affect mean arterial blood pressure
Although the volume of blood is usually relatively constant, changes in blood volume can affect mean arterial blood pressure
if blood volume increases, blood pressure increases
fluid intake
if blood volume decreases, blood pressure decreases
fluid loss
Relative distribution of blood between the venous and arterial sides of circulation is an important factor in regulating arterial blood pressure
when arterial blood pressure falls, vasoconstriction of the veins redistributes blood to the arterial side
As blood moves through the vessels, pressure is lost due to friction between the blood and the vessel walls
As blood moves through the vessels, pressure is lost due to friction between the blood and the vessel walls
The low pressure blood in veins inferior to the heart (arms, abdominopelvic cavity and legs) must flow against gravity to return to the heart
To assist venous flow, these veins have internal one way valves to ensure that blood passing the valve cannot flow backward
The movement of blood through veins is also assisted by the contraction of skeletal muscle
Veins located between skeletal muscles are squeezed during contraction
This increases the venous pressure enough to move the blood through the valves, back towards the heart
For fluid flowing through a tube, resistance is influenced by 3 parameters:
For fluid flowing through a tube, resistance is influenced by 3 parameters:
the radius (r) of the tube (half of the diameter)
the length (L) of the tube
the viscosity (η) or thickness of the fluid
Poiseuille’s Law relates these factors to resistance:
R Lη/r4
if the tube length increases, resistance increases
if the viscosity increases, resistance increases
if the tube’s radius increases, resistance decreases
Since blood viscosity remains relatively constant and blood vessel lengths can’t change, vessel diameter is the major determinant of resistance
Arteriolar constriction reduces blood flow through that arteriole and redirects the flow through all arterioles with a lower resistance
Arteriolar constriction reduces blood flow through that arteriole and redirects the flow through all arterioles with a lower resistance
total blood flow through all the arterioles of the body always equals cardiac output
Local control is accomplished by paracrines secreted by the vascular endothelium or by tissues to which the arterioles are supplying blood
Local control is accomplished by paracrines secreted by the vascular endothelium or by tissues to which the arterioles are supplying blood
low O2 and high CO2 dilate arterioles which increase blood flow into the tissue bringing additional O2 while removing excess CO2
can be caused by an increase in metabolic activity (active hyperemia) or by a period of low perfusion (reactive hyperemia)
Systemic control occurs by sympathetic innervation
tonic release of norepinephrine which binds to α-adrenergic receptors on vascular smooth muscle helps maintain tone of arterioles
if sympathetic release of norepinephrine decreases, the arterioles dilate, if the release of norepinephrine increases, arterioles constrict
Most cells are located within 0.1 mm of the nearest capillary over which diffusion occurs rapidly
Most cells are located within 0.1 mm of the nearest capillary over which diffusion occurs rapidly
The most common type are continuous capillaries
endothelial cells are joined by leaky junctions
Less common type are fenestrated capillaries
endothelial cells have large pores (fenestrations) that allow high volumes of fluid to pass quickly between the plasma and interstitial fluid
Exchange occurs either by:
movement of substances through the gaps between adjacent endothelial cells (paracellular movement)
movement of substances through/across the cell membrane of endothelial cells (transcellular movement)
Paracellular exchange occurs through endothelial cell junctions or fenestrations
Paracellular exchange occurs through endothelial cell junctions or fenestrations
solutes can move by diffusion
solutes can move by bulk flow which refers to the mass movement of a solvent as a net result of hydrostatic and or osmotic pressure gradients across the capillary wall
if the direction of bulk flow is out of the capillary the fluid movement is called filtration
if the direction of bulk flow is into the capillary the fluid movement is called absorption
Transcellular exchange occurs through the cell membrane of endothelial cells
nonpolar gasses and solutes can move by diffusion
large polar solutes can move by vesicular transport
2 forces regulate bulk flow in capillaries
2 forces regulate bulk flow in capillaries
hydrostatic pressure (Pcap)
lateral pressure component of blood flow that pushes plasma out through the capillary pores
decreases along the length of the capillary as energy is lost to friction
osmotic pressure (cap)
pressure exerted by solutes within the plasma
the main solute difference between plasma and interstitial fluid is due to proteins (present in plasma, but mostly absent in interstitial fluid)
the osmotic pressure created by plasma proteins is called colloid osmotic pressure
favors water movement by osmosis from interstitial fluid into plasma
is constant along the length of the capillary
Net Pressure = Pcap – cap
Net Pressure = Pcap – cap
Net Pressurearterial end = 32mmHg – 25mmHg = 7mmHg
favors filtration
Net Pressurevenous end = 15mmHg – 25mmHg = -10mmHg
favors absorption
In most capillaries there is more filtration than absorption
In most capillaries there is more filtration than absorption
90% the volume of fluid filtered out at the arterial end is absorbed back into the capillary at the venous end
the other 10% enters lymphatic vessels where it is returned back into circulation as the lymph vessels empty lymph fluid into blood at the right atrium
