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Terms in this set (23)
Arterial system: elastic arteries
Also known as conducting arteries
Largest in diameter (2.5 cm-1 cm)
Stretch to accommodate surge of blood when heart contracts. During diastole the vessels then recoil allowing blood to move forward.
Arterial system: muscular arteries
Diameter: 1 cm-0.3 mm
Medium-sized distributing arteries
Arterial system: arterioles
Nearly microscopic arteries that deliver blood to capillaries (diameter: 0.3 mm-10 um)
Arterioles play key role in regulating blood flow from arteries to capillaries via vasoconstriction and dilation (in response to chemical and neural stimuli)
Sympathetic nervous system--primary role in vasoconstriction
Nitric oxide, secreted by endothelial cells--major role in vasodilation
Diameter (8-10 um)
Permit exchange of nutrients and wastes between blood and tissue cells.
Distribution varies according to activity of tissue: muscle, nervous tissue, kidney, liver, lungs all have higher metabolism and require more O2 and nutrients, thus more capillaries
Tendons and ligaments poorly vascularized. Cartilage and epithelium are avascular.
What can cross blood capillary wall
1. Directly across endothelial membrane
2. Intercellular clefts
A. Vascular shunt (metarteriole-thoroughfare channel)
Metarteriole and its thoroughfare channel serve as low-resistance pathway. Bypasses capillary beds while sustaining blood flow through area when precapillary sphincters are contracted.
B. True capillaries
Branch off metarteriole into tissue for exchange. Regulating by precapillary sphincters, which are smooth muscle cells surrounding root of true capillaries. Tissue may be flooded with blood or bypassed.
Precapillary sphincters will contract in response to chemical conditions and nerve impulses. Flow is not continuous through true capillaries.
Join capillaries to veins
Large diameter lumens decrease resistance to blood return and flow. Also called capacitance vessels since they can hold up to 65% of blood volume.
Contain valves that prevent backflow of blood.
When you stand, gravity pulling blood down is barely overcome by pressure pushing blood up.
Valves most abundant in extremities and absent in veins of the ventral body cavity.
Respiratory pump: Diaphragm contracts, increasing abdominal mmHg while decreasing thoracic mmHg
- Negative thoracic pressure draws blood toward R.A.
- Abdominal contents compress inferior vena cava
Muscular pump: Action of muscle contraction milks blood back toward heart while the valves prevent back flow
Quantity of blood that passes a given point in the circulation in a given time
1. Difference in pressure in two ends of a tube. Blood flow is proportional to pressure difference (delta P)
2. Frictional resistance. Blood flow is inversely proportional to resistance. There are three factors that affect resistance: Length of system, viscosity of blood, radius of vessels.
Blood Flow (F)=Delta P/R
Finding Delta P and Blood Flow
To find P1, you must determine MAP.
To Find MAP:
MAP=diastolic pressure+pulse pressure (PP)/3
Assuming BP of 120/80
P2=0 (average pressure of vena cava)
Factors influencing blood flow
1. Vessel diameter
2. Cross-sectional area
3. Auto-regulation of blood flow
4. Distribution of blood flow
Factors influencing blood flow: vessel diameter
Vasoconstriction increase frictional resistance
Vasodilation decreases frictional resistance
Extrinsic mediators of vessel diameter (vascular smooth muscle):
1. NE acting on alpha 1 receptors
- Antidiuretic hormone (vasopressin)
- Angiotensin II
1. NE acting on VSM beta 2 receptors (heart, lungs, skeletal muscles)
- Nitric oxide
- Atrial natiuretic peptide
Summary: regulation of microcirculation is an interplay between vasoconstrictors and vasodilators needed to maintain local perfusion
Factors influencing blood flow: Cross-sectional area
Velocity of blood flow is inversely related to cross-sectional area. Velocity DECREASES as cross-sectional area INCREASES.
While blood flow is volume of blood that passes a given point in a given period of time, velocity of blood flow is how quickly the blood travels through the system.
Total blood flow across the system remains constant.
Velocity=Flow rate/cross-sectional area
Factors influencing blood flow: Auto-regulation of blood flow
Local control of blood flow is largely independent of systemic factors. It is governed by metabolic needs of a given tissue.
1. Decreased O2=relaxation of precapillary sphincters and vasodilation (short term)
2. Increased CO2=increased H+=relaxtion of precapillary sphincters and vasodilation (short term)
3. Myogenic reflex=increased vessel stretch=vasoconstriction (short term)
4. Increased capillarization (angiogenesis) (long term)
Factors influencing blood flow: Distribution of blood flow
Systemic venous system: 65-70%
Heart, arteries, capillaries: 30-35%
Blood flow can be controlled systemically by ANS
Blood flow at rest
Skeletal muscle: Vasodilation largely autregulated
Brain: Stays constant in both parasympathetic and sympathetic
Lungs: Decrease in O2 vasoconstriction; ventilation-perfusion coupling
Heart: Myocardial demand increases as HR increases
Refers to opposition to blood flow. Inversely proportional to frictional resistance of blood flow through vessels.
Poiseuille's law: Resistance (R)=8 L n / pi r^4
R=L n / r^4
L=length of vessel
n=viscosity of blood
r^4=radius of vessel
Resistance to blood flow
1. Total blood vessel length (L): Longer a blood vessel, the greater the resistance as blood flows through it (fairly constant)
2. Blood viscosity (n): Viscosity, or thickness, of blood depends largely on ratio of RBC to plasma. To smaller extend on concentration of proteins in plasma. Blood plasma viscosity is about 1.5 times that of water (fairly constant)
3. Blood vessel radius (r^4): Primary factor affecting blood flow.
The smaller the radius of blood vessel, the greater the resistance it offers to blood flow. Blood flows in concentric rings with outer ring touching vessel wall and inner rings flowing more rapidly in smaller vessels. The inner stream does not exist since essentially all blood is in contact with the walls.
Velocity of flow in each ring is different from that in other rings. In smaller vessels, a relatively larger percent of blood contacts vessel wall which increases frictional resistance to blood flow.
Resistance is inversely proportional to fourth power of radius of blood vessel. Thus vasoconstriction and vasodilation effectively control blood flow and the shunting of blood to various areas.
Blood flow=Delta P/R
The major physiological regulators of BF are:
1. MAP (P1)
Refers to pressure in systemic arteries. It is measure of the pressure exerted by blood on wall of blood vessel. Three most important variable affecting arterial blood pressure are:
1. Cardiac rate
2. Blood volume
3. Total peripheral resistance
An increase in any of the three will increased BP
Hypotension: systolic mmHg below 100
Orthostatic: experienced when standing from sitting or lying. ANS does not correct quickly.
Hypertension: related to increased peripheral resistance (140/90)
Blood pressure regulation: Nervous system controls
Maintains blood pressure within normal limits, and alters blood distribution to achieve specific functions via:
Located in aortic arch and carotid sinuses. Stretch receptors that detect changes in arterial pressure and protect against short-term change in BP.
2. Vasomotor center
Group of sympathetic neurons located in medulla. Send out signals to keep arterioles in state of partial constriction: called VASOMOTOR TONE.
HOW IT WORKS:
Decrease in sympathetic AP=vasodilation
Increase in sympathetic AP=vasoconstriction
BP increase=stretching of arterial walls of these regions=stetches receptors=increase in rate of sensory nerve firing to VMC in medulla=decrease rate of impulses sent by medulla to arterioles (decrease vasomotor tone)=vasodilation=decrease in BP
BP decrease=decrease in frequency of sensory A.P.s sent to VMC=increased rate of AP from VMC to arterioles=vasoconstriction of systemic arterioles and increase in systemic BP
Blood pressure regulation: Chemoreceptors
Located in aorta and carotid bodies.
Increase in CO2/Decrease in O2/Decrease in pH=increase in sympathetic activity=vasoconstriction and increase in BP=increase in HR=faster return of blood to heart and lungs=faster oxygenation of blood in pulmonary circulation
Blood pressure regulation: Chemical controls
Antidiuretic hormone (ADH)
ADH stimulates kidneys to conserve water=increase BP
High osmolarity of blood activiates osmoreceptors in hypothalamus to signal release of ADH. ADH stimulates production of H2O channels in apical surface of principle cells to increase H20 reabsorption.
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