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Questions and Answers
What role does the macula densa play in the kidneys?
What role does the macula densa play in the kidneys?
What physiological change occurs in response to increased CO2 or H+ ion concentration in the brain?
What physiological change occurs in response to increased CO2 or H+ ion concentration in the brain?
How is skin blood flow primarily regulated?
How is skin blood flow primarily regulated?
Which of the following is a mechanism for blood flow control in the kidneys?
Which of the following is a mechanism for blood flow control in the kidneys?
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What happens when there is excessive fluid filtering from the blood in the kidneys?
What happens when there is excessive fluid filtering from the blood in the kidneys?
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What is the primary reason tissues regulate their own local blood flow?
What is the primary reason tissues regulate their own local blood flow?
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Which of the following substances can influence local blood flow control?
Which of the following substances can influence local blood flow control?
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What does not usually happen due to precise local blood flow control?
What does not usually happen due to precise local blood flow control?
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Why is it important for tissues to have minimal blood flow regulation?
Why is it important for tissues to have minimal blood flow regulation?
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Which one of the following is a need of tissues for blood flow?
Which one of the following is a need of tissues for blood flow?
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How do tissues maintain proper ion concentrations?
How do tissues maintain proper ion concentrations?
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What is a consequence of tissues not experiencing oxygen nutritional deficiency?
What is a consequence of tissues not experiencing oxygen nutritional deficiency?
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Which mechanism assists tissues in controlling their blood flow?
Which mechanism assists tissues in controlling their blood flow?
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What is the primary role of the precapillary sphincter?
What is the primary role of the precapillary sphincter?
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Which of these structures is characterized by weak muscular coats?
Which of these structures is characterized by weak muscular coats?
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What occurs at the intercellular clefts of capillaries?
What occurs at the intercellular clefts of capillaries?
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Which type of capillary has tight junctions allowing passage of only small molecules?
Which type of capillary has tight junctions allowing passage of only small molecules?
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What is vasomotion?
What is vasomotion?
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Which feature allows the liver capillaries to be more permeable than others?
Which feature allows the liver capillaries to be more permeable than others?
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How do substances filter through glomerular capillaries?
How do substances filter through glomerular capillaries?
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What effect do local tissue conditions have on metarterioles and precapillary sphincters?
What effect do local tissue conditions have on metarterioles and precapillary sphincters?
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What unique feature do muscle capillaries exhibit compared to other capillary types?
What unique feature do muscle capillaries exhibit compared to other capillary types?
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Which of the following substances has the highest ability to diffuse through capillary walls?
Which of the following substances has the highest ability to diffuse through capillary walls?
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What does a positive net filtration pressure indicate?
What does a positive net filtration pressure indicate?
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What does the capillary filtration coefficient (Kf) measure?
What does the capillary filtration coefficient (Kf) measure?
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Which factors contribute to the calculation of net filtration pressure (NFP)?
Which factors contribute to the calculation of net filtration pressure (NFP)?
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What occurs when the net filtration pressure is negative?
What occurs when the net filtration pressure is negative?
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In the given formula $NFP = Pc - Pif - IIp - IIif$, what does $Pc$ represent?
In the given formula $NFP = Pc - Pif - IIp - IIif$, what does $Pc$ represent?
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Which component is considered free of proteoglycan molecules?
Which component is considered free of proteoglycan molecules?
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How is fluid filtration calculated based on net filtration pressure?
How is fluid filtration calculated based on net filtration pressure?
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What is a characteristic of free fluid mentioned in the content?
What is a characteristic of free fluid mentioned in the content?
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What is the average mean pressure in the aorta?
What is the average mean pressure in the aorta?
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What is the pulse pressure?
What is the pulse pressure?
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Which type of blood vessel has the strongest muscular walls?
Which type of blood vessel has the strongest muscular walls?
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What percentage of total blood volume is found in systemic veins?
What percentage of total blood volume is found in systemic veins?
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How does the body regulate blood flow to different tissues?
How does the body regulate blood flow to different tissues?
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What is the average pulmonary capillary pressure?
What is the average pulmonary capillary pressure?
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Which statement describes turbulent flow?
Which statement describes turbulent flow?
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What is the relationship between conductance and vessel diameter?
What is the relationship between conductance and vessel diameter?
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What is the main driving force for blood flow through a vessel?
What is the main driving force for blood flow through a vessel?
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What happens to blood flow when the radius of a blood vessel increases?
What happens to blood flow when the radius of a blood vessel increases?
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What is the average mean pulmonary arterial pressure?
What is the average mean pulmonary arterial pressure?
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How does arterial pressure regulation occur when pressure falls below normal?
How does arterial pressure regulation occur when pressure falls below normal?
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What is total peripheral vascular resistance?
What is total peripheral vascular resistance?
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What is the definition of resistance in the circulatory system?
What is the definition of resistance in the circulatory system?
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What is the average blood flow rate through the total circulation of an adult at rest?
What is the average blood flow rate through the total circulation of an adult at rest?
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Study Notes
Physical Characteristics of the Circulation
- The circulatory system consists of arteries, arterioles, capillaries, venules, and veins.
- Arteries transport blood under high pressure to tissues.
- Arterioles act as conduits through which blood is released into the capillaries.
- Capillaries exchange fluids, nutrients, and substances between the blood and interstitial fluid.
- Venules collect blood from the capillaries and coalesce into progressively larger veins.
- Veins transport blood from the tissues back to the heart and serve as blood reservoirs.
Pressures in the Various Portions of the Circulation
- Mean pressure in the aorta is high, averaging around 100 mmHg.
- Aortic pressure fluctuates due to the heart's pulsatile action.
- Systolic pressure is the highest point during systole.
- Diastolic pressure is the lowest point at the end of diastole.
- Pulse pressure is the difference between systolic and diastolic pressure.
- Blood pressure progressively falls to about 0 mmHg at the termination of the vena cavae in the right atrium.
- Pulmonary capillary pressure averages 8 mmHg due to the lower vascular resistance of the pulmonary blood vessels.
- The lower pressure in the pulmonary system is necessary for gas exchange in the pulmonary alveoli.
Volumes of Blood in the Different Parts of the Circulation
- Systemic circulation accounts for 84% of total blood volume.
- Veins hold 64% of systemic blood volume.
- Arteries hold 13% of systemic blood volume.
- Arterioles and capillaries hold 7% of systemic blood volume.
- The heart holds 7% of total blood volume.
- Pulmonary circulation holds 9% of total blood volume.
Blood Flow
- Blood flow is the quantity of blood passing a given point in the circulation over a specific timeframe.
- Blood flow is measured in milliliters per minute (mL/min).
- The overall blood flow at rest in an adult is approximately 5000 mL/min.
- Laminar flow occurs when blood flows in streamlines with each layer maintaining its distance from the vessel wall.
- Turbulent flow occurs when blood flows in various directions within the vessel, leading to mixing.
- Laminar flow exhibits a parabolic profile for blood flow velocity, with a faster central stream and slower flow at the edges.
- Turbulent flow results in greater resistance due to friction caused by eddy currents.
- Eddy currents are whirlpools that increase friction and resistance.
- Tendency for turbulent flow increases with velocity, diameter, and density and decreases with viscosity.
- Reynold's number measures the tendency for turbulence to occur.
- A Reynold's number above 200-400 produces turbulence at vessel branches but dies out in smooth portions.
- A Reynold's number above 2000 usually produces turbulence in straight, smooth vessels.
- High Reynold's numbers can occur during ventricular ejection, causing turbulence in the proximal aorta and pulmonary artery.
Resistance
- Resistance is the impediment to blood flow in a vessel.
- Resistance is measured in peripheral resistance units (PRUs).
- One PRU is defined as the resistance when the pressure difference is 1 mmHg and the flow is 1 mL/sec.
- Resistance is expressed in CGS units of dyne sec/cm5.
Total Peripheral Vascular Resistance & Total Pulmonary Vascular Resistance
- Total peripheral resistance is the resistance of the entire systemic circulation.
- At rest, total peripheral resistance is approximately 1 PRU.
- Constriction of blood vessels increases total PRU, potentially reaching 4 PRU.
- Dilation of blood vessels decreases total PRU, potentially going as low as 0.2 PRU.
- Total pulmonary vascular resistance is the resistance of the entire pulmonary circulation.
- The mean pulmonary arterial pressure difference across the pulmonary circulation is approximately 14 mmHg.
- For a normal cardiac output of 100 mL/sec, total pulmonary vascular resistance is about 0.14 PRU.
Conductance
- Conductance measures the blood flow through a vessel at a given pressure difference.
- Conductance is expressed in milliliters per second per millimeter of mercury pressure or other units of blood flow and pressure.
- Conductance is the reciprocal of resistance.
- Conductance increases proportionally to the fourth power of the diameter.
Blood Pressure
- Blood pressure is the force exerted by blood against the vessel wall.
- Blood pressure is measured in millimeters of mercury (mm Hg).
- Blood pressure can also be measured in centimeters of water (cm H2O).
Effects of Pressure on Vascular Resistance and Tissue Blood Flow
- Increased arterial pressure increases the force that pushes blood through the vessels and triggers compensatory increases in vascular resistance.
- Reduced arterial pressure generally reduces vascular resistance and maintains blood flow at a constant rate.
- Blood flow autoregulation allows each tissue to adjust its vascular resistance and maintain normal blood flow despite changes in arterial pressure between 70 and 175 mmHg.
- Sympathetic stimulation can constrict blood vessels and reduce blood flow transiently.
- Vasoconstrictor hormones (norepinephrine, angiotensin II, vasopressin, endothelin) can reduce blood flow.
- Local autoregulatory mechanisms override most of the effects of vasoconstrictors to maintain appropriate blood flow based on tissue needs.
- Increased arterial pressure can distend elastic vessels, decreasing vascular resistance.
- Reduced arterial pressure can lead to vessel collapse and increased resistance.
- Critical closing pressure is the pressure below which flow ceases due to vessel closure.
- Sympathetic inhibition dilates vessels and increases blood flow.
- Strong sympathetic stimulation constricts vessels.
Clinical Methods for Measuring Systolic & Diastolic Pressures
- Direct measurement is the most accurate method, involving a needle or catheter inserted directly into a blood vessel at a specific site.
Effects of Blood Hematocrit & Viscosity on Vascular Resistance
- Hematocrit is the percentage of blood volume occupied by red blood cells.
- Viscosity, which affects blood flow resistance, increases dramatically as hematocrit increases.
- Typical hematocrit results in blood viscosity 3-4 times greater than water's.
- Polycythemia, a condition with abnormally high hematocrit (60-70%), significantly increases blood viscosity and flow resistance.
- Plasma protein concentration and types can also influence blood viscosity.
- Plasma viscosity is about 1.5 times that of water.
Resistance to Blood Flow in Series and Parallel Vascular Circuits
- Blood vessels are arranged in series, with blood flow through each vessel being the same.
- Total resistance in a series circuit is the sum of resistances of each vessel.
- Blood vessels extensively branch into parallel circuits, allowing each tissue to regulate blood flow independently.
- Total resistance in a parallel circuit is much less than the resistance of any single vessel.
- Blood flow through each parallel vessel is determined by its own resistance and the pressure gradient, not the resistance of other parallel vessels.
- Increased resistance in a parallel circuit increases total vascular resistance.
- Parallel blood vessels create multiple pathways (conductance) for blood flow, making it easier for blood to flow through the circuit.
- Total conductance for blood flow is the sum of conductance in each parallel pathway.
Interrelationships of Pressure, Flow, and Resistance
- Blood flow through a vessel is determined by the pressure difference (pressure gradient) and vascular resistance.
- Ohm's law describes the relationship between pressure difference, flow, and resistance.
- The pressure difference between the ends of a vessel drives blood flow.
- Vascular resistance is the impediment to blood flow, resulting from friction between blood and the vessel wall.
- Poiseuille's Law is used to calculate the flow through a vessel.
- The vessel's radius plays a significant role in flow rate.
- Poiseuille's Law reveals that conductance increases in proportion to the fourth power of the diameter.
Capillary Fluid Exchange
- Capillary hydrostatic pressure is the force that pushes fluid out of capillaries (Pc)
- Interstitial fluid hydrostatic pressure is the force that pushes fluid into capillaries (Pif)
- Plasma colloid osmotic pressure is the force that pulls fluid into capillaries (IIp)
- Interstitial fluid colloid osmotic pressure is the force that pulls fluid out of capillaries (IIif)
- Net filtration pressure (NFP) is the difference between the forces pushing fluid out and the forces pulling fluid in
- Formula for NFP: NFP = Pc - Pif - IIp - IIif
- Positive NFP results in net fluid filtration
- Negative NFP results in net fluid absorption
- Capillary filtration coefficient (Kf) measures a capillary's ability to filter water
- Formula for Filtration: Filtration = Kf × NFP
Capillary Structure & Function
- Precapillary sphincters are smooth muscle fibers that regulate capillary blood flow
- Venules have weaker muscular coats than arterioles, allowing for contraction even with low pressure
- Capillary walls are thin and composed of a single layer of endothelial cells
- Capillary pores allow for fluid exchange between blood and interstitial fluid
- Intercellular clefts are gaps between endothelial cells that allow small molecules to pass through
- Caveolae are small pockets on the surface of endothelial cells that help transport plasma and extracellular fluid
- Vesicular channels are formed by coalescing caveolae, which help to transport large molecules
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Special types of pores:
- Brain capillaries: Tight junctions restrict passage of large molecules
- Liver capillaries: Wide open clefts allow for free passage of most substances, including plasma proteins
- Gastrointestinal capillaries: Porosity is intermediate between muscle and liver capillaries
- Glomerular capillaries: Fenestrae allow for rapid filtering of small molecules and ions
Vasomotion
- Vasomotion is the intermittent contraction of metarterioles, which regulates blood flow in capillaries
Local & Humoral Control of Tissue Blood Flow
- Tissues regulate their own blood flow based on metabolic needs
- Blood flow needs include:
- Oxygen delivery
- Nutrient delivery
- Carbon dioxide removal
- Hydrogen ion removal
- Maintaining ion concentrations
- Humoral control involves substances like hormones and locally produced factors that influence local blood flow.
- Local control ensures tissues receive adequate oxygen and nutrients while minimizing workload on the heart.
Mechanisms of Blood Flow Control
- Tubuloglomerular feedback in the kidneys regulates blood flow and filtration
- CO2 and H+ ions concentration in the brain regulate cerebral blood flow
- Skin blood flow is controlled by the central nervous system through sympathetic nerves, primarily for temperature regulation
Special Mechanisms for Acute Blood Flow Control in Specific Tissues
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Kidneys:
- Macula densa in the distal tubule senses fluid composition
- Increased fluid filtration triggers constriction, reducing renal blood flow and filtration
-
Brain:
- Increased CO2 or H+ ions dilate cerebral vessels, removing excess CO2 and H+
-
Skin:
- Blood flow is regulated primarily by the sympathetic nervous system, primarily for temperature regulation
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