WEEK X (Veins) Flashcards
__________ have little tone and resistance
Venules
What is Venuloarteriolar signalling important for?
Extensive communication through chemical signals between venues and nearby arterioles is crucial for matching capillary inflow and outflow within an organ
What are the functions of veins?
- Low-resistance pathways for blood return from tissues to the heart
- Blood reservoir
Describe the structure of veins
- LARGE RADIUS -> minimal resistance to flow
- THINNER WALLS & LESS SMOOTH MUSCLE
- LITTLE MYOGENIC TONE
- DISTENSIBLE & STRETCHABLE -> minimal elastic recoil
What happens to the total cross-sectional area of the Venous system?
Gradually decreases as smaller veins converge into larger vessels -> causes blood flow to speed up as it approaches the heart
What happens during period of rest when capillary beds are closed?
Capacity of the venous reservoir increases
Extra blood bypasses the closed capillaries and enters the veins, stretching them -> Blood moves forward MORE SLOWLY due to INCREASED CROSS-SECTIONAL AREA -> Slower transit time leads to the storage of extra volume of blood as it takes longer to reach the heart for pumping
What is the Frank-Starling law of the heart?
If too much blood pools in the veins instead of being returned to the heart cardiac output is abnormally diminished
What happens during exercise/when stored blood is needed?
Extrinsic factors reduce the capacity of the venous reservoir -> Reduced capacity drives the extra blood from the veins to the heart for pumping to the tissues
What does Venous capacity depend on?
- Distensibility of vein walls
- Externally applied pressure on the veins
What do increased and decreased venous capacity lead to?
Increased venous capacity = more blood remaining in the veins, decreasing the effective circulating blood volume
Decreased venous capacity = more blood to be returned to the heart and subsequently pumped out
How does blood move despite low blood pressure in the veins?
A small bur adequate driving pressure exists due to the near-zero atrial pressure promoting blood floe through the large-radius, low-resistance veins
What enhances venous return?
- Sympathetic-induced venous constriction
- Skeletal muscle pump
- Venous valves
- Respiratory pump
- Cardiac suction
What is Capacitance?
Capacitance describes the distensibility of blood and how volume changes in response to a change in pressure
What is the equation for determining Capacitance and explain it?
C = V/P
C = Capacitance/Compliance (mL/mm Hg)
V = Volume (mL)
P = Pressure (mm Hg)
Capacitance is directly proportional to volume and inversely proportional to pressure
What are the properties of Capacitance?
- Inversely proportional to ELASTANCE/STIFFNESS -> The greater the amount of elastic tissue there is in a blood vessel, the higher the elastance is and the lower the compliance is
- Much greater for veins than for arteries -> More blood volume is contained in veins (UNSTRESSED VOLUME) than in arteries (STRESSED VOLUME)
- Capacitance of arteries decreases with age
[person ages -> arteries become stiffer -> less distensible]
What happens when veins are sympathetically stimulated?
Venous vasoconstriction -> Elevates venous pressure -> Increases the pressure gradient for enhanced venous return -> Increased cardiac output (CO)
What is the difference between Arteriolar vasoconstriction and Venous vasoconstriction?
Arteriolar vasoconstriction = reduces flow immediately due to increased resistance
Venous vasoconstriction = increases flow immediately by decreasing venous capacity and squeezing out more blood from the veins
Describe the Skeletal muscle pump
Large veins in the extremities are situated between skeletal muscles -> Muscle contraction compresses the veins externally -> Reduces VENOUS CAPACITY and increases VENOUS PRESSURE -> Propels blood in veins towards the heart
What happens when a person stands?
The blood in the vessels below heart level is subjected to increased pressure due to the gravitational force acting on the blood column
What effects does the increased pressure resulting from gravitational effects have?
- Vein distensibility and increased capacity
- Fluid filtration and localised oedema
Describe Vein distensibility and increased capacity
Under increased hydrostatic pressure, DISTENSIBLE VEINS expand increasing their capacity -> Veins undergo significant expansion -> allows blood from capillaries to collect in EXPANDED VEINS of the lower legs instead of being efficiently returned to the heart -> VENOUS RETURN is reduced which increases cardiac output and a shrinking of the effective circulating volume
Describe Fluid filtration and localised oedema
The increased hydrostatic pressure promotes fluid leakage out of capillaries into interstitial spaces -> Swelling and oedema in areas of the body affected by gravity
What happens when Mean Arterial Pressure (MAP) drops when a person stands up?
- Sympathetically induced vasoconstriction (drives pooled blood forward)
- Skeletal muscle pump activity interrupts the column of blood by emptying specific vein segments (prevents weight of the entire venous column from affecting a particular portion of a vein)
What does fainting do?
Fainting is a response to the reduced blood flow to the brain which returns the person to a horizontal position, eliminating gravitational effects and restoring effective circulation
