Lectures 9+10 - Venous Circulation Flashcards
Explain how complexity varies in the venous system?
Venous anatomy more complex as vessel size ↓ terminating in branching networks
Returns blood from organ systems and musculature
Complexity pronounced in periphery (i.e. hands, feet and cerebral circulation)
More variable between individuals than arterial anatomy – due to redundancy of venous return
Why can veins act as a blood resevoir?
Venous compliance -
They’re larger and thinner than arteries
They contain less smooth muscle
What is the model used for venous collapse?
thin-walled elastic cylinder
Circular vein will buckle under a uniform pressure load due to elastic instability
Why cant you use the linear approximation for venous collapse?
Linear approx. neglects the large changes in compliance of the veins which arise over the range of transmural pressures experienced by these vessels (vessel changes shape ‘dumbbell shape’ which changes compliance
What happens to the resistance of the vessel during collapse? Why?
Significantly increases due to increased viscous losses and vessel closes
Collapsed veins present a high resistance to flow —> dynamic mechanism for flow limitation
What happens in the vessels of the foot as someone moves from sitting to standing? Why?
If the subject becomes upright, the excess pressure above hydrostatic which drives the mean flow remains the same
The transmural pressure remains the same only at the level of the heart.
The veins of the foot will fill from the microcirculation, until the internal pressure has increased by about 104 N m-2
Explain venous return
Volume of blood returning to right atrium from systemic veins
Provides blood for pulmonary circulation
What is total venous return dictated by?
pressure gradient
resistance of the venous vascular bed
influence of transient muscle pump activation
effects of the respiratory pump
When do venous valves form?
During the development of the cardiovascular system.
How do venous valve leaflets initially form?
As a bulging in the venous endothelial layer.
What are venous valve leaflets made of?
A thin elastic layer on the luminal surface and underlying collagen with very little connective tissue.
Where are smooth muscle cells found in venous valves?
Only in the valvular agger, where the valve attaches to the vein wall.
What is the most common structure of venous valves?
Most venous valves are bicuspid (having two leaflets).
Some tricuspid valves (with three leaflets) are observed.
How can the sinus of venous valves be observed?
Using MRI or X-ray imaging.
How does the sinus geometry of venous valves change with pressure?
The venous wall at the sinus is more distensible and thinner than elsewhere.
Where are valves found within the venous system?
In all components, including superficial, deep, and perforating veins.
How does the number of valves change from central to peripheral circulation?
The number of valves increases from the central circulation to the peripheral circulation of the arms and legs.
Which major vein has no valves?
The vena cava.
What is the incidence of venous valves associated with?
The importance of the skeletal muscle pump in the peripheral circulation.
What imaging technique is used to characterize the hemodynamics of venous valves?
B-mode ultrasound
What are the three distinct phases of valve leaflet motion?
Opening, closing, and resting.
What happens to the valve leaflets during the resting phase?
They undergo oscillatory motion when flow is constant.
To what can the oscillatory motion of valve leaflets during the resting phase be compared?
Self-excited oscillations during steady flow in compliant tubes.
What occurs during the opening phase of valve leaflets?
The leaflets do not open fully to the vein wall, causing local stenosis, velocity augmentation, and fluid recirculation in the sinus, leading to valvular thrombosis
What is the protective mechanism against thrombus formation related to valve function?
Regular valve motion.
How do changes in thoracic and abdominal pressures during inspiration and expiration affect venous return?
During inspiration, thoracic pressure decreases and abdominal pressure increases compressing the vena cava in the abdomen and causing blood to flow from the abdomen to the thorax and extremities.
The valves in the leg veins and the decrease in thoracic pressure promote flow towards the heart.
During expiration, thoracic pressure increases and abdominal pressure decreases, allowing the vena cava to refill as the valves below the abdomen open.
The vena cava is then primed for the next inspiration.
Why is the vena cava not compressed within the thoracic region during the respiratory cycle?
The thoracic pressure remains lower than atmospheric pressure throughout the respiratory cycle, preventing the vena cava from being compressed within this region.
How have respiratory effects on venous flow in the legs been observed despite the presence of valves?
The effects of respiration on venous flow in the legs have been observed using Doppler ultrasound, even with valves between the abdomen and periphery.
How does exercise affect cardiac output and venous return?
Exercise increases cardiac output, which requires an increase in venous return.
What muscle pumps contribute to increased venous return from the legs during exercise?
The foot, calf, and thigh muscle pumps contribute to increased venous return from the legs during exercise.
Which muscles form the calf pump, and what is their action?
The gastrocnemius and soleus muscles form the calf pump. When these muscles contract, they force blood from the deep calf veins (anterior tibial, posterior tibial, and peroneal veins) into the popliteal vein at the knee.
How do the deep veins refill after blood is forced out during muscle contraction?
The deep veins refill through inflow from the arterial circulation and flow between the superficial and deep venous systems via perforating veins.
What is the role of perforating veins in the venous circulation?
Perforating veins extend through the fascia to connect the deep and superficial venous circulations.
How do competent venous valves contribute to venous return?
Competent venous valves ensure that blood ejected from the calf cannot return under gravity, maintaining unidirectional flow from the superficial to the deep system.
How does the distensibility of the venous system affect venous volume in response to changes in hydrostatic pressure?
Changes in hydrostatic pressure lead to significant changes in venous volume due to the distensibility of the venous system.
What is the equation that represents the change in pressure within a vein, and what do the variables represent?
ΔP=ρgh, where
ΔP is the change in pressure,
ρ is the density of blood,
g is the acceleration due to gravity, and
h is the height of the blood column.
Why do deep veins not collapse in the same manner as superficial veins?
Because they are supported by the surrounding muscular tissue.
What impedes the instantaneous change in hydrostatic pressure when posture changes, and what is the timescale for this adjustment?
Fluid shifts, which are impeded by venous valves, prevent the instantaneous change in hydrostatic pressure. The timescale for the adjustment when moving from sitting to standing is around 20 seconds.
What transient effect occurs within the veins during walking, and how does pressure change when walking stops?
During walking, there is a transient reduction in venous pressure as blood is expelled from the calf. When walking stops, the pressure gradually increases to the value associated with the hydrostatic column due to refilling from the arterial circulation.
What is deep vein thrombosis (DVT) and where does it typically form?
DVT is the formation of a thrombus within the deep veins, typically in the calf or thigh.
What are some potential symptoms of DVT?
DVT is often asymptomatic, but it can cause swelling of the limb and changes in skin color.
What serious complication can arise if a thrombus from DVT travels to the pulmonary circulation?
It can result in a potentially fatal pulmonary embolism.
Where do post-mortem studies indicate that DVT often forms, and what is this linked to?
DVT often forms at valve sites, linked to vortices within the valve sinus.
Why is there cellular damage at valve sites in the context of DVT?
Cellular damage occurs because valve leaflets are avascular and have reduced PO2 tension.
What biological response is triggered due to cellular damage at the valve sites?
The biological response involves platelet aggregation and clot formation behind the leaflets.
During what periods is the risk of DVT higher, and why?
The risk of DVT is higher during periods of prolonged inactivity, such as long-haul air travel or bed rest following surgery, because the muscle and respiratory pumps are relatively inactive.
Why is the risk of deep vein thrombosis (DVT) higher during periods of prolonged inactivity?
Because the muscle and respiratory pumps are relatively inactive during periods of prolonged inactivity, such as long-haul air travel or bed rest following surgery.
What can B-mode ultrasound be used for in the context of venous assessment?
B-mode ultrasound can be used to assess venous anatomy and perform vein mapping, identifying anatomical features such as congenital absence of valves, valve incompetence, and outflow obstruction that contribute to poor venous return.
How is Doppler ultrasound used in the assessment of venous flow?
Doppler ultrasound measures the direction and magnitude of blood flow during clinical tests, including assessing reflux magnitude following manual calf compression, muscle pump activation, or the Valsalva maneuver. It can be repeated along the vein length to identify specific valve sites associated with reflux for targeted intervention.
What does M-mode ultrasound evaluate in the venous system?
M-mode ultrasound evaluates variations in valve geometry and valve incompetence.
