Lectures 9+10 - Venous Circulation

Question 1

Explain how complexity varies in the venous system?

Answer 1

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

Question 2

Why can veins act as a blood resevoir?

Answer 2

Venous compliance -
They’re larger and thinner than arteries
They contain less smooth muscle

Question 3

What is the model used for venous collapse?

Answer 3

thin-walled elastic cylinder

Circular vein will buckle under a uniform pressure load due to elastic instability

Question 4

Why cant you use the linear approximation for venous collapse?

Answer 4

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

Question 5

What happens to the resistance of the vessel during collapse? Why?

Answer 5

Significantly increases due to increased viscous losses and vessel closes

Collapsed veins present a high resistance to flow —> dynamic mechanism for flow limitation

Question 6

What happens in the vessels of the foot as someone moves from sitting to standing? Why?

Answer 6

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

Question 7

Explain venous return

Answer 7

Volume of blood returning to right atrium from systemic veins

Provides blood for pulmonary circulation

Question 8

What is total venous return dictated by?

Answer 8

pressure gradient
resistance of the venous vascular bed
influence of transient muscle pump activation
effects of the respiratory pump

Question 9

When do venous valves form?

Answer 9

During the development of the cardiovascular system.

Question 10

How do venous valve leaflets initially form?

Answer 10

As a bulging in the venous endothelial layer.

Question 11

What are venous valve leaflets made of?

Answer 11

A thin elastic layer on the luminal surface and underlying collagen with very little connective tissue.

Question 12

Where are smooth muscle cells found in venous valves?

Answer 12

Only in the valvular agger, where the valve attaches to the vein wall.

Question 13

What is the most common structure of venous valves?

Answer 13

Most venous valves are bicuspid (having two leaflets).

Some tricuspid valves (with three leaflets) are observed.

Question 14

How can the sinus of venous valves be observed?

Answer 14

Using MRI or X-ray imaging.

Question 15

How does the sinus geometry of venous valves change with pressure?

Answer 15

The venous wall at the sinus is more distensible and thinner than elsewhere.

Question 16

Where are valves found within the venous system?

Answer 16

In all components, including superficial, deep, and perforating veins.

Question 17

How does the number of valves change from central to peripheral circulation?

Answer 17

The number of valves increases from the central circulation to the peripheral circulation of the arms and legs.

Question 18

Which major vein has no valves?

Answer 18

The vena cava.

Question 19

What is the incidence of venous valves associated with?

Answer 19

The importance of the skeletal muscle pump in the peripheral circulation.

Question 20

What imaging technique is used to characterize the hemodynamics of venous valves?

Answer 20

B-mode ultrasound

Question 21

What are the three distinct phases of valve leaflet motion?

Answer 21

Opening, closing, and resting.

Question 22

What happens to the valve leaflets during the resting phase?

Answer 22

They undergo oscillatory motion when flow is constant.

Question 23

To what can the oscillatory motion of valve leaflets during the resting phase be compared?

Answer 23

Self-excited oscillations during steady flow in compliant tubes.

Question 24

What occurs during the opening phase of valve leaflets?

Answer 24

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

Question 25

What is the protective mechanism against thrombus formation related to valve function?

Answer 25

Regular valve motion.

Question 26

How do changes in thoracic and abdominal pressures during inspiration and expiration affect venous return?

Answer 26

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.

Question 27

Why is the vena cava not compressed within the thoracic region during the respiratory cycle?

Answer 27

The thoracic pressure remains lower than atmospheric pressure throughout the respiratory cycle, preventing the vena cava from being compressed within this region.

Question 28

How have respiratory effects on venous flow in the legs been observed despite the presence of valves?

Answer 28

The effects of respiration on venous flow in the legs have been observed using Doppler ultrasound, even with valves between the abdomen and periphery.

Question 29

How does exercise affect cardiac output and venous return?

Answer 29

Exercise increases cardiac output, which requires an increase in venous return.

Question 30

What muscle pumps contribute to increased venous return from the legs during exercise?

Answer 30

The foot, calf, and thigh muscle pumps contribute to increased venous return from the legs during exercise.

Question 31

Which muscles form the calf pump, and what is their action?

Answer 31

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.

Question 32

How do the deep veins refill after blood is forced out during muscle contraction?

Answer 32

The deep veins refill through inflow from the arterial circulation and flow between the superficial and deep venous systems via perforating veins.

Question 33

What is the role of perforating veins in the venous circulation?

Answer 33

Perforating veins extend through the fascia to connect the deep and superficial venous circulations.

Question 34

How do competent venous valves contribute to venous return?

Answer 34

Competent venous valves ensure that blood ejected from the calf cannot return under gravity, maintaining unidirectional flow from the superficial to the deep system.

Question 35

How does the distensibility of the venous system affect venous volume in response to changes in hydrostatic pressure?

Answer 35

Changes in hydrostatic pressure lead to significant changes in venous volume due to the distensibility of the venous system.

Question 36

What is the equation that represents the change in pressure within a vein, and what do the variables represent?

Answer 36

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.

Question 37

Why do deep veins not collapse in the same manner as superficial veins?

Answer 37

Because they are supported by the surrounding muscular tissue.

Question 38

What impedes the instantaneous change in hydrostatic pressure when posture changes, and what is the timescale for this adjustment?

Answer 38

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.

Question 39

What transient effect occurs within the veins during walking, and how does pressure change when walking stops?

Answer 39

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.

Question 40

What is deep vein thrombosis (DVT) and where does it typically form?

Answer 40

DVT is the formation of a thrombus within the deep veins, typically in the calf or thigh.

Question 41

What are some potential symptoms of DVT?

Answer 41

DVT is often asymptomatic, but it can cause swelling of the limb and changes in skin color.

Question 42

What serious complication can arise if a thrombus from DVT travels to the pulmonary circulation?

Answer 42

It can result in a potentially fatal pulmonary embolism.

Question 43

Where do post-mortem studies indicate that DVT often forms, and what is this linked to?

Answer 43

DVT often forms at valve sites, linked to vortices within the valve sinus.

Question 44

Why is there cellular damage at valve sites in the context of DVT?

Answer 44

Cellular damage occurs because valve leaflets are avascular and have reduced PO2 tension.

Question 45

What biological response is triggered due to cellular damage at the valve sites?

Answer 45

The biological response involves platelet aggregation and clot formation behind the leaflets.

Question 46

During what periods is the risk of DVT higher, and why?

Answer 46

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.

Question 47

Why is the risk of deep vein thrombosis (DVT) higher during periods of prolonged inactivity?

Answer 47

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.

Question 48

What can B-mode ultrasound be used for in the context of venous assessment?

Answer 48

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.

Question 49

How is Doppler ultrasound used in the assessment of venous flow?

Answer 49

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.

Question 50

What does M-mode ultrasound evaluate in the venous system?

Answer 50

M-mode ultrasound evaluates variations in valve geometry and valve incompetence.