Venous System

Question 1

What are some characteristics of VEINS?

Answer 1

• Have little elasticity as the venous connective tissue contains more collagen than elastin fibers
• Low pressure system

Question 2

Why are veins highly stretchable and have little elastic recoil?

Answer 2

So they can easily distend to accommodate additional volumes of blood with only a small increase in venous pressure.

Question 3

List the factors affecting venous return

Answer 3

1. Sympathetic Activity
2. Respiratory Activity
3. Cardiac Suction
4. Skeletal Muscle Activity
5. Venous Valves

Question 4

How does SYMPATHETIC ACTIVITY affect venous return?

Answer 4

• Produces venous vasoconstriction which mildly elevates venous pressure
• This increases the pressure gradient so that more of the stored blood from the veins can be driven to the right atrium, enhancing venous return
• Venous vasoconstriction increases blood flow through vessels due to reduced capacity
• Reduced capacity causes less blood to remain in the veins as they flow toward the heart.
• This leads to increased cardiac output by elevated heart rate and contractility due to increased EDV

Question 5

How does RESPIRATORY ACTIVITY affect venous return?

Answer 5

• Pressure in the chest cavity is slightly lower than atmospheric pressure
• Venous system returns blood to heart from lower regions of the body which is at normal atmospheric pressure
• This creates a pressure gradient which pushes blood from lower veins to chest veins, increasing venous return

Question 6

How does CARDIAC SUCTION affect venous return?

Answer 6

• During ventricular contraction: AV valves drawn downward and atrial cavities are enlarged, causing the atrial pressure to reduce. This increases vein-to-atria pressure gradient, enhancing venous return.
• During ventricular relaxation: Transient negative pressure created in the ventricles so that blood is ‘sucked in’ from the atria and veins. This increases vein-to-atria-to-ventricle pressure gradient, enhancing venous return.

Question 7

How does SKELETAL MUSCLE ACTIVITY affect venous return?

Answer 7

• Large veins in the extremities: Muscle contraction compresses the vein which decreases venous capacity and increases venous pressure, squeezing the blood in the veins forward, toward the heart
• Counteract the effects of gravity on the venous system

Question 8

What are the effects of gravity on the venous system? (Skeletal muscle)

Answer 8

• Lying down, force of gravity is applied uniformly
• Standing, gravitational effects are not uniform
• Vessels below heart level are subject to pressure from the weight of the column of blood extending from the heart to the level of the vessel.
• Veins yield under the increased hydrostatic pressure which further expands to allow increased capacity. Blood entering from capillaries pool in expanded lower leg veins instead of returning to the heart. This reduces venous return, cardiac output and effective circulatory volume shrinks
• Excessive fluids filter out of capillary bed in the lower extremities and causes oedema.

Question 9

What are compensations to counteract the gravitational effects?

Answer 9

• Sympathetically induced venous vasoconstriction to drive the pooled blood forward
• Skeletal muscle pump empties given veins segments intermittently so that a particular portion of a vein is not subjected to the weight of the entire venous column from the heart to that portion’s level.

Question 10

How does VENOUS VALVES affect venous return?

Answer 10

• Blood can only be driven forward as the large veins have one-way valves
• Valves let blood move forward toward the heart but keep it from moving back toward the tissues
• This minimises backflow of blood and temporarily supports portions of the column of blood when skeletal muscle are relaxed.