Venous and lymphatic systems

Question 1

Learning outcomes

Answer 1

1. Briefly describe vein structure in relation to function
2. Describe the pressure gradient in the cardiovascular tree
3. Discuss the factors that effect venous return
4. Explain venous compliance
5. Explain the function of veins as capacitance vessels
6. Describe the normal distribution of blood volume at rest
7. Describe the regulation of venous tone
8. Explain how veins differ from arteries
9. Describe the functions of the lymphatic system

Question 2

Veins- structure and function

Answer 2

Thin walls with Smooth muscle & some elastic tissue
Large radii
One-way valves

Function

1. Transport - Passageway back to the heart – conduit vessels
2. Storage - Store blood & mobilise when required –capacitance vessels veinartery

Question 3

What is venous return?

Answer 3

Velocity of flow is inversely proportional to the total cross-sectional area. Therefore-
The volume of blood returning to heart from systemic veins is known as venous return

Determined by 1) the pressure gradient & 2) venous resistance
Venous system
1. Low pressure system  ~ 18 mmHg
Right atrial pressure ~ 0 mmHg
So, small pressure gradien
t2.Resistance to flow is low

Other factors promote venous return primarily by increasing the pressure gradient

Question 4

Factors that facilitate venous return

Answer 4

Venous valves- (mechanically preven tbackflow of blood)
Cardiac suction effect (↓ pressure in heart –↑ pressure gradient)
Pressure of blood due to cardiac contraction
↑ Venous tone (↑ venous pressure –↑ pressure gradient)
Skeletal muscle pump (↑ venous pressure –↑ pressure gradient)
Respiratory pump (↓ venous pressure –↑ pressure gradient)
↑ Blood volume (↑ venous pressure –↑ pressure gradient)

Increase in venous return (blood pumped into heart pm) means
- ↑ End diastolic volume > ↑ stroke volume > ↑ cardiac output (amount of blood pumped from heart pm)

Question 5

Factors that ↑ venous tone & lead to venoconstriction

Answer 5

Venous tone- degree of smooth muscle contraction in veins
1. Sympathetic stimulation (e.g. during haemorrhage, exercise)
2. Activated venous stretch receptors
Reflex response is ↑ venous smooth muscle tone
3. Hormones
Circulating adrenaline, noradrenaline

Question 6

Venous storage

Answer 6

Venous Compliance: the rate of change in volume (V) with changing pressure
(P)C = ΔV/ΔP
Characteristics of veins
-highly distensible: high vascular compliance accommodate changes in blood volume without much change in P
large storage capacity
–blood reservoir
- capacitance vessels: at rest most blood volume will be in venous circulation (high compliance, accomodate large volumes of blood w/out large changes in pressure)
Capacitance vessels– low resistance, high volume

Question 7

Venous system as a blood resevoir

Answer 7

Baroreflex

• In haemmhorrage, drop in V return, drop in C. output, drop in stroke volume and arterial pressure
• baroreceptors detect drop in p, increase a.pressure by inc heart rate and contractility, increased c.output, increased arterial pressure
• Increased symp NS activity, arteriolar constriction, inc total peripheral resistance and inc in art. pressure
• Venoconstriction, inc in TP.resistance and reduces venous capacity- blood pushed back to RHS of heart, attempt to restore central BV and C.output

CBV drop detected by atrial receptors, increased arterial pressure
Decrease in arterial pressure in Haemhorrage> reduced regional blood flow, activation in peripheral and central chemoreceptors> medulla oblongata, increased arterial pressure and decreased venous capacity

Venous system v. important in mobilising blood, venoconstriction helps restore CBV.
-Specific blood reservoirs:The spleen (<100ml), the liver (<400-600ml), large abdominal veins (<300ml) & venous plexus (<400-600ml)
Diagram lecture 28

Question 8

Venous pressure

Answer 8

2 components-
Hydrostatic pressure
Pressure generated by heart

Pressure decreases in venous system from the feet up- gravity acting on veins and weight of column of blood from heart to foot is much greater than for, e.g heart and hand (volume differences)
see practical manual+ notes

Question 9

Central venous pressure

Answer 9

Pressure in the thoracic vena cava near right atrium.
Depends on
1) venous return &
2) right ventricular function

Normal right atrial pressure: 0 – 5 mmHg
Abnormal right atrial pressure e.g.
1. serious heart failure, large blood transfusion: 20-30 mmHg
2.when heart pumps vigorously, after haemorrhage: -3 to -5 mmHg

Question 10

How do veins and arteries differ?

Answer 10

A- Transport blood away from heart
V- Transport blood towards heart

A- Pulsatile flow
V- Smooth flow

A- Carry Oxygenated Blood bar PA
V- Carry De-oxygenated Blood bar PV

A- Small lumen
V- Large lumen

A- More muscle/elastic tissue
V- Less muscle/elastic tissue

A- Transports blood under high pressure
V- Transports blood under lower pressure

A- No valves
V- Valves

Arteries have a low compliance- veins have a high compliance

Question 11

Pathological conditions affecting venous system

Answer 11

Varicose veins-
Most commonly occur in the leg-
Include:
Valve incompetence- gravity pulling blood into legs can affect valves, can be genetic
Venous obstruction- can be caused by clot: increase in VP due to obstructed blood flow, and if thrombus breaks away from wall of femoral vein it is known as an embolism

Question 12

The lymphatic system- structures and organs

Answer 12

Structures
Lymph nodes- filter and attack antigens- found throughout body
MALT (mucosa associated lympathic tissue) -filter and attack antigens in air, food or urine- found in GI walls, resp, genital and urinary tracts
Tonsils- prtotect against inhaled/ingested materials- found within pharynx

Organs
Thymus- site of T- lymphocyte maturation/differentiation- stores maturing lymphocytes- found in superior mediastinum in adults, ant+sup in children
Lymph nodes- filter lymp- mount immune response if required- found throughout body, frequently in clusters in axillary, inguinal and cervical regions
Spleen- filters blood/recycles ages erythrocytes and platelets, serves as blood resevoir, houses lymphocytes, mounts IR to foreign blood antigens- In left upper quadrant of abdomen, near 9th-11th ribs and inferior to diaphragm

Question 13

The lymphatic system

Answer 13

Runs in parallel to circulatory system
Transports lymph (clear, yellowish fluid containing white blood cells, proteins & fats)

Lymphatic capillaries, lymphatic vessels, lymph nodes, - lymphatic trunk drain into lymphatic ducts, drain into lymph drains into central veins
return to RHS of heart

Question 14

3 primary functions of the lymphatic system

Answer 14

1. Fat absorption
2. Defense against disease
3. Fluid balance– collects & returns interstitial fluid, including plasma protein to blood
   Also important in other ECF compartments- intravascular compartment, blood and IS fluid