L8: Lymphatic and venous circulation

Question 1

Why are veins called capacitance vessels?

Answer 1

• The veins conduct the blood from the tissues to the heart. They accommodate the main bulk of blood volume (about 60%)

Question 2

What are the functions of veins? “Drain blood/lymph - reservoir - pump”

Answer 2

1. Drainage of blood from all parts of the body to the heart. Also the lymph drainage opens into the venous system.
2. They act as blood reservoirs; they contain about 3 liters of blood.
3. By means of venous pump, it aids in propelling the blood forward (towards the heart) and helps to regulate the cardiac output.

Question 3

What is the hydrostatic indifferent level or (HIP)?

Answer 3

• It is a point in the circulation, lies 5-7 cm below the diaphragm, at which the venous pressure (VP) is kept constant at 10-11 mmHg, independent of the body posture.
• VP below this point increases and above this point decreases.

Question 4

What is central venous pressure and what are its values??

Answer 4

• It is the venous pressure in big veins at their entrance with the right atrium, or the intrathoracic portions of vena cavae.
• It averages 4.6 mmHg in recumbancy and 2 mmHg on standing It is 2 mmHg during inspiration and 6 mmHg during expiration.

Question 5

What increases CVP? “Bad thing”

Answer 5

• Positive intrathoracic pressure like straining
• Congestive heart failure “Due to fullness with blood”
• hypervolemia (blood transfusion)
• Sympathomimetics due to venoconstriction and increasing the venous return

Question 6

What decreases CVP?

Answer 6

• Negative intrathoracic pressure during respiration
• Standing (CVP decreases to 2 mm mercury)
• Hypovolemia (hemorrhage)
• Sympatholytics due to venodilation and decreasing the venous return

Question 7

What are the factors affecting the venous pressure?

Answer 7

1- Gravity or posture
2- Rate of blood inflow into veins
3- Rate of blood outflow from veins
4- Venomotor tone
5- Blood volume
6- Muscular exercise
7- Respiratory movement

Question 8

How does gravity or posture affect venous pressure?

Answer 8

• In recumbent position gravity has no effect on circulation.
• On standing, PVP is increased in lower limbs to 90 mmHg in feet veins and CVP is decreased from 4.6 mmHg to 2 mmHg→ Dec. venous return→ Dec COP → decABP (postural hypotension) in prolonged standing.

Question 9

How does the rate of blood flow into veins affect venous pressure?

Answer 9

• Vasodilatation of arterioles without capillary dilatation→Inc CVP and PVP and vice versa.
• If the capillaries are dilated, they accommodate blood and decrease the blood inflow to veins→ Dec CVP and PVP.

Question 10

How does the rate of blood outflow from Veins affect venous pressure?

Answer 10

• Normally, the rate of blood inflow into veins equals the rate of its outflow. “Due to venous congestion or due to inc in intrathoracic pressure”
• If the outflow is less than the inflow→ Dec the venous return→Inc CVP and PVP

Question 11

How does the venomotor tune affect venous pressure?

Answer 11

• Tonic contraction in veins maintains normal VP.

- inc venous tone due to sympathetic stimulation or noradrenaline→ Inc CVP and PVP are vice versa.

Question 12

How does blood volume affect venous pressure?

Answer 12

• CVP and PVP are increased in hypervolaemia (blood transfusion).
• CVP and PVP are decreased in hypovolaemia (haemorraghe).

Question 13

How does muscular exercise affect venous pressure?

Answer 13

• Contracting skeletal muscle press on veins, increasing the VP.
• Arteriolar dilatation of active muscle, increasing the VP.
• However VP is not much increased in exercise, due to increase the outflow from veins because the heart rate and the stroke volume are increased during exercise.

Question 14

How do respiratory movement affect venous pressure?

Answer 14

• Inspiration: The diaphragm descends → inc VP in abdominal veins and dec VP in intrathoracic veins→ Inc VR.
• Expiration: the opposite effect occurs.

Question 15

What is the nature of lymph flow in lymphatic vessels?

Answer 15

■ One-way flap valves permit interstitial fluid to enter, but not leave, the lymph vessels.

■ Flow through larger lymphatic vessels is also unidirectional.

Question 16

What is the function of lymphatic system? “DAI”

Answer 16

1. Drainage of excess interstitial fluid filtered from capillaries back to blood.
2. Drainage of filtered proteins.
3. Absorption of fat from the intestine.
4. The lymph nodes have the following functions:
   - Filtration of lymph (i.e. by removing foreign substances from it).
   - Formation of lymphocytes.
   - Take part in the formation of immune bodies.

Question 17

What are the factors that maintain lymph flow?

Answer 17

Normally, the lymph flows towards the thorax; it is maintained by the following factors:

1. Rhythmic contraction of smooth muscles in the wall of the lymphatic vessels. “Like veins and arteries”
2. Skeletal muscle contraction compresses the lymph vessels → inc lymph flow.
3. Arterial pulsations are mechanically conducted to lymph vessels, which may help lymph flow.
4. Gravity helps the lymph flow from parts above the level of the heart, but antagonizes it from parts below the level of the heart.
5. The hydrostatic pressure of the interstitial fluid is normally negative (- 3 mmHg), this helps the lymph flow. “Suction force”

Question 18

What are the factors that increase lymph flow? “Inc filtration”

Answer 18

The lymph flow increases as a result of increased rate of tissue fluid formation. This occurs in the following conditions:

1. Dilatation of the capillaries and arterioles ➔ increase capillary filtration and increase lymph flow.
2. Venoconstriction➔increase capillary pressure➔increase filtration and lymph flow.
3. Increased tissue activity→accumulation of metabolites→dilatation of the arterioles and precapilalry sphincters.
4. Lymphagogues: These are substances that increase the lymph flow. They act mainly by increasing fluid filtration into the tissue spaces as histamine and bradykinin.

Question 19

What is the definition of edema?

Answer 19

Edema is the accumulation of fluid in the interstitial space

Question 20

What are the two forms of edema?

Answer 20

Intracellular non-pitting and extracellular fitting

Question 21

What causes intracellular edema? “Dangerous”

Answer 21

Means edema due to increased intracellular fluid (i.e. intracellular swelling). It results from disturbance of the membrane permeability.

“May be Due to a problem in sodium potassium pump”

Question 22

What causes extracellular Edema?

Answer 22

Occurs when there is excess fluid accumulation in the extracellular spaces i.e. increased interstitial fluid.

Question 23

What are the factors that cause extracellular edema?

Answer 23

Significant alterations in the Starling forces, which then tip the balance toward filtration, increase capillary permeability and/or interrupt lymphatic function, resulting in edema.

Question 24

What causes increase in the hydrostatic pressure of the capillaries?

Answer 24

▪ Arteriolar dilatation as in fever.

▪ Increased venous pressure as in congestive heart
failure, venous obstruction and pregnancy. “Due to enlarged uterus”

▪ Extracellular volume expansion.

Question 25

What causes decrease in osmotic pressure of the capillaries?

Answer 25

Decreased plasma protein concentration to 5 gm% as in:

▪ Severe liver disease (failure to synthesize
proteins).

▪ Protein malnutrition.

▪ Nephrotic syndrome (loss of protein in urine).

Question 26

What increases the capillary permeability?

Answer 26

It leads to excessive fluid and protein filtration, so edema develops as in:
▪ Burn
▪ Inflammation (release of histamine; cytokines)
▪ Allergy (allergic edema) due to histamine release
▪ Vitamin C deficiency.
▪ Bacterial toxins.

Question 27

What causes lymphatic obstruction?

Answer 27

Accumulation of tissue fluid and protein in tissue spaces produces edema as in:

▪ Infections (e.g. filaria) produce edema called elephantiasis and it is non pitting.

▪ Cancer produces cancer edema and it is non pitting.

▪ Surgical: due to interruption of lymphatic vessels.

▪ Congenital absence of lymphatics.