L8: Lymphatic and venous circulation Flashcards
Why are veins called capacitance vessels?
- The veins conduct the blood from the tissues to the heart. They accommodate the main bulk of blood volume (about 60%)
What are the functions of veins? “Drain blood/lymph - reservoir - pump”
- Drainage of blood from all parts of the body to the heart. Also the lymph drainage opens into the venous system.
- They act as blood reservoirs; they contain about 3 liters of blood.
- By means of venous pump, it aids in propelling the blood forward (towards the heart) and helps to regulate the cardiac output.
What is the hydrostatic indifferent level or (HIP)?
- 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.
What is central venous pressure and what are its values??
- 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.
What increases CVP? “Bad thing”
- 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
What decreases CVP?
- Negative intrathoracic pressure during respiration
- Standing (CVP decreases to 2 mm mercury)
- Hypovolemia (hemorrhage)
- Sympatholytics due to venodilation and decreasing the venous return
What are the factors affecting the venous pressure?
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
How does gravity or posture affect venous pressure?
- 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.
How does the rate of blood flow into veins affect venous pressure?
- 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.
How does the rate of blood outflow from Veins affect venous pressure?
- 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
How does the venomotor tune affect venous pressure?
- Tonic contraction in veins maintains normal VP.
- inc venous tone due to sympathetic stimulation or noradrenaline→ Inc CVP and PVP are vice versa.
How does blood volume affect venous pressure?
- CVP and PVP are increased in hypervolaemia (blood transfusion).
- CVP and PVP are decreased in hypovolaemia (haemorraghe).
How does muscular exercise affect venous pressure?
- 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.
How do respiratory movement affect venous pressure?
- Inspiration: The diaphragm descends → inc VP in abdominal veins and dec VP in intrathoracic veins→ Inc VR.
- Expiration: the opposite effect occurs.
What is the nature of lymph flow in lymphatic vessels?
■ One-way flap valves permit interstitial fluid to enter, but not leave, the lymph vessels.
■ Flow through larger lymphatic vessels is also unidirectional.
What is the function of lymphatic system? “DAI”
- Drainage of excess interstitial fluid filtered from capillaries back to blood.
- Drainage of filtered proteins.
- Absorption of fat from the intestine.
- 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.
What are the factors that maintain lymph flow?
Normally, the lymph flows towards the thorax; it is maintained by the following factors:
- Rhythmic contraction of smooth muscles in the wall of the lymphatic vessels. “Like veins and arteries”
- Skeletal muscle contraction compresses the lymph vessels → inc lymph flow.
- Arterial pulsations are mechanically conducted to lymph vessels, which may help lymph flow.
- Gravity helps the lymph flow from parts above the level of the heart, but antagonizes it from parts below the level of the heart.
- The hydrostatic pressure of the interstitial fluid is normally negative (- 3 mmHg), this helps the lymph flow. “Suction force”
What are the factors that increase lymph flow? “Inc filtration”
The lymph flow increases as a result of increased rate of tissue fluid formation. This occurs in the following conditions:
- Dilatation of the capillaries and arterioles ➔ increase capillary filtration and increase lymph flow.
- Venoconstriction➔increase capillary pressure➔increase filtration and lymph flow.
- Increased tissue activity→accumulation of metabolites→dilatation of the arterioles and precapilalry sphincters.
- Lymphagogues: These are substances that increase the lymph flow. They act mainly by increasing fluid filtration into the tissue spaces as histamine and bradykinin.
What is the definition of edema?
Edema is the accumulation of fluid in the interstitial space
What are the two forms of edema?
Intracellular non-pitting and extracellular fitting
What causes intracellular edema? “Dangerous”
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”
What causes extracellular Edema?
Occurs when there is excess fluid accumulation in the extracellular spaces i.e. increased interstitial fluid.
What are the factors that cause extracellular edema?
Significant alterations in the Starling forces, which then tip the balance toward filtration, increase capillary permeability and/or interrupt lymphatic function, resulting in edema.
What causes increase in the hydrostatic pressure of the capillaries?
▪ Arteriolar dilatation as in fever.
▪ Increased venous pressure as in congestive heart
failure, venous obstruction and pregnancy. “Due to enlarged uterus”
▪ Extracellular volume expansion.
What causes decrease in osmotic pressure of the capillaries?
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).
What increases the capillary permeability?
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.
What causes lymphatic obstruction?
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.
