Physiology
Lec: 8 د. زيـد الاطرقجيCardiovascular system
Lymphatic:
The lymphatic system is an accessory route by which interstitial fluid can be returned to the blood. Even under normal circumstances slightly more fluid is filtered out the capillaries into the interstitial fluid than the reabsorbed from the interstitial fluid back into the plasma.
On average the net ultra-filtration pressure starts at 11mmHg at the beginning of the capillary whereas the net reabsorption pressure only reaches 9mmHg by the vessel end.
Because of this pressure differential on average more fluid is filtered out of the first half of the capillary than is reabsorbed in its last half. The extra fluid filtered out as a result of this filtration-reabsorption imbalance is picked up by the lymphatic system.
The lymphatic system is an extensive network of one way vessels provide an accessory route by which fluid can be returned from the interstitial fluid to the blood
Pick and flow of lymph:
Small blinded terminal lymph vessels known as initial lymphatics permeate almost every tissue of the body. The endothelial cells forming the walls of initial lymphatics slightly overlap like shingles on a roof, with their overlapping edges being free instead of attached to the surrounding cells. This arrangement creates one-way valve like openings in the vessel.
Fluid pressure on the outside of the vessel pushes the innermost edge of a pair of overlapping edges inward creating a gap between the edges [that is opening the valve]this opening permits interstitial fluid to enter. Once interstitial fluid enters a lymphatic vessel it is called lymph. Fluid pressure on the inside forces the overlapping edges together closing the valves so that lymph does not escape.
These lymphatics valve like opening are much larger than the pores in blood capillaries. Consequently large particles in the interstitial fluid such as escaped plasma proteins and bacteria can gain access to initial lymphatics.
Initial lymphatics converge to form larger and larger lymph vessels which eventually empty into the venous system near where blood enters the right atrium.
Lymph flow is accomplished by two mechanisms:
1. Lymph vessels beyond the initial lymphatics are surrounded by smooth muscle which contract rhythmically as a result of myogenic activity. When this muscle is stretched because the vessel is distended with lymph the muscle inherently contracts more forcefully pushing the lymph through the vessel. This intrinsic lymph pump is the major force for propelling lymph. Stimulation of lymphatic smooth muscle by the sympathetic nervous system further increases the pumping activity of the lymph vessels
2. Because lymph vessels lie between skeletal muscles, contraction of these muscles squeezes the lymph out of the vessels.
One way valves spaced at intervals within the lymph vessels direct the flow of lymph toward its venous outlet in the chest. The average rate of flow through the lymph vessels is 3 liters per day compared with 7200 liters per day through the circulatory system.
Functions of the lymphatic system
1. Return of excess filtered fluid
Normally capillary filtration exceeds reabsorption by about 3 liters per day [20liters filtered 17 liters reabsorbed ] the cumulative effects of this process being repeated with every heart beats results in the equivalent of more than the entire plasma volume being left behind in the interstitial fluid each day.
2. Defense against disease
The lymph percholate through lymph nodes located en-route with in the lymphatic system, passage of this fluid through the lymph nodes is a defense mechanism against disease for example bacteria picked up from the interstitial fluid are destroyed by special phagocytes within the lymph nodes.
3. Transport of absorbed fat.
The lymphatic system is important in the absorption of fat from the digestive tract. The end products of the digestion of dietary fat are packed by cells lining the digestive tract into fatty particles that are too large to gain access to the blood capillaries but can easily enter the initial lymphatic.
4- Return of filtered protein
Most capillaries permit leakage of some plasma proteins during filtration these proteins cannot readily be absorbed back into the blood capillaries but can easily gain access to the initial lymphatic. if the proteins were allowed to accumulate in the interstitial fluid rather than being returned to the circulation via the lymphatic the interstitial fluid colloid osmotic pressure [an outward pressure] would progressively increase while the plasma colloid osmotic pressure [an inward pressure] would progressively falls.
As a result filtration forces would gradually increase and reabsorption forces would gradually decrease, resulting in progressive accumulation of fluid in the interstitial spaces at the expense of loss of plasma volume, the result will be Edema (which occurs when too much interstitial fluid accumulates).
Veins:
Blood leaving the capillary beds enters the venous system to be transported back to the heart. Capillaries drain into venules which progressively converge to form veins that exit the organ. In contrast to arterioles venules have little tone and resistance, which is controlled by chemical signals between venules and nearby arterioles, this venulo-arteriolar signaling is vital to matching capillary inflow and outflow within an organ.
Veins serve as a blood reservoir as well as passage ways back to the heart. Veins have a large radius so they offer little resistance to flow.
Veins in contrasts to arterioles have very little elasticity because venous connective tissue contains considerably more collagen fibers than elastic fibers. Unlike arteriolar smooth muscle venous smooth muscle has little inherent myogenic tone. Because of these features veins are highly distensible to accommodate additional volumes of blood with only a small increase in venous pressure
Veins containing an extra volume of blood simply stretch to accommodate the additional blood without tending to recoil. In this way veins serve as a blood reservoir that is when demand for blood are low the veins can store extra blood in reserve because of their passive distensibility. Under resting conditions the veins contain more than 60% of the total blood volume.
In addition to the driving pressure imparted by cardiac contraction five other factors enhance venous return:
1. Sympathetically induced venous vasoconstriction.
2. Effect of venous valves.
3. Skeletal muscle activity.
4. Respiratory activity.
5. The effect of cardiac suction.
Most of these secondary factors affect venous return by influencing the pressure gradient between the veins and the heart.
Effect of sympathetic activity on venous return:
Veins are not very muscular and have little inherent tone but venous smooth muscle is abundantly supplied with sympathetic nerve fibers. Sympathetic stimulation produces venous vasoconstriction which modestly elevates venous pressure this in turn increases the pressure gradient to drive more of the stored blood from the veins into the right atrium thus enhancing venous return.
Effect of skeletal muscle activity on venous return:
Many of the large veins in the extremities lie between skeletal muscles so muscle contraction compresses the veins. This external venous compression decreases venous capacity and increases venous pressure in effect squeezing blood in the veins forward toward the heart. this pumping action known as the skeletal muscle pump in one way extra-blood stored in the veins is returned to the heart during exercise . Increased sympathetic activity and the resultant venous vasoconstriction also accompany exercise further enhancing venous return . The skeletal muscle pump also counters the effect of gravity on the venous system.
Venous valves:
Large veins are equipped with one way valves spaced at 2-4cm intervals, these valves let blood move forward toward the heart but keep it from moving back toward the tissues.
These venous valves also play a role in counter acting the gravitational effects of upright posture by helping minimize the backflow of blood that tends to occur when a person stands up and by temporarily supporting portions of the column of blood when skeletal muscle are relaxed.
Varicose- veins occurs when the venous valves become incompetent and can no longer support the column of blood above them.
Effect of respiratory activity on venous return:
As a result of respiratory activity the pressure within the chest averages 5mmHg less than atmospheric pressure. As the venous system returns blood to the heart from the lower regions of the body it travels through the chest cavity where it is exposed to this sub-atmospheric pressure because the venous system in the limbs and abdomen is subject to normal atmospheric pressure, an externally applied pressure gradient exists between the lower veins [At atmospheric pressure]and the chest veins [at 5mmHg less than atmospheric pressure]. This pressure difference squeezes blood from the lower veins to the chest veins promoting increased venous return this is called respiratory pump.
During exercise there will be :
1. Increase respiratory activity.
2. Increase in skeletal muscle activity.
3. Venous vasoconstriction.
All the above enhance venous return during exercise.
The heart functions as a suction pump to facilitate cardiac filling:
During ventricular relaxation the AV valves are drawn downward enlarging the atrial cavities .as a result the atrial pressure transiently drop below 0mmHg thus increasing the vein to atria pressure gradient so that venous return is enhanced . In addition the rapid expansion of the ventricular chambers during ventricular relaxation creates a transient negative pressure in the ventricles so that blood is sucked in from the atria and veins that is the negative ventricular pressure increases the vein to atria to ventricular pressure gradient, further enhancing venous return.