In this video, formation of lymph will be
discussed. In the last video we discussed on lymphatic vessels. Hope you guys watched it.
If not, please do watch the video for better understanding, before proceeding to this video.
The link of the video is given in the description. So, let's start with the
microcirculation and capillary system. Microcirculation is the most important
part of the circulatory system, as transport of nutrients to the tissue
and removal of cell waste occurs. Each nutrient artery that enters the organ,
branches 6 to 8 times to form the arterioles, then each arteriole branches 2 to
5 times to form the capillaries. The branch of the arteriole which
connects directly with capillary is called a meta-arteriole. Meta arterioles
are different from the arterioles. In meta arterioles the smooth muscle coat is not
continuous, as in arterioles. It circles the meta arterioles at intermittent points. At the point
where capillaries arise from the meta arteriole, there is smooth muscle fibre encircling it, to
form precapillary sphincter. This precapillary sphincter, opens and closes to regulate the blood
flow into the capillaries, according to the local condition of the tissue. The most important
factor is oxygen concentration in the tissue. When there less oxygen concentration
in the tissues, these sphincters open, and allows more blood in to the capillaries, so
that more oxygen can be delivered to the tissues. From the capillaries, venules arise
and then drains into larger veins. Capillaries is made of a single layer of
endothelial cells. And it is surrounded by a thin basement membrane. There are two very
small pathways in the endothelium for transport of nutrients to the interstitial spaces. One
of the pathway is the intercellular cleft. This cleft allows fluid, small solutes, and
water soluble ions to freely pass from the capillaries into the interstitial spaces.
But albumin molecules cannot freely pass, as the cleft space is slightly narrow
for the protein molecules to escape. Second pathway is the plasmalemmal vesicles.
It can imbibe small amount of plasma or extracellular fluid, and transport
across the endothelium, as shown. These vesicles can sometime combine to
form vesicular channel for the transport of nutrients from plasma to tissues, or
waste products from tissues into the plasma. Interstitium, is made of solid and fluid parts.
The solid part is formed by the collagen fiber bundles, and proteoglycan filaments. Collagen
bundles gives the Interstitium tensional strength. The fluid in these solid spaces
is called interstitial fluid. The interstitial fluid is derived from the
blood capillaries by diffusion and filtration. So, the interstitial fluid is similar to
plasma, but contains less of proteins. As capillaries, doesn t allow proteins
to escape easily from the circulation. There are four pressures, that regulate the
fluid movement through the capillary membrane. First pressure is capillary pressure. The
capillary pressure, pushes the fluid outwards through the capillary membrane. The second
pressure is, interstitial fluid pressure, it tends to push the fluid into the capillaries. The third
pressure is, capillary, plasma colloid osmotic pressure, which tends to cause osmosis of fluid
into the capillaries from the interstitial spaces. The fourth pressure, is interstitial
fluid colloid osmotic pressure, which tends to cause osmosis of fluid into
the Interstitium from the capillaries. So, when all these forces are summed up,
it is called the net filtration pressure. If the net filtration pressure is positive,
then fluid flows across the capillaries into the Interstitium. If the net filtration pressure
is negative, the interstitial fluid is absorbed into the capillaries. In normal condition, the
net filtration pressure is slightly positive, so that fluid flows across the
capillaries, into the Interstitium. Now, we will discuss, how the lymph is formed. The
fluid that is filtered by the arterial end of the capillaries, supplies the nutrients to tissues,
and collects the metabolic waste from the tissues. Most of the fluid is reabsorbed back into
the venous end of the blood capillaries. This excess fluid that is left behind, when enters
the lymphatic capillaries, is called the lymph. On an average per day, about 20 litres of
fluid is filtered at the arterial end of the blood capillaries, and about 18 litres of
this fluid is reabsorbed back at the venous end of the blood capillaries. So, if this excess 2
litres of interstitial fluid are left behind, it causes tissue edema. This is where
the lymphatic system come in to role. It absorbs the excess 2 litres of the fluid, and
drains back into the circulatory system as lymph. So, lymphatic system forms an accessory
pathway to the circulatory system, in aiding in drainage of the
tissue fluid back into circulation. What regulates this lymph flow. This can
easily be summarised into two factors. Namely, interstitial fluid pressure, and activity
of the lymphatic pump. When more fluid accumulates in the interstitial spaces, the interstitial
pressure increases, it causes tissue to swell, the anchoring filaments, attached to the
endothelial cells of the lymphatic capillaries, move apart, and allowing the excess fluid to
enter the lymphatic capillaries. Once entered, the fluid cannot re-enter the Interstitium, due to
overlapping arrangement of the lymphatic capillary endothelium, as it acts as a flap valve. Once the
excess fluid is inside the lymphatic system, the flow now depends on the lymphatic pump. Lymphatic
pump can be divided into intrinsic and extrinsic. Intrinsic pump is due to contraction of the
lymphatic vessels by its smooth muscle fibres. Which help in aiding the forward flow of
the lymph. It should be noted that the backflow of the lymph in lymphatic vessels
is prevented by the presence of many valves. The external pump is due to the following factors.
Contraction of the surrounding skeletal muscle. Movements of the parts of the body.
Pulsation of the adjacent arteries. And compression of the
tissues by external objects. The pathway of the lymphatic vessels was already
discussed in the previous video. We request you to watch the video for better understanding. The
link of the video is given in the description. We conclude the video here. Hope it was useful.
In the next video, we will discuss on the lymphoid organs.
See you soon, in the next video.