Tissue fluid and lymph formation at capillary beds

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at the arterial end of the capillary bed the blood is still at high pressure so blood plasma is forced out through the permeable walls of the capillary. Cells and proteins are too big to leave the capillary so they remain in the blood so tissue fluid is formed by FILTRATION NOT DIFFUSION.

this fluid now forms tissue fluid surrounding the cells. materials are exchanged between the tissue fluid and the cells by all 4 methods of transport across a cell membrane:

gases and lipid soluble substances such as steroids cross by lipid diffusion

water crosses by osmosis

ions cross by facilitated diffusion

glucose and amino acids cross by active transport

at the venous end of the capillary bed the blood is at low pressure since it has lost so much plasma. the blood and tissue fluid are now around the same pressure so tissue fluid returns by diffusion, not MASS FLOW.

solutes such as carbon dioxide, urea, salts etc enter the blood by diffusion, down their concentration gradients

water returns to the body by osmosis down its water potential gradient. the blood has lost a lot o water but retained soluble proteins so has a low water potential

not all the fluid that left the blood returns to it, so there is excess tissue fluid. this excess drains into lymph vessels which are found in capillary beds. lymph vessels have very thin walls, like capillaries and tissue fluid can diffuse easily inside forming lymph.

THE LYMPHATIC SYSTEM

consists of a network of lymph vessels flowing alongside the veins. the vessels lead towards the heart where the lymph drains back into the blood system near the superior vena cava. there is no pump, just numerous semi-lunar valves and lymph is helped along by contraction of body muscles just as in veins.

the lymphatic system has three different functions:

it drains excess tissue fluid

it absorbs fats from the small intestine via the lacteals inside each villus

it is part of the immune system. the network of lymph vessels at various places in the body sich as tonsils and armpits called lymoh nodes where white blood cells develop. these become swollen if more white blood cells are required to fight an infection.

differences between;

PLASMA - THE LIQUID PART OF BLOOD. IT CONTAINS DISSOLVED GLUCOSE, AMINO ACIDS, SALTS AND VITAMINS AND SUSPENDED PROTEINS AND FATS

SERUM - PURIFIED BLOOD PLASMA, WITH BLOO DCLOTTING PROTEINS REMOVED USED IN HOSPITALS FOR BLOOD TRNASFUSIONS

TISSUE FLUID - THE SOLUTION SURROUNDING CELLS, COMPOSITION SIMILAR TO PLASMA BUT WITH FEWER PROTEINS AS THEY REMAIN IN THE CAPILLARIES

LYMPH - THE SOLUTION INSIDE LYMPH VESSELS, COMPOSITION SIMILAR TO TISSUE FLUID BUT WITH MORE FATS FROM THE DIGESTIVE SYSTEM.

water transport in plants

plants have mass transport system for carrying water and dissolved solutes to different parts of the plant

both diffusion and mass flow are used to move substances.

xylem tissue

water is transported in plants through xylem vessels

ligin makes these vessels very strong so they dont collapse under pressure

movement through the roots - diffusion

the symplast pathway: consist of living cytoplasms of the cells in the root. water is absorbed into the root hair cells by osmosis since hte cels have a lower water potential than teh water in the soil. water then diffuses from the epidermis thropugh the root to the xylem down a water potential gradient