Blood, Tissue Fluid & Lymph

Blood, Tissue Fluid & Lymph

1. Blood

   • Liquid held in heart and blood vessels
   • Consists of blood cells in a watery fluid called plasma
   • Contains many dissolved substances such as oxygen, CO2, salts, glucose, fatty acids, amino acids, hormones and plasma proteins

2. Tissue Fluid

   • Similar to blood, but does not contain most of the cells found in the blood nor the plasma proteins
   • Transports oxygen and nutrients from blood to cells and CO2 and other wastes back to the blood
   • Formation
     • At the arterial end of a capillary, the blood is under higher pressure due to the contraction of the heart muscle
     • This forces blood fluid and small molecules out of the capillaries through tiny gaps in the capillary wall (pressure filtration)
     • Platelets, plasma proteins & red blood cells remain in the blood, as they are too large to be pushed out through the gaps
     • The tissue fluid surrounds the cells and exchange of gases and nutrients occurs by diffusion and facilitated diffusion
     • Tissue fluid itself has some hydrostatic pressure, which tends to push the fluid back into the capillaries
     • Both blood & tissue fluid contains some solutes, giving them a low water potential
     • The water potential of the tissue fluid is less negative than that of blood, so water tends to move back into the blood from the tissue fluid by osmosis, DOWN the water potential gradient
     • At the venule end of the capillary, the blood has lost its hydrostatic pressure so fluid moves back into the capillary

3. Lymph

   • Liquid held within the lymphatic system
   • Contains same solutes as tissue fluid, but less oxygen and fewer nutrients (more CO2 & waste)
   • Contains fatty materials that have been absorbed from the intestines
   • Formation
     • Some tissue fluid is drained into the lymphatic system to prevent tissues filling up with too much fluid
     • Lymph vessels are similar to capillaries in structure. They start in the tissues and drain the excess fluid into larger vessels, which eventually re-join the blood system in the subclavian veins and the vena cava at the thoracic duct