Tissue Fluid

Learning Intentions Explain the differences between blood, tissue fluid and lymph. Describe how tissue fluid is formed from plasma.

Blood: Is the liquid held in our blood vessels. It consists of blood cells such as red blood cells (erythrocytes), white blood cells (Leucocytes) and fragments called platelets in a watery fluid called plasma. Plasma contains oxygen, carbon dioxide, salts, glucose, amino acids, hormones and plasma proteins

Blood – a connective tissue Blood cells are made from stem cells, mainly in the bone marrow and foetal liver Plasma (liquid part – 55%) – 90% water + 10% substances Transports substances around the body CO2 from cells to lungs; urea from liver to kidneys; hormones; enzymes; antibodies; fibrinogen; heat White blood cells (Leucocytes) – 5000 – 7000 per ml of blood Defence Lymphocytes (produce antibodies and antitoxins) Phagocytes – engulf and destroy pathogens (microbes) Platelets (Thrombocytes) – 200 000 – 300 000 per ml of blood blood clotting – cell fragments - contain enzymes – released a the site of a cut – converts soluble blood protein fibrinogen to insoluble fibrin – forms blood clot – prevent loss of blood and prevent entry of microorganisms Red blood cells (Erythrocytes) – 4 -6 million per ml of blood 45% of blood volume (termed the haematocrit) – lower in anaemia Transport oxygen from lungs to cells Contain blood group antigens on surface of membrane 5 – 6 litres

Tissue Fluid What is the role of tissue fluid? It is the fluid which allows the exchange of substances between the blood and cells. It bathes the cells of the tissues. What substances are found in tissue fluid? glucose, amino acids, fatty acids, salts and oxygen = all delivered to the cells. carbon dioxide and other waste substances = removed from the cells.

Hydrostatic pressure

Hydrostatic Pressure As the capillaries are narrower than the arterioles, a pressure builds up which forces tissue fluid out of the blood plasma = hydrostatic pressure. Overall, pressure pushes tissue fluid and small molecules out of the capillary, leaving cells and large proteins behind = This exchange occurs by diffusion and facilitated diffusion

Return of tissue fluid Most tissue fluid is returned to the blood plasma via the capillaries. Hydrostatic pressure at the venule end of the capillary is higher outside the capillary and tissue fluid is forced back in. Osmotic forces (resulting from the proteins in the plasma) pull water back into capillaries. Remaining tissue fluid enters the lymph vessels – drain back into the veins close to the heart.

Lymph The lymphatic system consists of vessels that drain the excess fluid to rejoin the blood system in the chest cavity Lymph fluid contains the same solutes as tissue fluid. There will be less oxygen and nutrients but more carbon dioxide and wastes. Lymph fluid also contains lymphocytes that engulf and destroy bacteria and foreign particles

Lymph System

Comparison of blood, tissue fluid and lymph. Feature Blood Tissue Fluid Lymph Cells Proteins Fats Glucose Amino acids Oxygen Carbon dioxide