What’s the Question? Come up with some questions where the key words below are the answers: Open circulatory system Blood vessels Body cavity Ostia Low pressure Tissue fluid Lumen Endothelium Elastic fibres Smooth muscle Collagen fibres High pressure

Blood, Tissue Fluid and Lymph By the end of the lesson you will be able to: Explain the differences between blood, tissue fluid and lymph. (E) Describe how tissue fluid is formed from plasma. (C) Explain how effective blood pressure determines the direction of fluid movement. (A)

Key definition Blood is the liquid held in our arteries, veins & heart. Tissue fluid bathes the cells of individual tissues Lymph is held within the lymphatic system

Blood composition

Plasma -55% RBC -40% WBC -5%

Tissue fluid Bathes the cells of a tissue (hence the name!) Similar to blood (plasma) but without the cells or plasma proteins. Its job is to transport oxygen and nutrients to the cells from the blood and carbon dioxide and other wastes back to the blood from the cells.

Artery linked to a vein by a capillary bed

How is tissue fluid formed? Blood flows into a capillary in an organ The blood is under high pressure at the arteriole end of the capillary bed This is ‘hydrostatic pressure’ This tends to push the blood fluid out of the capillaries through tiny gaps in the wall The fluid consists of plasma with dissolved oxygen and nutrients RBC & WBC are too large to be pushed out-as are plasma proteins Exchange of gases and nutrients takes place across plasma membranes by diffusion or facilitated diffusion

Artery linked to a vein by a capillary bed

A single capillary showing the relative hydrostatic and osmotic (water potential) forces Therefore fluid pushed out of capillary Effective HP = 4.3 – 1.1 = 3.2kPa Effective SP = -3.3 – (-1.3)= -2kPa Effective blood pressure = 3.2 – 2 = 1.2kPa Arteriole End Worked Example

How does fluid return to the blood? The hydrostatic pressure of the blood is not the only force acting on the fluid. The fluid itself has some hydrostatic pressure. This tends to push the fluid BACK into the capillaries. Both the blood and tissue fluid contain solutes giving them a negative water potential. The water potential of the tissue fluid is less negative than that of the blood. Therefore water moves back into the blood from the fluid by OSMOSIS.

Now calculate the effective blood pressure and direction of flow for the venous end: Venous End Values (kPa): In capillary: HP = 1.6 SP = -3.3 In tissue fluid: HP = 1.1 SP = -1.3

A single capillary showing the relative hydrostatic and osmotic (water potential) forces Therefore fluid pushed into the capillary Effective HP = 1.6 – 1.1 = 0.5kPa Effective SP = -3.3 – (-1.3)= -2kPa Effective blood pressure = 0.5 – 2 = -1.5kPa Venous End Worked Example

Formation of lymph Not all tissue fluid is returned to the blood capillaries Excess fluid drains into the lymphatic system Eventually the lymph rejoins the blood It is similar in composition but with less oxygen and fewer nutrients, and fatty material that has been absorbed from the intestines Lymph contains ‘lymphocytes’ which are produced at lymph nodes and which filter bacteria from the fluid This is part of our immune system which protects against infection