state the functions of tissue fluid and describe the formation of tissue fluid in a capillary network

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – The Circulatory System

The Circulatory System

Objective

State the functions of tissue fluid and describe the formation of tissue fluid in a capillary network.

Functions of Tissue Fluid

  • Provides a medium for the exchange of nutrients, gases, and waste products between blood and cells.

  • Maintains a stable extracellular environment (homeostasis) for cellular metabolism.

  • Facilitates transport of hormones, enzymes, and immune cells to sites of action.

  • Acts as a reservoir for plasma proteins that can re‑enter the bloodstream via the lymphatic system.

Formation of Tissue Fluid in a Capillary Network

The movement of fluid across the capillary wall is explained by Starling’s forces. The net filtration pressure (NFP) determines whether fluid leaves (filtration) or enters (reabsorption) the capillary.

StepProcessKey Forces Involved
1Blood enters the capillary at the arterial end.High hydrostatic pressure (\$P_c\$) pushes plasma out of the capillary.
2Filtration of plasma (water + small solutes) into the interstitial space.\$Pc > \pic\$ (oncotic pressure of plasma).
3As blood moves toward the venous end, \$P_c\$ falls.Reduced hydrostatic pressure.
4Reabsorption of fluid back into the capillary.\$\pic > Pc\$; oncotic pressure of plasma exceeds hydrostatic pressure.
5Excess interstitial fluid that is not reabsorbed enters lymphatic capillaries.Formation of lymph.

The net filtration pressure can be expressed as:

\$\$

\text{NFP} = (Pc - Pi) - (\pic - \pii)

\$\$

where \$Pc\$ = capillary hydrostatic pressure, \$Pi\$ = interstitial hydrostatic pressure, \$\pic\$ = capillary oncotic pressure, and \$\pii\$ = interstitial oncotic pressure.

Summary Flowchart (Suggested diagram)

Suggested diagram: Flow of blood through a capillary network showing filtration at the arterial end, reabsorption at the venous end, and collection of excess fluid by lymphatics.