What is the primary difference in protein concentration between blood plasma and tissue fluid?

Blood plasma has a high concentration of large proteins (like albumin), whereas tissue fluid contains very few. This is because plasma proteins are too large to pass through the pores in the capillary walls during filtration.

How does the lymphatic system differ from the circulatory system in terms of fluid transport?

The circulatory system is a closed loop, while the lymphatic system is a one-way drainage system. It collects excess tissue fluid (lymph) and returns it to the blood near the heart, rather than pumping it in a continuous circle.

Compare the hydrostatic pressure at the arterial end versus the venous end of a capillary.

Hydrostatic pressure is high at the arterial end due to heart contraction, forcing fluid out. It is significantly lower at the venous end due to resistance and distance from the heart, allowing oncotic pressure to dominate and pull fluid back in.

What happens to tissue fluid volume if a person has a severely low-protein diet?

Tissue fluid volume increases, leading to oedema. A low-protein diet reduces the concentration of plasma proteins, lowering the oncotic pressure and preventing the efficient reabsorption of water at the venous end.

Why is it incorrect to say that tissue fluid contains red blood cells?

Red blood cells are much larger than the gaps (fenestrations) in the capillary endothelium. Only water, small solutes, and some flexible white blood cells can exit the capillary to enter the tissue fluid.

What is the consequence of a blockage in the lymphatic vessels?

A blockage prevents the drainage of the 10% of tissue fluid that is not reabsorbed by capillaries. This leads to a buildup of fluid in the tissues, causing significant swelling or lymphedema.

Define 'Oncotic Pressure' in the context of capillaries.

Oncotic pressure is a form of osmotic pressure exerted by proteins (mainly albumin) in the blood plasma. It acts to pull water from the tissue fluid back into the capillaries because the proteins lower the water potential of the blood.

Oedema is the abnormal accumulation of tissue fluid in the intercellular spaces. It occurs when the rate of fluid filtration out of the capillaries exceeds the rate of reabsorption and lymphatic drainage.

Define 'Hydrostatic Pressure'.

Hydrostatic pressure is the pressure exerted by a stationary fluid (blood) against the walls of its container (capillary). In the circulatory system, it is the force that drives the filtration of plasma out into the tissues.

Why does most tissue fluid return to the capillary at the venous end?

At the venous end, the blood's hydrostatic pressure drops below the oncotic pressure. This creates a net inward pressure gradient, moving water back into the blood by osmosis.

Tissue Fluid | OCR AS-Level Biology

3. Exchange & Transport

Tissue Fluid

Summary

Tissue fluid is a vital intermediary medium that bathes the cells of the body, facilitating the exchange of nutrients, gases, and wastes between the blood and the tissues. Its formation and reabsorption are governed by the dynamic balance between hydrostatic pressure and oncotic pressure across the capillary walls.

1. Definition & Core Concepts

Tissue fluid is a straw-colored liquid that escapes from the blood capillaries to fill the spaces between cells, known as the intercellular or interstitial spaces.
It acts as a bridge for diffusion, allowing oxygen and nutrients to move from the blood to the cells, while carbon dioxide and metabolic wastes move from the cells back into the blood.
While its composition is derived from blood plasma, it lacks large molecules like plasma proteins and red blood cells, which are too large to pass through the small gaps in the capillary endothelium.

Diagram showing a capillary with fluid moving out at the arterial end and moving back in at the venous end.

2. Underlying Principles: Pressure Dynamics

Hydrostatic Pressure: This is the physical pressure exerted by the blood against the capillary walls, primarily generated by the contraction of the heart. It tends to push fluid out of the capillary into the surrounding tissue.
Oncotic (Osmotic) Pressure: This is the pressure created by plasma proteins (like albumin) that remain in the blood. Because these proteins lower the water potential of the blood, they create an osmotic pull that draws water into the capillary.
The Net Filtration Pressure (NFP) is the result of these two opposing forces. If hydrostatic pressure exceeds oncotic pressure, fluid leaves the vessel; if oncotic pressure is higher, fluid is reabsorbed.

3. Methods of Formation and Reabsorption

At the Arterial End: The hydrostatic pressure is significantly higher than the oncotic pressure. This creates a net outward force, squeezing water and small solutes through the capillary pores to form tissue fluid.
At the Venous End: As blood moves through the capillary, hydrostatic pressure drops due to friction and distance from the heart. Meanwhile, the concentration of plasma proteins increases relatively as fluid is lost, maintaining a constant oncotic pressure.
Net Reabsorption: At the venous end, the oncotic pressure becomes greater than the hydrostatic pressure, causing approximately 90% of the fluid to be drawn back into the bloodstream by osmosis.

4. Key Distinctions: Plasma vs. Tissue Fluid vs. Lymph

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions