A-Level Biology: Transport in Mammals

Front

Explain the relationship between the structure and function of arteries, veins, and capillaries.

Back

**Arteries:** Thick muscle/elastic walls to withstand high pressure & maintain flow. **Veins:** Valves and wide lumen to return low-pressure blood against gravity. **Capillaries:** One-cell thick walls for short diffusion distance and narrow lumen to reduce flow rate for exchange.

Front

Define tissue fluid and describe its composition compared to blood plasma.

Back

Tissue fluid is the watery liquid surrounding cells in tissues. It has the same composition as blood plasma **except** it lacks red blood cells, large plasma proteins (which are too big to leave the capillary), and contains lower oxygen concentration.

Front

What is the role of ultrafiltration in the formation of tissue fluid at the arteriole end of a capillary?

Back

Hydrostatic pressure from the heart is higher inside the capillary than outside. This forces fluid out (containing water, glucose, amino acids, and oxygen). Large proteins remain, creating a lower water potential inside to aid reabsorption later.

Front

Explain the mechanism of tissue fluid return at the venule end of the capillary.

Back

At the venule end, hydrostatic pressure is low. The oncotic pressure (created by plasma proteins remaining in the capillary) draws water back into the vessel via osmosis. This returns metabolic waste products to the blood.

Front

Describe the structure and function of a lymphatic vessel.

Back

**Structure:** Blind-ended tubes with one-way valves. **Function:** Drains excess tissue fluid that doesn't return to capillaries, filters it through lymph nodes, and returns it to the bloodstream near the heart (subclavian vein).