4.6 - Transfer of materials between circulatory system & cells

What happens at the capillaries?

• Metabolic materials (e.g. oxygen, carbon dioxide & glucose) are exchanged between the blood & body cells

• Capillaries can’t serve every cell directly → the final journey is through tissue fluid

What is tissue fluid?

The fluid that surrounds all cells of the body & is made from blood plasma:

• contains water, glucose, amino acids, fatty acids, ions & oxygen → supplies these to the cells, whilst also removing waste materials

What is the difference in proteins between plasma & tissue fluid?

• Plasma: proteins are more & larger

• Tissue fluid: proteins are fewer & smaller

Why are urea & carbon dioxide levels higher in tissue fluid than plasma?

They are produced by cells, enter the tissue fluid first, & then gradually diffuse into the plasma

Why are oxygen, glucose & amino/fatty acids levels lower in tissue fluid than plasma?

• They are delivered in plasma, diffuse out of capillaries into tissue fluid & are used up by cells for energy/growth

• Less of these substances are left in the tissue fluid compared to plasma

What are fenestrations in capillaries?

• Small gaps in the endothelial cell lining layer → fluid is forced through these gaps at the arterial end

• Erythrocytes & large plasma proteins are too big to leave the plasma & enter the tissue fluid

What is hydrostatic pressure?

The residual pressure from the heartbeat that is still present at the arterial end of the capillary bed → tends to force fluid out through the leaky capillary walls

What is osmotic (oncotic) pressure?

The tendency for water to move back into the capillaries by osmosis

How is tissue fluid formed?

• Capillaries have small gaps in their walls (fenestrations), so that liquid & small molecules can be forced out

• As blood enters the capillaries from arterioles (arteriole end), the smaller diameter results in a high hydrostatic pressure, so water, glucose, amino acids, fatty acids, ions & oxygen are forced out → ultrafiltration

What is forced out & what remains in the capillary during ultrafiltration?

What is forced out:

• Water molecules

• Dissolved minerals/salts

• Glucose & oxygen

• Fatty acids

• Small proteins & amino acids

What remains in the capillary:

• Red blood cells & platelets

• Large proteins

How is tissue fluid reabsorbed?

1. Large molecules (e.g. proteins & red blood cells) remain in the capillaries, as they are too big to pass through & therefore, lower the water potential

2. Towards the venule end of the capillaries, the hydrostatic pressure is lowered due to the loss of liquid & the water potential is very low

3. Water re-enters the capillaries by osmosis, as osmotic pressure is greater than hydrostatic pressure

What happens to excess tissue fluid?

It drains into lymph vessels into the lymphatic system & the lymph returns to the blood near the heart

What is the lymphatic system?

Consists of a network of lymph vessels flowing alongside the veins:

• the vessels lead towards the heart, where the lymph drains back into the blood system near the vena cava

What are the three different functions of the lymphatic system?

• Drains excess tissue fluid

• Absorbs fats from the small intestine, via the lacteals inside each villus

• Part of the immune system → networks of lymph vessels (lymph nodes) where white blood cells develop

What is plasma?

The liquid part of blood → contains dissolved glucose, amino acids, salts/vitamins & suspended proteins & fats

What is serum?

Purified blood plasma, with blood clotting proteins removed → used in hospitals for blood transfusions

What is lymph?

The solution inside lymph vessels:

• its composition is similar to tissue fluid, but with more fats from the digestive system