Role of Plasma in transportation

Blood is not just one substance — it is made of cells floating in a liquid. That liquid is called plasma.

• Plasma is the straw-coloured (pale yellow) liquid part of blood.
• It is mostly water (about 90% water).
• The blood cells — red blood cells, white blood cells and platelets — are carried floating in the plasma.

The water in plasma is very useful for transport. Many important substances dissolve in this water, and once dissolved they can be carried around the body as the blood flows.

Exam tip. Two phrases score reliably for describing plasma: "straw-coloured liquid" and "mostly water". A common one-mark question is simply "What is plasma?" — those two ideas answer it.

For spec point 2.60 you must know the five things plasma transports. A simple memory list:

1. Carbon dioxide — a waste gas from respiration.
2. Digested food — mainly glucose and amino acids.
3. Urea — a waste substance made when the body breaks down excess protein.
4. Hormones — chemical messengers made by glands.
5. Heat energy — warmth carried in the water of the plasma.

The first four are dissolved substances carried in the watery plasma. Heat energy is slightly different — it is not a substance, but the warm plasma carries heat as it flows.

Exam tip. If a question asks you to "list substances transported by plasma", give the named examples (glucose, amino acids, urea) rather than vague words like "food" or "waste".

Marks are often given for the route — where a substance is carried from and to. Learn each one as a "from → to" pair:

• Carbon dioxide: from the body cells / tissues (where it is made by respiration) → to the lungs (to be breathed out).
• Glucose (digested food): from the small intestine (where it is absorbed after digestion) → to the body cells (used in respiration for energy).
• Amino acids (digested food): from the small intestine → to the body cells (used to build proteins).
• Urea: from the liver (where it is made) → to the kidneys (where it is removed in urine).
• Hormones: from the glands that make them → to their target organs (where they have an effect).

Heat energy does not have a single "from–to" — instead, the flowing plasma spreads heat around the whole body, moving it from warmer areas (like working muscles or the liver) to cooler areas. This helps keep the body's temperature even.

Exam tip. Two routes are commonly muddled: carbon dioxide goes to the lungs (it is breathed out), while urea goes from the liver to the kidneys. Keep "liver makes it, kidneys remove it" firmly in your memory.

Plasma is such a good transport liquid because it is mostly water, and water is an excellent solvent (it dissolves many things).

• Substances such as glucose, amino acids, urea and carbon dioxide can dissolve in the water of the plasma.
• Once dissolved, they are carried in the moving blood to wherever they are needed (or removed).
• Because the plasma reaches every part of the body, it can both deliver useful substances (food, hormones) and collect waste substances (carbon dioxide, urea) for removal.

Heat energy is carried slightly differently — the warm watery plasma flows around the body and shares out the heat, stopping one part from getting much hotter than another. This is part of how the body keeps a steady temperature.

Exam tip. If asked why plasma can carry so many substances, the key idea is: it is mostly water, and these substances are soluble (dissolve) in water, so they are carried dissolved in the plasma.