Science (086) — AI-generated practice question

The dialysing fluid must have the same osmotic pressure as blood so that useful substances like glucose, salts, and water are not lost from the blood by osmosis or diffusion — they remain in balance across the semi-permeable membrane.

The fluid must be devoid of nitrogenous wastes (e.g., urea) so that a concentration gradient is maintained between the patient's blood (high urea) and the dialysing fluid (no urea). This causes urea and other waste products to move out of the blood into the dialysing fluid by diffusion, thereby cleaning the blood.

If urea were present in the fluid, no gradient would exist and waste removal would stop. If osmotic pressure differed, the patient would lose vital substances.

Source: Chapter 5, Section 5.5.1 — Artificial Kidney (Hemodialysis)

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Examiners look for two distinct logical points linked to the two features of the dialysing fluid:

1. Same osmotic pressure → prevents loss of useful blood components (glucose, salts, water).
2. No nitrogenous wastes → maintains concentration gradient → wastes diffuse out by diffusion.

Both points must be connected to the mechanism (osmosis/diffusion across semi-permeable membrane). A common mistake is explaining only one feature. The textbook explicitly states: "This fluid has the same osmotic pressure as blood, except that it is devoid of nitrogenous wastes… waste products pass into dialysing fluid by diffusion." Use this as your anchor.