Question

Using the principle of diffusion, a dialysis machine removes waste solutes from a patient's blood. Imagine blood runs through a cylinder wherein diffusion can occur across an artificial selectively permeable membrane to a saline solution on the other side. Which of the following statements is correct? a. Solutes move from lower to higher concentration. b. The concentration gradient is lower in the patient's blood than in the saline solution wash. c. The solutes are transported through a symport in the blood cell membrane. d. The saline solution has a lower concentration gradient of solute than the blood. e. The waste solutes are actively transported from the blood.

Short answer

The correct answer is (d). The saline solution has a lower concentration gradient of solute than the blood, which allows for diffusion of solutes from the blood into the saline solution.

Step-by-step solution

Understand diffusion in dialysis

Recognize that diffusion is a passive process where particles move from an area of higher concentration to an area of lower concentration until an equilibrium is reached. In dialysis, this principle is used to remove waste solutes from a patient's blood by having them diffuse across a selectively permeable membrane into a saline solution.

Evaluate each statement

Assess each provided statement to determine if it aligns with the principle of diffusion. For example, statement (a) is incorrect because solutes do not move from a lower to higher concentration in diffusion. Statement (e) is incorrect because the movement of waste solutes in dialysis does not involve active transport; it’s passive.

Identify the correct statement

The correct answer must align with the principle of diffusion as applied to a dialysis machine. Therefore, any statement that suggests movement of solutes from lower to higher concentration, active transport, or that fails to acknowledge the gradient difference between blood and saline solution in favor of blood is incorrect.

Key concepts

Concentration Gradient

In the world of chemistry and biology, a concentration gradient represents the difference in the concentration of a substance between two regions. In dialysis, it is this gradient that facilitates the movement of waste solutes from the blood into the dialysate, which is typically a saline solution.

Let's envision solute particles as tiny hikers at different camps on a mountain. At the top of the mountain (the blood), there are many hikers (solute particles) while the basecamp (saline solution) has fewer. Naturally, the hikers will move downhill from an area of overcrowding to one with more space, akin to how solutes move from high to low concentration zones. This downhill journey will continue until the number of hikers is balanced at both locations – reaching what we call an equilibrium.

Selectively Permeable Membrane

Picture a selectively permeable membrane as a smart sieve or gatekeeper. It can choose which molecules or ions to pass through while blocking others. In the case of a dialysis machine, this membrane mimics the natural filtration properties of the kidneys.

This sophisticated gatekeeper allows small waste solutes and water to cross over, but it prevents larger molecules like proteins and blood cells from escaping the blood. This selectivity is crucial as it ensures that only the unwanted wastes diffuse into the saline solution while essential components of the blood like plasma proteins, which are vital for bodily functions, are kept intact and remain within the bloodstream.

Passive Transport

Passive transport is the cell's laid-back way of moving substances without using energy. Think of it as a leaf floating along with the current of a stream. In dialysis, diffusion is a form of passive transport that allows waste solutes to cross the selectively permeable membrane.

The solutes simply ride the wave of the concentration gradient from an area of higher concentration within the blood to a lower concentration in the saline solution. No cellular energy is expended in this process; nature's inherent tendency to reach equilibrium does all the heavy lifting. This is in stark contrast to active transport, where the cell would have to expend energy, akin to a boat paddling against the current.

Equilibrium in Dialysis

Equilibrium in dialysis can be thought of as an evenly balanced scale. When the concentration of waste solutes in the blood matches that in the saline solution, equilibrium is achieved. At this point, there is no net movement of solutes in either direction.

Although the goal of dialysis is not to reach complete equilibrium, as that would mean the blood and saline solution have the same concentration of wastes, it's useful to monitor the process. The aim is to reduce the concentration of waste in the blood to safe levels, not to equalize it with the dialysate. The process is stopped before equilibrium is reached, making sure that a gradient is maintained and waste removal continues efficiently.