How does hemodialysis remove urea?

Urea is removed by the process of diffusion from the blood (high concentration) into the dialysing fluid (zero concentration) across a semi-permeable membrane.

What is added to blood before it enters the dialysis machine?

An anticoagulant called heparin is added to prevent blood clotting during the procedure.

Given below are two statements: One is labelled as Assertion (A) and other is labelled as Reason (R). Assertion (A): During the process of hemodialysis, the dialysing fluid has the same composition as that of plasma except the nitrogenous wastes. Reason (R): Nitrogenous wastes from the blood move into the dialysing fluid by the process of osmosis. In light of the above statements, choose the most appropriate answer from the options given below: (A) A is true but R is false (B) A is false but R is true (C) Both A and R are true and R is the correct explanation of A (D) Both A and R are true but R is NOT the correct explanation of A

Assertion: During the process of hemodialysis, the dialysing fluid has the same composition as that of plasma except the nitrogenous wastes.

Q. Given below are two statements: One is labelled as Assertion (A) and other is labelled as Reason (R).

Assertion (A): During the process of hemodialysis, the dialysing fluid has the same composition as that of plasma except the nitrogenous wastes.
Reason (R): Nitrogenous wastes from the blood move into the dialysing fluid by the process of osmosis.

In light of the above statements, choose the most appropriate answer from the options given below:

(A) A is true but R is false
(B) A is false but R is true
(C) Both A and R are true and R is the correct explanation of A
(D) Both A and R are true but R is NOT the correct explanation of A

Correct Answer: (A) A is true but R is false

Detailed Explanation

Let's analyze the statements based on the principles of hemodialysis used for patients with uremia:

Assertion (A) is True: The dialysing fluid (dialysate) is designed to have the same osmotic pressure and mineral composition as normal plasma, but it lacks nitrogenous wastes like urea, uric acid, and creatinine. This concentration gradient is essential for treatment.
Reason (R) is False: Nitrogenous wastes move from the blood into the dialysing fluid based on a concentration gradient across a semi-permeable membrane. This process is called Diffusion, not osmosis. Osmosis specifically refers to the movement of solvent (water) molecules, whereas the movement of solutes (like urea) is diffusion.

Since the assertion is a factually correct description of the dialysate and the reason uses the wrong biological term for solute transport, the correct choice is (A).

Related Theory

Hemodialysis is an artificial process of eliminating waste (diffusion) and unwanted water (ultrafiltration) from the blood. This process is used when the kidneys are in a state of failure (Uremia).

1. The Mechanism of Hemodialysis

Blood from a convenient artery is pumped into a dialysing unit (artificial kidney) after adding an anticoagulant like heparin. The unit contains a coiled cellophane tube surrounded by a fluid (dialysing fluid) having the same composition as plasma except the nitrogenous wastes.

2. The Cellophane Membrane

The porous cellophane membrane of the tube allows molecules to pass through based on a concentration gradient. As nitrogenous wastes are absent in the dialysing fluid, these substances freely move out, thereby clearing the blood. The anti-heparin is added to the blood before pumping it back into the body through a vein.

3. Diffusion vs. Osmosis in Dialysis

Diffusion: The movement of solutes (Urea, Creatinine, K+) from high concentration (blood) to low concentration (dialysate). This is the primary method of waste removal.
Osmosis/Ultrafiltration: The movement of water. In dialysis, hydrostatic pressure is often used to "push" excess water out of the blood into the dialysate, a process known as ultrafiltration.

4. Clinical Significance

Hemodialysis is a lifesaver for thousands of uremic patients all over the world. However, it is a temporary solution. For a permanent cure, Kidney Transplantation is the ultimate method in the correction of acute renal failures.

5. Common Mistakes to Avoid

Confusing Diffusion and Osmosis: Always remember that waste removal (solutes) is diffusion. If the question says wastes move by osmosis, it is factually incorrect.
Composition of Dialysate: It must be identical to plasma minerals to prevent the loss of essential ions from the patient's blood.
Heparin vs. Anti-heparin: Heparin is added *before* entering the machine; anti-heparin is added *after* the blood is cleaned but *before* it re-enters the patient.

6. Exam Relevance

This topic is part of "Excretory Products and Their Elimination" in NCERT Class 11. NEET often focuses on the physical principles (gradients) and the anticoagulants used during the procedure.

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Frequently Asked Questions (FAQs)

1. What is the principle of hemodialysis?

The principle is simple diffusion across a semi-permeable membrane based on a concentration gradient.

2. Why is heparin added during dialysis?

Heparin is an anticoagulant that prevents the blood from clotting while it is outside the body in the dialysing unit.

3. What is the composition of dialysing fluid?

It is identical to blood plasma in terms of osmotic pressure and electrolytes but contains zero nitrogenous wastes like urea.

4. Is hemodialysis a permanent cure for kidney failure?

No, it is a management technique. Kidney transplantation is the only permanent cure for end-stage renal disease.

5. What is uremia?

Uremia is a clinical condition characterized by the accumulation of urea and other nitrogenous wastes in the blood due to kidney malfunction.

6. How is blood returned to the patient after dialysis?

The cleared blood is mixed with anti-heparin and returned to the body through a vein.

7. What is the semi-permeable membrane used in the artificial kidney?

A coiled cellophane tube acts as the semi-permeable membrane.

8. Does dialysis remove proteins from the blood?

No, the pores in the cellophane membrane are too small to allow large plasma proteins to pass through.

9. Why is the dialysing fluid kept at the same osmotic pressure as plasma?

To prevent the gain or loss of water and essential electrolytes from the patient's blood cells.

10. What is ultrafiltration in the context of dialysis?

Ultrafiltration is the removal of excess fluid (water) from the blood by applying pressure across the dialyzer membrane.

Author: Jee neet experts

10 year experience in Human Physiology and Renal Sciences. NEET Biology content specialist.