Explain how filtration, secretion, and reabsorption affect the renal plasma clea

Question

Explain how filtration, secretion, and reabsorption affect the renal plasma clearance of a substance. Use this information to explain how Inulin can be used to measure the GFR. [3.3]
Explain how filtration, secretion, and reabsorption affect the renal plasma clearance of a substance. Use this information to explain how Inulin can be used to measure the GFR. [3.3]
Explain how filtration, secretion, and reabsorption affect the renal plasma clearance of a substance. Use this information to explain how Inulin can be used to measure the GFR. [3.3]

Explanation / Answer

Renal plasma clearance is defined as the amount of plasma which is cleared off a substance in a given unit of time and is also known as renal clearance. It is based on Fick's principle

A graph of amount filtered vs amount excreted gives information about reabsorption and secretion of substance. But it does not give information on specific regions of the nephron or on individual nephrons

Relation between filtration, secretion and reabsorption: Glomerular Filtration Rate - Reabsorption rate + Secretion rate = Excretion rate / clearance

Measurement of Glomerular filration rate by (GFR) Inulin:

A substance that is completely filtered but neither reabsorbed or secreted is generally used to measure the GFR. Inulin is a substance which is completely filtered and is neither absorbed or secreted. So, Inulin is the ideal substance used to measure GFR.

Inulin clearance:

A known amount of inulin is injected into the body. After certain time period, the concentration of inulin in plasma and uring and the volume of urine excreted are estimated.

For example, the concentration of Inulin in urine is 125 mg/dL. The plasma concentration is 1 mg/dL. The volume of urine output is 1 mL/min.

Glomerular filtration rate = UV/ P

U = Concentration of substance in urine

V= Volume of urine flow

P= Concentration of the substance in plasma

Thus GFR or clearance of inulin in this case = (125 x 1)/ 1 = 125 mL/ min

Please note that GFR = Clearance as there is no reabsorption or excretion

Additional information: Creatine clearance is also used to measure the clearance or GFR more accurately as creatine is already present in body fluids and its plasma concentration is steady all through out the day. It is completely filtered and being a metabolite it is neither reabsorbed nor secreted. The normal value of GFR by this method is approximately the same as determined by inuin clearance

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