Kidney: A smart, sensible and sensitive organ

Kidney: A smart, sensible and sensitive organ

Often the students of medicine have a lop-sided perception about the

organ systems. CNS is obviously considered to be the most important

organ system for the body. (After all, cessation of life is considered

final after a person is declared ‘brain dead’.) Cardiorespiratory system

supplies vital nutrients and oxygen to each cell; digestive system

makes the nutrients available in readily usable forms. Each system

performs its function in a straightforward simplistic manner. The

wisest of them all, the kidney, is often neglected even while studying

the body physiology.

It is thought to be an organ that simply eliminates the waste products

from the body. There is much more to kidney than what is apparent.

Let’s see some examples of the smartness and sensitivity with which

kidney handles the needs of the body, almost on a minute-to-minute

basis.

(1) If you were to set up a cupboard, how will you proceed? First step

would be to take everything out from the cupboard. Then, the

things which are needed by you (books, etc) will be kept back, new

things (if any) are added, and the articles and objects not needed

anymore will be discarded. Kidney performs its excretory function

in almost the similar fashion. First, by glomerular filtration,

everything in the plasma is filtered out. Then, the substances

needed by the body are given back by reabsorption, and the wastes

are excreted out. The substances that are to be removed, but

couldn’t be adequately filtered, are added for excretion by tubular

secretion.

(2) Tubuloglomerular (TG) feedback and glomerulo-tubular (G-T)

balance ~ A smart and sensitive way of maintaining the ECF volume

and blood pressure:- If the Na + handling is not done smartly, there

would be wide fluctuations in the ECF volume and blood pressure,

an undesirable consequence for the homeostasis. By the

aforementioned mechanisms, kidney ensures the steady levels of

Na + (and indeed other ions and electrolytes) in the body.

(3) Body’s water balance and osmolarity of body fluids ~ A certain

amount of excretory solutes have to be excreted everyday. And, to

dissolve them, certain volume of water is needed (which will be

excreted out). Sensing the body’s water content and osmolarity of

the fluids, kidney first separates the solute and water in the thick

ascending limb of loop of Henle (the part that removes solutes but is

impermeable to water). First, the excretory solutes will be

eliminated in an iso-osmotic fashion, and then the decision is taken

about the remaining solute-free water. If body needs this water, it

will be reabsorbed under the guidance of ADH; else, there would be

diuresis and water removed.

(4) Precise regulation of pH ~ Kidney very smartly titres the H + with

HCO 3 - , and then takes a decision whether to eliminate H + or HCO 3 - ,

based on the current pH status. (Intercalated cell in the collecting

duct can secrete either of the two.)

(5) Many more contributions by the kidney ~ Cortical interstitial cells in

kidney have oxygen sensors (O 2 -sensitive K + channels); these cells

synthesize erythropoietin, and it will be secreted when body faces

hypoxia, so as to hasten the process of RBC production. Final step in

the formation of vitamin D is handled by the kidney; thus it is

involved in Ca ++ homeostasis in the body (remember, calcium is

needed for almost every other function in the body). By the way of

neoglucogenesis, kidney can also provide glucose to the body, if and

when needed.

Apart from these functions, kidney smartly ‘autoregulates’ its own blood

flow.

The list is long. However, in summary, kidney is not ‘just an excretory organ’;

it is much much more than that. It is sensitive to body’s many needs, and it

takes care of them in a smart way.

- Dr. Vivek Nalgirkar.