urine formation

INTRODUCTION

Every one of us depends on the process of urination for the removal of certain waste products in the body. The production of urine is vital to the health of the body. We could not survive if we did not produce urine and eliminate it. Urine is composed of water, certain electrolytes, and various waste products that are filtered out of the blood system.. A major part of “cleaning" of the blood takes place in the kidneys and, particularly in the nephrons, where the blood is filtered to produce the urine. Both kidneys in the body carry out this essential blood cleansing function. Normally, about 20% of the total blood pumped by the heart each minute will enter the kidneys to undergo filtration. This is called the filtration fraction. The rest of the blood (about 80%) does not go through the filtering portion of the kidney, but flows through the rest of the body to service the various nutritional, respiratory, and other needs that are always present.

                            Urine formation involves three main processes namely, glomerular filtration, re absorption and secretion, that take place in different parts of the nephron

           

The three steps of urine formation are

A.    GLOMERULAR FILTRATION,

2. TUBULAR REABSORPTION, and

3. TUBULAR SECRETION.

o   GLOMERULAR FILTRATION occurs in the GLOMERULUS,

o   TUBULAR REABSORPTION occurs through the PROXIMAL CONVOLUTED TUBULE,

o   And TUBULAR SECRETION occurs through the DISTAL CONVOLUTED TUBULES.

o   GLOMERULAR FILTRATION is defined as the process of primary filtration taken place from the glomerulus in to the Bowmans capsule.

o   TUBULAR REABSORPTION is the process of reclaiming water and solutes from the tubular fluid and returning them in to the blood.

o   TUBULAR SECRETION is defined as the process of movement of  “wastes and excesses”  from the blood in to  the Distal and collecting tubule.

Filtration

Urine formation begins with the process of Glomerular filtration,. The water, waste products, salt, glucose, and other chemicals that have been filtered out of the blood are collectively known as glomerular filtrate. The glomerular filtrate consists primarily of water, excess salts (primarily Na+ and K+), glucose, and a waste product of the body called urea. Urea is formed in the body to eliminate the very toxic ammonia products that are formed in the liver from amino acids. Since humans cannot excrete ammonia, it is converted to the less dangerous urea and then filtered out of the blood. Urea is the most abundant of the waste products that must be excreted by the kidneys. The total rate of glomerular filtration (glomerular filtration rate or GFR) for the whole body  is normally about 125 ml per minute. That is, about 125 ml of water and dissolved substances are filtered out of the blood per minute. The following calculations may help you visualize how enormous this volume is. The GFR per hour is: 125 ml/min X 60min/hr= 7500 ml/hr.  Reabsorption The GFR per day is: 7500 ml/hr X 24 hr/day = 180,000 ml/day or 180 l