Introduction:
The kidney plays a crucial role in filtering waste products and excess substances from the blood to maintain homeostasis in the body. The glomerular filtration rate (GFR) is a measure of the amount of filtrate formed by the kidneys per minute. It is an important indicator of kidney function and can vary depending on various factors.

Explanation:
There are several reasons for the large difference between the average blood filtered by the kidneys per minute and the glomerular filtration rate:

1. Blood flow: The amount of blood pumped by the heart per minute, known as cardiac output, is typically around 5 liters. However, only a portion of this blood flows through the kidneys. The renal blood flow constitutes only about 20-25% of the cardiac output. Therefore, the amount of blood available for filtration is significantly lower than the total blood volume.

2. Renal plasma flow: Not all blood flowing through the kidneys is filtered. The renal plasma flow is the portion of renal blood flow that is actually available for filtration. It is estimated to be around 600-700 ml per minute. This further reduces the amount of blood available for filtration.

3. Renal blood vessels: The renal blood vessels, including the afferent arteriole, efferent arteriole, and glomerular capillaries, have different diameters and resistances. The afferent arteriole is larger in diameter than the efferent arteriole, resulting in increased blood flow into the glomerulus. This difference in diameters helps in maintaining the pressure needed for filtration. As a result, not all blood entering the glomerulus is filtered.

4. Filtration fraction: The filtration fraction is the ratio of the GFR to the renal plasma flow. It represents the percentage of plasma that is actually filtered. The normal filtration fraction is approximately 20%. This means that only a fraction of the plasma flowing through the kidneys is filtered, while the rest is returned to the circulation.

5. Regulation: The GFR is tightly regulated by various mechanisms to ensure proper kidney function. Factors such as hormonal control, autoregulation, and feedback mechanisms help maintain a relatively constant GFR. These regulatory mechanisms adjust the filtration rate based on the body's needs and maintain the balance of fluid and solutes.

Conclusion:
In summary, the large difference between the average blood filtered by the kidneys per minute and the glomerular filtration rate can be attributed to factors such as the amount of blood available for filtration, the renal plasma flow, the different diameters of renal blood vessels, the filtration fraction, and the regulatory mechanisms involved in maintaining kidney function. These factors ensure that the kidneys effectively filter the blood while maintaining homeostasis in the body.

Not all blood has to be excreted! Only blood plasma minus proteins get filtered into the renal tubule. Also the net filtration pressure 10-15 mm Hg drives the entry.