650 ml of plasma passes through the glomeruli each minute. Of the 650 ml plasma, about 125 ml is forced to the renal tubules. Doing basic math, it entails that the entire plasma volume will be filtered for more than 60 times each day. Taking into consideration its small size and the momentous task it undertakes, the kidneys consume 20-25% of oxygen supplied by the body.
Difference of Filtrate and Urine
Some may misunderstand urine as filtrate or vice versa. Filtrate is the product after the plasma has been filtered. It contains almost all that can be seen in the plasma, except for the protein component. As the filtrate passes through the collecting ducts, its water, nutrients and ion components are absorbed back. What is left is the metabolic waste which we call urine. Every day, the kidneys filter roughly 180 litres of fluid and close to 1.5 litres leaves the body as urine. Depending on the needs of the body, the collecting ducts work with the nephrons in order to dilute or concentrate the formed urine.
As the blood flows, hydrostatic pressure forces the solutes and the fluids through a membrane. As this happens, the blood is filtered and filtrate is formed. During this process, no energy is being consumed by the body; hence it can be viewed as a simple filter.
The glomerulus is a very efficient filter because the filter membrane present on it has a large surface area and is more permeable to water and other solutes. The glomerulus also yields higher net filtration pressure because the glomerular blood pressure is considerably higher compared to other areas in the body. With these reasons laid out, the kidneys are able to product roughly 180 L of filtrate every day.
Water, amino acids, nitrogenous wastes and water can pass with relative ease from the blood to the glomerular capsule while larger molecules have great difficulty doing so. Molecules like protein or blood cells usually are not able to pass through the glomerular capsule, thereby the presence of these molecules in the urine signify a problem with the filtration membrane.
Tubular reabsorption is the reclamation process that prevents the draining of all our plasma. Every 22 minutes, the total plasma volume enters the renal tubules and as is passes through, plasma contents are quickly reclaimed and are absorbed for circulation. Tubular reabsorption occurs as soon as the filtrate enters the proximal tubules. The reabsorbed substances have to pass through three different barriers in order to reach the blood. These three barriers are the luminal and basolateral membranes, together with the endothelium.
Failure may be seen as a bad sign but it is not the case with the tubules. The failure of the tubules in reabsorbing some components of the filtrate serves useful in clearing the plasma of unwanted substances. Thus, tubular secretion is the process of secreting the metabolic waste of the body. The urine then now contains both secreted and filtered substances.
Tubular secretion is also important in controlling the pH of the blood. When the pH of the blood is moving towards the acidic end, the tubules secrete more hydrogen and retain more carbonate. At the other end of the spectrum, when the pH of the blood is moving towards the alkaline end, Chlorine is reabsorbed instead of carbonate.