The renal system

biology MCAT
By Bayan

Many questions about the renal system in the B/B section of the MCAT require a strong understanding of terminology that can often be confused or mixed up.

Let’s break down two sets of vocabulary that are key to tackling any renal system question! 

Set #1

The term Starling forces is used to describe the various forces that help regulate the glomerular filtration process, including hydrostatic pressure and oncotic pressure. 

(Note: these forces act generally within any capillary, not just the glomerulus specifically.) 

Hydrostatic pressure: Hydrostatic pressure is the pressure pushing out within a vessel. Think of a flattened, unfilled garden hose. Once you turn the water on, it blows the hose up into a round shape that bounces back after deformation. That is a result of the hydrostatic pressure. 

Oncotic pressure: Oncotic pressure is the pressure sucking into a vessel. It’s a bit like having a sponge sitting in water. The less water already in the sponge, the more water it draws up, and vice versa. 

The hydrostatic pressure of the glomerulus forces water and solutes out into Bowman’s capsule. However, large solutes, like serum proteins, are unable to pass through, meaning that as the water continues to leave, the osmolarity of the blood increases. To balance this, the oncotic pressure sucks water back into Bowman’s capsule.  

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These two forces act as competing pressures at the glomerulus. Ultimately, the hydrostatic pressure is greater than the oncotic pressure of the glomerulus, which causes a net movement of water and solutes from the blood to the nephron – a process known as glomerular filtration. That brings us to our second set of terms, which focus on distinguishing between filtration, secretion, reabsorption, and excretion! 

Set #2:  

The key to remembering the meaning of these terms is to remember that the blood is the starting point of the water and solutes. So, if a term relates to release, it is being released from the blood to the tubule. Conversely, if a term relates to reuptake, it is being taken back to its original location, namely the blood. More specifically, the four terms to remember are: 

Filtration: Filtration is the initial movement of water and solutes from the blood to the nephron at the Bowman’s capsule. When blood enters the afferent arteriole and flows into the glomerulus, water and solutes move into Bowman’s capsule. 

Secretion: Secretion is the movement of water and solutes from the blood to the nephron after the Bowman’s capsule. Substances like urea are removed by secretion.  

Reabsorption: Reabsorption is the movement of water and solutes from the nephron back into the blood. This can be thought of as the opposite of secretion because substances are being taken back to the blood.  

Excretion: Excretion refers to what ends up in the urine. The amount excreted can be indicated as a function of the 3 previous terms: excretion = filtration - reabsorption + secretion. In other words, the urine that is excreted is made up of the water and solutes that were initially filtered, minus those that were reabsorbed and taken back into the blood, plus those that were secreted into the nephron.  

In short, just remember that blood is in and the nephron is out. Understanding that little detail will make it much easier to remember the directions of secretion versus reabsorption! 

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