The Kidneys and Osmosis

I believe that one of the greatest satisfactions we human beings experience is the instant we excrete a yellow substance called urine. Or in a more colloquial manner, the moment we pee. I know that this is perhaps not the best way to begin a scientific essay, but I want you to be relaxed when you receive this wonderful piece of information. This might seem like a tough question to many, and it is, but here you will know the answer: What is the relationship between the kidneys and osmosis?

First, let’s understand what osmosis is. Basically, osmosis is when water or any other sort of liquid moves across a semipermeable membrane from an area of high concentration to an area of low concentration. At one side you have lots of solutes to which water will be bound to and on the other the water is “free”. Therefore, because water has an adhesion trait, the “free” one will pass through the semipermeable membrane and bind to the solutes on the other side and dilute them to create an equal amount of concentration on both sides. The side of “free” water and little solutes is known as hypoosmotic and the other that attracts water and has many solution is hyperosmotic.

Now, the kidneys are very complex organs that are shaped like a bean. They are extremely essential for they serve as removers of excess organic molecules from the blood and of waste products in our metabolism. Without them, the environment within our body would be looking something like Chernobyl. In other words, they maintain homeostasis (their specialty) in us by regulating electrolytes (e.g. salt), maintaining the pH balance, and regulating the blood pressure by equalizing the amount of water and sodium. In practical terms, they serve as a filter of the blood and remove water soluble wastes such as urea (nitrogen-filled) and ammonium through urine. Kidneys also produce hormones and reabsorb water, glucose, and amino acids into the body.

Okay, so now that we understand the basics, let’s look at the organisms inside the kidneys that carry out the process and then relate them with osmosis. These include, the nephron, loops of Henle, the adrenal cortex, and the renal medulla.

Containing the structures of the nephrons in charge of keeping the balance of water and sodium in the blood is the renal medulla. The nephron holds the proximal tubule, the loops of Henle which, the distal tubule, and the collecting tubule.

Things function like this. A ball or network of capillaries called the glomerulus passes through a dome shape known as Bowman’s capsule. The cells belonging to Bowman’s capsule are permeable to small solutes and water, but not to large molecules, proteins, or large blood cells. This enables blood to be filtered correctly and nutrients to not be excreted in urine. This means that the renal medulla is hypertonic (adds higher osmotic pressure) to the filtrate in the nephron and this aids in the reabsorption of water. Having less concentrations of vitamins, salt, glucose and other solutes equal to the concentrations in the blood, the resulting filtrate will pass to the proximal tubule.

Inside the proximal tubule, reabsorption will occur. Epithelial cells will return sodium ions to interstitial fluid by active transport. Negative ions and water will passively follow sodium and diffuse back into the capillaries. Glucose, amino acids, and other desired compounds will do similarly in an active and passive transport pattern. To maintain the pH balance, transport epithelium cells actively pump hydrogen ions to lower acidity, while adding ammonia to buffer it. Bicarbonate reabsorption is aided by the proximal tubules to increase the pH and keep stability. As water and other useful solutes are reabsorbed, urea becomes more concentrated in proximal tubules.

Here comes the important part, the Henle Loop. The loop of Henle is a U-shaped tube that extends from the proximal tubule and leads to the distal tubule. It consists of a descending limb and ascending limb. It begins in the cortex, receives filtrate from the proximal tubule, extends into the medulla as the descending limb, and then returns to the cortex as the ascending limb to empty into the distal tubule. The primary role of the loop of Henle is to concentrate the salt in the interstitium (tissue surrounding loop).

As the interstitial fluid descends in the loop, it becomes progressively more concentrated. Since it is permeable to water, this descending limb will cause more water to be reabsorbed. The transport epithelium is permeated by aquaporins (water channels), which easily let water through by osmosis, allowing salt to continue flowing into the ascending loop. The ascending limb epithelium is impermeable to water (very rare in biology) and is responsible of absorbing ions as it is dotted with specialized channels for it. Finally, salt diffuses out of the tubule near its thinner tip and as it moves farther up into a thick membrane, salt is removed via active transport. All of this causes the resulting filtrate to be more dilute as it enters the distal tubule.

The hypoosmotic fluid in the distal tubule will receive a secretion of potassium for the regulation of concentration of salt and potassium in bodily fluids. Aditionally, it will receive hydrogen ions secreted by the tubule to regulate pH and absorb bicarbonate. Much of the ion transport in the distal tubule is regulated by the endocrine system. Here is when the adrenal cortex comes in as it produces the hormone aldosterone to reabsorb more sodium and secrete more potassium in the distal tubule. To conclude the process, the filtrate is transported to the collecting duct. Here the filtrate will pass through the medulla towards the renal pelvis, where it will ultimately be turned into urine. Hormones will aslo control the permeability of the epithelial tissue and regulate the urine’s concentration.

In very short terms, if there is too much water, the kidney uses more water in the urine. If there is not enough water, the kidney uses less in the urine. This is why we make less pee when we are dehydrated. Osmosis and diffusion help regulate the whole process without utilizing too much energy. While the filtrate of the blood travels into the kidney, the concentration of sodium increases and water dilutes it via osmosis. Then, the ascending Henle loop becomes water resistant to not allow the water to dilute the salt. The end result is urine in the collecting duct ready to be flushed away. All of this happens so that we do not lose more water than we need to.