Nephron Definition

A nephron is the basic unit of structure in the kidney. A nephron is used separate to water, ions and small molecules from the blood, filter out wastes and toxins, and return needed molecules to the blood. The nephron functions through ultrafiltration. Ultrafiltration occurs when blood pressure forces water and other small molecules through tiny gaps in capillary walls. This substance, lacking the blood cells and large molecules in the bloodstream, is known as an ultrafiltrate. The ultrafiltrate travels through the various loops of the nephron, where water and important molecules are removed, and into a collecting duct which drains into the bladder.

The glomerulus is the specialized configuration of capillaries within the nephron that make kidneys possible. Vertebrates are the only group to have developed kidneys, which is mostly used to conserve water in terrestrial environments. Fish and other primitive vertebrates excrete ammonia as a byproduct of protein reactions. Ammonia is toxic in the bloodstream, and must be removed. Reptiles and birds excrete uric acid, which is a more concentrated form of ammonia. Mammals have even more derived nephrons, which contain an extended loop, called the loop of Henle. Mammals produce urea from ammonia, and concentrate the urea in the urine to a high extent. This promotes the extraction of water from the ultrafiltrate, and allows mammals to live in some of the driest environments on Earth. A camel, for instance, will continually filter most of the water from its blood, recollect a large majority of that water, and reuse it continually.

Function of a Nephron

A nephron is responsible for removing waste products, stray ions, and excess water from the blood. The blood travels through the glomerulus, which is surrounded by the glomerular capsule. As the heart pumps the blood, the pressure created pushes small molecules through the capillaries and into the glomerular capsule. This is the, more physical function of the nephron. Next, the ultrafiltrate must travel through a winding series of tubules. The cells in each part of the tube have different molecules that they like to absorb. Molecules to be excreted remain in the tubule, while water, glucose and other beneficial molecules work their way back into the bloodstream. As the ultrafiltrate travels down the tubules, the cells become more and more hypertonic compared to the ultrafiltrate. This causes a maximum amount of water to be extracted from the ultrafiltrate before it exits the nephron. The blood surrounding the nephron returns to the body via the interlobular vein, free of toxins and excess substances. The ultrafiltrate is now urine, and moves via the collecting duct to the bladder, where it will be stored.

Structure of Nephron

The picture below is of a general nephron. This nephron contains a loop of Henle, so it is a mammalian nephron. While the loop of the nephron is special to mammals, the rest of the structure is seen in all vertebrate animals. The glomerulus is the net of capillaries inside of the glomerular capsule (aka Bowman’s capsule). While the picture shows the glomerular capsule and the rest of the renal tubule look to be the same in the graphic below, they are in fact composed of a wide variety of cell types, intended to extract and retain certain chemicals within the tubules.

Blood Flow in the Nephron

Each nephron consists of one main interlobular artery feeding a single renal tubule. Each kidney in a vertebrate has hundreds to millions of nephrons, each of which produces urine and sends it to the bladder. The cells in each nephron are arranged so that the most concentrated cells are at the bottom of the nephron, while the cells at the top are less concentrated. The cells near the exit of the nephron are the most concentrated, and therefore extract as much water as possible from the ultrafiltrate before it is sent to the bladder.

  • Kidney – An organ containing hundreds, or millions of individual nephrons, used to concentrate urine from the blood.
  • Liver – An organ in the body which metabolizes certain molecules in the blood, and controls the levels of a variety of important blood components.
  • Renal – Relating to the kidneys.


1. Imagine this scenario in a nephron. The cells at the start of the loop of Henle are more concentrated than the cells at the end. Which of the following would be true, if this were the case?
A. The organism would retain more water
B. The organism would retain less water
C. This would not affect the amount of water retained

Answer to Question #1
B is correct. This hypothetical organism would retain much less water than an organisms with a normally functioning loop of Henle. The loop of Henle always has the most concentrated cells at the end, which promotes more and more water to be extracted from the ultrafiltrate. This process allows almost 100% of the water to be recycled by the kidney, although some is needed to flush urea from the body.

2. Why do mammals require a loop of Henle in each nephron, while fish do not?
A. Mammals create more waste
B. The urine mammals create is more concentrated
C. Fish have access to as much water as they need to process waste

Answer to Question #2
C is correct. If you’ve ever heard the expression that fish live in their own excrement, the saying isn’t wrong. However, the vast amounts of water in the ocean ensure that the waste is diluted so quickly that it never affects the fish. So, while B is true, mammals create a more concentrated urine out of necessity. The loop of Henle allows them to do this, by extracting much more water from each nephron. Animals have more concentrated urine because they have less water to wash it away with. They must store the waste as urea, instead of ammonia, because it is less toxic in that form.

3. Why can you survive with only 1 kidney?
A. Each nephron is extremely efficient
B. Your other organs will pick up the task of filtration
C. Two kidneys actually over filter the blood

Answer to Question #3
A is correct. Each nephron is able to process an enormous amount of blood each day, while it is very small. This means that lacking an entire kidney, a person can still survive. However, there are side-effects to having only one kidney, and some toxins and molecules may take longer to filter out of the blood. However, this is still good news for people with no functional kidneys in need of one. A kidney transplant is one of a small number of organ transplants that can take place while both patients are still alive.