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Describe and explain the function of the loop of henle and suggest how its structure may differ in organisms adapted to a desiccated environment.

The human kidney contains an outer layer called cortex that has many capillaries, along with an inside layer that is called medulla. The cortex has cortical nephrons that have a loop of Henle which reaches medulla only.

The loop of Henle is in a close contact with capillaries, and both of them form a water potential gradient between filtrate and medulla which makes the water to be reabsorbed from the distal tube and collecting duct. So, the main function of the loop of henle is to absorb sodium chloride and water from the urine which is the filtrate. This will allow the urine to become more concentrated, hence more water will stay in the body that will limit dehydration. If we talk about the structure, it is divided into three parts: thin descending limb, thin ascending limb and thick

ascending limb. The thick ascending limb connects with the distal convoluted tubule that connects with urine collecting duct.

The whole process is referred as the countercurrent multiplication:

·   To start with, in the thick ascending limb sodium ions are pumped out actively using active transport and chloride ions follow, making medulla a concentrated and salty region (1).

·   The thick ascending limb is permeable to salts only and not to the water. Moreover, the salty or concentrated medulla is the reason why water is able to move out passively from the collecting duct (2), and be reabsorbed into the blood, leaving behind concentrated urine.

·   Then, water leaves passively by osmosis from the thin descending limb because of the surrounding salty environment (3).

·   As the water leaves, the filtrate in the descending limb becomes significantly more concentrated and is the most in the bottom at the loop of henle (4), and the descending limb is permeable to water only.

·   Then, in the thin ascending limb sodium and chloride ions diffuse out of the loop, down their concentration gradient by diffusion, hence making the filtrate less concentrated as they move upwards towards the thick ascending limb (5).

The desiccated environment lacks moisture and hence an increased spell of dryness. Many organisms living in such areas have adaptations in the loop of henle that helps them to survive in such harsh conditions. Such organisms are the tetrapods that have a longer loop of henle which penetrates deep down the medulla. This means that more ions can be pumped into the medulla, enabling more reabsorption of the water. So, more water will be removed from the filtrate and more will be absorbed back into the blood.   Hence, urine produced will be less concentrated and less dehydration will happen.

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