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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Water Reabsorption
Our bodies are roughly 60% water and maintaining the right amount is crucial for survival. The kidneys play a vital role in this process through water reabsorption - a key function that helps maintain water balance in our bodies. Without this process, we would quickly become dehydrated as our kidneys filter about 180 litres of fluid every day!
Key Definitions:
Water is essential for nearly all bodily functions. It helps transport nutrients, remove waste, regulate body temperature and provide the environment for chemical reactions. Too much or too little water in your body can be dangerous and even life-threatening. That's why your kidneys work constantly to maintain the perfect balance!
Your kidneys filter about 180 litres of fluid from your blood every day, but you only produce about 1-2 litres of urine. This means that approximately 99% of the water filtered by your kidneys is reabsorbed back into your bloodstream. Without this reabsorption, you would need to drink an impossible amount of water daily!
The nephron is the functional unit of the kidney and each kidney contains about one million of them. Each nephron is a tiny filtering unit that consists of several parts, each playing a role in water reabsorption.
Understanding the structure of the nephron is essential for grasping how water reabsorption works. Let's look at the key parts involved:
This cup-shaped structure surrounds the glomerulus. Blood is filtered here, but water reabsorption doesn't occur at this stage.
About 65% of water reabsorption occurs here. Water follows sodium and glucose as they are actively transported out of the filtrate.
Creates a concentration gradient in the medulla of the kidney. The descending limb allows water to leave, while the ascending limb is impermeable to water.
Some water reabsorption occurs here, but this section is primarily involved in ion regulation.
The final site of water reabsorption. The amount of water reabsorbed here is regulated by ADH, making it crucial for water balance control.
The peritubular capillaries surround the tubules and collect the reabsorbed water, returning it to the bloodstream.
Water reabsorption relies on osmosis - the movement of water from an area of lower solute concentration to an area of higher solute concentration. In the kidney, this process is carefully controlled to maintain the right amount of water in your body.
Most water reabsorption happens passively through osmosis. As solutes like sodium, glucose and amino acids are actively transported out of the tubule, water follows the concentration gradient. This is especially important in the proximal tubule, where about two-thirds of water reabsorption occurs.
In the collecting duct, water reabsorption is regulated by ADH. When ADH is present, special water channels called aquaporins are inserted into the membrane of the collecting duct cells, allowing water to be reabsorbed. Without ADH, these channels aren't present and water passes out in the urine.
Antidiuretic Hormone (ADH), also known as vasopressin, is the key hormone that regulates water reabsorption in the kidneys. It's produced by the hypothalamus and released from the posterior pituitary gland in response to changes in blood concentration.
The body uses a negative feedback system to maintain water balance:
Diabetes insipidus is a condition where the body cannot properly control water balance due to problems with ADH. People with this condition produce large volumes of dilute urine (up to 20 litres per day!) and experience extreme thirst. This highlights the crucial role of ADH in water reabsorption. Without proper ADH function, the collecting ducts cannot reabsorb water effectively, leading to water being lost in the urine instead of being returned to the bloodstream.
Our kidneys are remarkably adaptable and can respond to different water conditions in the body:
When you're dehydrated (like after exercise or not drinking enough):
When you have too much water (like after drinking a lot of fluids):
Water reabsorption is vital for several reasons:
Camels don't store water in their humps (that's fat!), but they have incredibly efficient kidneys that can produce very concentrated urine. This adaptation allows them to conserve water in desert environments. Their kidneys have extra-long loops of Henle that create a stronger concentration gradient, enabling more water reabsorption. Humans who are dehydrated can concentrate urine about 4 times more than blood, but camels can concentrate it up to 8 times more!
Let's follow a water molecule through the nephron to understand the reabsorption process:
Through this remarkable process, your kidneys can precisely control water levels in your body, adapting to changing conditions and maintaining the delicate balance needed for health and survival.
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