Why is selective reabsorption important

Chromosomes 3. Meiosis 4. Inheritance 5. Genetic Modification 4: Ecology 1. Energy Flow 3. Carbon Cycling 4. Climate Change 5: Evolution 1.

Evolution Evidence 2. Natural Selection 3. Classification 4. Cladistics 6: Human Physiology 1. Digestion 2. The Blood System 3. Disease Defences 4. Gas Exchange 5. Homeostasis Higher Level 7: Nucleic Acids 1. DNA Structure 2. Glucose, amino acids, and salts can be reabsorbed against a concentration gradient and their transport across the nephron into the capillaries requires energy see active transport.

Other components, such as ammonia and urea, are secreted rather than absorbed see secretion , while certain ions, including potassium, can be both secreted and absorbed by the tubules according to the overall ionic balance throughout the body.

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Oxford Reference. The kidney works in a number of different stages:. Blood passes through the nephron inside the kidneys, there are many capillaries inside the kidney, and the blood is under high pressure at the start of the nephron, which aids the ultrafiltration of the blood. Small molecules are filtered out and pass into the nephron tubule. These small molecules include ureas, water, ions, and glucose.

However, large molecules, such as blood proteins, are too big to fit through the capillary wall and remain in the blood. Having filtered out small essential molecules from the blood - the kidneys must reabsorb the molecules which are needed, while allowing those molecules which are not needed to pass out in the urine.

Therefore, the kidneys selectively reabsorb only those molecules which the body needs back in the bloodstream. The reabsorbed molecules include:. The molecules which are not selectively reabsorbed the urea, excess water and ions continue along the nephron tubule as urine.