1. **Filtration:** Blood is filtered at high pressure in the glomerulus. Small molecules like glucose, urea, water, and mineral ions pass into the Bowman's capsule. Large molecules like proteins are too big to pass through and remain in the blood.
2. **Selective Reabsorption:** As the filtrate passes through the kidney tubule, all of the glucose is reabsorbed back into the blood by active transport.
3. **Excretion:** Urea is a waste product. Most of it is not reabsorbed and passes along the tubule, becoming part of the urine which is then excreted from the body.
The kidneys filter blood and produce urine in a three-step process occurring in millions of tiny tubes called nephrons.
- **Glucose:** Glucose is a small molecule, so it is easily filtered out of the blood in the glomerulus and enters the kidney tubule. However, glucose is a valuable source of energy for the body, so it needs to be conserved. All of the filtered glucose is reabsorbed back into the bloodstream from the tubule by the process of active transport. Therefore, there should be no glucose in the urine of a healthy person.
- **Protein:** Proteins are very large molecules. During filtration in the glomerulus, they are too big to be forced through the filter and into the Bowman's capsule. They remain in the blood. Consequently, there should be no protein in the urine of a healthy person.
- **Urea:** Urea is a small waste molecule produced in the liver from the breakdown of excess amino acids. Like glucose, it is small enough to be filtered out of the blood into the kidney tubule. However, unlike glucose, it is a waste product that the body needs to remove. Very little urea is reabsorbed; most of it remains in the filtrate and becomes a major component of urine, which is then excreted.
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