secretion
Summary
Secretion is the export of a cell-made product, such as a hormone, enzyme, or mucus, usually by exocytosis. In the kidney, it means moving solutes from blood into the filtrate. Either way, material is added to a lumen or extracellular fluid, unlike reabsorption.
Detail
Meaning: Secretion is the regulated release of a substance a cell has made into the extracellular space, a duct, or the blood. Proteins are made on rough ER, modified in the Golgi, packaged into vesicles, and released by exocytosis, often after a signal such as Ca²⁺ entry. Endocrine glands secrete hormones into blood; exocrine glands secrete products such as digestive enzymes, sweat, or mucus into ducts. In the nephron, tubular secretion is the transport of solutes such as H⁺, K⁺, and some organic acids from the peritubular capillaries into the tubule lumen.
Equation: Excretion rate = filtration rate − reabsorption rate + secretion rate, each in mol/s.
On the MCAT: Passages ask you to trace a protein through the secretory pathway (rough ER → Golgi → vesicle → plasma membrane), to separate endocrine from exocrine release, or to predict how a nephron change alters urine. For example, aldosterone increases K⁺ secretion in the distal nephron, so K⁺ excretion rises. Digestive examples include HCl from the stomach and bicarbonate and enzymes from the pancreas.
Common confusion: Secretion is not excretion. Excretion is removal from the body in urine, and renal secretion is only one input to it. Renal secretion also runs opposite to reabsorption, which moves solute from filtrate back into blood.
Sources
- Campbell Biology
- Molecular Biology of the Cell
- Human Physiology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.