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Tubular secretion occurs simultaneously during re-absorption of filtrate. Substances, generally produced by body or the by-products of cell metabolism that can become toxic in high concentration, and some drugs (if taken). These all are secreted into the lumen of renal tubule. Tubular secretion can be either active or passive or co-transport.
Drugs are excreted from the kidney by glomerular filtration and by active tubular secretion following the same steps and mechanisms as the products of intermediate metabolism. Therefore, drugs that are filtered by the glomerulus are also subject to the process of passive tubular reabsorption .
Each adult human kidney contains around 1 million nephrons, while a mouse kidney contains only about 12,500 nephrons. The kidneys also carry out functions independent of the nephrons. For example, they convert a precursor of vitamin D to its active form, calcitriol; and synthesize the hormones erythropoietin and renin.
The unipapillary kidney with a single renal pyramid is the simplest type of kidney in mammals, from which the more structurally complex kidneys are believed to have evolved. [17] [6] [18] Differences in kidney structure are the result of adaptations during evolution to variations in body mass and habitats (in particular, aridity) between species.
The basic physiologic mechanisms of handling fluid and electrolytes by the nephron - filtration, secretion, reabsorption, and excretion - are labelled. Assessment of kidney function occurs in different ways, using the presence of symptoms and signs, as well as measurements using urine tests, blood tests, and medical imaging.
The nephron is the functional unit of the kidney. [3] This means that each separate nephron is where the main work of the kidney is performed. A nephron is made of two parts: a renal corpuscle, which is the initial filtering component, and; a renal tubule that processes and carries away the filtered fluid. [4]: 1024