Tubular reabsorption is the mechanism that transports water and solid particles out of the filtrate composition back into the blood stream. This is usually the second time they are absorbed after the original absorption from the alimentary canal that carries digestion materials. Blood from Bowmans capsule that passes through glomerulus resembles plasma of the blood that lacks proteins. Tubular Reabsorptions ensures the return of water and solutes from the nephron back into the extracellular fluid of the circulatory system. The extracellular body fluid consists of plasma and interstitial fluid. In addition to the reabsorption, the nephron secretes unwanted harmful materials from the bloodstream to join the filtrate which comes together to form the liquid part of the waste products called urine. The nephron in our body is specifically meant to keep our extracellular fluid of the body volume in a constant ph. condition. This process is called homeostasis. The nephrons do this in two steps, which involve one, active or passive transportation of water and substances that are already dissolved from the intercellular fluid into space outside and finally movement of water and the substances via the capillary into the blood through active transport whereas (Marieb & Keller, 2016).
Tubular secretion involves the relocation of materials from the capillaries to the lumen. This relocation is through either active transport or diffusion (passive). The mechanisms through which secretion occurs include the transportation of molecules into the interstitial fluid from the capillaries. Also, there is a concurrent movement of molecules again from the renal epithelial cells into the lumen of the nephron through ATPase, which is an energy-rich molecule. The secretion happens through all the parts of the nephron and moves from the proximal convoluted tubule to collecting duct which is positioned at the end of the nephron Secretion of the tubule involves hydrogen ion secretion. The hydrogen ion comes from the blood into the fluid of the tubules, and this is important for regulation of blood ph. Secretion of these substances also helps in the conservation of sodium bicarbonate. The final process of urine selection involves the removal of urine through the processes of filtration, reabsorption and subsequent secretion. The urine then leaves the kidney via the ureter pending storage in the bladder before expulsion through the urethra. The process of secretion is then complete (Steiness, 2009).
The difference between secretion and absorption are the exact processes that take place in the two mechanisms. While tubular reabsorption involves reclaiming back of certain substances that are useful for the body from the glomerular filtrate back to the blood tubular secretion involves physical movement of materials from the capillaries to the lumen. Secondly, absorption of useful substances and secretion of useful substances are the exact opposite of each other and takes place concurrently time after time (Marieb & Keller, 2016). Absorption is enhanced by active transport while in secretion it involves diffusion and sometimes active transport. Although in secretion, there is involvement of a rich molecule called ATPase, which initiate the movement molecules from renal to, the lumen, in reabsorption there is no involvement of an energy rich molecule. Secretion also differs from reabsorption since whereas it enhances filtering and cleaning substances from the blood rather than retaining them reabsorption takes useful substances from the waste back into the blood. Finally, reabsorption happens in two stages that involves active transport movement of already dissolved substances from the intercellular fluid into the space outside and movement of water into the capillary, secretion is only a one-stage process but also involves filtration.
Steiness, E. V. A. (2009). Renal Tubular Secretion of Digoxin. Circulation, 50(7), 103-107.
Marieb, E., & Keller, S. (2016). Essentials of human anatomy & physiology (9th ed.).
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