How the Kidney Maintains Osmolarity Gradient and Uses it in Waste Elimination
Osmolarity gradient is central to regulation of excretion, a process that takes place in the kidney of vertebrates. Osmolarity gradient refers to the gradient created in the loop of Henle between the tubules and the surrounding interstitial fluid. The gradient is by solutes exiting and reentering the tubules at different sections, a process made possible by selective permeability of the tubules. Osmolarity gradient increases from the cortex to the inner medulla.
Osmolarity gradient is established a countercurrent mechanism that uses energy to establish the concentration gradient. Osmolarity gradient is maintained through critical processes: the pump, equilibration, and shift steps. The pump, located in the ascending loop of Henle acts as a Na+/K+/2Cl− transporter contributes in the creation of the gradient by shifting Na+ into medullary interstitial. This step is crucial in conversion of the isomolar osmolarity to plasma in the proximal tubule, which in a hypothetical scenario with a pump absent, would remain at 300 mOsm/L (Sembulingam & Sembulingam, 2012).
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The ascending loop of Henle in the medulla has thick walls that lack aquaporin, a transporter protein for water channels, and their impermeability to water and the high concentration of Na+/K+/2Cl− creates a hypoosmolar tubular fluid and hyperosmolar fluid in the interstitium. According to Sembulingam and Sembulingam (2012), equilibration occurs because of the permeability of the descending loop of Henle to water leading to a hyperosmolar fluid inside. The shift occurs as the fluid moves through the tubules causing the hyperosmotic fluid to further down the loop. As the process is repeated through many cycles, it causes the fluid at the top of the loop of Henle to be isomolar and highly concentrated at the bottom of the loop.
The process of removal of waste from the body and maintain solute and water balance is dependent on the need to excrete excess water or conserve water. Excess water in the body leads to reduction in fluid osmolarity causing the kidney to excrete dilute urine with osmolarity as low as 50 mOsm/L, which is a sixth of the normal osmolarity of extracellular fluid. A deficit of water in the body leads to high osmolarity of extracellular fluid and the kidney can excrete urine with a concentration nearly five times that or normal extracellular speed (1200 to 1400 mOsm/L). The kidney can also excrete large volumes of dilute urine or small volumes of concentrated urine without significant changes in the levels of sodium or potassium solutes.
Formation of concentrated urine requires high levels of ADH, excreted by the posterior pituitary gland, stimulated by high osmolarity form concentration of solutes in extracellular fluid (Sands, & Layton, 2009). ADH increases permeability of distal tubules and collecting ducts to water allowing reabsorption of water by tubular segment. The high osmalarity at the top of the loop of Henle in the medulla provides the osmolarity gradient needed for reabsorption of the water, a process aided by high levels of ADH. ADH functions by activating kinases on the walls of the distal tubules, and subsequently stimulates aquaporin and rapid diffusion of water through the cells. The processes can either trigger absorption of solutes including wastes for secretion from the body through urine.
Processes and Enzymes Involved in Digestion of Turkey, Mashed Potatoes with Gravy, and Pumpkin Pie
The human cells cannot use food in its raw form and hence must be broken down for it to be of use to the body. Turkey is meat rich in proteins; while potatoes and pumpkin are all rich in starch or carbohydrates. Once food is placed into the mouth, digestion is both mechanical and chemical. Mechanical digestion occurs for all these foods. Mastication involves breaking of large food particles into smaller ones using teeth. The process is aided by the tongue, which involuntarily turns and mixes food with saliva from three different glands (parotid gland, submandibular gland, and sublingual gland). Saliva lubricates the food and makes it stick together for easier swallowing. Saliva begins the process of chemical digestion in the mouth where the enzyme, salivary amylase, starts the breakdown of carbohydrates in the potatoes and pumpkin (starch into sugars). Once the food is ready in the mouth, the tongue rolls it into a bolus and initiates the swallowing process (Hall, 2015).
Swallowing is both a voluntary and involuntary. The tongue moves the food bolus to the back of the mouth. The soft palate raises and pushes the bolus against the wall of the pharynx which closes of the nasal cavity and simultaneously triggers the involuntary swallowing response. The larynx is stimulated to raise, raising the glottis in the process which flaps against the epiglottis keeping the bolus from the respiratory tract and directing it to the esophagus. Hall (2015) observes that peristalsis moves the food rhythmically down the esophagus and the resultant pressure forces the esophageal sphincter linking it to the stomach to open, and closes once the food is in the stomach.
In the stomach, food stays for hours and is churned and mixed with gastric juices secreted from gastric glands located in the mucosa to form a paste called chyme. Chemical digestion of proteins starts here where pepsin enzyme acts on protein from the turkey into its constituent amino acids. Gastric juice is also abundant in hydrochloric acid which activates pepsinogen into pepsin, regulates the pH and kills any ingested microorganisms. Absorption of vitamin B12 present in the ingredients used to prepare the turkey, potatoes, and pumpkin is aided in the stomach by the intrinsic factor. Chyme leaves the stomach to the small intestine via the pyloric sphincter.
In the small intestine, terminal chemical digestion of food components is done. The process is aided by enzymes secreted by the pancreas, liver, and gall bladder. Pancreatic juice contains trypsin and chymotrypsin enzymes that digest proteins in turkey meat, pancreatic amylase which completes digestion of starch from potatoes and pumpkin, and lipases that digest fats present in the meat and that used to prepare the food. The bicarbonates present in the pancreatic juice neutralize hydrochloric acid from the stomach. Assuming the food is fatty, its arrival in the duodenum stimulates release of bile salts for emulsification of fats in readiness for action of lipases. The proteins from turkey meat and carbohydrates from potatoes and pumpkin are broken down into amino acids and monosaccharides monomers respectively ready for absorption.
References
Hall, J. E. (2015). Guyton and Hall Textbook of Medical Physiology E-Book . Elsevier Health Sciences.
Sands, J. M., & Layton, H. E. (2009, May). The physiology of urinary concentration: an update. In Seminars in nephrology (Vol. 29, No. 3, pp. 178-195). WB Saunders.
Sembulingam, K., & Sembulingam, P. (2012). Essentials of medical physiology . JP Medical Ltd.
