Introduction
Francis L. S. Tse, James M. Jaffe in Preclinical Drug Disposition, 2017
The amount of material entering the tubular lumen by filtration is dependent on the filtration rate and the degree of plasma-protein binding. Active tubular secretion occurs in the proximal renal tubule and involves the carrier-mediated transfer of anions or cations from the renal interstitial fluid to the tubular fluid. Weak organic acid and bases are usually involved in this process. In the proximal and distal tubules passive reabsorption of compounds from the glomerular filtrate back into the blood (see Fig. 1.1) is influenced by the intrinsic lipid solubility of the compound, its ionization constant, and the pH of the urine. Thus, compounds of high lipid solubility do not appear in the urine in large proportions because most of the molecules filtered at the glomerulus return to blood by diffusing across the lipidlike boundary of the tubular cells. Conversely, compounds of low lipid solubility are readily excreted in the urine because they are poorly reabsorbed in the tubule. Because of this, the pH of the tubular fluid and the dissociation constant of the compound being excreted often influence the renal tubular transfer of many weak acids and bases, just as these same factors influence the absorption of compounds in the gastrointestinal tract as discussed previously.
Sodium Intake and Hypertension
Austin E. Doyle, Frederick A. O. Mendelsohn, Trefor O. Morgan in Pharmacological and Therapeutic Aspects of Hypertension, 2020
In addition to the reabsorption of sodium ions which is associated with the reabsorption of bicarbonate, there is also absorption of sodium chloride. Sodium ions are actively transported, and the absorption of chloride ion is coupled electrogenically. Alternatively, there may be a neutral sodium chloride pump. In the proximal tubule, approximately 70% of sodium ions filtered are absorbed, whereas only about 60% of the chloride ions filtered are absorbed (Table 1). At the end of the proximal tubule, the tubular fluid is isoosmotic and consists mainly of sodium chloride. The volume amounts to approximately 30% of the volume filtered. Reabsorption of sodium chloride probably continues in the straight portion of the proximal tubule, which, however, is not accessible for study.
Control of blood vessels: extrinsic control by nerves and hormones
Neil Herring, David J. Paterson in Levick's Introduction to Cardiovascular Physiology, 2018
At normal physiological concentrations, angiotensin II and III help stimulate the secretion of the steroidal hormone aldosterone by the outermost adrenal cortex (zona glomerulosa). Over the course of an hour or so, aldosterone stimulates the renal distal convoluted tubules to increase the rate of Na+ reabsorption, in exchange for K+ and H+. The response is slow because it requires the synthesis and insertion of Na+/K+ pumps into the basal membrane and epithelial Na+ channels into the apical membrane of the tubule cells. The Na+ reabsorbed from the tubular fluid draws water with it by osmosis. In this way, the renin-angiotensin-aldosterone system (RAAS) maintains the plasma volume and therefore arterial BP. Failure of the adrenal cortex to secrete aldosterone leads to severe hypotension and hyperkalaemia, a potentially fatal condition called Addison’s disease. Conversely, an adrenal tumour may secrete excessive amounts of aldosterone, leading to salt and water retention and hypertension (Conn’s syndrome).
Kidney physiology and pathophysiology during heat stress and the modification by exercise, dehydration, heat acclimation and aging
Published in Temperature, 2021
Christopher L. Chapman, Blair D. Johnson, Mark D. Parker, David Hostler, Riana R. Pryor, Zachary Schlader
The kidneys are vital in regulating body fluid volume and composition and there are many systemic and intrarenal mechanisms underlying water and electrolyte regulation. The ability of the kidneys to concentrate urine arises from interactions among the active transport of NaCl from the thick ascending limbs (i.e., sodium reabsorption), water permeability of the collecting ducts, delivery rate of NaCl and water to the loop of Henle, and the volume of tubular fluid delivered to the medullary collecting ducts, which increases osmotic water transport across the epithelium of the collecting duct [203]. Most of the techniques used to quantify changes in water and electrolyte regulation require collection of timed urine samples and, depending on the measurements, blood draws (e.g., fractional excretion calculations, hormones). Thus, the measurements presented in this section typically reflect a systemic response to changes in water and electrolyte regulation.
Advances in understanding the role of angiotensin-regulated proteins in kidney diseases
Published in Expert Review of Proteomics, 2019
Ana Belén Sanz, Adrian Mario Ramos, Maria Jose Soler, Maria Dolores Sanchez-Niño, Beatriz Fernandez-Fernandez, Maria Vanessa Perez-Gomez, Marta Ruiz Ortega, Gloria Alvarez-Llamas, Alberto Ortiz
Renin is secreted by kidney juxtaglomerular cells in response to sodium content in the tubular fluid. Renin enzymatic activity processes circulating angiotensinogen to angiotensin I, which in turn is converted to angiotensin II by the angiotensin-converting enzyme (ACE) (Figure 1). The RAS may be activated in the systemic circulation or within tissues. Angiotensin II binds to and activates the angiotensin II type 1 and type 2 receptors (AT1R and AT2R, respectively), among others. Through AT1R activation, angiotensin II promotes vasoconstriction, water intake, sodium retention, and increases oxidative stress, inflammation, fibrosis, and cell growth. This has been termed the classical RAS [6]. A key action is promoting secretion of the mineralocorticoid aldosterone by adrenal glomerulosa cells. Aldosterone activates the mineralocorticoid receptor and promotes sodium reabsorption and potassium secretion in distal tubules, in addition to other actions that promote tissue injury and fibrosis.
Kidney stone proteomics: an update and perspectives
Published in Expert Review of Proteomics, 2021
Paleerath Peerapen, Visith Thongboonkerd
CaOx is the most dominant type of kidney stones found in approximately 80% of the stone formers (patients carrying kidney stones) [7,34,35]. The crystalline compositions of CaOx stones include CaOx monohydrate (COM) and/or CaOx dihydrate (COD) crystals [7,34,35]. CaOx kidney stones can initiate primarily inside renal tubular lumens or outside (at the renal interstitium). Within the renal tubules, crystalline components are crystallized as the result of supersaturation of ions in the renal tubular fluid [36,37]. The tiny crystals can adhere onto apical surface of renal epithelial cells, and this process is facilitated by cellular injury [36–38]. In addition, free crystals can further grow and self-aggregate [36,39,40]. When their sizes are big enough, they cannot pass through the intratubular lumens and then get stuck inside the renal tubular segments [36]. The deposited crystals then act as the nidus for further enlargement and the stone starts to form. For the interstitium mechanism, supersaturation of calcium phosphate (CaP) is common within the interstitial compartment leading to the formation of plaque, namely Randall’s plague [41,42]. Together with inflammatory response, some plaques erode into the urinary space or pelvis, where they are exposed to the supersaturated calcium and oxalate ions [41,42]. The Randall’s plaque then serves as the nidus for CaOx crystals to deposit and to form the stone [42–44].