Glomerular Filtration
Blood enters each kidney through the renal artery that ramifies (splits) into progressively smaller vessels that ultimately become thin hair-like afferent arterioles. These arterioles branch out as even thinner vessels, known as glomerular capillaries, whose function is to filtrate from the blood large amounts of fluid and solutes (but virtually no proteins). Bunches of glomerular capillaries form glomeruli in the kidney of vertebrates, which are enclosed in a structure termed Bowman's capsule. Each glomerulus filters water and dissolved substances (i.e., solutes) from the blood, through the endothelium of the capillaries, and through the epithelium of the capsule, delivering the filtrate into the peritubular capillaries that surround the renal tubules of the Loop of Henle. From the Loop of Henle, the fluid is ultimately transported through a series of tubular renal structures to the medullary collecting tubule and, from there, to the collecting duct.
Urine formation involves the dynamic interaction among glomerular filtration, tubular reabsorption of variable amounts of water and some solutes (calcium, magnesium, sodium, etc.), and tubular excretion, with final delivery into the bladder. Each kidney contains about one million functional structures termed nephrons, each unit constituted by the glomerular capsule, its glomerulus, and a long tubule in which the filtrate is converted in urine during its transport to the pelvis of the kidney. The glomerular filtrate has almost the same composition of blood plasma, except for plasma protein concentrations.
However, when the filtrate leaves the Bowman's capsule and passes through the tubules, its ion concentrations are modified due to the hormonal controls and osmotic differences that cause reabsorption of specific solutes back into the blood. Approximately 80% of the solutes and water filtrated from the blood by the glomeruli are reabsorbed through the proximal tubule, changing the glomerular filtrate into a solution isotonic with the plasma. However, urine is a hypertonic solution, mainly because the collecting tubule becomes progressively permeable to water and urea as it gets closer to the collecting duct. The matrix that surrounds the tubules in the Loop of Henle is hypertonic, due to the sodium concentrations of 4-6,000 millimole (mM) around the cortical portion, and due to urea concentrations of about 500mM around the lower portions of the collecting tubule. The total osmolality at the bottom of the Loop of Henle is 1,000 milliosmole (mOsm). As a result, the urine becomes progressively more and more hypertonic, with urea concentrations reaching approx. 750mM. This process of tubular selective reabsorption and excretion is vital to the many regulatory and physiological functions executed by the kidneys, such as the maintenance of normal blood pressure levels, prevention of dehydration and electrolytic imbalance, body fluid acid-base balance, and elimination of metabolic waste.
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