This increase results in reabsorption of up to 15 percent of the water entering the nephron. Most of the solutes that were filtered in the glomerulus have now been recovered along with a majority of water, about 82 percent. As the filtrate enters the ascending loop, major adjustments will be made to the concentration of solutes to create what you perceive as urine.
The ascending loop is made of thin and thick portions. The thick portion is lined with simple cuboidal epithelium without a brush border. It is relatively impermeable to water due to the absence of aquaporin proteins. Approximately 80 percent of filtered water has been recovered by the time the dilute filtrate enters the DCT. The DCT will recover another 10—15 percent before the filtrate enters the collecting ducts.
Under hormonal action, additional water and solutes can be reabsorbed into the peritubular capillaries and returned to the circulation. Finally, calcitriol 1,25 dihydroxyvitamin D, the active form of vitamin D is very important for calcium recovery. These binding proteins are also important for the movement of calcium inside the cell and aid in exocytosis of calcium across the basolateral membrane.
Tubular secretion occurs mostly in the PCT and DCT where unfiltered substances are moved from the peritubular capillary into the lumen of the tubule. In the next chapter we will discuss how the kidney controls acid-base balance, but it important to understand the reabsorption and secretion mechanisms that the kidney uses to maintain this balance.
The amino acid glutamine can be deaminated by the kidney. Ammonia and bicarbonate are exchanged in a one-to-one ratio. This exchange is yet another means by which the body can buffer and excrete acid. Solutes move across the membranes of the cells of the collecting ducts, which contain two distinct cell types, principal cells and intercalated cells. A principal cell possesses channels for the recovery or loss of sodium and potassium. An intercalated cell secretes or absorbs acid or bicarbonate.
As in other portions of the nephron, there is an array of micromachines pumps and channels on display in the membranes of these cells.
The DCT and collecting ducts contain two distinct cell types, principal cells and intercalated cells. Principal cells function to control sodium and potassium balance. Intercalated cells play significant roles in regulating blood pH. This function lowers the acidity of the plasma while increasing the acidity of the urine.
The kidney regulates water recovery and blood pressure by producing the enzyme renin. It is renin that starts a series of reactions, leading to the production of the vasoconstrictor angiotensin II and the salt-retaining steroid aldosterone. Water recovery is also powerfully and directly influenced by the hormone ADH. Even so, it only influences the last 10 percent of water available for recovery after filtration at the glomerulus, because 90 percent of water is recovered before reaching the collecting ducts.
Mechanisms of solute recovery include active transport, simple diffusion, and facilitated diffusion. Most filtered substances are reabsorbed. Urea, NH 3 , creatinine, and some drugs are filtered or secreted as wastes. Movement of water from the glomerulus is primarily due to pressure, whereas that of peritubular capillaries and vasa recta is due to osmolarity and concentration gradients.
The PCT is the most metabolically active part of the nephron and uses a wide array of protein micromachines to maintain homeostasis—symporters, antiporters, and ATPase active transporters—in conjunction with diffusion, both simple and facilitated.
Almost percent of glucose, amino acids, and vitamins are recovered in the PCT. Learning Objectives Describe the purpose of tubular secretion in kidney physiology.
Key Points The substance that remains in the collecting duct of the kidneys following reabsorption is better known as urine. Secreted substances largely include hydrogen, creatinine, ions, and other types of waste products, such as drugs. Tubular secretion is the transfer of materials from peritubular capillaries to the renal tubular lumen and occurs mainly by active transport and passive diffusion.
Urine leaves the kidney though the ureter following secretion. Key Terms collecting duct : A system of the kidneys that consists of a series of tubules and ducts that connect the nephrons to the ureter. Mechanisms of Secretion The mechanisms by which secretion occurs are similar to those of reabsorption, however these processes occur in the opposite direction.
Passive diffusion—the movement of molecules from the peritubular capillaries to the intersitial fluid within the nephron. Active transport—the movement of molecules via ATPase pumps that transport the substance through the renal epithelial cell into the lumen of the nephron.
Following Secretion Urine that is formed via the three processes of filtration, reabsorption, and secretion leaves the kidney through the ureter, and is stored in the bladder before being removed through the urethra. Authored by : Boundless. The shape and cross-sectional structure of the different parts of the tubules differs, according to their functions.
First, the proximal convoluted tubule - which is the longest part of the renal tubule - has a simple tall cuboidal epithelium, with a brush border microvilli.
The epithelium almost fills the lumen, and the microvilli increases the surface area by fold. This has a thick descending portion pars recta , a thin descending portion, a thin ascending portion, and a thick ascending portion. The lumen is made up of simple squamous epithelium. This part of the nephron is hard to tell apart from adjacent capillaries, except that there are no red blood cells in the lumen.
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