Tubular Reabsorption

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Chapter: Anatomy and Physiology for Health Professionals: Urinary System

Two other processes contribute to urine formation. Tubular reabsorption selectively moves substances from the tubular fluid into the blood within the per-itubular capillary.

Formation of Urine

The purposes of urine formation are to cleanse the blood and balance the body’s chemical substances. Three steps are involved in urine formation and the regulation of blood composition: glomerular filtra-tion, tubular reabsorption, and tubular secretion.

Tubular Reabsorption

Two other processes contribute to urine formation. Tubular reabsorption selectively moves substances from the tubular fluid into the blood within the per-itubular capillary (FIGURE 22-8). This occurs in the renal tubules and collecting ducts and is a transepithe-lial process. The kidney reclaims the correct amounts of water, electrolytes, and glucose as required by the body. All amino acids and glucose that were filtered are reabsorbed. Approximately, 99% of water, sodium, and components are also reabsorbed. What is left over becomes urine.

Tubular reabsorption begins as soon as filtrate enters the proximal tubules. If not for tubular reab-sorption, all the body’s plasma would drain out as urine in under 30 minutes. In healthy adults, the total plasma volume filters into the renal tubules nearly every 22 minutes. Reabsorbed substances use either a paracellular or transcellular route to reach the blood. The paracellular route between the tubule cells is less extensive because tight junctions connect these cells. These tight junctions are, however, not as tight in the proximal nephron and allow water and certain ions to pass through via the paracellular route. These ions include calcium, magnesium, potassium, and less amounts of sodium.

Nearly all organic nutrients such as glucose or amino acids are totally reabsorbed by healthy kid-neys. This can maintain or restore normal plasma concentrations. However, water and specific ion reab-sorption is regularly controlled as a result of signals from hormones. The reabsorption process is either active or passive, based on the actual substances trans-ported. Adenosine triphosphate is required for active tubular reabsorption. This may be direct, as in primary active transport, or indirect, as in second-ary active transport. The process of passive tubular reabsorption uses diffusion, facilitated diffusion, and osmosis. In these processes, substances move down their own electrochemical gradients.

Substances remaining in the renal tubule become more concentrated as water from the filtrate is reab-sorbed by osmosis. Water reabsorption increases if sodium reabsorption increases and vice versa. Active transport reabsorbs nearly 70% of sodium ions in the proximal renal tubule. Passive, negatively charged ions move along with the sodium ions. This is a form of passive transport because it does not require cel-lular energy expenditure. Movement of solutes and water into the peritubular capillary reduces fluid vol-ume inside the renal tubule. Active transport reab-sorbs sodium ions continually as tubular fluid moves through the nephron loop, distal convoluted tubule, and collecting duct. Nearly all sodium ions and water from the renal tubule via the glomerular filtrate are reabsorbed before urine excretion. Tubular reabsorp-tion of sodium, nutrients, water, and ions is explained in TABLE 22-2.

All the renal tubule is involved in reabsorption in varying degrees. However, the proximal convoluted tubule cells are the most active in this regard. These cells normally reabsorb all amino acids and glucose in the filtrate. They also normally absorb 65% of the water and sodium in the filtrate. By the time filtrate reaches the nephron loop, most electrolytes have been reabsorbed. In the proximal tubule, almost all uric acid and about 50% of urea are reabsorbed. However, both substances are eventually secreted back into the filtrate.

Once reaching the nephron loop, permeability of tubule epithelia changes greatly. At this point, water reabsorption is no longer related to solute reabsorp-tion. Although water can leave the nephron loop’s descending limb, it cannot leave the ascending limb. This is where the aquaporins are at very low levels or absent in the tubule cell membranes. These differences in permeability are vitally important parts of how the kidneys form urine that is either concentrated or dilute. For solutes, a reverse situation compared with that of the water is true. They leave the ascend-ing limb of the nephron loop but not the descending limb. Nearly no reabsorption of solutes occurs in the descending limb, yet they are both actively and pas-sively reabsorbed in the ascending limb.

Reabsorption in the distal convoluted tubule and collecting duct is different. It is adjusted by hormones. Because most filtered water and solutes have already been reabsorbed, only a small part of the filtrate is affected. Approximately, 10% of originally filtered sodium chloride and 25% of water experience hor-monal adjustment. The hormones that are involved include antidiuretic hormone (ADH), aldosterone, atrial natriuretic peptide, and parathyroid hormone. These hormones are explained in greater detail in TABLES 22-3 and 22-4 summarizes how ADH helps reg-ulate urine concentration and volume.


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