25.3 Physiology of Urine Formation: Overview
By the end of this section, you will be able to:
- Broadly explain how the kidney creates urine using glomerular filtration, reabsorption, and secretion
Having reviewed the anatomy and microanatomy of the urinary system, now is the time to focus on the physiology. Recall that the glomerulus produce a simple filtrate of the blood and the remainder of the nephron works to modify the filtrate into urine. You will discover that different parts of the nephron utilize three specific processes to produce urine: filtration, reabsorption, and secretion. You will learn how each of these processes works and where they occur along the nephron and collecting ducts. The physiologic goal is to modify the composition of the plasma and, in doing so, produce the waste product urine.
EDITOR’S NOTE: Add a figure like 24.10 from McKinley, 2nd ed.
Glomerular filtration occurs as blood passes into the glomerulus producing a plasma-like filtrate (minus proteins) that gets captured by the Bowman’s (glomerular) capsule and funneled into the renal tubule. This filtrate produced then becomes highly modified along its route through the nephron by the following processes, finally producing urine at the end of the collecting duct.
As the filtrate travels along the length of the nephron, the cells lining the tubule selectively, and often actively, take substances from the filtrate and move them out of the tubule into the blood. Recall that the glomerulus is simply a filter and anything suspended in the plasma that can fit through the holes in the filtration membrane can end up in the filtrate. This includes very physiologically important molecules such as water, sodium, chloride, and bicarbonate (along with many others) as well as molecules that the digestive system used a lot of energy to absorb, such as glucose and amino acids. These molecules would be lost in the urine if not reclaimed by the tubule cells. These cells are so efficient that they can reclaim all of the glucose and amino acids and up to 99% of the water and important ions lost due to glomerular filtration. The filtrate that is not reasbsorbed becomes urine at the base of the collecting duct.
Tubular secretion occurs mostly in the PCT and DCT where unfiltered substances are moved from the peritubular capillary into the lumen of the tubule. Secretion usually removes substances from the blood that are too large to be filtered (ex: antibiotics, toxins) or those that are in excess in the blood (ex: H+, K+). These substances secreted into the tubule are destined to leave the body as components of urine.
The kidney glomerulus filters blood mainly based on particle size to produce a filtrate lacking cells or large proteins. Most of the ions and molecules in the filtrate are needed by the body and must be reabsorbed farther down the nephron tubules, resulting in the formation of urine. Many substances that need to be removed from the body still remain in the blood. The tubule cells remove them from the blood and secrete them into the filtrate, thereby removing them from the body.
|Substances Secreted or Reabsorbed in the Nephron and Their Locations (Table 25.5)|
|Substance||PCT||Loop of Henle||DCT||Collecting ducts|
|Glucose||Almost 100 percent reabsorbed; secondary active transport with Na+|
|Oligopeptides, proteins, amino acids||Almost 100 percent reabsorbed; symport with Na+|
|Urea||50 percent reabsorbed by diffusion; also secreted||Secretion, diffusion in descending limb||Reabsorption in medullary collecting ducts; diffusion|
|Sodium||65 percent actively reabsorbed||25 percent reabsorbed in thick ascending limb; active transport||5 percent reabsorbed; active||5 percent reabsorbed, stimulated by aldosterone; active|
|Chloride||Reabsorbed, symport with Na+, diffusion||Reabsorbed in thin and thick ascending limb; diffusion in ascending limb||Reabsorbed; diffusion||Reabsorbed; symport|
|Water||67 percent reabsorbed osmotically with solutes||15 percent reabsorbed in descending limb; osmosis||8 percent reabsorbed if ADH; osmosis||Variable amounts reabsorbed, controlled by ADH, osmosis|
|Bicarbonate||80–90 percent symport reabsorption with Na+||Reabsorbed, symport with Na+ and antiport with Cl–; in ascending limb||Reabsorbed antiport with Cl–|
|H+||Secreted; diffusion||Secreted; active||Secreted; active|
|NH4+||Secreted; diffusion||Secreted; diffusion||Secreted; diffusion|
|HCO3–||Reabsorbed; diffusion||Reabsorbed; diffusion in ascending limb||Reabsorbed; diffusion||Reabsorbed; antiport with Na+|
|Some drugs||Secreted||Secreted; active||Secreted; active|
|Potassium||65 percent reabsorbed; diffusion||20 percent reabsorbed in thick ascending limb; symport||Secreted; active||Secretion controlled by aldosterone; active|
|Calcium||Reabsorbed; diffusion||Reabsorbed in thick ascending limb; diffusion||Reabsorbed if parathyroid hormone present; active|
|Magnesium||Reabsorbed; diffusion||Reabsorbed in thick ascending limb; diffusion||Reabsorbed|
|Phosphate||85 percent reabsorbed, inhibited by parathyroid hormone, diffusion||Reabsorbed; diffusion|
The entire volume of the blood is filtered through the kidneys about 300 times per day, and 99% of the water filtered is recovered. Resabsorption reclaims most filtered substances in the PCT in association with active transport of sodium. Secretion adds unfiltered molecules to the filtrate before the filtrate exits the nephron.
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