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This chapter discusses the physiology of the kidneys, including their role in filtration, urine production, and various functions such as maintaining acid-base balance, regulating electrolytes, clearing toxins, and producing hormones. The nephron is the basic unit of the renal system, responsible for filtering the blood and reabsorbing water and small molecules. Kidney function is measured by the glomerular filtration rate (GFR), and there are three main categories of kidney function: secretion of hormones, gluconeogenesis, and extracellular homeostasis. The kidneys also play a crucial role in maintaining acid-base balance and regulating various substances such as glucose, amino acids, urea, sodium, chloride, water, bicarbonate, potassium, calcium, magnesium, and carboxylate. Kidney failure can be acute or chronic, and it impairs the kidneys' ability to filter waste products and reabsorb nutrients. Various tests, including GFR, are used to evaluate kidney function. Chapter 10, Kidney Physiology. This chapter is all about the physiology of the kidneys. The kidneys are responsible for the filtration of the blood and the making of urine. The kidneys are operating all the time and filter all the blood so that waste products can be eliminated from the body. Basic Renal Physiology. Renal Physiology is the study of the inner workings of the kidney. The kidney has many major functions including the maintenance of acid-base balance, the regulation of sodium and potassium, the regulation of fluid balance, the regulation of other electrolytes, the clearance of toxins, the absorption of small molecules like amino acids and glucose, the regulation of blood pressure, the production of erythropoietin and other hormones, and the activation of vitamin D. The nephron is the basic unit of the renal system. Each nephron has a part that filters the blood flowing through the kidneys. The filtrate flows through the nephron which is lined by specialized cells and capillaries. The specialized cells reabsorb water into the capillaries and take up small molecules from the filtrate. The water and small molecules leave the filtrate and enter the bloodstream through the capillaries. Waste products are left in the filtrate and are excreted in the urine. The kidneys can only function when blood is adequately flowing through the nephrons. Hundreds of thousands of renal corpuscles exist on a microscopic level. Each renal corpuscle consists of a glomerulus and a Bowman's capsule. Kidney function is measured by the glomerular filtration rate or GFR. There are three main categories of kidney function including the secretion of hormones, gluconeogenesis, and the extracellular homeostasis of pH and the components of blood. The nephron is considered the functional unit of the kidney. Kidney function secretion is the first category of kidney function. It secretes erythropoietin which increases red blood cell production in the bone marrow. It is also responsible for the secretion of renin which is one of the major players in the renin-angiotensin-aldosterone system. It also secretes prostaglandins and the activated form of calcitriol which is vitamin D. The next category of kidney function is gluconeogenesis. This involves the making of glucose from lactate, glutamine, and glycerol. The regulation of glucose production in the kidney involves the input of hormones like catecholamines and insulin. This part of kidney function takes place in the renal cortex. The renal medulla cannot produce glucose without the input of key enzymes. The third category of kidney function is known as extracellular homeostasis. This involves a wide variety of functions. The kidney balances glucose metabolism. Glucose needs to be reabsorbed by the kidneys during filtration. If it is not reabsorbed by the kidneys, it shows up in the urine in a condition known as glucocerea. The reabsorption of glucose is almost 100% and takes place using sodium glucose transport proteins. The kidneys are responsible for the regulation of amino acids and oligopeptides. These molecules are filtered by the kidneys and reabsorbed nearly completely. Urea is a molecule under the control of the kidneys. The kidneys regulate osmolality per the secretion of antidiuretic hormone. Urea is reabsorbed in the medullary collecting ducts. Sodium and chloride are under the control of the nephrons. There is a sodium hydrogen antiport and a sodium ion channel that allows for 65% of the reabsorption of sodium by the kidneys initially. The rest of the absorption of sodium happens utilizing the sodium potassium chloride symporter, which takes up 25% of the sodium, 5% is reabsorbed by the sodium chloride importer, and the remaining 5% is taken up under the regulation of aldosterone. Chloride is regulated by the kidneys. Chloride usually follows sodium in the kidneys. There is active reabsorption of chloride by the kidneys as well as passive reabsorption, which happens because of chloride's affinity for sodium. Reabsorption of chloride happens in the syn and syc ascending tubules through the sodium potassium chloride symporter. Water is reabsorbed by the kidneys through water channels. It is also passively absorbed through osmosis along with the solutes. The reabsorption of water occurs under the regulation of antidiuretic hormone and the arginine vasopressin receptor 2. Bicarbonate is important in acid-base balance in the kidneys. 80-90% is absorbed initially with the rest reabsorbed in the syc ascending tubules. Interchelated cells reabsorb a small amount of bicarbonate. Protons are regulated by the kidneys via the vacuolar hydrogen ATPase enzyme. They are also secreted by the interchelated cells. Potassium absorption varies per dietary needs. 65% is reabsorbed initially with 20% reabsorbed in the syc ascending tubules under the direction of the sodium potassium chloride symporter. Secretion of potassium also happens in the kidneys under the direction of the sodium potassium ATPase. Calcium reabsorption makes use of the calcium ATPase enzyme as well as the sodium calcium exchanger. It is reabsorbed in the syc ascending tubules utilizing passive transport. It is reabsorbed in response to parathyroid hormone which increases its absorption. Thiazide diuretics also control calcium absorption. Magnesium competes with calcium and an excess of one of these ions leads to the secretion of the other ion. Magnesium is reabsorbed by the syc ascending tubules. It is excreted as a titratable acid with 85% reabsorbed by the sodium phosphate cotransporter. Its absorption is blocked by parathyroid hormone. Carboxylate is reabsorbed in the kidneys using carboxylate transporters. The body is extremely sensitive to the pH level of the body. Outside of a specific pH range the body cannot survive and proteins are digested and denatured. Enzymes are unable to function and the body is unable to sustain itself. The kidneys can help maintain acid base homeostasis by helping to regulate the pH of the blood plasma. The losses and gains of bases and acids need to be balanced. There are two kinds of acids including volatile acids and non-volatile acids. The major homeostatic control point that helps maintain this stable pH balance is the act of renal excretion. The kidney can excrete or retain sodium using aldosterone, antidiuretic hormone, atrial natriuretic peptide and several other hormones. The kidney's ability to function depends on three major abilities of the kidneys including filtration of blood, reabsorption of nutrients and ions, and secretion of hormones. The excretory function of the kidneys is the equivalent of urination. The filtration of urine happens when blood is filtered by the nephrons. The nephron is considered the functional unit of the kidney. The excretory nephron begins with a renal corpuscle which is divided into a glomerulus and Bowman's capsule. Through the process of ultrafiltration, cells, proteins and other large molecules are filtered out of the nephron, leaving an ultrafiltrate that is similar to plasma without the proteins found in plasma. Filtration is driven by Starling forces. The ultrafiltrate passes through the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and a series of collecting ducts where urine is finally made. The reabsorption of urine happens in the tubules. Solutes and water are reabsorbed there and are sent back to the blood. These are nutrients and water that have been absorbed by the intestinal tract and are still needed by the body, so the kidneys need to reabsorb these things. There are two steps to reabsorption. The first is the active or passive extraction of tubular fluid that enters the interstitium of the kidneys. This is the connective tissue portion of the kidneys around the nephrons. Then the substances are sent from the interstitium to the blood. Active transport, diffusion, and Starling forces aid in the transport of these molecules. Reabsorption can be indirect. One good example of this is bicarbonate. It doesn't have a transporter protein, so it needs to be reabsorbed using a series of reactions in the lumen of the tubule and in the tubular epithelium. These cellular reactions involve the sodium hydroxide exchanger, which performs reactions that eventually lead to the absorption of bicarbonate. Reabsorption of renal substances does not happen without some control over the process. There are important regulatory hormones, such as aldosterone, which stimulates the active reabsorption of sodium and water. Also, the antidiuretic hormone is available, which stimulates passive reabsorption of water. The kidneys are also involved in secretion. Tubular secretion happens using active transport. Materials from the capillary surrounding the tubules have toxins, drugs, or poisons that are actively transported into the tubules and are cleared from the body through tubular secretion. Renal function evaluates each kidney's ability to secrete substances, hang on to water, and reabsorb essential nutrients and water. Tests of renal function include the serum creatinine level, the blood pH, the blood urea nitrogen level, and electrolytes, such as sodium, potassium, chloride, and bicarbonate. One test of kidney failure is the glomerular filtration rate, or GFR. The various stages of kidney failure are determined by measuring this value. Kidney Failure. Kidney failure is also referred to as renal insufficiency or renal failure. This involves an impairment in kidney function so that the kidneys cannot filter waste products from the blood or let nutrients go into urine without being reabsorbed. There are two major types of kidney disease. There is acute renal failure, in which there is an insult to the kidneys and temporary dysfunction of the kidneys. This is reversible and the patient will recover with treatment. In chronic renal failure, the insult is permanent and there is no chance for recovery. There are many causes of chronic renal failure, including unchecked diabetes, high blood pressure, kidney diseases, unchecked kidney infections, and long-term use of some medications. Doctors determine the stage of renal failure by the glomerular filtration rate, which is the rate at which blood is filtered by the kidneys glomeruli. The decrease in GFR signifies ongoing renal failure. The absence of urine function can detect renal failure, as well as findings of elevated urea or elevated creatinine in the blood. These tests will show kidney failure, but are not the best measurement of renal failure. The GFR is the gold standard in the testing of renal failure. Hematuria and proteinuria can be seen in some types of renal failure, but are not specific to renal failure. Other things that can be seen in renal failure include increased acid levels, decreased calcium levels, increased potassium levels, increased phosphate levels, and anemia from a lack of erythropoietin secretion. Specific renal failure is linked to a greater-than-average risk of heart disease. Urine One of the most common tests done in medicine is a urinalysis to measure the composition of urine. A urinalysis can detect things like uncontrolled diabetes, which will show glucose in the urine, diabetic ketoacidosis, which will show ketone bodies in the urine, and pregnancy, which will reveal elevated levels of human chorionic gonadotropin, or HCG, in the urine. The average urine output is 2 liters per day, although this can vary widely from person to person. The intake of water affects urine output. The diet, body temperature, ambient temperature, blood pressure, mental status, and other things can affect how much urine is put out per day. Urine is typically yellow to amber in color, although the color depends on how much a person drinks, which affects the concentration of the urine. Normal urine is clear. Cloudy urine can happen if there are bacteria in the urine or other non-absorbable substances. Urine usually has no smell, but can have a fruity odor in diabetic ketoacidosis. Urine left alone for a while will smell like ammonia. Urine has a pH of 4.6 to 8, with an average pH of 6. High protein diets will lead to acidic urine, and vegetarian diets will lead to a more alkaline urine. The specific gravity of urine is tested, which measures the concentration of the urine, which contains mostly water. The closer the specific gravity is to 1.0, the more dilute is the urine. About 95% of urine is water, with the remainder being dissolved solutes. Most dissolved solutes are ions, such as sodium and chloride, but there may be organic molecules, such as proteins, in the urine. The waste products found in urine include creatinine, uric acid, and urea. A urinalysis will measure the specific gravity of the urine, the pH, the presence or absence of protein, the presence or absence of glucose, the presence or absence of leukocyte esterase, which may mean an infection is going on, the presence or absence of blood, the presence or absence of ketones, the presence or absence of bilirubin, and the presence or absence of nitrites, found in bacteria. A microscopic evaluation of the urine may show leukocytes, blood cells, bacteria, and casts, which indicate a possible kidney infection. Key takeaways. The kidneys are the major organ for elimination of metabolic waste products. The kidneys filter blood all the time and reabsorb nutrients, water, and ions the body needs, leaving behind waste products such as creatinine, uric acid, urea, drugs, poisons, and toxins. The basic unit of the kidney is the nephron, which does all the work inside the kidneys. Kidney failure is best measured using the glomerular filtration test. Urine is normally yellow to amber, clear, and contains small amounts of ions, organic molecules like proteins, and waste products. A urinalysis can detect many diseases, including diabetes, pregnancy, kidney dysfunction, liver dysfunction, and bacterial infections of the bladder or kidneys. Quiz. Number 1. Which kidney hormone affects red blood cell production? A. Renin. B. Angiotensin. C. Erythropoietin. D. Aldosterone. Answer, C. The kidneys secrete erythropoietin to stimulate red blood cell production when it detects a low hemoglobin level. Number 2. How do the kidneys participate in acid-base balance? A. They reabsorb hydrogen ions, increasing the acidity of the blood. B. They regulate bicarbonate absorption, depending on the pH of the blood. C. They increase water absorption if the pH is too high. D. They respond to aldosterone when the pH is low and increase bicarbonate excretion. Answer, B. The pH balance in the kidneys is regulated by the reabsorption of bicarbonate, which is an alkaline substance. When the pH is low, bicarbonate reabsorption is high, and when the pH is high, bicarbonate reabsorption is low. Number 3. What happens to the kidney function when a person has prolonged hypertension? A. More sodium is reabsorbed by the kidneys. B. More glucose is excreted by the kidneys. C. The urine becomes cloudy. D. The GFR decreases. Answer, D. Prolonged hypertension can lead to decreased renal function as evidenced by a reduction in GFR. Number 4. Which is the best test to detect chronic kidney failure? A. Creatinine in the blood. B. Blood urea nitrogen level in the blood. C. Glomerular filtration rate. D. Creatinine in the urine. Answer, C. The glomerular filtration rate is the best test to detect chronic renal failure. It decreases as the kidneys start to fail and goes lower and lower with worsening kidney failure. Number 5. How is bicarbonate reabsorbed by the kidneys? A. Through active transport in the thick ascending tubules. B. Through passive transport in the loop of Henle. C. It is not reabsorbed by the kidneys. D. It is reabsorbed through a series of chemical reactions. Answer, D. Bicarbonate cannot be directly absorbed because it has no transporter, so it needs to be taken up through a series of chemical reactions. Number 6. The patient gives a urine specimen that is significant for a high nitrite level on the stick and a high leukocyte esterase level on the stick. What condition is suspected? A. Acute renal failure. B. Urinary tract infection. C. Kidney stones. D. Chronic renal failure. Answer, B. A positive nitrite level and a positive leukocyte esterase level on the urinalysis stick is suspicious for a urinary tract infection. Number 7. The patient gives a urine specimen that shows a normal specific gravity, negative bilirubin, positive protein, positive ketones, positive glucose, negative nitrites, acidic urine, and negative leukocyte esterase. What disease is suspected? A. Acute renal failure. B. Urinary tract infection. C. Hypoglycemia. D. Diabetic ketoacidosis. Answer, D. The patient has acidic urine with glucose and ketones spilling out in the urine. This means the patient has diabetic ketoacidosis. Number 8. The urine specimen shows a positive human chorionic gonadotropin level. What condition do you suspect from this urine specimen? A. Diabetic ketoacidosis. B. Hypertension. C. Pregnancy. D. Acute renal insufficiency. Answer, C. Human chorionic gonadotropin, or HCG, is a measure of pregnancy. It is an extremely sensitive test for pregnancy. Number 9. What does aldosterone do in the kidneys? A. It stimulates passive absorption of water. B. It allows for active transport of potassium in the tubules. C. It stimulates phosphate reabsorption and calcium excretion. D. It stimulates water and sodium reabsorption. Answer, D. Aldosterone stimulates the active reabsorption of water and sodium by the kidneys. Number 10. What does antidiuretic hormone do in the kidneys? A. It stimulates the active reabsorption of sodium by the kidneys. B. It stimulates the passive transport of water in the kidneys. C. It stimulates the active transport of sodium and water in the tubules. D. It increases water excretion by the kidneys. Answer, B. Antidiuretic hormone stimulates the passive transport of water in the kidneys.
