Block 6 Renal Physio Prt 3

The kidneys and lungs are the two organs most responsible for acid-base balance in the body. The kidneys help regulate the levels of bicarbonate ions and hydrogen ions in the blood, which helps maintain the pH balance. They can excrete excess acid or base through urine and reabsorb bicarbonate ions to maintain the balance. On the other hand, the lungs help regulate the levels of carbon dioxide in the blood. Carbon dioxide is an acidic waste product that can be expelled through exhalation, helping to maintain the acid-base balance. Together, the kidneys and lungs work to ensure that the body's pH remains within a normal range.

When the concentration of antidiuretic hormone (ADH) rises in the blood, water channels called aquaporins appear in the cell membranes of the collecting duct epithelial cells. These aquaporins promote the reabsorption of water from the filtrate, allowing it to be reabsorbed back into the body. This leads to a decrease in the permeability of the collecting ducts to dissolved solutes and water, resulting in the concentration of the ultrafiltrate becoming more hypotonic. As a result, a greater volume of dilute urine is excreted from the body.

Para-aminohippuric acid (PAH) is both filtered and secreted by the nephron, making it an ideal substance for measuring renal plasma flow and estimating total renal blood flow. PAH is freely filtered at the glomerulus and then actively secreted by the proximal tubules into the tubular fluid. This means that PAH is completely cleared from the blood as it passes through the kidneys. By measuring the clearance of PAH, it is possible to determine the rate at which blood is being delivered to the kidneys, providing an estimate of renal plasma flow and total renal blood flow. In contrast, inulin is only filtered and not secreted, while urea and glucose are reabsorbed to varying degrees.

Iron is not primarily regulated by the kidneys because its concentration is mainly regulated by the body's iron storage and absorption mechanisms. The kidneys do play a role in iron homeostasis by excreting a small amount of iron in the urine, but the majority of iron regulation occurs through the gastrointestinal absorption and release from iron stores in the liver, spleen, and bone marrow. Therefore, iron concentration is not primarily dependent on renal function.

Glucose is the substance that is filtered and then completely reabsorbed by the nephron. Glucose is filtered from the blood in the glomerulus and then reabsorbed by the renal tubules back into the bloodstream. This is because glucose is an essential nutrient that the body needs to function properly, so it is important to reabsorb it and prevent its loss in the urine.

Loop diuretics are the most powerful diuretics because they inhibit salt and water reabsorption by as much as 25%. This means that they prevent the kidneys from reabsorbing as much salt and water, leading to increased urine production and ultimately reducing fluid retention in the body. Loop diuretics work by blocking the sodium-potassium-chloride co-transporter in the thick ascending limb of the loop of Henle in the kidneys, preventing the reabsorption of these ions. This mechanism of action makes loop diuretics highly effective in treating conditions such as edema and hypertension.

Diabetes insipidus is a condition that occurs when the body is unable to properly regulate fluid balance due to inadequate secretion or action of antidiuretic hormone (ADH). ADH is responsible for controlling the amount of water reabsorbed by the kidneys, so when there is a deficiency of ADH, excessive amounts of diluted urine are produced, leading to increased thirst and frequent urination. This condition is different from diabetes mellitus, which is characterized by high blood sugar levels and is treated with insulin.

ADH is actually released when osmoreceptors in the hypothalamus sense an increase in the blood osmolality (blood becomes more hypertonic).

In the descending limb of the loop of Henle, water easily diffuses out of the tubular filtrate and into the medulla through osmosis. This helps in concentrating the tubular filtrate and decreasing its volume. However, the removal of NaCl from the filtrate occurs in both the ascending limb and the distal convoluted tubule, not in the descending limb. In the ascending limb, NaCl is actively transported out of the tubular filtrate, while in the distal convoluted tubule, it is removed by passive diffusion. Therefore, the event that does not occur in the descending limb of the loop of Henle is the removal of NaCl from the filtrate both by active transport and by passive diffusion.

In a person with high blood volume, the kidneys would most likely not decrease atrial natriuretic peptide (ANP) secretion to restore normal blood volume. ANP is a hormone released by the heart in response to increased blood volume and pressure. It acts on the kidneys to increase sodium and water excretion, thereby reducing blood volume. Therefore, in hypervolemia, it is unlikely that ANP secretion would decrease as it is a compensatory mechanism to restore normal blood volume.

Potassium-sparing diuretics are the correct answer because they competitively block the aldosterone-induced stimulation of Na+ reabsorption and K+ secretion in the distal tubule. This means that they prevent the reabsorption of sodium and the secretion of potassium, leading to increased sodium and water excretion while sparing potassium loss. This mechanism of action distinguishes potassium-sparing diuretics from other types of diuretics such as carbonic anhydrase inhibitors, loop diuretics, and thiazides.

Potassium ion (K+) is filtered, reabsorbed, and secreted by different regions of the nephron tubules. Filtration occurs at the glomerulus, where small molecules including K+ are filtered into the tubules. Reabsorption of K+ takes place in the proximal tubule, loop of Henle, and distal tubule, where most of the filtered K+ is reabsorbed back into the bloodstream. Secretion of K+ occurs in the distal tubule and collecting ducts, where excess K+ is actively transported from the bloodstream into the tubules for excretion in urine. Therefore, K+ undergoes all three processes in the nephron tubules.

Glucose and amino acids are filtered by the glomeruli into the renal tubules of the nephrons, which makes the first statement true. The second statement is also true as glucose and amino acids are normally not found in the urine. The fourth statement is also true as glucose and amino acids are reabsorbed completely until their concentrations in the blood and in the filtrate exceed their transport maximum (Tm). However, the third statement is false. Glucose and amino acids are reabsorbed into the nephron tubule cells by secondary active transport mechanisms, not primary.

Inulin is a polymer of fructose and is found in some plants. It is not easily reabsorbed by the walls of the nephron and returned to the blood. Instead, it is easily filtered by the glomerulus of the nephron and is measurable in the urine. This property of inulin allows for an estimation of the glomerular filtration rate (GFR) as it is not removed from capillary blood and is secreted by the walls of the nephron into the filtrate.

The kidneys excrete bicarbonate ion and reabsorb hydrogen ion. This statement is false because the kidneys actually reabsorb bicarbonate ions and excrete hydrogen ions. The reabsorption of bicarbonate ions helps to maintain the blood pH by preventing the loss of bicarbonate ions in the urine. On the other hand, the excretion of hydrogen ions helps to eliminate excess acid from the body and regulate blood pH.

During alkalosis, the pH of the blood is already high, so there is no need to reabsorb more bicarbonate to stabilize the pH. In fact, during alkalosis, the kidneys excrete more bicarbonate to decrease the pH and restore the acid-base balance. This statement contradicts the normal physiological response to alkalosis, making it false.

The statement that is false is "Normally some H+ is excreted each day in the urine, thereby raising the urine pH value above that of the blood, which is normally 7.4." In reality, the kidneys work to maintain a stable blood pH of around 7.4 by regulating the excretion and reabsorption of hydrogen ions (H+). The kidneys do excrete H+ ions, but they do so in a controlled manner to maintain acid-base balance, and the urine pH is typically lower than the blood pH, not higher.

The interaction between ascending and descending tubular flow in the loop of Henle is actually an example of positive feedback, not negative feedback. In this system, as more salt is extruded, the tissue fluid becomes more concentrated, which in turn causes more water to be reabsorbed from the descending limb. This further increases the concentration of the tissue fluid, leading to a continuous cycle of increasing concentration. This positive feedback mechanism helps in the formation of a hypertonic filtrate in the loop of Henle.

The statement that the role of the macula densa region of the distal tubule is to secrete the enzyme renin is false. The macula densa region of the distal tubule is responsible for sensing changes in the sodium concentration of the filtrate and signaling the granular cells to release renin. Renin is actually secreted by the granular cells within the afferent arteriole, not the macula densa. Renin plays a crucial role in the renin-angiotensin-aldosterone system, which regulates blood pressure and fluid balance.

Artificial dialysis membranes are unable to reabsorb Na+, K+, glucose, and other molecules. Dialysis is a process that helps in removing waste products and excess fluids from the blood when the kidneys are not functioning properly. However, artificial dialysis membranes are not capable of performing the function of reabsorbing essential molecules like sodium, potassium, glucose, and others, which are necessary for maintaining the body's balance and normal functioning.

Aldosterone is indeed a major steroid (mineralocorticoid) hormone, but it is not secreted by the kidney. Instead, it is secreted by the adrenal glands, specifically the adrenal cortex. The adrenal glands are located on top of the kidneys, which might be the reason for the confusion. Aldosterone plays a crucial role in regulating electrolyte balance by promoting the reabsorption of sodium and the secretion of potassium in the distal tubules of the kidneys.