Chapter 14 (Kidneys)

Explain the concept of net glomerular filtration pressure by listing the forces at play and how those forces interact to mandate the direction of filtration from the glomerulus.

Net glomerular filtration pressure is the difference between the hydrostatic pressure in the glomerular capillaries and the opposing forces of capsular hydrostatic pressure and colloid osmotic pressure. This pressure gradient drives the movement of water and solutes from the blood into the Bowman’s capsule, facilitating the filtration process essential for urine formation.

Define GFR and explain the effects of vasodilation/vasoconstriction of both the afferent and efferent arterioles on GFR.

GFR, or glomerular filtration rate, is the rate at which blood is filtered in the glomeruli of the kidneys. Vasodilation of the afferent arteriole increases GFR by increasing blood flow into the glomerulus, while vasoconstriction of the efferent arteriole also increases GFR by increasing pressure within the glomerulus.

(how much you can filter over time) If GFR decreases so does excretion of solutes.

What is the effect of activation of the sympathetic nervous system on GFR and why?

Activation of the sympathetic nervous system generally results in a decrease in GFR due to vasoconstriction of the afferent arterioles, which reduces blood flow to the glomeruli. This response prioritizes blood flow to vital organs during stress and helps conserve fluids.

Where is the macula densa located and what is its function?

The macula densa is located in the distal convoluted tubule of the nephron, adjacent to the glomerulus. Its primary function is to sense sodium chloride concentration in the filtrate and regulate glomerular filtration rate and renal blood flow by signaling the juxtaglomerular cells to adjust renin secretion.

How is the macula densa connected to intrinsic regulation of GFR?

The macula densa plays a crucial role in the intrinsic regulation of GFR by detecting changes in sodium chloride concentration in the distal convoluted tubule. When sodium levels are low, it signals the juxtaglomerular cells to release renin, which helps in the modulation of glomerular filtration rate and maintains overall renal function.

What is the renin-angiotensin system?

The renin-angiotensin system is a hormone system that regulates blood pressure and fluid balance. It involves the conversion of angiotensinogen, produced by the liver, to angiotensin I by renin released from the kidneys, which is then converted to angiotensin II, a potent vasoconstrictor that also stimulates aldosterone secretion.

How does RAS cause the juxtaglomerular cells to release renin?

The renin-angiotensin system causes juxtaglomerular cells to release renin in response to low blood pressure, low sodium chloride concentration, or sympathetic nervous system stimulation. This process initiates a cascade leading to increased blood pressure and fluid retention.

Define tubular reabsorption and identify which sections of the renal tubule are involved.

Tubular reabsorption is the process by which the kidneys reclaim water, ions, and other substances from the filtrate back into the bloodstream. This occurs primarily in the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting ducts.

Explain why and how Na+ and H20 are coupled during tubular reabsorption.

During tubular reabsorption, sodium (Na+) and water (H2O) are coupled due to osmotic principles. As Na+ is reabsorbed into the blood, it creates an osmotic gradient that facilitates water reabsorption, allowing the kidneys to efficiently conserve water and maintain fluid balance in the body.

Define the countercurrent multiplier system. Thoroughly explain how the countercurrent multiplier system works to produce the medullary osmotic gradient.

The countercurrent multiplier system refers to the mechanism in the nephron that creates a concentration gradient between the ascending and descending limbs of the loop of Henle. As filtrate moves down the descending limb, water is reabsorbed, concentrating the filtrate, while in the ascending limb, sodium and chloride are actively transported out, diluting the filtrate and contributing to the osmotic gradient necessary for water reabsorption in the collecting ducts.

Where does the countercurrent multiplier system take place and why is the system at work?

The countercurrent multiplier system takes place in the loop of Henle within the nephron. This system operates to establish a medullary osmotic gradient, which is crucial for the kidneys' ability to concentrate urine and conserve water.

Define osmolarity in terms of urine.

Osmolarity in terms of urine refers to the concentration of solutes in urine. It reflects the kidneys' ability to concentrate or dilute urine based on the body's hydration status and solute balance.

Define hyperosmotic in terms of urine.

Hyperosmotic in terms of urine refers to urine that has a higher concentration of solutes compared to the surrounding plasma. This condition indicates that the kidneys are effectively conserving water and concentrating waste products.

Define hypoosmotic in terms of urine.

Hypoosmotic in terms of urine refers to urine that has a lower concentration of solutes compared to the surrounding plasma. This condition indicates that the kidneys are excreting excess water and diluting waste products.

Define iso-osmolarity in terms of urine.

Iso-osmolarity in terms of urine refers to a state where urine has the same concentration of solutes as the surrounding plasma. This indicates that the kidneys are excreting urine that is balanced in solute concentration, reflecting neither significant water retention nor excessive water loss.

Explain the use of ADH in utilizing the countercurrent multiplier to produce hyperosmotic/concentrated urine.

ADH, or antidiuretic hormone, enhances the reabsorption of water in the kidneys' collecting ducts, facilitating the countercurrent multiplier effect. This process helps establish an osmotic gradient within the kidney medulla, allowing for the concentration of urine by drawing water out of the filtrate, resulting in hyperosmotic urine.

Define tubular secretion and identify which sections of the renal tubule are involved, as well as example solutes that are being secreted at those locations.

Tubular secretion is the process by which substances are moved from the blood into the renal tubule, allowing for the excretion of waste products and the regulation of electrolyte levels. This process mainly occurs in the proximal convoluted tubule and the distal convoluted tubule, with example solutes including potassium, hydrogen ions, and certain drugs being secreted at these sites.

Define proximal convoluted tubule.

The proximal convoluted tubule is the first segment of the renal tubule, located immediately after the Bowman’s capsule, where the majority of reabsorption of water, ions, and nutrients occurs.

Define distal convoluted tubule.

The distal convoluted tubule is a segment of the renal tubule located after the loop of Henle, playing a key role in the regulation of sodium and potassium balance as well as the reabsorption of calcium, under the influence of hormones like aldosterone.

Explain why K+ ion regulation is so important and how it is tied to Na+ reabsorption in the renal tubule.

Potassium ion (K+) regulation is crucial for maintaining normal cellular and electrical functions in the body. It is intimately tied to sodium (Na+) reabsorption, as changes in Na+ levels can influence K+ secretion; increased reabsorption of Na+ often results in increased secretion of K+ in the distal convoluted tubule.

A patient arrives with severe dehydration after working outside in high
heat. Blood pressure is markedly low. Explain how the afferent and
efferent arterioles would respond to restore GFR, identify the hormonal
system activated, and describe how each hormone in that system
contributes to restoring blood volume and pressure.

In response to severe dehydration and low blood pressure, the afferent arterioles would constrict to decrease blood flow to the glomerulus, while the efferent arterioles would constrict to increase glomerular pressure, thereby helping to maintain glomerular filtration rate (GFR). The hormonal system activated is the renin-angiotensin-aldosterone system (RAAS). Renin is secreted by the juxtaglomerular cells in response to low blood pressure, leading to the production of angiotensin II, which constricts blood vessels and stimulates aldosterone secretion from the adrenal cortex. Aldosterone promotes sodium reabsorption in the distal convoluted tubule and collecting ducts, which leads to water retention, increasing blood volume and pressure.

A physiologist blocks sodium reabsorption in the ascending limb of the
loop of Henle in a juxtamedullary nephron. Predict how this
manipulation will affect the medullary osmotic gradient, water
movement in the descending limb, and the final concentration of urine.
Explain the physiological principles behind each outcome

Blocking sodium reabsorption in the ascending limb will hinder the establishment of the medullary osmotic gradient, resulting in a decreased osmolarity in the renal medulla. This change will reduce water reabsorption in the descending limb, causing decreased concentration of urine. The physiological principle behind this is that the osmotic gradient is crucial for the counter-current multiplication mechanism, which facilitates water reabsorption in the collecting ducts.

During a prolonged period of low dietary sodium intake, describe how
Na⁺ reabsorption changes along the renal tubule. Explain how
aldosterone and ADH each contribute to restoring homeostasis, and
clarify why Na⁺ and water movement are linked but not always
proportional.

During low sodium intake, Na⁺ reabsorption increases primarily in the proximal tubule and is enhanced by aldosterone, which stimulates reabsorption in the distal tubule and collecting duct. Antidiuretic hormone (ADH) promotes water reabsorption independently, allowing for fine-tuned regulation of blood volume and osmolarity, illustrating the link between Na⁺ and water movement where reabsorption is not always proportional due to differing regulatory mechanisms.

Define ACE-inhibitor and what is its purpose.

An ACE-inhibitor is a medication that blocks the action of angiotensin-converting enzyme (ACE), preventing the conversion of angiotensin I to angiotensin II. Its purpose is to lower blood pressure by reducing vasoconstriction and decreasing aldosterone secretion, thereby promoting sodium and water excretion.

ACE-inhibitor medications are known to potentially cause
hyperkalemia. Using the nephron processes, explain why inhibiting
ACE can lead to elevated K⁺ levels. Include the role of aldosterone and
how changes in Na⁺ handling influence K⁺ secretion

ACE-inhibitor medications can lead to hyperkalemia because they reduce the secretion of aldosterone, which normally promotes sodium reabsorption and potassium secretion in the distal tubule and collecting duct. With decreased aldosterone levels, less Na⁺ is reabsorbed, and K⁺ secretion is diminished, leading to elevated K⁺ levels in the blood.

Define hyperkalemia

Hyperkalemia is a medical condition characterized by elevated levels of potassium (K⁺) in the blood, which can lead to serious health issues such as cardiac arrhythmias.

A person drinks a large volume of water very quickly. Describe how
osmoreceptors respond, how ADH secretion changes, and how these
hormonal changes alter aquaporin availability and urine osmolarity in
the collecting duct. Explain why Na⁺ excretion does not change
significantly in this scenario.

When a person drinks a large volume of water quickly, osmoreceptors in the hypothalamus detect the decrease in plasma osmolarity. This triggers a reduction in antidiuretic hormone (ADH) secretion from the posterior pituitary. With lower ADH levels, there are fewer aquaporins inserted into the collecting duct's membranes, leading to decreased water reabsorption and dilute urine. Despite increased plasma volume, Na⁺ excretion remains relatively stable due to homeostatic mechanisms that maintain sodium balance.