Regulation of Homeostasis by the Kidney: Fluid Balance

Question 1

What are the functions of the kidneys?

Answer 1

The kidneys have several functions, including:

Regulation of volume and composition of extracellular fluid
Excretion of endogenous waste products of metabolism
Excretion of foreign substances and their derivatives (e.g., drugs and metabolites)
Synthesis of prostaglandins and kinins that act within the kidney
Production of hormones, such as renin, erythropoietin, and calcitriol

Question 2

What are the consequences of renal dysfunction?

Answer 2

Renal dysfunction can lead to fluid overload and metabolic derangement, reduction in renal excretory function (resulting in uremia and acidosis), reduction in renal excretory function (increasing the risk of drug toxicity), and reduced hormone function (which can cause anemia, hypertension, and other issues).

Question 3

How do the renal system, cardiovascular system, and respiratory system work together to maintain fluid and acid-base homeostasis?

Answer 3

The renal system, cardiovascular system, and respiratory system closely interact to maintain fluid and acid-base balance. The kidneys control extracellular fluid volume, body fluid osmolality, and the balance of water, electrolytes, hydrogen ions (H+), and bicarbonate ions (HCO3-). This coordination helps maintain the balance between fluid gained and fluid lost each day.

Question 4

What can the kidneys control?

Answer 4

The kidneys can control the following:

Extracellular fluid volume, specifically plasma fluid volume and effective circulating volume (ECV)
Body fluid osmolality through water and electrolyte control
The amount of ultrafiltrate produced in the glomeruli
The amount of water, electrolytes, hydrogen ions (H+), and bicarbonate ions (HCO3-) reabsorbed in the nephron and tubules
Fluid, electrolyte, and acid-base balance by regulating the amount gained versus the amount lost each day

Question 5

How does the nephron control the osmolality and volume of urine?

Answer 5

The nephron controls the osmolality and volume of urine through a countercurrent mechanism in the loop of Henle.

Question 6

What is the countercurrent mechanism in the loop of Henle?

Answer 6

The descending limb of the loop of Henle is permeable to water but not solutes, while the ascending limb is permeable to solutes but not water. This allows for a predominantly passive process that efficiently produces either a dilute or concentrated urine.

Question 7

What is glomerular filtration rate (GFR)?

Answer 7

Glomerular filtration rate is the rate at which plasma is filtered by the glomerulus per unit of time. It is an important measure of kidney function and is typically expressed in milliliters per minute (ml/min).

Question 8

What four effector pathways act on the kidney to control ECV?

Answer 8

The four effector pathways that act on the kidney to control ECV are:

Renin-Angiotensin-Aldosterone System
Sympathetic nervous system
Antidiuretic Hormone (ADH) release
Atrial Natriuretic Peptide (ANP) release, which acts to reduce ECV

Question 9

What do these effector pathways change in the kidney?

Answer 9

Together, these effector pathways change renal hemodynamics and Na+ transport by renal tubule cells. Most pathways use changes in Na+ excretion to regulate effective circulating volume, except for ADH which plays a role in water reabsorption.

Question 10

What is the Renin-Angiotensin-Aldosterone system (RAAS)?

Answer 10

The Renin-Angiotensin-Aldosterone system (RAAS) is an important hormonal pathway involved in regulating ECV. It involves the release of renin from the juxtaglomerular cells of the kidney in response to decreased blood flow or decreased Na+ delivery to the distal tubule.

Question 11

What is the role of the macula densa in ECV regulation?

Answer 11

The macula densa is a group of specialized cells located in the distal tubules of the kidney. It senses sodium delivery to the distal tubule and plays a role in regulating ECV.

Question 12

What is the main function of the Renin-Angiotensin-Aldosterone system?

Answer 12

The main function of the Renin-Angiotensin-Aldosterone system is to increase blood volume and blood pressure by stimulating the release of aldosterone, which enhances sodium reabsorption and water retention in the kidney.

Question 13

What are the different types of baroreceptors involved in detecting changes in ECV?

Answer 13

The different types of baroreceptors involved in detecting changes in ECV include central vascular sensors, blood volume receptors in large systemic veins, cardiac atria, pulmonary vasculature, peripheral stretch receptors in the carotid sinus and aortic arch, and the renal afferent arteriole (the renal baroreceptor). There are also sensors in the central nervous system (CNS) and liver, although they are less important.

Question 14

What is the role of Anti Diuretic Hormone (ADH) in ECV regulation?

Answer 14

ADH is released by the posterior pituitary gland in response to hyperosmolality and volume depletion. Its antidiuretic effect is mediated by V2 receptors, acting on renal collecting ducts to increase water reabsorption. ADH also increases vascular resistance through V1 receptors.

Question 15

How does ADH respond to a decrease in ECV?

Answer 15

A decrease in ECV is detected by peripheral baroreceptors, which signal to the hypothalamus in the brain. This leads to the release of ADH into circulation, which increases water reabsorption in the kidneys, ultimately increasing ECV.

Question 16

What is the role of Atrial Natriuretic Peptide (ANP) in ECV regulation?

Answer 16

ANP is synthesized and stored in atrial myocytes. Increased ECV causes atrial stretch, leading to the release of ANP into circulation. ANP promotes natriuresis (increased sodium and water excretion from the kidney) and renal vasodilation, which increases blood flow and glomerular filtration rate (GFR). It also reduces the release of renin by the Juxtaglomerular apparatus (JGA), counteracting the effects of Angiotensin II (Ang-II).

Question 17

What are the sensors involved in hormonal regulation of salt and water balance?

Answer 17

In osmoregulation, plasma osmolality is sensed by hypothalamic osmoreceptors. In volume regulation, the macula densa in the juxtaglomerular apparatus (JGA) acts as a sensor for circulating/vascular fluid volume. Baroreceptors in the atria, carotid sinus, central veins, pulmonary vasculature, and renal afferent arterioles also contribute to sensing fluid volume.

Question 18

What are the effectors involved in hormonal regulation of salt and water balance?

Answer 18

The effectors include Antidiuretic hormone (ADH), which acts on the ADH receptors in the collecting tubules of the nephron to promote water reabsorption. The Renin-Angiotensin-Aldosterone system is also involved. Additionally, Atrial Natriuretic Peptide (ANP) and Brain Natriuretic Peptide (BNP) play a role in promoting natriuresis and opposing the effects of Angiotensin II (Ang-II).

Question 19

What are the consequences of antidiuretic hormone (ADH) release?

Answer 19

ADH promotes water reabsorption in the collecting ducts of the nephron and stimulates thirst. It leads to a reduction in urinary sodium excretion.

Question 20

What is the main solute that governs plasma osmolality?

Answer 20

Sodium is the main solute that governs plasma osmolality, which refers to the number of solutes in a solution.

Question 21

How does water behave in relation to sodium across an osmotic gradient?

Answer 21

Water follows sodium across an osmotic gradient. When there is low osmolality, water tends to move from an area of low solute concentration to an area of high solute concentration, and vice versa.

Question 22

Are sodium and water regulated independently?

Answer 22

Yes, sodium and water are regulated independently in the body. They can be affected separately, leading to conditions such as hyponatremia (excess water) or hypernatremia (insufficient water).

Question 23

What is the difference between osmolality and tonicity?

Answer 23

Osmolality refers to the number of particles in a solution, while tonicity refers to the number of “osmotically” active particles in a solution. Larger molecules such as urea and glucose do not significantly contribute to plasma osmotic pressure (osmolality), as they freely cross the cell membrane through facilitated diffusion.

Question 24

How is plasma osmolality calculated?

Answer 24

Plasma osmolality is calculated using the formula: 2(Na + K) + Urea (mmol/L) + Glucose (mmol/L). The normal range for plasma osmolality is 280-300 mosmol/kg.

Question 25

Why do we calculate the difference between measured and calculated plasma osmolality?

Answer 25

The calculation of the osmolar gap helps identify alcohol poisoning, such as the ingestion of ethanol, methanol, or substances like antifreeze (ethylene glycol) by the patient.

Question 26

What is the “Rule of Third” in water distribution across body compartments?

Answer 26

The “Rule of Third” states that approximately two-thirds of the total body water is located in the intracellular fluid (ICF) compartment, while one-third is in the extracellular fluid (ECF) compartment. The ECF is further divided into the interstitial fluid (ISF) and plasma compartments.

Question 27

What happens to cells when the effective osmolality of the ECF changes?

Answer 27

If the effective osmolality of the ECF increases, cells shrink as water moves out of the intracellular fluid (ICF). If the effective osmolality of the ECF decreases, cells swell as water moves into the ICF.

Question 28

What are the major electrolytes distributed in the extracellular and intracellular compartments?

Answer 28

Extracellular compartment: Sodium (Na+), Chloride (Cl-), Bicarbonate (HCO3-), and Calcium (Ca++)
Intracellular compartment: Potassium (K+), Phosphate (PO4-), and Magnesium (Mg++)

Question 29

What does the Darrow-Yannet diagram illustrate regarding the effect of adding or losing salt and water?

Answer 29

The Darrow-Yannet diagram shows the relationship between changes in volume and osmolality when salt (NaCl) is added or lost, as well as the effect of isotonic saline administration. Adding salt increases extracellular fluid (ECF) volume and osmolality, while losing salt decreases ECF volume and osmolality. The effect on intracellular fluid (ICF) volume depends on the specific situation.

Question 30

What are the common causes of hyponatremia?

Answer 30

The common causes of hyponatremia include:

Increased serum osmolality: High blood sugar (hyperglycemia), gastrointestinal losses (e.g., diarrhea, vomiting), hyperlipidemia, burns, IV glucose infusion
Reduced serum osmolality: Renal losses (e.g., diuretic therapy, hypoaldosteronism/Addison’s disease), oedematous states (heart failure, renal failure, liver failure, nephrotic syndrome), syndrome of inappropriate ADH (SIADH), hypothyroidism, psychogenic polydipsia

Question 31

What happens with rapid correction of hyponatremia?

Answer 31

Rapid correction of hyponatremia can lead to intracellular dehydration, where water moves out of the intracellular fluid (ICF) compartment due to the rapid increase in osmolality. This can result in complications such as central pontine myelinolysis.

Question 32

What happens with rapid correction of hypernatremia?

Answer 32

Rapid correction of hypernatremia can lead to cerebral edema, as water moves rapidly into the brain cells to correct the osmotic imbalance. This can be a potentially dangerous consequence of rapid correction.

Question 33

What are some causes of hypernatremia?

Answer 33

Hypernatremia can be caused by dehydration, certain drugs (e.g., lithium), diabetes insipidus (either due to deficiency or renal tubular resistance to ADH).