Renal System 2 Lecture 24

Question 1

Key functions of the kidney to homeostasis

Answer 1

1. Regulation of water and electrolyte balance
2. Maintenance of blood osmolarity: By regulating solute concentration (osmolarity), the kidneys ensure proper hydration levels and prevent cellular swelling or dehydration.
3. Regulation of blood volume: The kidneys control blood volume by adjusting the amount of water reabsorbed into the bloodstream, which directly influences blood pressure.
4. Regulation of blood pressure
5. Excretion of metabolic waste products or foreign chemicals: The kidneys filter out waste products like urea, creatinine, and toxins from the blood, which are excreted in urine.
6. Regulation of blood pH: The kidneys maintain acid-base balance by excreting hydrogen ions (H+) and reabsorbing bicarbonate (HCO₃⁻), helping to prevent metabolic acidosis or alkalosis.
7. Regulation of erythrocyte (red blood cell) production: The kidneys produce erythropoietin, a hormone that stimulates the production of red blood cells in the bone marrow, especially in response to low oxygen levels.
8. Regulation of hormone production: In addition to erythropoietin, the kidneys produce and regulate hormones like renin (involved in blood pressure regulation) and calcitriol (active vitamin D, important for calcium homeostasis).
9. Regulating blood glucose levels: The kidneys contribute to glucose homeostasis through processes like gluconeogenesis (production of glucose from non-carbohydrate sources) and reabsorption of glucose.

Question 2

Peritoneal dialysis

Answer 2

Type of dialysis treatment that uses the lining of the abdomen, called the peritoneum, as a natural filter to remove waste products and excess fluid from the blood when the kidneys are not functioning properly. Unlike hemodialysis, which uses an external machine to filter the blood, peritoneal dialysis allows the filtration process to occur inside the body.

Question 3

Osmosis

Answer 3

Movement of water across a selectively permeable membrane from an area of low solute concentration (high water concentration) to an area of high solute concentration (low water concentration).

Question 4

Key points of osmosis

Answer 4

Water molecules move from the side with fewer solutes (lower concentration of sucrose) to the side with more solutes (higher concentration of sucrose).
Differentially permeable membrane: This membrane allows only certain molecules (like water) to pass while preventing larger solute molecules (like sucrose) from moving across.
The movement of water continues until the concentration of solutes on both sides of the membrane reaches equilibrium or other balancing forces act upon it.

Question 5

Osmotic pressure

Answer 5

Amount of pressure needed to stop the net movement of water across a membrane. It is directly related to the concentration of solutes: the more solutes present, the higher the osmotic pressure required to prevent water movement.

Question 6

Key stages illustrated

Answer 6

Initial Condition (Left Arm and Right Arm, Start of Experiment):
The left arm contains a solution with fewer solutes (more water), while the right arm contains a solution with more solutes (less water).
Water moves through a selectively permeable membrane from the left arm to the right arm by osmosis, attempting to balance the concentration of solutes on both sides.

Equilibrium (Middle Diagram):
As water moves to the right arm, the volume on the right side increases. This movement increases the hydrostatic pressure (the pressure exerted by the fluid) on the right side.
Eventually, the movement of water into the right arm is balanced by the increasing hydrostatic pressure, reaching equilibrium.

Osmotic Pressure (Right Diagram):
To prevent further water movement, pressure (equal to the osmotic pressure) is applied to the right arm.
Once the applied pressure equals the osmotic pressure, no further water will move between the arms, and the volumes will remain equal.

Question 7

Hyperosmotic

Answer 7

A solution with a higher osmolarity (Osm) than another solution. This means that the hyperosmotic solution has more solute particles per unit of water compared to the other. For example, a solution with 300 mM/L NaCl would be hyperosmotic compared to a solution with 300 mM/L urea, because NaCl dissociates into more particles (Na⁺ and Cl⁻) than urea.

Question 8

Isosmotic

Answer 8

Two solutions that have the same osmolarity. This means they have an equal concentration of solute particles. For example, a solution with 150 mM/L NaCl and a solution with 300 mM/L urea are isosmotic because NaCl dissociates into two ions, making the total concentration of solutes in both solutions equal.

Question 9

Hyposmotic

Answer 9

A solution with a lower osmolarity than another solution. This means that the hyposmotic solution has fewer solute particles per unit of water. For example, a solution with 150 mM/L urea would be hyposmotic compared to a solution with 150 mM/L NaCl, because NaCl produces more solute particles due to dissociation.

Question 10

Tonicity

Answer 10

Considers both the concentration of solutes (like osmolarity) and the permeability of those solutes to the membrane.

Question 11

Dehydration (Loss of Water)

Answer 11

ECF osmotic pressure rises: When water is lost from the body, the concentration of solutes in the extracellular fluid increases, causing osmotic pressure to rise.
Cells lose water: Water moves out of the cells into the extracellular fluid by osmosis in an attempt to balance the osmotic pressure. As a result, the cells shrink.
The image shows a person losing water, which can represent conditions like sweating, vomiting, or diarrhea that lead to dehydration.

Question 12

Hydration (Gain of Water)

Answer 12

ECF osmotic pressure falls: When too much water is gained (e.g., excessive drinking of water), the concentration of solutes in the extracellular fluid decreases, causing the osmotic pressure to fall.
Water moves into cells: Water enters the cells by osmosis to equalize the osmotic gradient, causing the cells to swell.
The image depicts a person drinking excessive water, leading to overhydration.

Question 13

Summary of dehy and hy

Answer 13

Dehydration leads to hyperosmotic conditions (high solute concentration) in the extracellular fluid, causing water to leave the cells, which shrink.
Hydration leads to hypoosmotic conditions (low solute concentration) in the extracellular fluid, causing water to move into the cells, which swell.

Question 14

Total Body Fluid

Answer 14

In males, about 60% of total body mass is fluid.
In females, about 55% of total body mass is fluid.
The remaining mass is made up of solids like bone, fat, and other tissues.

Question 15

Fluid Compartments

Answer 15

Intracellular Fluid (ICF):
This is the fluid within cells and accounts for 2/3 of the total body fluid.
For a 70 kg male, this equates to approximately 28 L of intracellular fluid.

Extracellular Fluid (ECF):
This is the fluid outside of cells, making up 1/3 of the total body fluid.
In a 70 kg male, this is about 14 L.
ECF is further divided into:
20% Plasma (the liquid component of blood): About 2.8 L.
80% Interstitial Fluid (the fluid surrounding tissue cells): About 11.2 L.

Question 16

Major sources of water intake and output in the human body

Answer 16

Water Intake Sources:
Beverages (64%): The largest portion of daily water intake comes from liquids consumed throughout the day, approximately 1600 mL on average.
Foods (28%): Many foods, especially fruits and vegetables, contain water, contributing about 700 mL of water intake daily.
Metabolism (8%): Water is also produced during metabolic processes, particularly during the breakdown of carbohydrates and fats, adding around 200 mL to daily water intake.
In total, the average daily water intake is about 2500 mL.

Water Output Sources:
Urine (60%): The largest portion of water loss occurs through the kidneys in the form of urine, averaging around 1500 mL per day.
Skin (24%): Water is lost through the skin, primarily through sweating, accounting for about 600 mL daily.
Lungs (12%): Water vapor is exhaled with each breath, contributing around 300 mL of water loss per day.
Feces (4%): A small amount of water is lost in feces, approximately 100 mL per day.
In total, the average daily water output also amounts to around 2500 mL.

Question 17

Electrolyte and protein anion concentrations in plasma, interstitial fluid and
intracellular fluid

Answer 17

In the human body the main electrolytes are sodium (Na+), chloride (Cl-), potassium (K+)
magnesium (Mg+2), calcium (Ca+2), hydrogen phosphate (HPO4 2-) and hydrogen carbonate
(HCO3−).

Na+, Ca2+ and Cl- ions have a higher concentration in the extracellular fluid than the intracellular fluid.

K+ ions have a higher concentration in the intracellular fluid than the extracellular fluid.

This asymmetric distribution of ions is of fundamental importance. These ion gradients are responsible for the likes of setting the membrane potential, generating electrical activity in nerve and muscle, providing the energy for the uptake of nutrients and the expulsion of waste products.

Specialised transport properties of the cell membrane are responsible for the asymmetric
distribution of ions between the intracellular and extracellular fluids.

(Intracellular Ion Concentrations (inside the cell):
Sodium (Na⁺): Low concentration at 10 mM.
Potassium (K⁺): High concentration at 140 mM.
Chloride (Cl⁻): Low concentration at 4 mM.
Organic compounds are also abundant within the cell, contributing to overall intracellular solute content.

Extracellular Ion Concentrations (outside the cell):
Sodium (Na⁺): High concentration at 142 mM.
Potassium (K⁺): Low concentration at 4 mM.
Chloride (Cl⁻): High concentration at 103 mM.)

Question 18

Facts and figures about kidney function

Answer 18

Proportional blood flow:
Despite making up only 0.5% of the body’s mass, the kidneys receive 20-25% of the cardiac output at rest, which is approximately twice as much as the brain.

Kidney blood flow:
For a 70 kg male, kidney blood flow is around 1100 mL/min or 360 mL/kg/min. By comparison, the brain receives only about 50 mL/kg/min.

Fluid filtration:
Approximately 180 L of fluid enters the renal tubules each day. This is about 32 times the total blood volume, showing how extensively the kidneys filter the blood.

Extracellular fluid filtration:
In the average adult, the entire extracellular fluid volume is filtered about 12 times per day.

Reabsorption:
Out of the 180 L of fluid filtered each day, approximately 178.6 L is reabsorbed by the kidneys, meaning very little fluid is lost.

Urine production:
Only about 1.4 L of urine is produced daily, which is the waste product containing excess water, solutes, and metabolic waste.

Question 19

Equation that determines the amount excreted in urine

Answer 19

Amount filtered: This is the volume of fluid and solutes that are filtered from the blood into the nephron at the glomerulus.

Minus (−) the amount reabsorbed: After filtration, a significant portion of water, electrolytes, glucose, and other important molecules are reabsorbed from the nephron back into the bloodstream via the peritubular capillaries.

Plus (+) the amount secreted: Some substances, such as excess ions (like potassium or hydrogen ions) or metabolic wastes, are actively secreted from the blood into the nephron tubule for excretion.

Equals (=) the amount excreted in urine: The result of this equation is the final composition of urine, which contains the body’s waste products and excess fluids.

Question 20

Function of the kidney: focusing on the processes of filtration

Answer 20

Filtration:
Blood enters the kidney through the afferent arteriole and reaches the glomerular capillaries. Here, fluid and small solutes are filtered into Bowman’s space (the start of the nephron).
Approximately 180 liters of fluid are filtered into the nephron tubules each day (around 125 mL/min).

Reabsorption:
After filtration, the majority of the filtered fluid and solutes (~178 liters per day) are reabsorbed from the nephron tubules back into the peritubular capillaries and eventually returned to the bloodstream. This process ensures that the body retains vital substances like water, electrolytes, and glucose.

Secretion:
In addition to reabsorption, certain substances are actively secreted from the blood into the nephron tubules. These substances include potassium (K⁺), hydrogen ions, and certain drugs, which are excreted to maintain electrolyte balance and remove toxins.

Excretion:
After filtration, reabsorption, and secretion, the remaining fluid in the nephron, which contains waste products, excess electrolytes, and water, is excreted as urine.
Only about 1.4 liters of urine is produced each day, which is the fluid that the body does not need to retain.

Question 21

Glomerular Filtration

Answer 21

Filtration Rate:
The glomerular filtration rate (GFR) is about 25% of total renal plasma flow, which equals approximately 125 mL/min or 180 L/day.
This filtration rate indicates how much blood is filtered by the kidneys each minute, serving as a key indicator of kidney function.
Filtrate Composition:

The filtrate formed by the glomerulus has a similar solute concentration to plasma, meaning it contains glucose, electrolytes (like sodium and chloride), and other small molecules.
However, the filtrate lacks proteins and other large molecular weight compounds because they are too large to pass through the filtration barrier.
Exclusion of Blood Cells and Proteins:

The glomerular filtration barrier prevents the passage of blood cells and large molecules like proteins, ensuring that only small molecules and water pass into Bowman’s space for further processing in the nephron.

Question 22

overview of the production of urine in the kidney

Answer 22

Filtration (Step 1):
Blood enters the kidney through the afferent arteriole and passes into the glomerulus, a network of capillaries within the glomerular capsule (Bowman’s capsule).
Filtration occurs at the glomerulus, where water and small solutes (like glucose, ions, and urea) are filtered from the blood into the renal tubule. The filtrate is similar in composition to plasma but lacks larger molecules like proteins and blood cells.

Reabsorption (Step 2):
As the filtrate moves through the renal tubule, many substances such as water, glucose, and essential ions are reabsorbed from the tubular fluid back into the blood. This process occurs in the peritubular capillaries and helps the body retain needed substances while excreting waste.

Secretion (Step 3):
Secretion is the process where additional substances (such as potassium ions, hydrogen ions, and certain toxins) are actively transported from the blood into the renal tubule. This helps the body get rid of unwanted substances and maintain pH balance.

Excretion:
The final composition of urine results from the combination of filtered, reabsorbed, and secreted substances.
The urine is collected in the collecting duct and eventually exits the kidney, containing water, waste products, and excess ions.

Equation Summary:
Excretion = Filtration - Reabsorption + Secretion
This formula represents the balance of what is excreted in the urine, reflecting the amounts filtered, reabsorbed, and secreted throughout the nephron.