Lecture 12
List two substances that normally pass into the filtrate at the glomerulus.
Two substances that normally pass into the filtrate at the glomerulus are:
Describe the main difference in water permeability between the descending and ascending limbs of the nephron loop.
The descending limb of the nephron loop is highly permeable to water (due to aquaporins) but relatively impermeable to solutes, so water leaves the tubule by osmosis and the filtrate becomes more concentrated.
In contrast, the ascending limb (especially the thick ascending limb) is impermeable to water but actively reabsorbs solutes like Na⁺, K⁺, and Cl⁻, so the filtrate becomes more dilute as it moves upward.
Define glycosuria and name the most likely associated condition.
Glycosuria is the presence of glucose in the urine, which occurs when blood glucose levels exceed the renal threshold for reabsorption in the proximal tubule.
The most likely associated condition is diabetes mellitus.
Explain why Na⁺ is considered the major determinant of ECF osmolarity and how this affects water movement.
Na⁺ is the major determinant of ECF osmolarity because it is the most abundant effective osmole in the extracellular fluid and is largely restricted to the ECF due to the Na⁺/K⁺ ATPase maintaining high Na⁺ outside cells.
Because water moves by osmosis toward higher solute concentration, changes in Na⁺ concentration directly change ECF osmolarity and therefore drive water movement between compartments:
Thus, Na⁺ effectively “controls” ECF osmolarity, and water follows Na⁺ to maintain osmotic balance between ECF and ICF.
What is the primary cause of respiratory alkalosis, and how does it change CO₂ levels in the blood?
The primary cause of respiratory alkalosis is hyperventilation (increased breathing rate or depth), which can occur due to anxiety, high altitude, fever, or sepsis.
Hyperventilation causes the lungs to remove CO₂ from the blood faster than it is produced, leading to a decrease in arterial CO₂ (PCO₂). This drop in CO₂ shifts the carbonic acid–bicarbonate equilibrium to the left, reducing H⁺ concentration and increasing blood pH.
Define GFR and describe one consequence of a GFR that is too high and one consequence of a GFR that is too low.
GFR (glomerular filtration rate) is the amount of filtrate formed per minute by both kidneys combined. It reflects how well the kidneys are filtering blood.
Name the cotransporter responsible for Na⁺ reabsorption in the thick ascending limb and briefly describe how it works.
The cotransporter in the thick ascending limb is the Na⁺/K⁺/2Cl⁻ cotransporter (NKCC2).
It works by simultaneously moving Na⁺, K⁺, and 2 Cl⁻ ions from the tubular fluid into the epithelial cells of the thick ascending limb. This process is driven by the low intracellular Na⁺ concentration created by the basolateral Na⁺/K⁺ ATPase, which indirectly powers the cotransporter. The thick ascending limb is impermeable to water, so solutes are reabsorbed without water following.
Match the odor to the correct condition:
A. Sweet/fruity odor
B. Musty (“mousy”) odor
C. Ammoniacal odor
A. Sweet/fruity odor → Diabetic ketoacidosis (DKA)
B. Musty (“mousy”) odor → Phenylketonuria (PKU)
C. Ammoniacal odor → UTI (urease-producing bacteria)
Explain why principal cells can either secrete or conserve K⁺ depending on the body’s needs.
Principal cells can either secrete or conserve K⁺ because their transport activity is hormonally and electrochemically regulated depending on the body’s K⁺ balance.
Thus, principal cells adjust K⁺ handling to maintain tight homeostasis of plasma potassium levels, which is critical for normal membrane excitability in muscles and nerves.
Give one cause each of:
Explain how the myogenic mechanism prevents damage to the glomerulus during high blood pressure.
The myogenic mechanism protects the glomerulus by responding to increased blood pressure through an intrinsic smooth muscle response in the afferent arteriole.
When blood pressure rises, the afferent arteriole stretches, which opens mechanically gated ion channels in the smooth muscle cells. This leads to Ca²⁺ influx, causing the smooth muscle to contract. The arteriole then vasoconstricts, which reduces blood flow into the glomerulus.
This constriction helps lower glomerular capillary pressure, preventing excessive filtration and protecting the glomerulus from damage while keeping GFR relatively stable.
Describe the role of ADH and aldosterone in the late DCT and collecting duct.
In the late distal convoluted tubule (DCT) and collecting duct, both hormones fine-tune fluid and electrolyte balance:
List the three layers of the ureter wall and briefly describe the function of each.
The three layers of the ureter wall are:
List the three major physiological responses to dehydration that help restore fluid balance.
The three major physiological responses to dehydration are:
List the normal ranges for:
Name the three components of the juxtaglomerular apparatus and give one function of each.
The three components of the juxtaglomerular apparatus (JGA) are:
Explain how the loop of Henle creates the medullary osmotic gradient using countercurrent multiplication.
The loop of Henle creates the medullary osmotic gradient through countercurrent multiplication, which depends on opposite fluid flow in its two limbs and different transport properties.
In the thick ascending limb, Na⁺ and Cl⁻ are actively reabsorbed into the medullary interstitium via the NKCC2 cotransporter, but this segment is impermeable to water, so water cannot follow. This makes the surrounding interstitium increasingly hyperosmotic.
In contrast, the descending limb is permeable to water but not solutes, so water leaves the tubule by osmosis into the salty medulla, concentrating the filtrate as it moves downward.
Because fluid flows in opposite directions in the two limbs, this small stepwise difference is continuously “multiplied” along the loop, progressively increasing osmolarity deeper in the medulla. This establishes the corticomedullary gradient, which is essential for concentrating urine.
Compare the roles of the sympathetic and parasympathetic nervous systems in bladder control.
The sympathetic nervous system (T11–L2) promotes urine storage. It causes the detrusor muscle to relax (β3 receptors) and the internal urethral sphincter to contract (α1 receptors), allowing the bladder to fill without leaking urine.
The parasympathetic nervous system (S2–S4) promotes urine voiding (micturition). It causes the detrusor muscle to contract (M3 receptors) and the internal urethral sphincter to relax, enabling urine to be expelled from the bladder.
A patient has low blood pressure and high plasma osmolarity. Explain how RAAS and ADH work together to restore homeostasis.
With low blood pressure and high plasma osmolarity, the body activates both RAAS and ADH, which work together to restore both volume and osmolarity balance:
RAAS primarily restores blood volume/pressure (Na⁺ + water retention), while ADH primarily corrects osmolarity (water retention without Na⁺), leading to coordinated restoration of homeostasis.
Given:
Identify:
Trace the steps of the RAAS system from decreased blood pressure to angiotensin II formation.
When blood pressure decreases, the RAAS pathway is activated as follows:
➡ Angiotensin II is then responsible for increasing blood pressure and helping restore GFR.
Compare obligatory vs facultative water reabsorption, including where each occurs and whether it is hormonally controlled.
Obligatory water reabsorption is the automatic, constant reabsorption of water that occurs regardless of the body’s hydration status. It happens in the proximal convoluted tubule and the descending limb of the loop of Henle, where water follows solute reabsorption by osmosis. It is not hormonally controlled.
Facultative water reabsorption is variable and regulated based on the body’s needs. It occurs mainly in the late distal convoluted tubule and collecting duct and is controlled by ADH, which increases water permeability by inserting aquaporins. This allows the body to adjust urine concentration and conserve water when needed.
A patient has difficulty voiding and cannot fully empty the bladder due to loss of parasympathetic function.
Explain how this affects:
Loss of parasympathetic (S2–S4) function disrupts the micturition reflex and impairs bladder emptying:
Explain what happens in hypotonic hydration (water intoxication) in terms of:
In hypotonic hydration (water intoxication):
Differentiate between:
(What happens to pH and the other variables?)