flashcard 8
(50 cards)
What is the functional unit of the kidney, and what are its three main components?
The nephron, consisting of the renal corpuscle (glomerulus + Bowman’s capsule), the renal tubule (proximal tubule, loop of Henle, distal tubule), and the collecting duct.
How do cortical nephrons differ from juxtamedullary nephrons in structure and function?
Cortical nephrons have shorter loops of Henle that dip only slightly into the medulla, mainly carrying out filtration and bulk reabsorption. Juxtamedullary nephrons have long loops extending deep into the medulla, critical for generating and maintaining medullary concentration gradients.
What is the initial process that occurs in the renal corpuscle?
Glomerular filtration, where plasma is forced through the fenestrated capillaries of the glomerulus into Bowman’s space, creating filtrate.
Approximately what percentage of plasma entering the afferent arteriole is filtered into Bowman’s capsule?
Around 20% of plasma is filtered into Bowman’s capsule; the remaining 80% continues through the efferent arteriole to peritubular capillaries.
What prevents large proteins and blood cells from entering the filtrate during glomerular filtration?
The size- and charge-selective filtration barrier (fenestrated endothelium, basement membrane, and podocyte slit diaphragms) excludes large proteins (e.g., albumin) and cells, allowing only small molecules to pass.
What three pressures determine net filtration pressure in the glomerulus?
Glomerular capillary hydrostatic pressure (favors filtration), Bowman’s capsule hydrostatic pressure (opposes filtration), and glomerular capillary colloid osmotic pressure (opposes filtration).
How does constriction of the afferent arteriole affect glomerular filtration rate (GFR)?
Constricting the afferent arteriole reduces blood flow into the glomerulus, lowering glomerular hydrostatic pressure and decreasing GFR.
What is the effect of constricting the efferent arteriole on GFR?
Constricting the efferent arteriole increases hydrostatic pressure in the glomerulus, thereby increasing GFR (up to a limit before backpressure rises too much).
What is tubular reabsorption, and where does most of it occur?
Tubular reabsorption is the movement of filtered substances (water, electrolytes, nutrients) from filtrate back into peritubular capillaries. About 65–70% of filtered fluid is reabsorbed in the proximal convoluted tubule.
How does the permeability of the descending limb of the loop of Henle differ from that of the ascending limb?
The descending limb is permeable to water but not to solutes, causing the filtrate to become more concentrated as water is reabsorbed. The ascending limb is impermeable to water but actively reabsorbs Na⁺, K⁺, and Cl⁻, diluting the filtrate.
What mechanism creates and maintains the medullary osmotic gradient?
The countercurrent multiplier system: the loop of Henle’s descending limb loses water, concentrating filtrate; the thick ascending limb actively pumps out ions without water, diluting filtrate; vasa recta capillaries preserve the gradient by countercurrent exchange.
How is final urine osmolarity determined in the collecting duct?
By variable reabsorption of water and solutes under hormonal control (mainly ADH and aldosterone), in response to the body’s hydration status, which adjusts permeability of collecting duct water channels.
What hormone increases the water permeability of the collecting duct, and where is it produced?
Antidiuretic hormone (ADH or vasopressin), produced by the hypothalamus and released from the posterior pituitary, increases aquaporin insertion in the collecting duct, promoting water reabsorption.
What triggers aldosterone release, and what is its primary renal effect?
Aldosterone is released from the adrenal cortex in response to angiotensin II or high plasma K⁺. It increases Na⁺ reabsorption and K⁺ secretion in the distal tubule and collecting duct, indirectly promoting water retention.
What is the macula densa, and how does it contribute to tubuloglomerular feedback?
The macula densa is a specialized group of cells in the distal tubule that senses NaCl concentration. If NaCl delivery is high, they release paracrine signals causing afferent arteriole constriction to reduce GFR; if NaCl is low, they cause dilation to increase GFR.
Explain the myogenic mechanism of GFR autoregulation.
When systemic blood pressure rises, the afferent arteriole stretches and reflexively constricts (myogenic response) to maintain constant blood flow and GFR. If pressure falls, the arteriole dilates to preserve GFR.
Why is GFR normally maintained relatively constant despite fluctuations in systemic blood pressure?
Autoregulation via the myogenic response and tubuloglomerular feedback ensures stable GFR by adjusting afferent/efferent arteriole tone, preventing large swings in filtration.
How is GFR clinically estimated using inulin clearance?
Inulin is freely filtered, not reabsorbed, secreted, or metabolized. By measuring plasma inulin concentration, urine inulin concentration, and urine flow rate, GFR = ([Inulin]_urine × Urine flow) ÷ [Inulin]_plasma.
Why is creatinine clearance used as an endogenous estimate of GFR?
Creatinine is produced at a fairly constant rate by muscle metabolism, freely filtered, not reabsorbed, and minimally secreted. Measuring its plasma and urine concentrations gives an approximate GFR without requiring infusion.
What are limitations of using creatinine clearance to estimate GFR?
Approximately 7–10% of creatinine is secreted in the proximal tubule (overestimating GFR), and factors such as age, muscle mass, diet, exercise, and certain medications (e.g., trimethoprim, cimetidine) can alter serum creatinine independently of GFR.
What is the typical normal GFR in a young healthy adult, and how does it change with age?
Normal GFR is about 120–125 mL/min. GFR declines gradually with age (e.g., by 1 mL/min per year after age 40), so elderly individuals often have lower ‘normal’ GFR.
How do clinicians classify stages of chronic kidney disease (CKD) based on estimated GFR?
CKD stages are defined as: Stage 1 (GFR ≥90 with kidney damage), Stage 2 (60–89), Stage 3a (45–59), Stage 3b (30–44), Stage 4 (15–29), Stage 5 (<15, kidney failure).
What major functions, beyond filtration, do the kidneys perform to maintain homeostasis?
Control of blood composition (fluid/electrolyte balance), regulation of blood volume and pressure, endocrine functions (erythropoietin and calcitriol production), acid-base balance, and excretion of metabolic wastes (urea, uric acid, creatinine).
How do the kidneys contribute to acid-base balance through bicarbonate reabsorption?
In proximal tubule cells, filtered HCO₃⁻ combines with secreted H⁺ to form CO₂ + H₂O (via carbonic anhydrase). CO₂ diffuses into cells, is reconverted to HCO₃⁻, which is reabsorbed with Na⁺, while H⁺ is secreted back into lumen.
