Renal System Flashcards
Kidney Overview:
- Two kidneys on the posterior wall of the abdomen, outside the peritoneal cavity.
- Two ureters (about 25 cm) that transport urine from the kidneys to the bladder by peristalsis.
- The bladder stores urine (up to 700–800 mL) until micturition, which involves voluntary contraction of the bladder through the urethra
Kidney Functions:
- Water and Electrolyte Balance: Regulates Na⁺, K⁺, Cl⁻, Ca²⁺, phosphate, and magnesium ions.
- Acid-Base Balance: Adjusts pH by excreting hydrogen ions and reabsorbing bicarbonate.
- Waste Excretion: Eliminates metabolic waste products (e.g., nitrogen) and foreign substances (e.g., drugs, pesticides).
- Hormone Secretion:
1. Renin: Controls angiotensin formation.
2. Erythropoietin: Stimulates RBC production.
3. 1,25-dihydroxy Vitamin D3: Involved in calcium metabolism
Kidney Structure:
- Divided into cortex, medulla, and pelvis.
- Cortex: Outer layer containing glomeruli and convoluted tubules for filtration.
- Medulla: Inner region with the loop of Henle and collecting ducts.
- Pelvis: Funnel-shaped top part of the ureter where kidney tubules drain
Nephron Structure:
The nephron consists of vascular components (afferent and efferent arterioles, glomerulus, peritubular capillaries) and tubular components (proximal tubule, loop of Henle, distal tubule, collecting duct).
Filtration occurs in the glomerulus; tubular filtrate flows through renal tubules, where reabsorption and secretion occur
Nephron Filtration and Urine Formation:
Plasma volume (~3L) filters at a rate of 125 mL/min (GFR), totaling 180 L/day. Filtration, reabsorption, secretion, and excretion processes maintain homeostasis. Without tubular reabsorption, plasma and essential solutes would be lost quickly.
Reabsorption and Secretion in the Tubules:
Proximal Tubule: Reabsorbs 67% of Na⁺ and water, along with nutrients like glucose and amino acids.
Loop of Henle: Descending limb reabsorbs water; ascending limb is impermeable to water but reabsorbs Na⁺ and Cl⁻.
Distal Tubule & Collecting Duct: Fine-tunes Na⁺ and water reabsorption under hormonal control (aldosterone and ADH)
Hormones Regulating Tubular Reabsorption:
Antidiuretic Hormone (ADH): Increases water permeability in the collecting duct, reducing urine volume.
Aldosterone: Enhances Na⁺ reabsorption in the distal tubule, increasing blood volume and pressure
Tubular Secretion
Tubular secretion is reabsorption in reverse which is the movement of solutes from the peritubular capillaries into the tubules
Occurs almost completely in PCT (Proximal convoluted tubule)
Selected substances are moved from peritubular capillaries through tubule cells out into filtrate
– K+, H+, NH4+, creatinine, organic acids and bases
– Substances synthesized in tubule cells also are secreted (example:
HCO3–)
Tubular secretion is important for:
– Disposing of substances, such as drugs or
metabolites, that are bound to plasma proteins
– Eliminating undesirable substances that were passively reabsorbed (example: urea and uric acid)
– Ridding body of excess K+ (aldosterone effect)
– Controlling blood pH by altering amounts of H+ or HCO3– in urine
Glomerular Filtration Barrier:
Composed of:
- Fenestrated Endothelial Cells: Allow small molecules through but block larger ones.
- Basement Membrane: Negatively charged, repels proteins.
- Podocytes with Slit Diaphragms: Form the final filtration layer, allowing free passage of small solutes and blocking large molecules like proteins
Glomerular Filtration Rate
Normal GFR is ~125 mL/min.
GFR is determined by the net filtration pressure, membrane permeability, and surface area.
Creatinine clearance is a common clinical measure of GFR
Forces Governing GFR:
Hydrostatic Pressure in Glomerular Capillaries: Favors filtration (approx. 55 mm Hg).
Plasma Oncotic Pressure: Opposes filtration.
Bowman’s Capsule Hydrostatic Pressure: Also opposes filtration.
Starlings Law: GFR depends on net filtration pressure and the permeability of the glomerular membrane
Factors Affecting GFR:
Severe Burns: Decreased plasma oncotic pressure increases GFR.
Urinary Tract Obstruction: Increases Bowman’s capsule pressure, reducing GFR.
Severe Dehydration: Increases plasma oncotic pressure, reducing GFR
Autoregulation of GFR:
Maintains GFR by controlling glomerular capillary pressure, despite fluctuations in blood pressure.
Myogenic Mechanism: Afferent arteriole constricts in response to increased blood pressure, preventing excessive GFR.
Tubuloglomerular Feedback: Macula densa cells sense NaCl levels; high NaCl leads to afferent arteriole constriction, reducing GFR, while low NaCl dilates the arteriole, increasing GFR
Renal Clearance:
Clearance rate (C) is calculated by:
C=UV/P
where:
U = concentration in urine
V = urine flow rate
P = plasma concentration of the substance.
Inulin: Used as a precise measure of GFR; freely filtered, neither reabsorbed nor secreted.
Creatinine: Commonly used clinically to estimate GFR, though slightly less accurate than inulin
Loop of Henle and Countercurrent Mechanism:
Countercurrent Multiplier:
- Ascending Limb: Actively transports Na⁺ and Cl⁻ into the medulla, creating a hyperosmotic environment.
- Descending Limb: Permeable to water, which diffuses out to balance medullary osmolarity.
Result: The osmotic gradient in the medulla allows for the production of concentrated urine when necessary
Hormonal Control of Salt and Water Balance:
Vasopressin (ADH):
- Synthesized in the hypothalamus, stored in the posterior pituitary, released when plasma osmolarity is high.
- Inserts aquaporins into the collecting duct, increasing water reabsorption and concentrating urine.
Aldosterone:
- Secreted from the adrenal cortex in response to angiotensin II or high plasma K⁺ levels.
- Enhances Na⁺ reabsorption and K⁺ excretion in the distal tubule, contributing to increased blood volume and blood pressure
Renin-Angiotensin-Aldosterone System (RAAS):
Activation:
Renin is released from juxtaglomerular cells in response to low blood pressure or low NaCl sensed by macula densa cells.
Renin converts angiotensinogen to angiotensin I, which is converted to angiotensin II in the lungs by ACE.
Effects of Angiotensin II:
Vasoconstriction: Increases systemic blood pressure.
Aldosterone Release: Promotes Na⁺ and water reabsorption.
ADH Secretion: Enhances water retention
Factors Regulating Renin Release:
Renal Baroreceptors: Juxtaglomerular cells in afferent arterioles detect blood pressure; low pressure stimulates renin release.
Sympathetic Stimulation: β₁-adrenergic receptors increase renin secretion during sympathetic activation.
Macula Densa Feedback: Low NaCl concentration in distal tubules prompts renin release, triggering RAAS to increase Na⁺ reabsorption and blood pressure
Tubuloglomerular Feedback Mechanism:
Macula Densa Cells: Specialized cells in the distal tubule that monitor NaCl concentration.
Feedback Process:
- High NaCl in tubular fluid signals the afferent arteriole to constrict, reducing GFR.
- Low NaCl promotes afferent arteriole dilation, increasing GFR to maintain sodium balance
Describe the vascular components of the nephron:
Afferent Arteriole: Supplies blood to the glomerulus.
Glomerulus: A network of capillaries where blood filtration occurs.
Efferent Arteriole: Drains blood from the glomerulus to the peritubular capillaries or vasa recta, which facilitate exchange with the tubular fluid
Describe the tubular components of the nephron:
Proximal Tubule: Major site for reabsorption of water, ions, and nutrients.
Loop of Henle: Establishes an osmotic gradient in the medulla, allowing concentration of urine.
Distal Tubule: Involved in the selective reabsorption and secretion of ions, influenced by hormones.
Collecting Duct: Final site for water and electrolyte balance, regulated by ADH and aldosterone
Explain the difference between tubular reabsorption and secretion:
Reabsorption: Movement of substances from the tubular lumen back into the blood (e.g., Na⁺, water).
Secretion: Transfer of waste and excess ions from the blood into the tubular fluid for excretion (e.g., H⁺, K⁺)
Diagram to Show Sodium and Water Reabsorption in the Nephron:
In Image
Explanation of Sodium and Water Reabsorption (Roles of Vasopressin and Aldosterone):
Proximal Tubule: Reabsorbs 67% of Na⁺ and water, along with nutrients like glucose and amino acids.
Loop of Henle: Descending limb reabsorbs water; ascending limb is impermeable to water but reabsorbs Na⁺ and Cl⁻.
Distal Tubule & Collecting Duct: Fine-tunes Na⁺ and water reabsorption under hormonal control (aldosterone and ADH)
Vasopressin (ADH): Increases water reabsorption in the collecting duct by inserting aquaporins.
Aldosterone: Enhances Na⁺ reabsorption in the distal tubule, promoting water retention
Describe the Three Zones of the Kidney:
Cortex: Outer layer with glomeruli and convoluted tubules.
Medulla: Contains loops of Henle and collecting ducts.
Pelvis: Urine collection area that drains into the ureter
Diagram of Blood Flow into and out of the Kidney:
Blood flow: Renal artery → Segmental artery → Interlobar artery → Arcuate artery → Cortical radiate artery → Afferent arteriole → Glomerulus → Efferent arteriole → Peritubular capillaries/vasa recta → Renal vein
Structure and Function of the Nephron:
Structure: Composed of the glomerulus (filtration) and renal tubules (reabsorption, secretion, and excretion).
Function: Filters blood, reabsorbs essential nutrients, secretes waste products, and concentrates urine
Labelled Diagram of the Tubular Components of the Nephron:
Proximal Tubule: Bulk reabsorption.
Loop of Henle: Osmotic gradient.
Distal Tubule: Hormone-regulated adjustments.
Collecting Duct: Final concentration of urine
What is the Glomerular Filtration Rate (GFR)? How is it Measured?
GFR: Volume of plasma filtered per minute (~125 mL/min).
Measurement: Estimated using substances like creatinine or inulin that are freely filtered and not reabsorbed or secreted.
GFR = UV/P
(U = urine concentration, V = urine flow rate, P = plasma concentration)
Structure of the Glomerular Filtration Barrier:
Composed of endothelial cells (fenestrated), basement membrane (negatively charged), and podocytes (with slit diaphragms).
Role: Selective filter allowing water, ions, and small solutes but blocking large molecules like proteins
Forces Driving and Opposing Glomerular Filtration:
Driving Force: Glomerular capillary hydrostatic pressure (~55 mm Hg).
Opposing Forces: Plasma oncotic pressure and Bowman’s capsule hydrostatic pressure.
Net Filtration Pressure: Controls the rate of filtration
Factors Controlling Glomerular Capillary Pressure:
Arterial Pressure: Directly affects blood flow into the glomerulus.
Afferent & Efferent Arteriole Resistance: Constriction or dilation of these arterioles modulates glomerular pressure
Changes in GFR in Severe Burns, Urinary Tract Obstruction, and Dehydrating Diarrhea:
Severe Burns: Decreased plasma oncotic pressure, increasing GFR.
Urinary Tract Obstruction: Raises Bowman’s capsule hydrostatic pressure, decreasing GFR.
Dehydrating Diarrhea: Increases plasma oncotic pressure, reducing GFR
Myogenic Mechanism (with Diagram):
Mechanism: Afferent arteriole constricts in response to increased blood pressure, stabilizing GFR by reducing glomerular capillary pressure.
Tubuloglomerular Feedback Mechanism (with Diagram):
Mechanism: Macula densa cells detect increased NaCl, triggering afferent arteriole constriction to lower GFR; low NaCl has the opposite effect.
Renin-Angiotensin-Aldosterone System (RAAS):
Activation: Triggered by low blood pressure or NaCl. Renin converts angiotensinogen to angiotensin I, which is then converted to angiotensin II.
Effects of Angiotensin II: Increases blood pressure via vasoconstriction, stimulates aldosterone (enhancing Na⁺ reabsorption), and ADH release (increasing water reabsorption).
Factors Stimulating Renin Release:
Renal Baroreceptors: Detect low afferent arteriole pressure.
Sympathetic Stimulation: Increases renin via β₁-receptors on juxtaglomerular cells.
Macula Densa: Low NaCl levels increase renin release to maintain blood pressure.
Juxtaglomerular Apparatus (with Diagram):
Components:
- Macula Densa: Senses NaCl.
- Juxtaglomerular Cells: Release renin.
- Extraglomerular Mesangial Cells: Facilitate signaling within the apparatus.
Define Intrarenal Baroreceptors:
Definition: Juxtaglomerular cells in the afferent arteriole, acting as baroreceptors that detect blood pressure changes, modulating renin release
Countercurrent Multiplier System (with Diagram):
Mechanism: NaCl is actively transported out of the ascending limb of the loop of Henle, creating a hyperosmotic medullary interstitium that draws water from the descending limb. This mechanism concentrates urine and conserves water.
