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Questions and Answers
What percentage of tubular reabsorption occurs in the proximal convoluted tubule (PCT)?
What is the primary site of tubular secretion within the nephron?
What happens when there is an absence of antidiuretic hormone (ADH)?
Which hormone stimulates sodium retention and increases water reabsorption?
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What triggers the release of renin from juxtaglomerular cells?
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What is the role of angiotensin II in the regulation of blood pressure?
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Which substances are primarily secreted in the distal convoluted tubule?
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Which of the following substances is 100% reabsorbed in the nephron?
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What is the main reason for the pressure difference between afferent and efferent arterioles in the kidneys?
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Which of the following substances is NOT typically found in glomerular filtrate?
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What primarily influences sodium reabsorption in the distal convoluted tubule (DCT)?
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How is glucose reabsorbed in the proximal convoluted tubule (PCT)?
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What is the glomerular filtration rate (GFR) primarily dependent on?
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Which of the following pathways does not contribute to potassium reabsorption?
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Which ions follow sodium due to changes in charge during reabsorption?
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What percentage of plasma is filtered by the kidneys every minute?
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What is the primary function of the nephron?
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Where is the renal corpuscle primarily located?
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What type of cells primarily make up the visceral layer of Bowman’s capsule?
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What term is used for the plasma that is filtered through the glomerulus?
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Which segment of the nephron is primarily responsible for reabsorption?
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What occurs in the collecting ducts regarding urine production?
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What type of epithelium is found in the ascending loop of Henle?
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What is the role of peritubular capillaries?
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What is the consequence of sympathetic stimulation on renal blood vessels?
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What percentage of cardiac output is directed to the kidneys?
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How do afferent glomerular arterioles function?
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Which of the following is NOT a mechanism of renal action?
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What occurs at the capsular space of Bowman’s capsule?
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Study Notes
Microscopic Anatomy of the Kidney
- The nephron is the basic functional unit of the kidney.
- Nephrons are responsible for filtration, reabsorption, and secretion.
- The number of nephrons varies by species and size of the animal.
- Cats: 200,000/kidney
- Dogs: 700,000/kidney
- Sheep, pigs, humans: 1,000,000/kidney
- Cows: 4,000,000/kidney
- Nephrons are located in the cortex and medulla of the kidney.
Renal Corpuscle
- The renal corpuscle is located in the cortex and consists of the glomerulus and Bowman's capsule.
- Glomerulus: A tuft of glomerular capillaries.
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Bowman's capsule: A double-walled capsule surrounding the glomerulus.
- Inner layer (visceral layer): Made up of podocytes, which have spaces for filtration.
- Outer layer (parietal layer): Forms the outer boundary of the capsule.
- Capsular space: The space between the visceral and parietal layers, continuous with the proximal convoluted tubule.
- The renal corpuscle filters blood and creates urine.
Proximal Convoluted Tubule (PCT)
- The PCT is the longest part of the tubular system and continues from the capsular space of Bowman's capsule.
- The PCT contains cuboidal epithelial cells with microvilli, which increase surface area for absorption.
- In the tubules, glomerular filtrate becomes tubular filtrate.
Loop of Henle
- The Loop of Henle continues from the PCT and descends from the cortex into the medulla and back to the cortex.
- The histology of the Loop of Henle varies:
- Descending limb: Cuboidal epithelia with microvilli.
- U-turn: Simple squamous epithelia, no brush border.
- Ascending limb: Cuboidal epithelia, no microvilli.
Distal Convoluted Tubule (DCT) & Collecting Ducts
- The DCT continues from the ascending loop of Henle and empties into collecting ducts.
- Collecting ducts transport tubular filtrate through the medulla to the calyces, renal pelvis, and ultimately to the ureter.
- Important functions occur within collecting ducts:
- Urine volume regulation (ADH action).
- Potassium (K) regulation.
- Acid-base balance.
Renal Nerve Supply
- Kidneys are innervated by the sympathetic nervous system.
- Sympathetic stimulation causes:
- Vasoconstriction of renal vessels.
- Increased renal blood pressure.
- Increased filtration.
Renal Blood Supply
- Approximately 25% of the cardiac output goes to the kidneys.
- Blood flow through the kidneys:
- Renal artery: Branches directly from the abdominal aorta and enters the kidney at the hilus.
- Afferent glomerular arterioles: Carry blood to the glomerular capillaries.
- Glomerular capillaries: Filter plasma to form glomerular filtrate.
- Efferent glomerular arterioles: Carry filtered blood (still oxygenated!) away from the glomerulus.
- Peritubular capillaries: Formed by branching of efferent glomerular arterioles; responsible for O2 transfer, tubular reabsorption, and secretion.
- Renal vein: Formed by the convergence of veins originating from the peritubular capillaries.
Mechanisms of Renal Action
- Filtration: The process of removing substances from the blood through the glomerular capillaries.
- Reabsorption: The process of returning filtered substances from the renal tubules to the bloodstream.
- Secretion: The process of adding substances to the tubular filtrate that were not adequately removed during filtration.
- Urine volume regulation: The ability to adjust urine output by altering the reabsorption of water.
- Regulation of blood pressure: Specialized mechanisms help maintain blood pressure within a normal range.
Filtration
- Filtration occurs in the renal corpuscle.
- The glomerular capillaries have high blood pressure due to the difference in size between the afferent and efferent arterioles (afferent is larger).
- Glomerular fenestrations (pores) are larger than in other capillaries, allowing filtration of plasma components.
- Glomerular filtrate is similar to plasma, but lacks proteins.
Glomerular Filtration Rate (GFR)
- GFR is the rate at which plasma is filtered through the glomerulus.
- GFR depends on renal blood flow.
- GFR is measured in mL/min.
- Approximately 25% of plasma is filtered each minute.
Reabsorption
- Reabsorption occurs as substances move from the renal tubules back into the bloodstream via peritubular capillaries.
- Mechanisms of reabsorption include:
- Osmosis
- Diffusion
- Active transport
- Tubular lumen → through tubular epithelium → interstitial fluid → peritubular capillary lumen.
Reabsorbed Substances
- Na (sodium)
- K (potassium)
- Ca (calcium)
- Mg (magnesium)
- Glucose
- Amino acids
- Cl (chloride)
- Bicarbonate (HCO3)
- Water
Reabsorption: Specific Pathways
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Sodium:
- PCT: Active transport pumps in and out of epithelial cells.
- Ascending loop of Henle: Exchanged for H, ammonium or K.
- DCT: Similar to ascending loop of Henle; exchange influenced by aldosterone.
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Glucose and amino acids:
- PCT: Co-transported with sodium into epithelial cells, then diffuse passively out.
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Potassium:
- Diffuses out of the tubule into peritubular capillaries; reabsorbed in the PCT, ascending loop of Henle, and DCT.
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Calcium:
- Movement is influenced by vitamin D, PTH, and calcitonin.
- Reabsorbed in PCT, ascending loop of Henle, and DCT.
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Magnesium:
- Reabsorbed in PCT, ascending loop of Henle, and collecting duct.
- PTH increases Mg reabsorption.
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Chloride:
- Follow Na to maintain electrical neutrality.
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Water:
- Follows sodium, glucose, amino acids, and chloride due to osmotic pressure.
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Other substances (e.g., urea):
- Reabsorbed by passive diffusion across the concentration gradient due to water loss.
Reabsorption Summary:
- PCT: 65% of tubular reabsorption; 80% of water, sodium, chloride, and bicarbonate; 100% of glucose and amino acids.
- Loop of Henle, DCT, Collecting ducts: Contribute to remaining reabsorption.
Secretion
- Secretion removes substances not adequately eliminated by filtration alone.
- Peritubular capillaries → interstitial fluid → tubular epithelial cells → tubular filtrate.
- DCT: Primary site of tubular secretion.
- Secreted substances include:
- H (hydrogen ions)
- K (potassium)
- Ammonia
- Antibiotics (e.g., penicillin, some sulfas)
Urine Volume Regulation
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Antidiuretic hormone (ADH):
- Released from the posterior pituitary gland.
- Increases water reabsorption in the DCT and collecting ducts.
- No ADH = decreased water reabsorption = larger urine volume.
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Aldosterone:
- Released from the adrenal cortex.
- Influences sodium reabsorption in the distal tubule and collecting ducts, thereby affecting water reabsorption.
- Increased aldosterone = increased sodium and water reabsorption.
Regulation of Blood Pressure
- Kidneys detect changes in blood pressure via the renin-angiotensin-aldosterone system (RAAS).
- Juxtaglomerular cells (in afferent glomerular arterioles) monitor blood pressure.
- Macula densa (in ascending loop of Henle) monitor NaCl concentration in tubular filtrate.
- If blood pressure is low or sodium concentration is low, juxtaglomerular cells release renin.
RAAS Mechanism
- Renin: An enzyme that converts angiotensinogen to angiotensin I.
- Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme (ACE).
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Angiotensin II:
- Causes arterial constriction, increasing blood pressure.
- Stimulates aldosterone release from the adrenal glands.
- Aldosterone stimulates sodium and water reabsorption, further increasing blood pressure.
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Description
Test your knowledge on the microscopic anatomy of the kidney, focusing on the structures and functions of nephrons and renal corpuscles. Explore the differences in nephron counts across various species and the roles of glomerulus and Bowman's capsule. This quiz is essential for understanding kidney physiology.
