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Which of the following is NOT a regulatory function of the kidneys?
What substance do the kidneys primarily excrete to regulate acid-base balance?
Which component of the nephron is primarily responsible for the filtration of blood?
Which hormone is most likely to increase renal blood flow?
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What is the primary function of the juxtaglomerular apparatus?
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What is the primary role of the kidneys in regulating plasma volume?
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Which of the following substances is considered a waste product excreted by the kidneys?
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Which mechanism is primarily responsible for autoregulation of renal blood flow?
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Which hormone is responsible for stimulating red blood cell production?
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What is the primary role of renin in the kidneys?
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Which type of nephron is primarily responsible for concentrating urine?
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What hormone secreted by the kidneys is essential for calcium absorption?
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During prolonged fasting, what metabolic function do the kidneys perform?
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What structural feature characterizes juxtamedullary nephrons?
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What function do the juxtaglomerular cells serve?
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Which of the following options is NOT a function of the kidneys?
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What role does the macula densa play in the kidneys?
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What percentage of cardiac output is directed to the kidneys in a resting adult?
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What is the primary purpose of the additional blood flow to the kidneys?
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What is autoregulation in the context of renal blood flow and glomerular filtration rate?
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Which mechanism involves the contraction of vascular smooth muscle in response to stretching?
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What substance does the macula densa secrete in response to increased sodium chloride loads?
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What happens to renal blood flow when arterial pressure exceeds 200 mm Hg?
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Which of the following structures is involved in the phagocytic activity in the kidneys?
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What is the primary role of autoregulation in relation to glomerular filtration rate (GFR)?
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Which hormone is primarily responsible for causing vasoconstriction of both afferent and efferent arterioles?
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How does the sympathetic nervous system influence renal blood flow (RBF) and glomerular filtration rate (GFR)?
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Which substance is produced within the kidneys and causes vasoconstriction of the afferent arteriole?
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What is the effect of nitric oxide on renal arterioles?
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What condition may lead to increased production of endothelin?
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What is a primary consequence of decreased renal blood flow (RBF)?
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Prostaglandins may play a role in which of the following scenarios?
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Study Notes
The Functions of Kidneys
- Control of electrolyte balance: Kidneys regulate the concentration and quantity of critical electrolytes, including sodium, chloride, potassium, and calcium. This ensures the body maintains balance and the proper levels of these substances in the extracellular fluid.
- Water balance regulation: The kidneys maintain water balance, which is crucial for regulating extracellular fluid osmolarity. This is essential for maintaining stable cell volume, preventing cells from shrinking or swelling due to water movement.
- Plasma volume regulation: Kidneys help maintain healthy plasma volume, a key factor in long-term blood pressure regulation. This is achieved by controlling salt balance in the body; sodium and chloride account for over 90% of the osmotic activity of the extracellular fluid.
- Acid-base balance regulation: The kidneys work alongside the lungs and body fluid buffers to maintain proper pH. This is accomplished by eliminating excess acid (H+) or base (HCO3-) in the urine.
- Excretion of metabolic waste products: The kidneys eliminate a wide range of waste products, including urea from protein breakdown, uric acid from nucleic acid breakdown, and creatinine from muscle creatine.
- Excretion of foreign chemicals: The kidneys eliminate numerous foreign substances from the body, such as drugs, food additives, pesticides, and other non-nutritive substances.
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Endocrine functions: Kidneys secrete three essential hormones:
- Erythropoietin (EPO): A hormone produced by the kidneys stimulates red blood cell production in the bone marrow. EPO production is crucial for ensuring the blood carries an adequate amount of oxygen.
- Renin: An enzyme produced by the kidneys initiating the renin-angiotensin-aldosterone system, instrumental in controlling sodium reabsorption in the kidney tubules. This system is vital for long-term regulation of blood pressure and plasma volume.
- 1,25-dihydroxycholecalciferol: The active form of vitamin D, which is essential for calcium absorption from the gastrointestinal tract. Calcium plays a vital role in numerous biological processes, so the kidneys play a vital function in calcium homeostasis.
- Metabolic functions: During extended fasting, kidneys participate in gluconeogenesis, producing glucose from amino acids and other precursors, releasing it into the bloodstream.
Structure of the Nephron
- Two distinct types of nephrons exist:
- Cortical nephrons: These nephrons originate in the outer cortex and have short loops of Henle that extend only slightly into the medulla.
- Juxtamedullary nephrons: These nephrons arise from deep within the cortex, near the medulla. They have long loops of Henle that extend into the entire medulla, essential for urine concentration when the body needs to conserve water.
The Juxtaglomerular Apparatus (JGA)
- The JGA is a critical structure located where the ascending limb of the loop of Henle passes between the afferent and efferent arterioles supplying the glomerulus. It comprises:
- Macula densa: Specialized cells in the wall of the ascending limb of the loop of Henle that detect flow and composition of tubular fluid.
- Juxtaglomerular cells (granular cells): These cells are located in the wall of the afferent arteriole, responsible for secreting renin.
- Lacis cells (extraglomerular mesangial cells): These cells have phagocytic activity and contribute to the overall function of the JGA.
The JGA's Functions
- The JGA plays a crucial role in:
- Regulation of renin secretion: Juxtaglomerular cells release renin, which is essential for blood pressure regulation and sodium balance.
- Regulation of glomerular filtration rate (GFR): The macula densa senses changes in tubular fluid flow and composition, adjusting GFR in response.
Renal Blood Flow (RBF and RPF)
- The combined blood flow through both kidneys in a resting adult is approximately 1200 ml/min, representing about 22% of the cardiac output. This robust blood flow is necessary to ensure adequate plasma supply for the high rates of glomerular filtration required for precise fluid and solute regulation.
Regulation of RBF and GFR
- The kidneys regulate their blood flow through adjusting vascular resistance, responding to changes in arterial pressure. This process, known as autoregulation, maintains a relatively constant blood flow even when arterial pressure fluctuates between 90 and 200 mm Hg.
- GFR is also regulated over the same range of arterial pressures.
Autoregulation of RBF and GFR
- Autoregulation is crucial for maintaining stable excretion rates of solutes and water in the face of fluctuating blood pressure. Without this mechanism, significant variations in GFR would occur, leading to imbalanced excretion.
- Autoregulation occurs in denervated kidneys (e.g., transplanted kidneys) and isolated, perfused kidneys, indicating that it is not dependent on nerve supply or blood-borne substances.
Mechanisms of Autoregulation
- Two key mechanisms contribute to the autoregulation of RBF and GFR:
- Myogenic mechanism: This mechanism is based on the intrinsic property of vascular smooth muscle to contract when stretched. When arterial pressure increases and the renal afferent arteriole stretches, the smooth muscle contracts, narrowing the vessel and resisting increased flow.
- Tubuloglomerular feedback (TGF): When arterial blood pressure increases within the autoregulatory range, both RBF and glomerular capillary pressure (PGC) rise, leading to increased GFR. As the flow through the macula densa increases, the macula densa senses the higher NaCl delivery and releases a vasoconstrictor substance, adenosine. Adenosine then constricts the afferent arterioles, reducing both RBF and GFR back to normal.
Importance of Autoregulation
- Autoregulation significantly reduces large changes in excretion rates of water and solutes that would otherwise occur due to blood pressure fluctuations.
Neural Control of RBF and GFR
- Sympathetic nervous system: The sympathetic nervous system exerts control over RBF and GFR through its innervation of the afferent arterioles. This extrinsic control plays a role in regulating blood pressure. For example, if plasma volume decreases, such as during hemorrhage, the fall in blood pressure is detected by arterial baroreceptors. These receptors trigger increased sympathetic activity to the heart and blood vessels. This results in vasoconstriction of the afferent arterioles in the kidneys, decreasing both RBF and GFR. This helps to maintain blood pressure but often leads to a reduction in urine production.
Hormonal and Paracrine Control of RBF and GFR
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Vasoconstrictors:
- Angiotensin II: A potent vasoconstrictor, angiotensin II constricts both the afferent and efferent arterioles, reducing RBF and GFR. Angiotensin II production is increased in situations of decreased arterial pressure or volume depletion (e.g., dehydration). This contributes to the body's effort to conserve fluid and increase blood pressure.
- Endothelin: Secreted by endothelial cells in the renal vessels, endothelin causes vasoconstriction of both the afferent and efferent arterioles, diminishing GFR and RBF. Endothelin production is elevated in disease states, such as diabetic nephropathy.
- Adenosine: Produced within the kidneys, adenosine has a vasoconstrictor effect on the afferent arteriole. This contrasts with its vasodilator effect on other vascular beds. Adenosine is critical in the tubuloglomerular feedback mechanism.
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Vasodilators:
- Nitric Oxide (NO): An endothelium-derived vasodilator, NO causes dilation of both the afferent and efferent arterioles. NO counteracts the vasoconstrictor actions of angiotensin II and catecholamines.
- Prostacyclin and Prostaglandin E2: These prostaglandins, while not critical regulators of RBF and GFR in healthy individuals, play a protective role in pathological conditions, such as hemorrhage. They act locally within the kidneys and can dampen the vasoconstrictor effects of the sympathetic nervous system and angiotensin II, preventing severe vasoconstriction and renal ischemia.
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Description
Explore the essential functions of kidneys in regulating electrolyte balance, water stability, plasma volume, and acid-base balance in the body. This quiz will help you understand how kidneys contribute to overall homeostasis and the significance of their role in health.
