Urinary System Structure and Function

Comprehensive practice flashcards detailing kidney anatomy, nephron histology, glomerular filtration dynamics, tubular countercurrent mechanisms, and endocrine regulation of urinary function.

What are the primary homeostatic functions performed by the urinary system?

Regulating blood volume and blood pressure; regulating plasma solute concentrations ($\text{Na}^+$, $\text{Cl}^-$, $\text{K}^+$, $\text{Ca}^{2+}$, $\text{HPO}_4^{2-}$); stabilizing extracellular fluid pH by secreting $\text{H}^+$; conserving nutrients; assisting the liver in detoxification; stimulating blood cell synthesis; and synthesizing vitamin D.

What is the complete sequence of arterial blood flow from the heart into the microscopic structures of the kidney, and the venous return back to the heart?

Aorta $\rightarrow$ Renal artery $\rightarrow$ Segmental artery $\rightarrow$ Interlobar artery $\rightarrow$ Arcuate artery $\rightarrow$ Cortical radiate artery $\rightarrow$ Afferent arteriole $\rightarrow$ Glomerulus $\rightarrow$ Efferent arteriole $\rightarrow$ Peritubular capillaries or Vasa recta $\rightarrow$ Cortical radiate vein $\rightarrow$ Arcuate vein $\rightarrow$ Interlobar vein $\rightarrow$ Renal vein $\rightarrow$ Inferior vena cava.

How do cortical nephrons and juxtamedullary nephrons differ in proportion, structure, and primary function?

Cortical nephrons make up $85\%$ of nephrons, reside primarily in the superficial cortex, have relatively short nephron loops, and perform reabsorption and secretion. Juxtamedullary nephrons make up $15\%$ of nephrons, have long loops extending deep into the renal medulla surrounded by vasa recta, and function in urine concentration.

What are the four layers found in the histological wall of the ureter and urinary bladder?

Transitional epithelium, connective tissue (lamina propria), smooth muscle layer (detrusor muscle in the bladder), and outer connective tissue (adventitia).

What are the three anatomical divisions of the male urethra?

The prostatic urethra, the membranous urethra, and the spongy (penile) urethra.

What are the three most abundant organic waste products removed by the kidneys, and what are their metabolic origins?

1. Urea: formed from amino acid breakdown (most abundant).\n2. Creatinine: generated by the breakdown of creatine phosphate in skeletal muscle.\n3. Uric acid: formed during the recycling of nitrogenous bases from RNA.

What are the four essential physiological processes involved in urine production?

1. Glomerular filtration: creates a plasma-like filtrate from blood.\n2. Tubular reabsorption: returns water and useful solutes from the filtrate back into the blood.\n3. Tubular secretion: transfers additional wastes from the blood into the tubular fluid.\n4. Water conservation: removes water from tubular fluid and concentrates wastes.

What three structural layers constitute the glomerular filtration membrane, and what are their filtration limits?

1. Fenestrated capillary endothelium: pores $60\text{--}100\,\text{nm}$ wide that prevent the passage of blood cells.\n2. Basement membrane (dense layer): blocks larger proteins.\n3. Filtration slits: gaps $6\text{--}9\,\text{nm}$ wide between podocyte foot processes that prevent the entry of small plasma proteins.

What is the formula and standard value for Net Filtration Pressure (NFP) across the glomerular capillaries?

$\text{NFP} = \text{GHP} - \text{BCOP} - \text{CsHP}$, which normally results in an average net outward filtration pressure of approximately $10\,\text{mm\,Hg}$.

What is Glomerular Filtration Rate (GFR), and how is it clinically estimated?

GFR is the total amount of filtrate produced each minute by both kidneys combined; it is clinically estimated using a plasma clearance test with creatinine or inulin.

What are the transcellular and paracellular routes of reabsorption in the proximal convoluted tubule (PCT)?

In the transcellular route, substances travel directly across the apical and basal membranes of PCT epithelial cells. In the paracellular route, substances pass between adjacent cells via leaky junctions, often carried along with moving water through solvent drag.

What are tubular maximum ($T_m$) and renal threshold, and what are the specific renal threshold values for glucose and amino acids?

Tubular maximum ($T_m$) is the maximum transport rate reached when all carrier proteins are saturated. Renal threshold is the plasma concentration at which a substance exceeds $T_m$ and begins spilling into urine; the renal threshold is approximately $180\,\text{mg/dL}$ for glucose and approximately $65\,\text{mg/dL}$ for amino acids.

What cellular transport mechanisms accomplish potassium ($\text{K}^+$) secretion in the distal convoluted tubule (DCT)?

Active transport moves $\text{K}^+$ from peritubular capillaries into tubular epithelial cells via basolateral $\text{Na}^+/\text{K}^+$ exchange pumps, followed by passive diffusion of $\text{K}^+$ into tubular fluid through apical potassium leak channels.

How do renal tubule cells regulate acid-base balance when peritubular fluid pH drops (acidosis)?

Tubule cells generate $\text{H}^+$ and $\text{HCO}_3^-$ via carbonic anhydrase; they actively secrete $\text{H}^+$ into tubular fluid (often coupled with $\text{Na}^+$ countertransport) to acidify urine while reabsorbing $\text{HCO}_3^-$ into peritubular blood to elevate systemic pH.

What defines clinical acidosis and alkalosis, and which condition occurs more frequently?

Acidosis is defined as a plasma $\text{pH} < 7.35$, whereas alkalosis is a plasma $\text{pH} > 7.45$. Acidosis is more common because normal cellular metabolic activities continually generate acids.

What is the difference between obligatory water reabsorption and facultative water reabsorption?

Obligatory water reabsorption cannot be prevented, recovers $85\%$ of filtrate volume, and occurs in the PCT and nephron loop. Facultative water reabsorption is hormonally regulated (by ADH, aldosterone, and natriuretic peptides), accounts for the remaining $15\%$ ($27\,\text{liters/day}$), and occurs in the DCT and collecting system.

How do the descending and ascending limbs of the nephron loop differ to create the countercurrent multiplier?

The thin descending limb is permeable to water but impermeable to solutes, allowing water to exit osmotically and concentrating tubular fluid up to $1200\,\text{mOsm/L}$. The thick ascending limb is impermeable to water and actively transports $\text{Na}^+$, $\text{K}^+$, and $\text{Cl}^-$ into the medullary ECF via $\text{Na}^+\text{--}\text{K}^+/2\text{Cl}^-$ cotransporters, diluting the remaining tubular fluid down to $100\,\text{mOsm/L}$.

How does urea recycling contribute to the osmotic concentration gradient of the deep renal medulla?

Urea diffuses out of concentrated tubular fluid in the deep collecting duct and papillary ducts into the medullary interstitium, then diffuses into the thin descending limb of the nephron loop, continually recycling solute through the inner medulla.

What role does the vasa recta play in countercurrent exchange without washing out medullary hyperosmolality?

As blood flows down the descending vasa recta, water diffuses out and solutes diffuse in; as blood flows up the ascending vasa recta, salt diffuses back into the interstitium and water is reabsorbed by blood colloid osmotic pressure (BCOP), yielding a net gain of water while preserving the medullary salt gradient.

How does the myogenic autoregulatory mechanism respond to an increase versus a decrease in renal blood pressure?

Increased renal blood pressure stretches vascular smooth muscle, causing afferent arterioles to constrict and decreasing glomerular blood flow to prevent GFR elevation. Decreased blood pressure reduces stretch, triggering afferent arteriole dilation and efferent arteriole constriction to sustain glomerular blood flow and GFR.

What is the sequence of events in tubuloglomerular feedback when GFR becomes elevated?

High GFR $\rightarrow$ Increased $\text{NaCl}$ delivery to the nephron loop $\rightarrow$ Macula densa senses $\text{NaCl}$ load and secretes ATP $\rightarrow$ Mesangial cells metabolize ATP to adenosine $\rightarrow$ Adenosine stimulates granular (juxtaglomerular) cells $\rightarrow$ Afferent arteriole constricts $\rightarrow$ Reduced GFR back to normal.

What effect does strong sympathetic nervous system stimulation have on kidney hemodynamics and GFR?

Norepinephrine and epinephrine trigger marked vasoconstriction of afferent arterioles, which dramatically lowers GFR, decreases urine output, redirects blood flow to vital organs, and stimulates renin release.

What cascade converts angiotensinogen to angiotensin II in the RAAS pathway?

Liver produces angiotensinogen ($453$ amino acids long) $\rightarrow$ Renin secreted by the kidney cleaves it to Angiotensin I ($10$ amino acids long) $\rightarrow$ Angiotensin-converting enzyme (ACE) from the lungs converts it to Angiotensin II ($8$ amino acids long).

How does Aldosterone alter transport mechanics in the distal convoluted tubule and collecting duct?

Aldosterone binds to intracellular receptors and stimulates the synthesis and membrane incorporation of basolateral $\text{Na}^+/\text{K}^+$ pumps and apical $\text{Na}^+$ channels, promoting sodium and water reabsorption while causing potassium excretion.

What are the stimuli for and actions of Natriuretic Peptides (NP) on renal function?

Stimulated by elevated blood volume and pressure, NPs dilate afferent arterioles and constrict efferent arterioles to increase GFR, while simultaneously inhibiting RAAS, ADH secretion, and collecting duct $\text{NaCl}$ reabsorption, thereby increasing sodium and water excretion.

What is the intracellular signaling pathway through which Antidiuretic Hormone (ADH) increases water permeability in collecting duct principal cells?

ADH binds basolateral V2 receptors $\rightarrow$ G protein activates adenylate cyclase $\rightarrow$ ATP is converted to cyclic AMP (cAMP) $\rightarrow$ Vesicles carrying aquaporin-2 are inserted into the apical membrane, allowing water to flow down the medullary osmotic gradient into the cell and back to peritubular capillaries via basolateral aquaporin-3 or aquaporin-4 channels.

What are the osmotic concentration differences in tubular fluid when urine is produced in the absence versus the presence of ADH?

Without ADH, collecting ducts are water-impermeable, causing fluid to remain dilute and resulting in large volumes of dilute urine ($\sim 100\,\text{mOsm/L}$). With ADH, water is reabsorbed, concentrating urine up to $1200\text{--}1400\,\text{mOsm/L}$ at the minor calyx.