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Describe the structure and function of the macula densa.
- Columnar epl thickening of distal tubule
- Responds to high volume dilute (low salt) urine (=diuresis)
- Leads to gap junction-mediated stimulation of juxtoglomerular cells
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What are juxtaglomerular cells?
- Modified pericytes of glomerular arterioles
- Secrete renin into the bloodstream
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What functions does renin have in the urinary system?
- Renin initiates the 'Angiotensin Cascade' which increases the amount of aldosterone and ADH/vasopressin in the blood
- Renin cleaves angiotensinogen into angiotensin
- Through its pathways, it decreases urine volume as water exits the collecting tubules to enter the increasingly salty medulla
- Leads to Anti-Diuretic state
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What is the function of aldosterone?
Aldosterone stimulates NaCl pumps in the ascending loop and distal tubule. This makes the medulla more salty, and decreases urine volume.
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What is the function of ADH/vasopressin?
ADH/vasopressin elevates the H20 permeability of the collecting tubules, reducing urine volume.
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What are the general functions of the kiddneys?
- Produce glomerular filtrate- similar to plasma minus the proteins
- Modify glomerular filtrate to produce urine
- Functions are carried out by the uriniferous tubules of the kidney
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What two categories of tubules make up the uriniferous tubules of the kidney, and where are they found?
- Nephron (in Cortex and Medulla)
- Collecting tubules (from cortex to medulla)
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What are the four components of a nephron?
- Renal corpuscles
- Proximal tubule
- Loop of Henle
- Distal tubule
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What is the function of the renal corpuscles?
in cortex, produce glomerular filtrate from blood; released into 'Bowman's space'
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What is the function of the proximal tubule?
move from cortex to medulla, resorbs majority of NaCl/H2O via membrane pumps and channels; resorbs majority of glucose via vesicular transport
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What is the function of the Loop of Henle?
in medulla, becomes increasingly H2O impermable as you move along the tube; NaCl actively pumped out of the tubule into peritubular CT of medulla
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What is the function of the distal tubule?
move from medulla into cortex, modifies urine via macula dense which is part of the juxtaglomerular apparatus producing dilute, high volume urine
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What is the function of the collecting tubules?
- Respond to ADH/vasopressin by inserting Aquaporin/water channels in plasma membrane that faciliate water flow out of tubule into salty medulla down osmotic gradient
- Aquaporins are inserted via a G-protein coupled signalling pathway triggered by ADH and involving cAMP/PKA
- Drain into Renal Pelvis in medulla which leads to the ureters that leave the kidney
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What type of epithelium lines the collecting tubules?
Simple cuboidal epl/low columnar
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What are vasa recta?
straight peritubular blood vessels in the medulla of the kidneys
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What is the renal pelvis?
- expanded, funnel-like opening of the ureter
- Collecting ducts drain into it
- Modification of urine is done by the time it reaches the renal pelvis- just storage and transportation from this point on
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Describe the structure of the ureters.
- Distensible mucosa: Transitional epithelium (urothelium) and fibroelastic l. propria
- Peristaltic muscularis externa: Longitudinal (inner) and circular (outer) layers of smooth muscle
- Adventitia
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What are three characteristics of transitional epithelium?
- Modulatable membrane domains (pleated/open/closed)
- Cytoskeleton is very dynamic to facilitate cell shape changes with distension
- Desmosomes prominent to hold/anchor cells together
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What are kidney stones?
- Calculi- Calcium salts/uric acid
- Appear white on X-ray/CT scan
- can pass due to peristaltic contraction
- Laser treatments, enzymatic treatments, or surgery
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Describe the embryology of the urinary system.
- Paired kidneys and ureters= mesoderm
- Midline bladder and urethra= endoderm
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What is the function of the bladder?
Stores urine until expulsion (micturition/urination) through urethra
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Describe the structure of the bladder.
- Distensible mucosa: transitional epithelium and fibroelastic l. propria
- No discernable submucosa
- Muscularis externa: interlaced smooth muscle - non-peristaltic contraction for micturition
- Adventitia
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Describe the structure of the urethra.
- Epl: Proximal has transitional epl, while distal has stratified squamous, non-keratinized epl
- L. propria: fibroelastic with lubricating mucous glands
- Submucosa: erectile tissue containing helicine arteries and venous sinuses
- Muscularis externa: inner longitudinal and outer circular layers of smooth muscle. Two thickenings (sphincters)
- Internal sphincter at the bladder-urethra opening is smooth muscle
- External sphincter wraps around the proximal urethra and is skeletal muscle
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Compare the female and male urethras.
- Female- conveys urine only, no direct contact with reproductive tract, less erectile tissue
- Male- conveys urine and semen, more erectile tissue
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