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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
What are juxtaglomerular cells?
- Modified pericytes of glomerular arterioles
- Secrete renin into the bloodstream
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
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.
What is the function of ADH/vasopressin?
ADH/vasopressin elevates the H20 permeability of the collecting tubules, reducing urine volume.
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
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)
What are the four components of a nephron?
- Renal corpuscles
- Proximal tubule
- Loop of Henle
- Distal tubule
What is the function of the renal corpuscles?
in cortex, produce glomerular filtrate from blood; released into 'Bowman's space'
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
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
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
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
What type of epithelium lines the collecting tubules?
Simple cuboidal epl/low columnar
What are vasa recta?
straight peritubular blood vessels in the medulla of the kidneys
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
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
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
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
Describe the embryology of the urinary system.
- Paired kidneys and ureters= mesoderm
- Midline bladder and urethra= endoderm
What is the function of the bladder?
Stores urine until expulsion (micturition/urination) through urethra
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
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
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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