Urinary System Flashcards
What type of organ is the kidney?
A compound tubular gland which separates metabolic wastes from blood and regulates composition of blood plasma
What covers the outside of the kidney?
A collagenous thin capsule of fibroblasts and myofibroblasts
What are renal corpuscles?
Part of the renal cortex, they include:
- Capillary glomerulus
- Bowman’s capsule
What is a glomerulus?
Capillary loop derived from afferent arterioles and drained by efferent arteriole
What are renal columns?
Cortical tissue found in the medulla, extending between pyramids
What are medullary rays?
Bundles of tubules in the cortex which extend into the medulla. Contains the proximal and distal straight tubules plus collecting ducts.
What comprises the medulla of the kidney? How is it organized / drained?
Inner portion composed of tubules and collecting ducts.
They group into pyramids, which converge to form a papilla. The papilla has a rounded apex which projects into a minor calyx?
What makes up a kidney lobe and how many are there?
A medullary pyramid and the cortical substance associated with it - 8-18 of these in humans
What makes up a kidney lobule?
A medullary ray (including straight portions of proximal and distal tubules and their collecting ducts) and the cortical tissue associated with it
What is a uriniferous tubule?
The nephron (from bowman’s capsule to distal convoluted tubule) + collecting duct
What are the parts of a nephron?
- Bowman’s capsule - blind-ended, indented by glomerular capillary
- Proximal thick tubule - convoluted and straight
- Thin limb - descending and ascending
- Distal thick tubule - straight and convoluted
What are the parts of the loop of Henle?
Proximal straight tubule, descending thin limb, ascending thin limb, distal straight tubule
What are the two types of nephrons and how do they differ? Where are their renal corpuscles?
- Cortical / subcapsular - renal corpuscles are in outer cortex, with short loops of Henle not reaching farther than outer medulla. They do not concentrate urine significantly.
- Juxtamedullary - renal corpuscles are in corticomedullary junction. They have long loops of Henle going deep into pyramids to concentrate urine.
What is the path of blood flow from renal artery to glomerulus?
- Renal artery - enters at hilus
- Interlobar arteries - run between pyramids
- Arcuate arteries - interlobar arteries bifurcate at corticomedullary junction to form arteries running between base of pyramid and cortex
- Interlobular arteries - run radially in cortex between medullary rays
- Afferent arterioles - from interlobular arteries
- Glomerular capillaries - from afferent arterioles
What is interlobar vs interlobular arteries?
Interlobar - between pyramids
Interlobular - between medullary rays in cortex
What are arcuate arteries?
They are arteries from interlobar arteries which run at corticomedullary junction between cortex and base of pyramids
What are efferent arterioles and why is their size important?
They accept the blood from glomerular capillaries and drain individual renal corpuscles.
They are smaller than afferent arterioles so that filtration pressure is maintained
What do efferent arterioles in subcapsular glomeruli form? Where do they drain?
Peritubular capillary networks around tubules in the cortex, and then drain into arcuate veins
What do efferent arterioles in juxtamedullary glomeruli form? Where do they drain? Why is this physiologically important?
They form vasae rectae arteriae (straight arteries) into the medulla where they make hairpin turns and ascend as venous vasae rectae and join arcuate veins. This forms the counter-current exchange system.
What is a renal corpuscle and what leaves at its two poles?
Place of filtration for plasma from glomerular capillary, which is fenestrated and branched.
Vascular pole: Site of entry of afferent arteriole and exit of efferent arteriole
Urinary pole: Proximal convoluted tubule
What are the layers of Bowman’s membrane and what is it continuous with?
Parietal / Capsular layer: Make outside of Bowman’s capsule made of simple squamous epithelial cells
Visceral layer: Partially surround the capillary
Parietal layer is continuous with beginning of proximal convoluted tubule
What does a Bowman’s capsule enclose?
Urinary space, which contains provisional urine. Bounded by parietal and visceral Bowman’s layers.
What makes up the visceral layer of Bowman’s membrane?
Podocytes, which have interdigitating pedicels separated by slit spores spanned by slit membranes
What defines the glomerular endothelium?
Endothelium cells with OPEN fenestrae (no diaphragm) though some have diaphragms also present. Do not allow passage of molecules greater than 70 kD into the glomerular basement membrane
What makes up the glomerular basement membrane (what fused layers + what compounds)?
Fused basal lamina of endothelial cells and podocytes.
Usually appears trilaminar, and contains type 4 collagen, heparan sulfate, laminin, and fibronectin, giving it a negative charge.
What is the function of the negatively charged glomerular basement membrane?
Limits movement of negatively charged proteins which manage to get through the open fenestrae
What is the slit membrane? What protein holds it together?
The membrane between pedicles formed by the adhesion protein nephrin, it has a role in filtration.
What is special about the glycocalyx of podocytes?
It is also polyanionic, to prevent passage of negatively charged molecules into the filtrate
What causes most severe form of glomerular nephritis?
Auto-antibodies to glomerular basement membrane. However, antigen-antibody complexes can accumulate in GBM to produce acute symptoms
What causes congenital nephrotic syndrome and what is the major symptom?
Defect in gene producing nephrin, so the slit membrane doesn’t work properly. Children have low albumin in blood but high in urine (it is being lost to the urine filtrate)
What are the locations of the two types of mesangial cells?
Intraglomerular - within the stalk of capillary tuft
Extraglomerular - At vascular pole, not within capillaries. Their function is unknown
What are mesangial cells derived from?
They are pericyte-like contractile cells derived from smooth muscle cells, NOT monocytes, despite having phagocytic properies
What are the functions of intraglomerular mesangial cells?
Maintenance of glomerular basement membrane through phagocytosis. Also structural support, regulation of blood flow through glomerulus, and secretion of IL-1 and platelet derived growth factor (PDGF).
What is the general function of the proximal tubule?
Recovery of 2/3 of glomerular filtrate - water, ions, glucose, amino acids, small proteins
How is water uptaken in the proximal tubule?
Active reabsorption of Na+ as well as passive diffusion of Cl- makes water follow paracellularly or transcellularly. Absorption is aided by aquaporin-1 channels in plasma membrane of proximal tubule cells.
Where are dyes, drugs, and creatinine secreted?
Proximal tubule
What is the morphology proximal tubule cells and why?
Simple cuboidal epithelium:
Well developed brush border (apical microvilli) containing peptidases for reabsorption
Basal striations and lateral interdigitating folds of plasma membrane - cell boundaries cannot be seen, filled with mitochondria at base
Prominent endocytotic apparatus with pinocytotic vesicles, vacuoles, and lysosomes for intracellular degradation
What are proximal tubule cells located in close proximity to?
Peritubular capillaries -> fenestrated capillaries with diaphragms for reabsorption of material
What defines the epithelium of the thin limb and what surrounds it?
Epithelium is simple squamous. It is surrounded by the vasa rectae (both venous and arteriae)
How do the ascending and descending thin limb differ?
Descending: Concentrates urine by allowing passive of water into surrounding medulla
Ascending: Impermeable to water, pumps ions to increase the osmotic gradient in the interstitium / reuptake ions
How do diuretics act on the thin limb?
Inhibiting the Na+/K+ ATPase prevents the ascending limb from reabsorbing ions, leading to increased excretion of NaCl and water by urination
How can the distal tubule be distinguished from the proximal tubule?
Distal tubule has few short apical microvilli and not nearly as extensive of a brush border (less reabsorption). Also, it i has much less endocytotic activity at this point.
However, it still has lateral interdigitations and basal striations
How is the urine acidified and how is it under hormonal control?
Aldosterone causes reabsorption of Na+ in the distal tubule.
Bicarbonate it reuptaken here and also K+ and H+ are secreted. The secretions of potassium and hydrogen acidify the urine
What blood vessels surround the distal tubule in its straight + convoluted portions?
Straight - vasa rectae
Convoluted - peritubular capillaries
What are macula densa and how are their oriented?
Tall, narrow cells in wall of distal tubule that contact afferent / efferent arterioles. They have a reversed polarity and release their products to the lumen of the capillaries via their basal surface (would normally be the apical surface of endothelial cells_
What is the function of macula densa?
Cells respond to low sodium content in filtrate and lowered blood pressure, and release ATP, adenosine, nitric oxide, and prostaglandins to form part of the juxtaglomerular apparatus.
What is the function of the juxtaglomerular apparatus? What cells does it include?
Acts to regulate sodium reabsorption and blood pressure
Includes: Macula densa cells and juxtaglomerular cells
What are juxtaglomerular cells and what is their function?
Modified smooth muscle cells in arteriole wall that produce the protease called renin in response to signalling from macula densa cells.
What is the purpose of renin?
Catalyzes hydrolysis of angiotensinogen (in plasma) to angiotensin I.
Angiotensin I is converted into the lungs to angiotensin II via angiotensin converting enzyme.
What is the function of angiotensin II?
It is a potent vasoconstrictor (increases BP), and stimulates adrenal cortex to produce aldosterone. Aldosterone, of course, acts on the distal convoluted tubule to increase Na+ absorption, increasing blood pressure and volume
What are ACE inhibitors / ACE receptor blockers?
Drugs taken by hypertensive patients which disrupts renin-angiotensin-aldosterone pathway to prevent the body from increasing blood pressure.
What is the progression of ducts from the distal convoluted tubule to the minor calyx?
Distal convoluted tubule -> arched collecting tubules -> straight collecting ducts -> papillary ducts -> minor calyx
What are the two cell types of collecting tubules and what histologically defines the tubules in general?
- Light (principle) cells
- Dark (intercalated) cells
Ducts are pale-staining with few organelles and distinct cell margins. They go from simple cuboidal to columnar as the ducts increase in size.
What are light (principal) cells? What do they respond to?
Cells of collecting tubules, reabsorb Na+ and secrete K+, mainly in response to antidiuretic hormone (ADH).
This increases water uptake by insertion of aquaporin-2.
What are dark (intercalated) cells?
Cells which function to regulate acid-base balance by reabsorbing K+ and secreting H+ or HCO3- as needed
What is the significance of the collecting ducts running through the medulla?
The sodium gradient created by the juxtamedullary nephrons leads to osmotic pressure out of the nephron. Thus, the primary function of the collecting duct is reuptake of water from urine.
More ADH -> more aquaporins for reuptake of water -> more concentrated urine
How is blood sodium raised?
RAA pathway leads to distal tubule cells increasing their uptake of Na+ (action of aldosterone). This is transferred to blood via the peritubular fenestrated capillary network
What is diabetes inspidus?
Increased water consumption (polydipsia) and increased urination (polyuria) due to defect in ADH effectiveness. There will be no glucose in the urine.
Why do you urinate more during diabetes mellitus?
High concentration of glucose in blood acts as an osmotic diuretic in nephron. Glucose will be present in urine.
Give three functions of interstitial cells of the cortex and medulla?
- Production of erythropoietin - stimulate RBC production
- Production of thrombopoietin - stimulate platelet production
- Hydroxylate vitamin D precursor to hormonally active form
What type of epithelium lines the excretory passages and how does it change down to the bladder?
Transitional epithelium (urothelium) starting in minor calyces. Walls gradually thicken as passages near the bladder
What are the layers of the ureter + renal pelvis? How does the ureter near the bladder change?
- Mucosa with transitional epithelium + dense lamina propria
- Muscularis: inner longitudinal + outer circular layer (flipped from GI)
Lower portion of ureter has a final outer longitudinal layer added. - Adventitia
How does the wall of the female urethra muscularis layer differ from the ureter?
Urethra is much more muscular.
Internal urethral sphincter: Smooth muscle around origin of urethra from bladder, formed by circular muscularis layer of bladder
External urethral sphincter: Skeletal muscle arising from UG diaphragm which is voluntary
What defines the mucosa of the bladder?
- Transitional epithelium - 3-10 layers but thicker than ureter based on contraction or distension
- Interdigitated borders of cells facilitate distension
- Surface cells have thick plasma membrane plaques / invaginations which look like vesicles in parietal cells. They are reserve membrane in the apical cytoplasm to accommodate distension.
What defines the muscularis layer of the bladder?
Same as lower ureter - inner longitudinal, middle circular, outer longitudinal
What defines the mucosa of the urethra?
- Transitional epithelium near bladder, becoming stratified squamous and even stratified / pseudostratified columnar in spots. The lamina propria has many elastic fibers.
- Mucous glands of Littre (less numerous in females, however)
