WEEK NINE - Functions of the urinary system and anatomy of the kidney Flashcards
State the components of the urinary system
SIX organs
2 x kidneys
2 x ureters
Urinary bladder
Urethra
Primary Function = prediction and excretion of urine
FVEBP
State and describe 5 functions of the kidneys other than urine formation
- Blood plasma filtration
Blood filter which excretes harmful toxic waste products - Regulation of blood volume
Conserves water or eliminates it in urine - Production of ERYTHROPOIETIN
Stimulate RBC production in red bone marrow - Regulation of BP
Kidneys secrete renin - helps bp by activating renin angiotensin-aldosterone pathway - Regulate blood pH
Kidneys excrete H+ and reabsorb HCO3 = helps maintain acid-base balance of body fluids
Describe the location and gross anatomy of the kidneys
kidneys = bean shaped retroperitoneal organs which lay lay against posterior abdominal wall at level of T12 –> L3
* R kidney = slightly lower due to R lobe of liver above
- lateral surface = convex
- medial surface concave w slit [hilum]
- ~150g
- 11cm long x 6cm wide x 3cm thick
THREE CT LAYERS
1. renal fascia [outer]
* binds to abdominal wall
- perirenal fat capsule [mid]
* cushions kidney and holds it in place - fibrous capsule [inner]
* encloses kidney - protects from trauma and infection
- the renal parenchyma consist of two zones = OUTER renal CORTEX + INNER renal MEDULLA
- renal columns divide the medulla into 6-10 pyramids, with each papilla found nestled in the minor calyx
- 2 or 3 minor calyx –> major calyx
- 2 or 3 major calyx –> **renal pelvis **
Name the major nitrogenous wastes and state their sources
- urea
made from protein catabolism of AA –> deamination of NH2 = ammonia [NH3] –> converted to urea by LIVER - uric acid
- nucleic acid catabolism - creatinine
creatine phospahte metabolism
Trace the flow of blood through the kidneys & identify the vessels
abdominal aorta > renal a. > segmental a. > interlobar a. > arcuate a. > cortical radiate a. > afferent arterioles > glomerulus > efferent arteriole > peritubular capillaries OR vasa recta > cortical radiate v./interlobular v > arcuate v > interlobar v > renal v. > IVC
Describe the nephron and its 2 parts (renal corpuscle and renal tubule)
NEPHRON = functional unit of kidneys - responsible for urine formation. found in renal pyramid ~ 1.2 million nephrons/kidney
- TWO types of nephrons
1. cortical
- found in OUTER 2/3. of cortex [~85% of nephrons]
- juxtamedullary
- inner 1/3 of cortex
- 15% of nephrons
**RENAL CORPUSCLE **
glomerulus
- specialised capillary network in phrone = filters plasma + produces filtrate [1st step in urine formation]
bowman’s capsule [around glomerulus]
- two layered capsule with capsular space
- visceral layer = comprised of podocyte cells which make up epithelial lining
**RENAL TUBULE **
- proximal convoluted tubule [PCT] = simple cuboidal epithelium w/ microvilli
- nephron loop
primarily in medulla w/ :
THICK segment –> simple cuboidal epithelium = active transport of saltsTHIN segment = simple squamous epithelium –> highly permeable to water - distal convoluted tubule [DCT]
found at end of nephron loop
simple cuboidal epithelium - NO microvilli - collecting duct
receives filtrate from multiple DCTs
CDs merge w/ eachother = papillary ducts
Describe the filtration membrane and the process by which the kidney filters the blood plasma
Almost any molecule < 3nm can pass freely through filtration membrane
Eg water, electrolytes, AAs, nitrogenous wastes and vitamins
- 99% reabsorbed = only 1/2L of urine is formed/day
THREE BARRIERS
1. Fenestrated endothelium of glomerular capillaries
- 70-90nm filtration pores
- HIGHLY permeable type of capilary
- allows passage of water, AAs, glucose, electroyltes
- EXCLUDES LARGER moleciles eg blood cells, plasma proteins from filtrate
- Basement membrane [proteoglycan gel]
- LARGER than 8nm
- repels some smaller molecules due to NEGATIVE CHARGE. eg albumin
- small protein bound solutes DONT pass - Filtration slits of podocyte
- pedicel extensions wrap around capillary to form barrier w/ 30nm filtration slits
- NEGATIVELY charged = repels anions
Explain the forces/factors that promote and oppose filtration
PROMOTES»_space;
Blood hydrostatic pressure
[BHP] = 60 mmHg outwards = much higher than most other capillaries. –> results from a large afferent arteriole vs small efferent arteriole at glomerulus
OPPOSES»
Capsular hydrostatic pressure
[CP]= 18 mmHg inwards pressure
- results from high rate of filtration and accumulation of fluid in capsule
Colloid Osmotic Pressure
[COP]
- 32 mmHg inwards pressure
- filtrate is almost protein free hence NO significant COP
Net filtration pressure [NFP] = 60-32-18=10 mmHg
- 60 mmHg OUT [blood hydrostatic pressure]
- 32 mmHg IN [colloid osmotic pressure]
-18 mmHg IN [capsular pressure]
= net filtration pressure [NFP] = 10 mmHg OUT
Describe how renal autoregulation controls glomerular filtration rate
= Ability of nephron to adjust to own BF and GFR w/o external control
TWO MECHANISMS
**1. Myogenic Mechanism **
- based on smoo.musc contraction tendency when stretched
- aims to regulate BF ensuring relatively constant production of urine
^ BP = stretches afferent arterioles = vasoconstriction
v BP in afferent arteriole = vasodilation
2. Tubuloglomerular Feedback
- HIGH GFRs = LESS reabsorption of NaCl in renal tubules
- Macula densa detects ^ in fluid flow and ^ NaCl levels in DCT
- Macula densa secretes paracrine signal –> stims juxtaglomerular cells [JG] = inhibit release of nitric oxide = vasoconstriction of afferent arteriole
Negative Feedback Control of
GFR
1. High GRF
2. Rapid flow of filtrate in renal tubules + ^ NaCl reabsoprtion
3. Sensed by macula densa cells
4. Paracrine secretion –> JG cells [inhibits release of nitric oxide]
5. Constriction of afferent arteriole
6. Reduced GFR
Describe how the nervous system controls glomerular filtration rate
symp. nerve fibres richly innervate renal blood vessels >
strenuous exercise, circulatory shock and fight/flight responses stimulate vasoconstriction of afferent arterioles
- because blood is redirected to heart/brain/skeletal muscles –> GFR/urine output = DECREASED
symp.control can also initiated renin release when BP is decreased
Describe how the renin-angiotensin-aldosterone mechanism acts to control blood pressure
if BP DECREASES dramatically > JG cells secrete renin
>renin converts angiotensinogen to angiotensin I
>angiotensin I converted to active hormone - angiotensin II by angiotensin-convertine-enzyme [ACE] in lungs
**angiotensin II **
> potent vasoconstrictor = ^ mean arterial BP bodywide
> constricts aff.art. = ^/maintains GFR desite low BP
> constriction of eff.art = v BP in peritubular caps = ^ NaCl reabsoprtion/H20 retention = ^ body wide BP
> stimulates adrenal cortex to secrete aldosterone in DCT and CD = ^ Na reabsorption/H2o retention
>stims post.pit to secrete ADH = stims H2o reab. in DCT, aquaporin synthesis in CD
> stimulates water intake via thirst
** ^ NaCl/Na/ H2O reabsorption/retention = ^ BP**
Describe tubular reabsorption and how it takes place
Removes useful water/solutes [eg NaCl, water, Glucose] from filtrate → returning them to blood
- PCT reabsorbs greater variety of chemicals than any other part of nephron and reabsorbs 65% of filtrate to preitubular capps
> great length, prominent microvilli and abundant mitochondria = active transport
Routes of tub.reab.
1. transcellular
- pass through cytoplasm via aquaporins
-
paracellular
- substances pass through leaky tight junctions between cells - water abosrbed by osmosis carries some other solutes [solvent drag]
NaCl reabsorption [two mechanisms]
> uses symports which bind Na+ to another solute eg AA, glucose
> antiports which pull Na+ into cell whilst H+ is pumped out [Na+H+ antiport is activated by** angiotensin II **= IMPORTANT INFLUENCE ON REABSOPRTION]
Water
>reabsorbed both by transcellular route via aquaporins
>also reabsorbed by paracellular route via osmosis
>2/3 of water is reabsorbed in PCT at constant rate [obligatory water reabsorption]
Glucose
> co-transported into epithelium w/ Na+ via Na-glucose transporteres
> 100% reabsorption in healthy subjects
Describe tubular secretion and state solutes that are excreted
tub.sec. = removes additional wastes from peritubular capillary blood –> secretes them to filtrate
> eg urea, creatinine, bile salts, ammonia, catecholamines, drugs [morphine, penicillin, aspirin]
tub.sec. = TWO functions
1. waste removal
2. acid-base balance [regulate pH]
- ^ secretion of H+ and ^ reabsorption of HCO3- = INCREASED PH
Describe how the nephron regulates water conservation
nephron = responsible for regulating water cons. reab useful substances and excreting waste products
- PCT
- majority [65%] of water is reab here due to great length, microvilli and mitochondria which aid active transport - loop of Henle/ nephron loop
- countercurrent exchange mechanism creates osmotic/salinity gradient from 300 mOsm/L in the cortex to 1200 mOsm/L in the medulla
- thin desc limb = permeable to water = water reab –> interstitial fluid = ^ osmo
- thick asc limb permeable to solutes NOT water = NaCl pumped out = ^ osmo
- as water moves from low–> high osmolarity, the countercurrent exchange mechanism promotes water reab
- think portion of loop reab. 15% water from filtrate
- reabsorbs 25% of electrolytes - Na+, K+, Cl- remaining in filtrate
[K+ = secreted back –> tubular fluid via Na+/K+ pump /// NaCl stays in tissue fluid of medulla]
DCT
- fluid arriving in DCT still contains 20% of the water and 7% of the salts from filtrate
- further water reab. occurs, as regulated by hormones such as ADH, aldosterone, angiotensin II, ANP, PTH
CD
- the permeability of the CD is controlled by ADH which is secreted in events of dehydration.
- ADH synthesises aquaporins in the CD making the membrane more permeable to water and promoting water reab.
- as the filtrate travels down the increasingly salty medulla water is reab. = via osmosis
DCT and CD reabsorb variable amounts of water/salt as regulated by hormones - aldosterone, atrial natriuretic peptides [ANP], ADH, parathyroid hormone [PTH]
Discuss the roles of aldosterone, atrial natriuretic peptide (ANP) and parathyroid hormone (PTH) on kidney function
ALDOSTERONE [salt retaining steroid hormone secreted by adrenal cortex on kidneys]
- acts to MAINTAIN/^ BP **
- secreted when Na+ FALLS or K+ rises in blood [LOW BP]
- also secreted in renin-angiotensin-aldosterone mechanism from drop in BP [angiotensin II **stimulates aldosterone secretion]
- acts on thick segment of nephron loop, DCT and cortical portion of CD
-
promotes Na+/water/Cl- reab. and K+ sec.
-effect = body retains NaCl/water = **maintains BP/volume **
ANP [sec. - atrial myocardium of heart from ^ BP]
- act to REDUCE BP/VOL by excreting more salt/water in urine
1 dilates aff.art, constricts eff.art = **^ GFR **
2 inhbit renin/aldosterone secretion
3 inhibit secretion of **ADH **
4 inhibits NaCl reabsorption by CD
PTH ** [secreted by parathyroid glands from Ca defieincy -hypocalcemia**]
- acts on PCT to **^ phosphate excretion **
- act on **thick **segment of ascending nephron loop + DCT to **^ Ca reabsoprtion **
- ^ phosphate content and **LOWERs Ca+ content in urine = ^ Ca. Conc in blood **= maintains calcium homeostasis
