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Regulate chemical composition of body fluids
Eliminates waste
Controls composition of bloods – ion levels and concentration
Help maintain PCO2 & acid/base balance [pH]
Help regulate blood pressure by secreting renin [renin-angiotensin system]
Contribute to metabolism
detoxify free radicals and drugs [with peroxisomes]
gluconeogenesis [during fasting]
produce erythropoietin – stimulates red blood cell production
activation of vitamin D [as calcitrol]
Metabolic waste – waste substance produced by the body [often lethal]
50% of N containing waste is urea [from protein – aa NH2 ammonia urea [by liver]
Uric acid – from nucleic acids
Creatinine – from creatine phosphate
BUN – typical = 10-20 mg/dL
Too high = azotemia [renal insufficiency]
Plasma creatinine increase above 1.5 mg/dL with decreased filtration normal = 0.6-1.2 mg/dL
Retroperitoneal in the superior lumbar region.
Extend from twelfth thoracic to third lumbar vertebra.
Right kidney is lower than left because it is crowded by the liver.
Renal fascia – outer layer of dense fibrous connective tissue that anchors the kidney to abdominal wall.
Adipose capsule – shock absorbing.
Renal capsule – fibrous cover that prevents kidney infection.
Cortex –outer - cortical zone and juxtamedullary zone
Medulla – renal pyramids [8-18]
Renal papillae – narrow ends of pyramids
Renal columns – between pyramids
~ one-fourth (1200 ml) of systemic cardiac output flows through the kidneys each minute.
Arterial flow into venous flow out of the kidneys follow similar paths.
Figure 25.3c
Renal Arteries segmental arteries interlobar arteries arcuate arteries cortical radial arteries afferent arterioles
Efferent arterioles also form vasa recta with deep juxtamedulary nephrons
Venules cortical radial veins arcuate veins interlobar veins segmental veins renal veins inferior vena cava
Figure 25.3c
NERVE SUPPLY -
Renal plexus of sympathetic division of ANS – to afferent & efferent arterioles [vasomotor nerves] - regulate flow and pressure
Figure 25.3c
Nephrons are the structural & functional units that form urine, consisting of:
Renal Corpuscle – glomerulus + Bowman’s capsule
Renal Tubule - PCT, loop of Henle, DCT
BOWMAN”S CAPSULE
Parietal layer – simple squamous epithelium.
Capsular space.
Visceral layer consists of modified, branching epithelial podocytes.
Functions – pressure filtration of blood – water and small solutes leave blood
vascular pole - blood in
urinary pole -urine out
Loop of Henle [nephron loop]:
Descending limb [thin] simple squamous epithelium – permeable to water [out], urea [in]; thick walls
Ascending limb [thick] – cuboidal to low columnar epithelium; thick at top, then thin
Distal convoluted tubule (DCT):
Principal cells:
Cuboidal cells without microvilli.
Help maintain water & salt balance.
Collecting Ducts - drains several DCT's
Combine to form papillary ducts calyces
Cuboidal epithelium, then columnar
All nephrons begin in the cortex. Where the loop of Henle reaches to determines type
Juxtamedullary nephrons:
Have loops of Henle that deeply penetrate medulla.
Cortical nephrons – 85% of nephrons:
Have loops of Henle that only slightly penetrate medulla.
Endothelium of glomerulus – open pores [fenestrations] – 70-90 nm diameter everything but cells and platelets pass through
Basal lamina [basement membrane] of glomerulus – serves as dialysis membrane –blocks large plasma proteins
Filtration slits - Endothelium of visceral layer of glomerular capsule – podocytes form filtration slits [spaces between pedicels] – negatively charged - repel anions -30 nm slit width
Glomerulus blood filtering depends on 3 main pressures –1 promotes, 2 oppose
Blood Hydrostatic Pressure [HPG] – about 60 torr – forces fluid out of capillaries
Capsular Hydrostatic Pressure [HPC] – about -18 torr – opposes –from fluid already in capsular space
Colloidal Osmotic Pressure of blood [OPC] about -32 torr – opposes
NFP = HPG – [HPC + OPC] = 55 – [15- 30]] = about 10 torr outward
The positive pressure moves fluid out of the glomerulus into Bowman’s capsule.
Pressure remains high throughout length so filtration continues. Especially sensitive to hypertension
Amount of filtrate formed in all areas of the renal corpuscles of both kidneys every minute
Directly related to pressures that determine NFP
Adult rate is about 125 mL/min – 180L/day [males]
If the GFR is too high:
Needed substances cannot be reabsorbed quickly enough and are lost in the urine.
If the GFR is too low:
Everything is reabsorbed, including wastes that are normally disposed of.
Three mechanisms control GFR:
Renal autoregulation (intrinsic system);
Neural controls;
Hormonal mechanisms (renin-angiotensin system).
Renal Autoregulation of GFR - blood flow autoadjustment
Myogenic Mechanism – Smooth muscle contracts when stretched reduces blood flow which reduces pressure downstream.
Tubuloglomerular feedback – negative feedback mechanism using the
Juxtaglomerular apparatus [respond to NaCl concentration]
juxtaglomerular cells - smooth muscle fibers of afferent arterioles – mechanoreceptors - dilate or constrict
with pressure change & secrete renin.
macula densa- chemoreceptors – at end of ascending limb tall crowded cells that monitor Na+ and Cl- concentration
Neural Regulation – sympathetic
Norepinephrine causes vasoconstriction
Affects smooth muscles of vessels – low input dilation, high constriction
Renin-angiotensin II pathway- JG cells release renin in response to
1 – decreased delivery of fluid and NaCl to macula densa
2 – decreased stretching of JG cells
3 – increased rate of stimulation by renal sympathetic nerves
Angiotensin II is the active hormone that
produces constriction of arterioles to increase GBHP and raise GFR
stimulates secretion of aldosterone, which enhances reabsorption of Na+ [and water] by principal cells in collecting ducts
stimulates the thirst center of the hypothalamus
stimulates release of ADH which increases water reabsorption increase in blood volume higher BP
Every nephron has 2 capillary beds: Glomerulus & Peritubular
Each glomerulus is:
Fed by an afferent arteriole
Drained by an efferent arteriole
Peritubular beds are low-pressure, porous capillaries.
Vasa recta – long, straight efferent arterioles of juxtamedullary nephrons.
99% of materials move from filtrate back into peritubular capillaries or vasa recta
Solutes are reabsorbed by active or passive transport
Water is reabsorbed by osmosis = facultative water reabsorption
Small peptides and proteins are reabsorbed by pinocytosis
Most reabsorption occurs in PCT’s.
Na+ reabsorption- by facilitated diffusion, symporters & antiporters.
Reabsorption of water
Helps establish concentration gradients
Promotes reabsorption of other substances
Substances reabsorbed in PCT:
100% of filtered glucose, lactate & amino acids
90% of bicarbonate ions
65% of Na+ & water
50% of Cl & K+
Transport maximum (Tm):
Reflects the number of carriers in the renal tubules available
Exists for nearly every substance that is actively reabsorbed
When the carriers are saturated, excess of that substance is excreted
Removes materials from blood and adds them into filtrate
Function – to rid body of certain materials and help control blood pH.
Except for K+, the PCT is the main site of secretion.
Removes urea, uric acid, bile salts catecholamines, prostaglandins, morphine, penicillin, etc.
Variable absorption based on need - 2 cell types
Principal cells – have infolding of basement membrane – maintain water and Na balance. Sensitive to ADH and aldosterone
Intercalated Cells - very few – lots of mitochondria - can reabsorb K+ and secrete H+ to rid body of excess acid
Hormonal Influences
Aldosterone - renin angiotensin system
ADH - in response to dehydration and high osmolality - acts on collecting ducts,increases water absorption
Parathyroid hormone stimulates Ca2+ uptake and increases phosphate excretion
Atrial Natriuretic Peptide [increases GFR]
Secreted by atria of heart when muscle is stretched [high Bp]
Promotes excretion of water and Na+
Inhibits ADH secretion & antagonizes renin system
Reduces blood volume and BP
Dilute Urine is hypotonic to blood plasma.
To produce dilute urine – just don’t remove any water after ascending loop of Henle
Dilution occurs in the absence of ADH – makes principal cells impermeable to water reabsorption
Concentrated urine is hypertonic to plasma.
ADH is present – water channels [aquaporins] form in principal cell membranes [increase water reabsorption]
More Complex than dilution.
Solute concentration is maintained by counter current mechanism
Based on anatomic arrangement of juxtamedullary nephrons & the vasa recta.
Get salinity gradient produced in ECF - very high at base.
Countercurrent Multiplier – loop of Henle - recaptures Na+ and returns it to deep medullary tissues keeping the gradient in place.
1. Descending – water leaves, Na+ & Cl- enter.
2. Ascending water enters, NaCl leaves.
Recycling of urea in renal medulla:
gets concentrated in tubules
diffuses out at collecting duct
into medulla
into tubular fluid in ascending loop of Henle - repeats.
Countercurrent exchanger – Vasa Recta - Blood flows in opposite direction from loop - Maintains a gradient – keep removing water and adding salt.
Appearance - clear, colorless to amber [pus, bacteria blood, etc. make cloudy and/or colored
Odor - slight - increases with standing due to bacterial ammonia production
Specific gravity - 1.001 - 1.028 [water is 1.000]
pH
Slightly acidic (pH 6) with a range of 4.5 to 8.2
Diet can alter pH
Osmolarity - 50 - 1200 mOsm/L
Composition - 95 water
1 -2L/da
Polyuria - excessive output, Oliguria - low, Anuria – none
Diabetes - 4 forms - I, II gestational and insipidus. In most - results from high sugar in tubule. Insipidus is from hyposecretion of ADH
Diuretics – enhance urinary output
Osmotic – not reabsorbed
ADH inhibitors [alcohol]
Na+ symporter inhibitors [Lasix]
The volume of plasma that is cleared of a particular substance in a given time.
Renal clearance tests are used to:
Determine the GFR
Detect glomerular damage
Follow progress of renal disease
RC = UV/P
RC = renal clearance rate
U = concentration (mg/ml) of the substance in urine
V = flow rate of urine formation (ml/min)
P = concentration of the same substance in plasma
1/kidney – retroperitoneal, 25 cm inches long
Valve-like region as enter bladder – prevents backflow
3 layer wall
mucosa [transitional epithelium]
muscularis –longitudinal, outer circular
adventitia – fibrous coat
Smooth, collapsible, muscular sac that temporarily stores urine.
In pelvic cavity
Trigone – triangular area outlined by openings for ureters and urethra.
3 layers: Transitional epithelial mucosa
Thick muscular layer
Serous coat
Storage – capacity 700-800mL
Muscular tube that drains urine from the bladder & conveys it out of the body.
Female much shorter than male.
The male urethra has 3 regions:
Prostatic
Membranous
Spongy
Stretch receptors stimulated by 200-400mL – signal to sacral spinal cord - some to
Sympathetic neurons that suppress urination others to
Parasympathetic - micturation reflex
Voluntary relaxation of external sphincter
Renal insufficiency = state in which kidneys cannot maintain homeostasis due to extensive destruction of nephrons
Causes - hypertension, infections, trauma, ischemia, poisoning, tubule blockage - protein, etc. atherosclerosis, glomerulonephritis