Urinary system
-A group of organs in the body that filters out excess fluid and other substances from the bloodstream.
-The substances are filtered out from the body in the form of urine.
Excretion
-Is the separation of wastes from body fluids and eliminating them.
4 body systems that carry out excretion
-respiratory, integumentary, digestive, and urinary.
Respiratory system
-CO2, small amounts of other gases, and water.
Integumentary system
-Water, inorganic salts, lactic acid, urea in sweat.
Digestive system
�Water, salts, CO2, lipids, bile pigments, cholesterol, other metabolic waste, and food residue.
Urinary system
-Many metabolic wastes, toxins, drugs, hormones, salts, H+, and water.
Organs of the urinary system
Functions of the kidney
-Filters blood plasma.
-Regulate blood volume and pressure.
-Regulate the osmolarity of the body fluids.
-Acid-base balance.
-Secrete enzyme renin.
-Hormone secretion: Erythropoietin and Calcitriol.
-Gluconeogenesis.
Kidney anatomy
Kidney anatomy
Renal circulation
Renal circulation
The Nephron
-Functional unit of kidney.
-Two principle parts: Renal corpuscle and renal tubule.
Renal corpuscle
-Consists of the glomerulus and a two-layered glomerular (Bowman) capsule.
Proximal convoluted tubule (PCT)
-longest and most coiled region.
-Prominent microvilli for majority of absorption.
Nephron loop (loop of Henle)
-Descending and ascending limb.
-thick and thin segments.
Distal convoluted tubule (DCT)
-Shorter and less coiled than PCT.
-Without microvilli.
Collecting duct
-Receives fluids from the DCT's of several nephrons.
-Leads to papillary duct.
Flow of fluid through renal tubule
-Glomerular capsule => proximal convoluted tubule => nephron loop => distal convoluted tubule => collecting duct => papillary duct => minor calyx => major calyx => renal pelvis => ureter => urinary bladder => urethra
Cortical nephrons
-Short nephron loops.
-Efferent arterioles branch into peritubular capillaries.
Juxtamedullary nephrons
-Long nephron loops.
-Maintain salinity gradient in the medulla.
-Efferent arterioles branch into vasa recta.
Renal plexus
-Nerves and ganglia wrapped around each renal artery.
Sympathetic
-Innervation from the abdominal aortic plexus.
-Stimulation reduces filtration and urine production.
-Respond to falling BP (stimulate the kidneys to secrete renin).
Parasympathetic
-Innervation from the vagus nerve.
-Increases rate of urine production.
Overview of urine formation
Kidneys convert blood plasma to urine in 3 stages:
-Glomerular filtration
-Tubular reabsorption and secretion
-Water conservation
Filtration Pressure
Glomerular Filtration Rate (GFR)
-The amount of filtrate formed per minute by Kidneys.
-Net filtration pressure (NFP).
-Filtration coefficient (Kf) depends on permeability and surface area of filtration barrier.
-Total amount of filtrate produced equals 50 to 60 times the amount of blood
GFR too high
-Fluid flows through the renal tubules too rapidly for them to reabsorb the usual amount of water and solutes.
-Increased urine output.
-Dehydration and electrolyte depletion.
GFR too low
-Wastes reabsorbed.
-Azotemia may occur.
-GFR controlled by adjusting glomerular blood pressure.
GFR control is achieved by 3 homeostatic mechanisms:
1.) Renal autoregulation
2.) Sympathetic control
3.) Hormonal control
Renal auto regulation of GFR
-Nephrons to adjust their own blood flow and GFR.
-GFR fluctuates slightly
-2 Methods of auto regulation: Myogenic mechanism and tubuloglomerular feedback.
Myogenic mechanism
-Smooth muscle contracts when stretched.
-Increased pressure stretches the afferent arteriole. which constricts and prevents change in blood flow.
-The afferent arteriole relaxes and allows blood flow more easily into glomerulus.
Tubuloglomerular feedback
-Adjust filtration to regulate the composition of the fluid, stabilize its own performance, and compensate for fluctuation in systemic blood pressure.
-Juxtaglomerular apparatus.
Macula densa
-Senses variations in flow or fluid composition.
-Stimulate JG cells.
Juxtaglomerular (JG) cells
-Enlarged smooth muscle cells in the afferent arteriole.
-Dilate or constrict the arterioles.
-Secrete renin.
Mesangial cells
-Between arterioles and among the capillaries of the glomerulus.
-Constrict or relax capillaries to regulate flow.
If GFR rises:
-Macula densa stimulates JG cells, which constrict afferent arteriole, reduces GFR.
-Mesangial cells may constrict capillaries, reducing filtration.
If GFR falls:
-Macula densa relaxes afferent arterioles and mesangial cells.
-Blood flow increases and GFR rises.
Sympathetic control of GFR
-Sympathetic nervous system and adrenal epinephrine constrict the afferent arterioles in strenuous exercise or acute conditions like circulatory shock.
-Reduces GFR and urine output.
-Blood redirected.
-GFR may be as low as a few mL/min.
Hormonal control
-Renin-Angiotensin System:
-Renin secreted in response to low BP.
-Angiotensinogen converted to angiotensin I
-ACE converts angiotensin I to angiotensin II
-Restores fluid volume and BP.
Tubular reabsorption
-Process of reclaiming water and solutes from the tubular fluid and returning them to the blood.
-PCT reabsorbes 65% of glomerular filtrate.
Reabsorption in the PCT
-Two routes of reabsorption: Transcellular route and paracellular route; solvent drag.
-Taken up by peritubular capillaries by osmosis and solvent drag.
Transport maximum of Glucose
-Limited by the # of transport proteins in the plasma membrane.
-If all transporters are occupied as solute molecules pass.
*Excess solutes appears in urine.
*Any blood glucose level above 220mg/dL results in glycosuria.
Tubular secretion
-Renal tubule extracts chemicals from the capillary blood and secretes them into tubular fluid.
-Two purposes in PCT and nephron loop: 1.) waste removal. 2.) acid-base balance
Waste removal
-Urea, uric acid, bile acids, ammonia, catecholamines, prostaglandins and a little creatinine are secreted into the tubule; removes pollutants and drugs.
Acid-base balance
-Secretion of hydrogen and bicarbonate ions help regulate PH.
Nephron loop
-Primary function is electrolyte reabsorption from filtrate.
-Generates salinity gradient.
-Thick segment reabsorbs 25% of NA+, K+, Cl-
*ions leave cells by active transport and diffusion.
Tubular Secretion
-Renal tubule extracts chemicals from the capillary blood and secretes them into tubular fluid.
-Two purposes in PCT and nephron loop; Waste removal and acid-base balance.
Nephron loop
-Primary function is electolyte reabsorption from filtrate.
-Generates salinity gradient.
-Thick segment impermeable and reabsorbs 25% of Na+, K+, Cl-