The Urinary System Flashcards
lecture 12 week 6
What is the function of the urinary system
- maintain control over the composition of the body fluids
removal of excess fluid and excess salt
removal of metabolic waste products
-endocrine function
involved in red blood cell production, calcium metabolism, regulation of blood pressure/flow and gluconeogenesis
What is the structure of the urinary system
Kidneys: produce urine
(high in the abdomen on posterior wall, 11cm high and 6cm wide)
Ureters: transports urine towards the urinary bladder
Urinary bladder: temporarily stores urine prior to urination
Urethra: conducts urine to exterior (also transports semen)
Kidneys
- cortex
- medulla
- renal pelvis
- ureter
medulla is divided into a series of renal pyramids
What is a nephron
nephron: functional unit of the kidneys, around 1.25 million nephrons per kidneys
cortical and juxtomedullary nephrons
What are the key processes that a nephron carries out
filtration (Glomerulus)
- plasma crosses epithelial cells, basal lamina, epithelial cells. (no cells + big proteins cross through but salts, glucose, amino acids, water can)
reabsorption
- water, salts, glucose, amino acids can be reabsorbed from the tubules from the blood
secretion
- directly from the blood to tubule, bypassing filtration site (vitamins)
Composition of blood and urine
- compared to blood, urine is enriched with metabolic waste products
- chemical composition, volume and osmolarity vary with fluid intake, fluid loss through sweating and diet
- composition of urine varies much more than composition of plasma
What is the renal corpuscle
(look art nephron diagram)
- water and solutes pass from the blood into renal tubules at a rate of 125 ml/min
Where does filtration occur
filtration occurs in the renal corpuscle
- blood is filtered under pressure through a filtration barrier made up of capillary endothelial cells, a basal lamina, an d podocytes in the extracellular space formed by Bowman’s capsule
- salts, glucose, amino acids, water and other small molecules can pass but cells and big proteins do not
- glomerular filtration rate is 180 l/day
- blood enter glomerulus by different arteriole and leaves via efferent arteriole
What is glomerular filtration pressure and how is it formed
amount of fluid passing from glomerulus into Bowman’s capsule is governed by the forces acting on glomerular capillaries
- glomerular filtration rate also depends on factors other than NET FILTRATION RATE, such as total surface area for filtration and filtration membrane permiability
How is glomerular filtration rate autoregulated
- a nearly constant rate is maintained for the glomerular filtration rate (GFR) when mean blood pressure is 80-180 mm Hg
- if GFR is too low it will be insufficient to regulate internal environment
- if GFR is too high may lead to loss of amino acids, glucose, ions and water
- too high hydrostatic pressure can damage blood vessels
when blood pressure increases the resistance of AFFERENT arteriole increases, by narrowing diameter of the blood vessel. this prevents an increase of pressure in glomerulus and resistance decreases when blood pressure is low and more blood enters glomerulus preventing big drops in pressure
What are the autoregulation mechanisms of GFR
Myogenic response
- stretch-activated cation channels in smooth muscle wall open with increased blood pressure
- depolarisation occurs from the entry of cations into the cell
- voltage-gated calcium channels open causing an influx of Ca2+
- causes contraction which narrows the tube
Tubuloglomerular feedback
- macula densa senses flow -from sensing concentration of Na+ and Cl- - increased flow –> increased concentration of ions in filtrate –> increased uptake of ions into cells —> stimulates release of paracrine signals to neighboring cells –> contraction of smooth muscle cells
What is the proximal tubule and its role
- cells have microvilli on surface and are rich in mitochondria
- reabsorption of 2/3 water, Na+, K+, Cl- and bicarbonate
- reabsorption of all glucose, lactate and amino acids
reabsorption
- transport from interstitial fluid to capillary occurs via bulk flow
- driven by sodium-potassium ATPase in basolateral membrane: active sodium transport creates concentration gradient that drives sodium entry at apical membrane, reabsorption of organic nutrients and some cations by cotransport. reabsorption of water by osmosis in aquaporins, lipid-soluble ions and urea move down gradients
Loop of Henle and reabsorption
- reabsorption of 20% filtered Na+, Cl-, H20
- generation of concentration gradient between inner medulla and cortex
Thick ascending limb: water impermeable. actively pumps out NaCl creating concentration gradient in interstitial fluid
- Descending limb: water permeable. as filtrate reaches areas of higher solute concentration in interstitial fluid, water leaves by osmosis
- water and salts released by tubules are taken by blood vessels
- urea contributes to solute concentration gradient
- some urea secreted by facilitated diffusion into tubules of inner medulla
What is the role of the distal tubules in reabsorption
- regulation of the ionic balance of the body by regulated reabsorption and secretion
reabsorption of sodium and potassium is driven by sodium-potassium ATPase
- aldosterone (secreted by adrenal glands) stimulates production of sodium channels and sodium potassium pumps
What is the Renin-Angiotensin system
low sodium: macula densa cells stimulate Renin secretion by Juxtaglomer cells
Angiotensinogen –> (renin) Angiotensis i —> (converting enzyme) Angiotensin ii
- angiotensin ii stimulates aldosterone production by adrenal glands
- as well as sodium and potassium, regulated reabsorption and secretion of other substances occurs in the distal tubule (eg. Ca2+ and H+)
What is the role of the collecting duct in reabsorption
- contributes to solutes concentration gradient between cortex and inner medulla
- uses concentration gradients to determine final concentration and volume of urine
