Action potential

Liver

-responsible for excreting many wastes by chemically modifying them and releasing them into the bile

-liver deals with hydrophobic or large waste products, which cannot be filtered by the kidney

-synthesizes urea and releases it into the bloodstream

Urea

-is a carrier of excess nitrogen resulting from protein breakdown

-excess nitrogen must be converted into urea because free ammonia is toxic

-excreted in urine

Large Intestine -reabsorbs water and ions from feces

-merely processes wastes already destined for excretion

-colon is capable of excreting excess ions into the feces using active transport

Roles of the Kidney

1) Excretion of hydrophilic wastes

2) Maintenance of constant solute concentration and constant pH

3) Maintenance of constant fluid volume (important for blood pressure & Cardiac output)

Filtration

-first step

-entails the passage of pressurized blood over a filter

-cells and proteins remain in the blood while water and small molecules are squeezed out into the renal tubule

-during filtration, water, waste products and also useful small molecules such as glucose are filtered into the renal tubuleFiltrate

-fluid in the renal tubule

Selective Absorption

-second process

-take back useful items (glucose, water, amino acids) while leaving wastes and some water in the tubuleSecretion

-third process

-involves the addition of substances to the filtrate

-can increase the rate at which substances are eliminated from the blood

-not only are substances filtered out more of them are added to the filtrate after filtration

Concentration & Dilution

-last step in urine formation

-involves the selective reabsorption of water and where the decision to make dilute or concentrated urine is made

-after this step, whatever remains in renal tubule --> excreted

Renal Artery

-blood enters the kidney from this artery

-is a direct branch of the lower portion of the aorta

Renal Vein

-purified blood is returned to the circulatory system via this vein

-empties into the inferior vena cava

Ureter

-urine leaves each kidney in a ureter

-empties into the urinary bladder

Urinary bladder

-is a muscular organ which stretches as it fills with urine

-when it becomes full, signals are sent to the brain

-two sphincters involved in the release of urine from the bladder

Internal Sphincter

-made of smooth (involuntary) muscle

-relaxes reflexively (bladder contracts) when the bladder is stretched

External Sphincter

-made of skeletal (voluntary) muscle

-person decides when to relax

Cortex

-outer region

Medulla

-inner region

Medullary Pyramids

-pyramid shaped striations within the medulla

-appearance is due the presence of many collecting ducts

Collecting Ducts

-urine empties from the collecting ducts and leaves the medulla at the tip of the pyramid

Papilla

-tip of the pyramid

-each papilla empties into a space called a calyx

Renal Pelvis

-formed by the convergence of calyces

-is a large space where urine collects

-empties into the ureter

Nephron

-functional unit of the kidney

-consists of two components: (1) capsule (2) renal tubule

Capsule

-rounded region surrounding the capillaries where filtration takes place

Renal Tubule

-receives filtrate from the capillaries in the capsule at one end and empties into a collecting duct at the other end

-the collecting duct dumps urine into the renal pelvisAfferent Arteriole

-blood from the renal artery flows into here

-branches into a ball of capillaries (glomerulus)

Glomerulus

-ball of capillaries

Efferent Arteriole

-blood flows from the glomerulus into the efferent arteriole

-constriction of the efferent arteriole results in high pressure in the glomerulus --> causes fluid (essentially blood plasma) to leak out of the glomerular capillaries

-fluid passes thru a filter known as the glomerular basement membrane and enters the Bowman's capsuleBowman's Capsule

-lumen of the bowman's capsule is continuous with the lumen of the rest of the tubule

-substances that are too large to pass thru glomerular basement membrane are not filtered

Peritubular Capillaries

-filtrate in tubule consists of water & small hydrophilic molecules such as sugars, amino acids and urea; some must be returned to bloodstream

-they are extracted from the tubule often via active transport and picked up by peritubular capillaries

-drain into the venules that lead to the renal vein

Proximal Convoluted Tubule

-most reabsorption occurs in the part nearest to the bowman's capsule the PCT

-all solute movement is accompanied by water movement --> a lot of water reabsorption occurs as well (~70% of filtrate volume is reabsorbed)

-reabsorption is selective (chooses what to reabsorb) but not overly regulated (as much as possible)

-reabsorption of most filtered water and ions

-reabsorption of glucose and AA by secondary active transport

-secretion of drugs, toxins and some ions

Secretion

-movement of substances into the filtrate (usually via active transport) --> increasing rate of removal from plasma

-secretion is "back-up" method that ensures what needs to be eliminated gets eliminated

-occurs all along the tubule

-most secretion takes place at the distal convoluted tubule and the collecting ductDistal Nephron

-last part of the tubule were adjustments made so that urine volume and osmolarity are appropriate before filtrate is discarded into the ureter as urine

-includes the DCT and the collecting duct

-controlled by two hormones ADH and aldosteroneDistal Convoluted Tubule (DCT)

-reabsorption of water and urea in response to ADH

-sodium reabsorption in response to aldosterone

-secretion of drugs, toxins, and some ions

Loop of Henle

-long loop that dips down into the renal medulla (inner part of kidney)Descending Limb - heads into the medulla

-thin walled

Ascending Limb - towards the cortex

-part thin part thick walled

-Loop of Henle becomes DCT which dumps into a collecting duct; many collecting ducts form large tributaries which empty into renal calycesDescending Limb

-heads into the medulla

-thin walled

-water exits tubule --> more concentrated filtrate

Ascending Limb

-towards the cortex-part thin walled

-part thick walled

-in thick ascending limb of loop of Henle: active transport of Na+, K+ and Cl- ions out of filtrate with subsequent passive return of K+ to filtrate causing: (1) dilution of tubular fluid (2) increased osmolarity of medullaInner Medulla

-high osmolarity drives water reabsorption by osmosis

Vasa Recta & Peritubular Capillaries

-reclamation of all reabsorbed substances

Countercurrent Multiplier

-significance of loop of Henle is that the ascending and descending limbs go in opposite directions and have different permabilities

-descending limb is permeable to water, but not to ions

-water exits descending limb flowing by diffusion into the high-osmolarity medullary interstitium --> more concentrated filtrate

-thin ascending limb in NOT permeable to water, but passively loses ions from high-osmolarity filtrate into the renal medullary interstitium

-thick ascending limb actively transports salt out of the filtrate into the medullary interstitium and medullary interstitium becomes very salty

-important because the medulla will suck water out of the collecting duct by osmosis whenever the collecting duct is permeable to water (presence of ADH)-loop of Henle is concurrent multiplier that makes the medulla very salty and that facilitates water reabsorption from the collecting duct --> Kidney is capable of making urine with a much higher osmolarity than plasma

Vasa Recta

-also forms a loop that helps maintain the high concentrations of salt in the medulla (like loop of Henle)-ascending portion is near the descending portion of loop of Henle and thus carry off water that leaves the descending limb

-are branches of efferent arterioles

-IMPORTANT: vasa recta return to the bloodstream any water that is reabsorbed from the filtrate

Glomerular Filtration Rate (GFR)

-depends directly on pressure

-

Juxtaglomerular Apparatus (JGA)

-is a specialized contact point between the afferent arteriole and the distal tubule

-at this contact point the cells in the afferent arteriole are juxtaglomerular cells and in the distal tubule are macula densa-JG cells are baroreceptors monitor systematic blood pressure

-decrease in B.P. --> JG cells secrete renin into blood

Renin

-secreted by JG cells in response to decrease in B.P.

-catalyzes conversion of angiotensinogen (plasma protein made by the liver) into angiotensin I which is further converted into angiotensin II by angiotensin converting enzyme (ACE) in the lungsAngiotensin II

-powerful vasoconstrictor that immediately raises the B.P.

-also stimulates the release of aldosterone --> raises B.P. by increasing Na+ retention (indirectly water retention)Macula Densa

-chemoreceptors and monitor filtrate osmolarity in the distal tubule

-when filtrate osmolarity decreases (indicating reduced filtration rate) --> Macula Densa cells stimulate JG cells to release renin

-also causes direct dilation of the afferent arteriole --> increasing blood flow to the glomerulus (and thus blood pressure and filtration rate)Renal Regulation of pH

-plasma pH is too high --> HCO3- is excreted in the urine

-plasma pH is too low --> H+ is excreted

-enzyme carbonic anhydrase is involved

-found in epithelial cells throughout the nephron except the flat (squamous) cells of the thin parts of the loop of Henle

-after carbonic anhydrase catalyzes the reaction the kidney can reabsorb or secrete bicarbonate or protons as needed (protons secreted, bicarbonate is reabsorbed)

-renal adjustments are slow requiring several days to return pH to normal after a disturbance