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Kidney Functions
- 1.The principal function of the kidney is to maintain homeostasis of Extracellular Fluid
- fluid volume
- ion concentrations
- acid-base balance
- 2.Kidneys also excrete metabolic wastes and many non-biological chemicals, and synthesize glucose during prolonged fasting
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Kidney Mode of operation:
filter the blood,
then reabsorb almost everything
- •The kidneys filter about 125 ml/min
- –180 L/day
- –About 11 L during this class
- •They reabsorb over 90% of the plasma that they filter
- –99% of water
- –99.5% of Na
- –100% of glucose
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The Urinary System
The paired kidneys form a filtrate of the blood that is modified by reabsorption and secretion; urine moves along the ureters to the bladder and out through the urethra.
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Kidney Structure
- The outer layerof the kidney is the renal cortex;it is the site ofglomerular filtrationas well as the convoluted tubules
- The inner part ofthe kidney is the renal medulla;
- this isthe location of thelonger loops of Henle, and the drainage of the collecting ducts into the renal pelvis and ureter.
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Each kidney has ~1 million functional units, called nephrons
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The functional unit of a kidney is the Nephron
- A NEPHRONconsists of:Renal Corpuscle Renal Tubule
- The Renal Corpuscle is inthe cortex;
- the renal tubule begins at the corpuscle,and descends into the medulla
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The Renal Corpuscle is the apparatus for filtration
The renal tubule is the apparatus for reabsorption
- Proximal Convoluted Tubule reabsorbs water, electrolytes,Glucose, amino acids …
- Loop of Henleproduces a concentration gradient between renal cortex and medulla
- Collecting Duct regulates water and ions through actions of hormones,Collects filtrate from many loops
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Basic Renal Processes
1. Glomerularfiltration
2. Tubular reabsorption
3. Tubular secretion
- 1.The process by whichcomponents of plasma in the glomerularcapillary are passed to Bowman’s space of the glomerulus.Filtration occurs because blood pressure exceeds the sum of two opposing pressures.
- •GFR is the rate at which fluid is filtered from renal glomerularcapillaries into Bowman’s capsule.
- •Clinically, GFR is estimated by measuring rate of appearance in urine of substances that are freely filtered, but not reabsorbed.
- –Inulin, an injected polysaccharide
- –Creatinine, a waste produce derived from creatinein muscles
- •Normally, GFR = 125 ml/min = 180 L/day
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tubular reabsorptionis the transfer of materials from the kidney tubule lumen to peritubularcapillaries.The renal tubule is the apparatus for reabsorption•Primary active transport- •Secondary active transport
- •Diffusion
- The proximal convoluted tubule is the site of the majority of reabsorption
- 65% of the glomerularfiltrate is reabsorbed before the descending loop of Henle
- Na+reabsorptionpermits H2O to flow down its concentration gradient (osmosis)
- Reabsorption requires lots of ATP
- Loops of Henle produce a concentration gradient in the interstitiumthat makes it possible to control urine osmolarity
- The Descending Limb of the Loop of Henle is thin and highly permeable to water
- The Ascending limb of the Loop of Henle actively transports Na into the interstitium
- 3.TUBULAR SECRETION
- Transfer of materials from peritubularcapillaries to kidney tubule lumen
- •Opposite direction from reabsorption: from peritubularcapillaries into tubular lumen
- •By transcellularactive transport or diffusion
- •Main examples: H+, K+, Organic ions (many of which are compounds foreign to the body)
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Fluid Compartment of the Body
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RENAL CONTROL: (4)
1.Regulation of water balance
2.Regulation of Na+
3.Regulation of K+
4.Regulation of acid/base balance
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FLUID BALANCE:
•Two factors are regulated to maintain fluid balance:
–ECF osmolarity
•Important to prevent swelling or shrinking of cells
•Control of water balance is of primary importance
–ECF volume
•Important to maintain blood pressure
•Control of salt balance is of primary importance
- -Extracellularfluid serves as the intermediary between cells and the extracellularenvironment.
- •Plasma is the only fluid that can be acted on directly to control its volume and composition.
- •Because of free exchange across capillary walls, ECF volume and composition are regulated when plasma volume and composition are.
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Control Of Water balance:
The kidneys balance water loss with water gain
- •Daily water input and output are well balanced.
- Two factors can be controlled:
- –Thirst influences the amount of fluid ingested
- –The kidneys can adjust how much water is excreted as urine
- •Water reabsorption is controlled (and can be regulated) in the medullarycollecting ducts
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Hypothalamic Osmoreceptors
- •Increased osmolarity(too little H2O) is sensed by osmoreceptors in the hypothalamus
- •These osmoreceptors signal thirst and cause release of vasopressin.
- •Vasopressin causes increased permeability of the collecting tubules, leading to more H2O reabsorption.
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Vasopressin
- Vasopressin modulates water reabsorptionin the medullarycollecting ducts
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Control of Na+Balance
- •The renal system adjust the amount of Na+excreted by controlling two processes:
- –Glomerularfiltration
–Tubular reabsorption
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Regulation of Na+Reabsorption
- •Na+reabsorptionis regulatedonly in distal tubule and (cortical) collecting ducts
- •The major factor determining Na+reabsorptionis the steroid hormone aldosterone.Aldosterone causes increase Na+reabsorptionby increasing synthesis of Na channels and Na/K pumps in the cortical collecting ducts
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The main Na+control system:
The renin-angiotensin-aldosteronesystem
- •Cells in the juxtaglomerularapparatus secrete the hormone renin
- •Reninactivates angiotensinogeninto angiotensinI
- •AngiotensinI is converted to angiotensinII by ACE in the pulmonary circulation
- •AngiotensinII causes secretion of aldosteronefrom the adrenal cortex
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Regulation of K+ by ALDOSTERONE
- •Only about 2% of K+is in ECF, but it is important for excitable cells
- •Most K+is reabsorbed in proximal tubule
- •[K+] secretionis controlled
- •Increased [K]Plasmacauses increased aldosteronesecretion, which increases K+secretion
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Control of Acid-Base Balance
- •Acid-base balance refers to regulation of free H+ ion concentration in the ECF.
- •Controlled by breathing and by renal system
- •Ventilatoryresponse in minutes
- •Renal response in hours to days
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