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- 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
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
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.
- 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.
Each kidney has ~1 million functional units, called nephrons
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
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
Basic Renal Processes
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
- 2.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
- 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)
Fluid Compartment of the Body
RENAL CONTROL: (4)
1.Regulation of water balance
2.Regulation of Na+
3.Regulation of K+
4.Regulation of acid/base balance
•Two factors are regulated to maintain fluid balance:
•Important to prevent swelling or shrinking of cells
•Control of water balance is of primary importance
•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.
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
- •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.
- Vasopressin modulates water reabsorptionin the medullarycollecting ducts
Control of Na+Balance
- •The renal system adjust the amount of Na+excreted by controlling two processes:
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
The main Na+control system:
- •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
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
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