Urine Formation 1 Flashcards
The main functions of the Kidney
Homeostasis
Regulation of body fluid osmolality and electrolyte concentrations
Regulation of arterial blood pressure
Regulation of acid-base balance
Regulation of erythrocyte production
Regulation of 1, 25-dihydroxy vitamin D3 (Calcitriol) production
Synthesize glucose from AA’s (Gluconeogenesis)
Production of hormones/-like substances (Somatomidin and prostaglandins)
Excretion of metabolic waste products and foreign chemicals
Excretion of Metabolic Wste Products, Foreign chemicals, drugs and Hormone Metabolites
The kidneys are the primary meand for eliminating waste products of metabolism that are no longer needed by the body.
These products include, Urea(AA’s), Creatinine(Muscle), uric Acid(Nucleic acids), end products of hemoglobin breakdown,bilirubin and metabolites of various hormones.
These products must be excreted as rapidly as they are produced
The kidneys also eliminate most toxins and other foreign substances that are either produced by body or ingested such as pesticides, drugs and food additives.
Regulation of Arterial Pressure
Kidneys play a dominant role in the long term regulation of arterial pressure by excreting variable amounts of Sodium and Water.
Short term arterial pressure regulation by secreting hormones and vasoactive factors or substances(Renin) that lead to the formation of vasoactive products(Angiotensin II)
Regulation of Acid-Base Balance
The kidneys contribute to acid-base regulation, along with the lungs and body buffers, by excreting acids and regulating the body fluid buffer stores
The kidneys eliminate mainly certain types of acids such as Sulphuric Acid and Phosphoric Acid generated by the metabolism of proteins
Regulation of EPO production
Kidneys secrete EPO,which stimulates the production of red blood cells by hematopoietic stem cells in the bone marrow.
Important stimulus for EPO secretion by by the kidneys is Hypoxia.
Kidney normally account for almost all the EPO secreted into circulation.
In people with severe kidney disease or who have had their kidneys removed and have been place of hemodialysis, develop severe aneamia as a result of decreased EPO production
Regulation of 1.25-Dihydroxyvitamin D3 Production
The kidneys produce the active form of Vit D, 1.25-Dihydroxyvitamin D3(Calcitriol) by hydroxylating this vitamin at the nr. 1 position.
Calcitriol is essential for the normal calcium deposition in bone and calcium reabsorption by the GIT tract.
Calcitriol plays an important role in calcium and phosphate regulation
Glucose Synthesis
Kidneys synthesis glucose from Amino acids and other precursors during prolonged fasting.
The kidneys capacity to add glucose to the blood during prolonged periods of fasting rivals that of the liver
With chronic kidney disease or acute failure of the kidneys, these homeostatic functions are disrupted and severe abnormalities of body fluids and composition rapidly occur.
With complege renal failure,enough potassium, aciss, fluid and other substances accumulae innthe body to cause death within a few days,unless clinical interventions such as hemodialysis are initiated to restore, at least partially, the body fluid and electrolyte balances
Discuss the renal blood supply specifically the two capillary networks
The renal circulation is unique in having two capillary beds the glomerular and peritubular capilaries, which are aranged in series and seperated by the efferent arterioles
These arterioles help regulate the hydrostatic pressure in both sets of capilllaries.
High hydrostatic pressure in the glomerular capillaries(60 mmHg) causes rapid fluid filtration whereas the low hydrostatic pressure in the peritubular capillaries(13) permits rapid fluid reabsorption
By adjusting thethe resistance of the afferent and efferent arterioles, the kidneys can regulate the hydrostatic pressurein both the glomerular and the peritubular capillaries,thereby changing the rate of glomerular filtration, tubular reabsortion or both in respones to body homeostatic demands
The peritubular capillaries empty into the vessels of the venous system which run parallel to the arteriolar vessels/
The blood vessels of the venous system progressively form the interlubular>arcuate>interlobar vein and then renal vein.
The nephron is the functional unit of the Kidney
Each human kidney contains nephrons which are capabale to formation of urine.
The kidney cannot regenerate new nephrons,therefore with renal injury, disease or normal aging, the number of nephrons gradually decrease.
Each nephron contains a Tuft of glomerular capilaries called the glomerulus,through which large amounts of fluid are filtered from the blood and a long tubule in which the filtered fluid is converted into urine on its way to the pelvis of the Kidney.
The glomerular capillaries are covered by epithelial cells and the total glomerulus is encased in the Bowman’s Capsule
Fluid from the glomerular capillaries flows into the bowmans capsule and then into the proximal tubule which lies in the cortex of the kidneys.
At the end of the thick ascending limb is a short segment that has in its walls a plaque of specialized epithelial cells known as Macula Densa-plays important role in controling the nephron function
Regional differences in Nephron structure: Cortical and Juxtamedullary Nephrons
They are classified depending on how deep the nephron is situated/lies within the kidney mass.
Cortical: Nephrons which are located in the outter cortex, they have short loops of henle that penetrate only a short distance into the medulla
Juxtamedullary: Nephrons which have glomeruli that lie deep in the renal cortex near the medulla,they have long loops of henle that dip deeply into the medulla, in some cases all the way to the tips of the renal papillae
The processes that are involved in urine formation
Glomerular Filtration
Tubular Reabsorption
Tubular Secretion
How is the Urinary Excretion Rate Calculated
=Filtration rate - Reabsorption rate + Secretion Rate
The different types of renal handling
- Filtration only: Creatinine
- Filtration and partial reabsorption: Electrolytes: Na+ and Cl-
- Filtration and complete reabsorption: Nutritional substances, Amino acids and chloride ions
- Filtration and Secretion: Organic acids and bases
The different types of renal handling
- Filtration only: Creatinine
- Filtration and partial reabsorption: Electrolytes: Na+ and Cl-
- Filtration and complete reabsorption: Nutritional substances, Amino acids and chloride ions
- Filtration and Secretion: Organic acids and bases,Para-aminohippurate (PAH)
Filtration, Reabsosrption and Secretion of different substances
Tubular reabsorption is quantitively more important than tubular secretion in the formation of urine, but secretion plays an important role in determining the amounts of potassium and hydrogen ions and a few substances
Most substances must be cleared from the blood esp. the end products of metabolism such as Urea, Creatinine, Uric acid and urates,are poorly reabsorbed but in add., are secreted from the blood into the tubules ,so their excretion rates are high
Conversely electrons such as sodium ions, chloride ions and bicarbonate ions are highly reabsorbed so only small amounts appear in urine
Certain nutritional substances such as amino acids and glucose are completely reabsorbed from the tubules and do not appear in the urine even though large amounts were filtered y the glomerular capillaries
What is the relationship between the processes of filtration, reabsorption and secretion, and the excretion of a substance
Each of the processes-Glomerular filtration, tubular reabsorption and tubular secretion is regulated according to the needs of the body.
Example: When there is excessive sodium in the body,the rate at which sodium is filtered usually increases and a smaller fraction of the filtered sodium is reabsorbed, causing increased urinary excretion
For most substances the rates of filtration and reabsorption are extremely large relative to the rates of excretion.
Therefore even the slight changes of filtration or reabsorption can lead to relatively large changes in renal excretion
Example: An incease in GFR of only 10% would raise the urine volume 13-fold( from 1.5 - 19.5 litre) if tubular reabsortion remained constant
In reality changes in glomerular filtration and tubular reabsorption usually act in a cordinated manner to produce the necessary changes in renal excretion.
Why are large amounts of solutes filtered and then reabsorbed by the Kidneys/Advantages of first filtering large amounts of solutes and then reabsorbing most of them back into the blood again
Advantages of high GFR:
- Is that it allows the kidneys to rapidly remove waste products from the body depend mainly on the GF for their excretion-Most waste products are poorly reabsorbed by the tubules and therefore, depend on a high GFR for effective removal from the body.
- It allows all the body fluids to be filtered and processed by the kidneys many times a day. Because the entire plasma volume is only about 3l, whereas the GFR is about 180 L/day, the entire plasma can be filtered and processed about 60 times a day. this high GFR allows the kidneys to precisely and rapidly control the volume and composition of the body fluids.
Briefly describe the qualitative and quantitative composition of the glomerular filtrate
The glomerular capillaries are relatively impermeable to proteins,which means that the filtered fluid(Glomerular filtrate) is essentialy proteins free and devoid of cellular elements including red blood cells.
The conc. of other constituents of the glomerular filtrate,including most salts and organic molecules are similar to the concentrations in the plamsa
Exceptions to this generelazation include a few low molecular weight substances such as calcium and fatty acids are partially bound to plamsa proteins
The normal glomerular filtration rate for the normal adult per minute and per 24-hours
The GFR is determined by the balance of the hydrostatic and colloid osmotic forces acting across the capillary membrane and the capillary filtration coefficient(Kf),The product of the permeabilty and filtering surface area of the capillaries.
The glomerular capillaries have a filtration rate as a result of a high glomerular hydrostatic pressure and large Kf
In the ave. adult human the GFR is about 125ml/min or 180l/day.
The fraction of the renal plamsa flow that is filtered(The filtrationn fraction) averages about 0.2 which means that about 20% of the plasma flowing through the kidneys is filteredd though the glomerular capillaries
How is the Filtration Fraction calculated
Filtration Fraction= GFR/Renal plasma flow
Glomerular Capillary Membrane
Similar to those of other capillaries except that it has 3 major layers.
The endothelium of the capillary
Basement Membrane
Layer of epithelia cells(Podocytes) surrounding the outer surface of the capillaries basement membrane
These layers make up the filtration barrier which despite the three layers,filteres several hundred times as much water and solutes as the usual capillary membrane.
The high filtration across the glomerular capillary membrane is partly due to its special characteristics.
The capillary Endothelium
It is perforated by thousands of small holes called fenestrae similar to those of the liver.
The fenestrations are relatively large, endothelial cell proteins are richly endowed with fixed negative charges that hinder the passage of plasma proteins.
The Basement Membrane
Contains of a meshwork of collagen and protoeglycan fibrilae that have large spaces through which large amounts of water and small solutes can filter.
The basement membrane effectively prevents filtration of plamsa proteins in part because of strong negative eletrical charges ass. with proteoglycans
Layer of epithelial cells
These cells are not contineous but have long footlike processes(Podocytes) that encircle the outer surface capilaries which are separatedby gaps-Slit Pors through which the glomerular filtrate moves.
The epithelial cells which also have negative charges provide additional restrictions to filtration of plasma proteins.
