renal system dr. zainab h.h dept. of physiology lec.1,2
TRANSCRIPT
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Renal system
Dr. Zainab H.HDept. of Physiology
Lec.1,2
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Objectives
• List the functions of kidney• Describe the morphology of the typical
nephron and its blood supply.• Describe the concept of clearance
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What is excretion?Throwing out of waste product is known as excretion.The organs through which excretion occurs.1. Kidneys: Excrete water and water soluble waste products.2. Lungs: Excrete carbon dioxide, water vapour and other volatile substances such as acetone.3. Skin: Excretes water and salts mainly in the form of sweat.4. Gastrointestinal tract: Excretes undigested food.
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The Role of the Kidneys in the Body Includes:
• Regulation of the volume and composition of the ECF, by
maintaining a balance between intake and output of water and electrolytes in the body.
• Excretion and elimination of waste products of metabolism, such as the excretion of urea, creatinine and uric acids; as well as the excretion of various toxins such as drugs and food additives.
• The kidneys act as endocrine glands producing hormones, such as “erythropoietin hormone” and renin.
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• Playing a dominant role in the long-term and short-term regulation of arterial blood pressure.
• Kidneys along with the respiratory system contribute to acid-base regulation.
• Finally, kidneys synthesize glucose from amino acids and other precursors
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Metabolic waste products excreted by kidneys are:
• Urea from protein.• Uric acid from nucleic acid.• Creatinine from muscle creatine.• End products of haemoglobin breakdown.
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STRUCTURE OF NEPHRON
• Nephron is a structural and functional unit of the kidney.
• Each nephron is capable of forming urine. • There are two types of nephrons:• 1. Cortical nephrons. Glomeruli are present near
the surface of the kidneys. – These nephrons constitute about 86% of total
nephrons.– The main function of cortical nephrons is absorption of
sodium.
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• 2. Juxtamedullary nephrons. Glomeruli lie at the junction of cortex and medulla of the kidney. – These constitute 14% of the nephrons.– The main role of juxtamedullary nephron is to
increase concentration of medullary interstitial fluid.
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Two kidneys together have two millions nephrons.Nephron consists of two major parts:– Glomerulus.– A long renal tubule.
1. Glomerulus. It is made up of tuft of capillaries which connect afferent arteriole with an efferent arteriole.
• Capillaries have single layer of endothelial cells attached to a basement membrane.
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• Bowman’s capsule encloses the glomerulus and is formed of two layers:– inner layer which covers the glomerular capillaries
is called visceral layer, – outer layer is called parietal layer. Space between visceral and parietal layers is
continued as the lumen of the tubular portion.
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–fluid-filled space, Bowman’s space, is formed within the capsule. –Blood of capillary and fluid of Bowman’s space
are separated by the glomerular membrane. – From the Bowman’s capsule, tubule of the
nephron extends, the lumen of which is continuous with the Bowman’s space.
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• 2. Renal tubule. It is mainly formed of three parts:
(a) Proximal convoluted tubule.(b) Loop of Henle consisting of:– Thin segment :walls of descending limb and lower
end of ascending limb are very thin. Therefore, they are termed thin segment.
– Hair pin bend.– Thick ascending limb or segment
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• (c) Distal convoluted tubules. • open into initial arched collecting ducts called
cortical collecting ducts present in renal cortex. – Seven to ten such ducts form straight collecting
duct which passes into medulla forming medullary collecting ducts.
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• In the inner zone of medulla they form papillary ducts or ducts of Bellini. – These open into papilla of minor calyces. – Three or four minor calyces unite to form one
major calyx.– The major calyces open into pelvis of ureter. – The pelvis is an expanded portion present in renal
sinus and it continues as ureter
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BLOOD FLOW TO KIDNEYS
• Rate of blood flow to kidneys is 1200 ml/min. • This is quite high as compared to their size. • State peculiarities of renal circulation.
1.Very high blood supply, about 21% of cardiac output.
2.Two sets of capillaries. – The glomerular capillaries.
• These combine to form efferent arteriole which in turn breaks into peritubular capillary network around the tubules of cortical nephrons.
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– vasa recta which are loop-shaped vessels in juxtamedullary nephrons• the efferent arterioles continue as these loops dip into the
medullary pyramids alongside the loops of Henle
3.Glomerular capillary bed has a high hydrostatic pressure because efferent arteriole is of a smaller diameter than afferent arteriole which offers considerable resistance to blood flow.
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4. Peritubular capillary bed is a low pressure bed.
5. Only 1 to 2% of blood flows through vasa recta. The flow is very sluggish.
6. Renal blood flow shows remarkable constancy in face of blood pressure changes due to autoregulation
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• The kidneys regulate the hydrostatic pressure in both capillary beds (glomerular & peitubular ) , by adjusting resistance of the afferent and efferent arterioles.
• High hydrostatic pressure in GC (60 mmHg) causes rapid fluid filtration; whereas a much lower pressure in the peritubular capillaries (13 mmHg) permits rapid fluid reabsorption.
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• The afferent arteriole is a short, straight branch of the interlobular artery.
• The efferent arteriole, that drains the GC, has a relatively high resistance than the afferent arteriole.
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The Process of Urine Formation:• Urine formation begins with the • Filtration of plasma through the GC into the Bowman’s
space.
• As the filtered fluid flows through the remaining portions of the tubule, its composition is altered as a result of two main processes:
1. Tubular reabsorption.
2. Tubular secretion. and both processes will produce the final product, urine.
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Concept of Clearance:
• The renal clearance of a substance is the volume of plasma that is completely cleared or cleaned of that substance by the kidney per unit of time. (usually expressed as mL/ minute).
• However, renal clearance provides a useful way of quantifying renal excretory functions.
• It can be used to quantify the rate at which blood flows through the kidneys, as well as to measure the basic kidney functions such as GFR.
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Concept of Clearance (continues):• Renal clearance of a substance (S) is calculated by
dividing the urinary excretion rate of (S) (US × V•) by its plasma concentration (PS), as expressed below:
US × V•
CS = PS
• Where: US = urine concentration of S.
V• = urine flow rate/ minute = (0.9 mL/ minute).
PS = plasma concentration of S.
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• Clearance of any substance depends on the behavior of the tubular cell towards that substance:
• If a substance like inulin, which is freely filtered at the glomerulus level and is neither reabsorbed nor secreted by the renal tubule, then its clearance equals to GFR.
• On the other hand, if a substance is reabsorbed by the renal tubule, its clearance is lower than the GFR.
• Finally, if a substance that is in addition to filtration is secreted by the renal tubule, then its clearance is higher than the GFR.
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