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Chapter 11

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Department
Physiology
Course
Physiology 1021
Professor
Tom Stavraky
Semester
Fall

Description
Introduction Renal system includes the kidneys, ureters, bladder, and urethra Principal function of the kidneys is the regulation of water balance, electrolyte levels, pH of the blood, and the long-term regulation of arterial pressure Functions of the Kidneys Basic function of the kidneys is to remove nonessential substances from the plasma, including waste metabolites, excess water, and electrolytes and to recover any essential substance like glucose Kidneys play a major role in regulating the water levels, the chemical concentration of the body fluid compartments, and pH (or acidity) of the blood Kidneys do not produce water or electrolytes but only conserve them by reducing the amount removed from the body The elimination of waste or foreign substances is an important function of the kidneys Includes the removal of drugs, food additives, and vitamins that are excreted in the urine. The kidneys also act as an endocrine gland, producing hormones or components of hormonal systems such as erythropoietin, renin, vitamin D, and stanniocalcin Anatomy of the Kidneys Roughly the size of a fist Consist of an outer renal cortex, a middle renal medulla, and inner calyces that drain into a central renal pelvis Renal pelvis then drains into the ureter Located within the renal pyramids are the functional units of the kidneysthe nephrons Each nephron drains through a collecting duct into a calyx AnatomyBlood Supply of the Kidneys Blood flows to the kidneys through the renal artery -> several interlobar arteries -> arcuate arteries Blood in the arcuate arteries flows through the interlobular arteries to supply the nephron Blood supply to the nephron drains into the interlobular vein, the arcuate vein, the interlobar vein, and then into the renal vein Anatomy The Nephron Over 1 million nephrons in each kidney whose function is to filter the blood, reabsorb essential substances, and excrete nonessential molecules and waste Each nephron is composed of a highly coiled hollow tube surrounded by a complex blood supply The glomerular capsule (also called Bowman's capsule) surrounds a very small, highly permeable capillary bed called the glomerulus These structures are often collectively referred to as the renal corpuscle The tubular portion of the nephron consists of the following structures in order: the proximal convoluted tubule (a highly coiled portion of the nephron), the descending and ascending limb of the loop of Henle, the distal convoluted tubule, and the collecting duct AnatomyBlood Supply of the Nephron Blood from the renal artery eventually reaches the interlobular artery that drains into the afferent arteriole The afferent arteriole gives rise to the glomerulus (where filtration takes place) The blood from the glomerulus enters the efferent arteriole Blood then enters the peritubular capillaries (a dense network of capillaries surrounding the tubes of the nephron), which then drains into the interlobular vein and eventually back to the renal vein The Renal Corpuscle The renal corpuscle is made up of the glomerular capsule (also called Bowman's capsule) and glomerulus This is the site where the blood is filtereda process called glomerular filtration The fluid that is filtered from the blood that enters the glomerular capsule (or capsular space) is called the filtrate Glomerular filtration, as we will see, is facilitated by a highly permeable capillary endothelium that is surrounded bypodocytes The larger diameter afferent arteriole and smaller diameter efferent arteriole also enhance glomerular filtration Processes along the NephronImportant Terms Filtration is the movement of fluid through the glomerular capillary due to hydrostatic pressures The filtrate is the solution created by filtration The filtrate is generally composed of water plus all the dissolved solutes in the blood (except for large proteins that are too big to be filtered) Reabsorption is defined as the movement of a substance from the lumen of the nephron back into the blood Secretion is the movement of a substance from the blood into the lumen of the nephron Excretion is the removal of a substance from the body Excretion = filtration + secretion reabsorption Glomerular Filtration bulk flow of fluid from the blood into the glomerular capsule This fluid, called the filtrate, contains the same substances as plasma with the exception of large proteins and red blood cells Glomerular filtration is affected by the extremely permeable capillaries, which make up the glomerulus, and Starling Forces Special epithelial cells called podocytes surround the capillaries Podocytes have large filtration slits that are formed between pedicles These structural features increase filtration at the glomerulus Glomerular FiltrationStarling Forces Starling Forces cause the bulk movement of fluid across capillaries due to a combination of hydrostatic and colloid osmotic forces The glomerular capillary is similar, although the pressure of each force will be different The blood hydrostatic pressure is roughly 60 mmHg almost twice that in a regular capillary, causing filtration of fluid into the glomerular capsule This pressure is principally due to the difference in diameter between the afferent (large) and efferent (small) arterioles The colloid osmotic pressure due to plasma proteins is 32 mmHg, causing reabsorption of fluid into the plasma The capsular hydrostatic pressure is 18 mmHg, causing the reabsorption of fluid There is no colloid osmotic force in the glomerular capsule since very few proteins are filtered Resulting net filtration pressure is 10 mmHg out of the glomerulus into the capsular space Glomerular Filtration Rate (GFR) and Filtered Load Kidneys filter a tremendous amount of fluid each dayroughly 180 L/day (48 gallons/day) This glomerular filtration rate (GFR) is the volume of fluid that is filtered by the glomerulus during a certain time period Kidneys filter many other substances, it is important to be able to calculate the amount of these substances filtered by the kidneys per day; this is called the filtered load: It is often important to be able to calculate not only the GFR and filtered load of a substance but also its urine concentration and the amount of solute excreted These values also tell the physician important information concerning the health and functioning of the kidneys Urine concentration is the amount of the solute that is excreted per unit volume of urine (g/L) The amount of solute excreted is the actual amount (in grams) of solute that is excreted in the urine and can be calculated using equation 2 The amount reabsorbed is the amount of filtered substance that is taken back up (reabsorbed) by the kidneys and can be calculated using equation 3. The fraction excreted is calculated using equation 4. Tubular Transport Mechanisms Introduction.
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