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Kidney & Urinary System Kidney & Urinary System Location and Structure of Kidney Lie against the dorsal body wall in a retroperitoneal position in the superior lumbar region. Receive some protection from lower part of the rib cage. Right kidney positioned slightly lower than the left due to its location near liver. Above each kidney is adrenal gland, it’s a part of endocrine system and separate organ. Other structure include ureters, renal blood vessels and nerves enter or exit the kidney at the hilus. Kidney enclosed by a fibrous, transparent renal capsule. Adipose capsule, a fatty mass surrounds each kidney and helps hold it in place against the muscles of the trunk wall. Kidney & Urinary System Listed here are other structures which forms human kidney system : Renal cortex : outer region contains blood vessels, glomeruli and urine tubes. Renal pyramids : The conical segments within the internal medulla. They contain the secreting apparatus and tubules. Renal column : structures which support the cortex, consist of lines of blood vessels and urinary tubes and a fibrous material. It separates the renal pyramids. Renal artery : Two renal arteries come from the aorta, each connecting to a kidney. The artery divides into five branches, each of which leads to a ball of capillaries. The arteries supply (unfiltered) blood to the kidneys. The left kidney receives about 60% of the renal blood flow. Renal vein : The filtered blood returns to circulation through the renal veins which join into the inferior vena cava. Renal pelvis : accepts the urine and channels it out of the hilus into the ureter. Kidney & Urinary System Kidney & Urinary System Nephrons Each kidney contains million of tiny structures called nephrons. It’s the structural and functional units of the kidneys. Responsible for forming urine. Two types of nephrons : cortical and juxtamedullary. They are classified based on location : cortical located almost entirely within the cortex, juxtamedullary situated close to cortex-medulla junction. Consists of two main structures : glomerulus and renal tubule. Glomerulus is a knot of capillaries which receives blood supply from an afferent arteriole of renal circulation. Renal tubule the portion of the nephron of the kidney containing the tubular fluid filtered through the glomerulus. After passing the tubule, the filtrate continues to the collecting duct system. Kidney & Urinary System Kidney & Urinary System Kidney & Urinary System Kidney & Urinary System Kidney & Urinary System Glomerular or Bowman’s capsule is a portion of renal tubule which completely surrounds the glomerulus. Inner layer of capsule made up of cells called podocytes. Podocytes have long branching processes called foot processes. Podocytes form a porous membrane around glomerulus because filtration slits exists between their extensions. From glomerular, renal tubule forms several other structure where each structure forms a region with different name. They are proximal convoluted tubule, loop of Henle and distal convoluted tubule. As tubule extends from glomerular, it coils and twists before forming a hairpin loop, then again become coiled and twisted before entering collecting ducts. Collecting ducts receives urine from many nephrons and run downward through medullary pyramids. Kidney & Urinary System Kidney & Urinary System Glomerulus is feed and drained by arterioles. Afferent arteriole is the feeder vessel. Efferent arteriole receives blood that has passed through glomerulus. These arterioles are high resistance vessels. Afferent arteriole has larger diameter than efferent. Blood pressure in glomerular capillaries is higher than other capillary beds. This extremely high pressure forces fluid and solutes out of the blood into the glomerular capsule. Collecting ducts receives urine from many nephrons and run downward through medullary pyramids. Peritubular capillaries arises from efferent arteriole that drain the glomerulus. These capillaries are low-pressure porous vessels that are adopted for absorption. Kidney & Urinary System Kidney & Urinary System Urine Formation Urine formation is a result of three processes : filtration, tubular reabsorption and tubular secretion. Filtration A nonselective, passive process. Glomerulus acts as a filter. Filtrate that is formed is blood plasma without blood proteins. Proteins and blood cells too large to pass through filtration membrane. But if either of these appear in urine, there might be problem with glomerular filters. As long as systemic blood pressure is normal, filtrate will be formed. If arterial blood pressure drop too low, glomerular pressure becomes inadequate to force substances out of blood into tubules, thus filtrate formation stops. Kidney & Urinary System Kidney & Urinary System Tubular Reabsorption Filtrate contains useful substances besides wastes that must be removed. These substances must be returned to the blood. Tubular reabsorption begins as soon as the filtrate enters the proximal convoluted tubule. Tubule cells are transporters, taking up needed substances from the filtrate. The substances are passed out into extracellular space from which they are reabsorbed into peritubular capillary blood. Reabsorption mostly depends on active transport which uses membrane carriers and very selective. Example of needed substances that entirely removed from filtrate are glucose and amino acids. Nitrogenous waste products are poorly reabsorbed. Kidney & Urinary System These include urea formed by liver as end product of protein breakdown. Tubular secretion Reabsorption in reverse. Some substances (e.g. hydrogen, potassium ions) move from blood of peritubular capillaries through tubule cells or from tubule cells into filtrate to be eliminated in urine. Important for getting rid of substances not already in filtrate or as additional means for controlling blood pH. Kidney & Urinary System Renal Circulation In a resting adult, kidney receive 1.2 – 1.3 liters of blood per minute. Just under 25% of cardiac output. Renal blood flow can be measured with EM flowmeter or other type of meter. Renal plasma flow equals amount substance excreted per unit time divided by renal arteriovenous difference. Renal plasma flow commonly measured by infusing para-aminohippuric acid (PAH) or iodopyracet and determining their urine and plasma concetrations. These substances filtered by glomeruli and secreted by tubular cells so that their extraction ratio is high. Extraction ratio = (CA – CRV/CA) where CA is arterial concentration and CRV is renal venous concentration. Effective renal plasma flow (ERPF) is amount of PAH in urine divide by amount of PAH in plasma. Kidney & Urinary System Example Given concentration PAH in urine = 14 mg/ml , concentration PAH in plasma = 0.02 mg/ml , urine volume = 0.9 ml/min ERPF U PAH V PPAH 14 0.9 0.02 630 ml / min