THE URINARY SYSTEM STRUCTURE AND FUNCTIONS OF NEPHRON • LECTURE - 1 • Dr. Zahoor
Urinary System • Consists of • Urine forming organs • kidneys • Structures that carry urine from the kidneys to the outside for elimination from the body • Ureters • Urinary bladder • Urethra
Kidney Functions • Urine formation • Maintain H2O balance in the body • Maintain osmolarityof body fluids, primarily through regulating H2O balance • Regulate the quantity and concentration of most ECF ions e.g. Na+, Cl-, K+, Ca2+, H+, HCO3-, PO43- • Maintain plasma volume
Kidney Functions (cont) • Help maintain acid-basebalance in the body • Excreting (eliminating) the end products (wastes) of bodily metabolism e.g. urea, uric acid, creatinine, bilirubin • Excreting - drugs, food additive • Producing erythropoietin • Producing Renin • Converting vitamin D into its active form
Excretionof Metabolic Waste Products • Eliminating waste products of metabolism that are not longer needed by the body • Urea (from protein metabolism) • Uric acid (from nucleic acid metabolism) • Creatinine (from muscle metabolism) • Bilirubin (from hemoglobin metabolism) • Metabolites of various hormones
Regulation of Acid-Base Balance Lungs Body fluid buffers Kidneys Excrete acids (kidneys are the only means of excreting non-volatile acids, such as sulfuric acid and phosphoric acid. Regulate body fluid buffers ( e.g. Bicarbonate)
Regulation and production of hormones and enzymes • The kidney produces: • Erythropoietin • Renin • 1,25 dihydroxycholecalciferol( also known as calcitriol) The active form of vitamin D that is important in calcium and phosphate metabolism.
Regulation of Erythrocyte Production Hypoxia Erythropoietin O2 Delivery Kidney Erythrocyte Production in Bone Marrow Kidneys produce Erythropoietin: a hormone that stimulates bone marrow to produce red cells.
Renin production • It is an enzyme secreted by the kidneys from granular cells of the juxtaglomerular apparatus. • It activates the renin-angiotensin system by converting angiotensinogen to angiotensin I.
Metabolism of Hormones Most peptide hormones are metabolized and excreted by the kidney (e.g., insulin, angiotensin II, etc.)
Kidneys • The kidneys are a pair of bean-shaped organs that lie at the back of abdominal cavity. • Kidneys are Retroperitoneal ( behind the peritoneum ) • Each kidney is about 4-5 inches (10-12 cm) in length
Kidneys (cont) • Each kidney is supplied with a renal artery and a renal vein • Acts on plasma, flowing through it to produce urine • Formed urine drains into the renal pelvis, Located at medial inner core of each kidney • Urine is drained into ureters, and stored temporarily in the urinary bladder before it’s emptied through the urethra to outside the body.
Ureters • Smooth muscle-walled duct • Exits each kidney at the medial border in close proximity to renal artery and vein • Carry urine to the urinary bladder
Urinary Bladder • Temporarily stores urine • Hollow, distensible, smooth muscle-walled sac • Periodically empties to the outside of the body through the urethra
Urethra • Conveys urine to the outside of the body • In females Urethra is straight and short about 4cm long • In males • Much longer about 19- 20cm and follows curving course from bladder to outside • Dual function • Provides route for eliminating urine from bladder • Passageway for semen from reproductive organs
APPLIED • In male, Prostate gland lies below the neck of bladder and completely encircles the urethra. • Prostate gland enlargement occurs during middle to older age and can occlude (block) the urethra, therefore, obstructing the flow of urine.
Nephron • Functional and structural unit of the kidney • Approximately 1 million nephrons/kidney • Each nephron has two components • Vascular component • Tubular component
Nephron (cont) • Arrangement of nephrons within kidney gives rise to two distinct regions • Outer cortex • Renal cortex (granular in appearance) • Inner medulla Renal medulla -made up of striated triangles called renal pyramids
Nephron (Vascular component) • Dominant part is the glomerulus • Glomerulus is a tuft of glomerular capillaries. • large amounts of fluid & solutes are filtered from the blood.
Vascular Component • Renal Artery enters the kidney and forms afferent arteriole, which supplies each nephron. • Afferent arteriole delivers blood to the glomerulus. • Glomerular capillaries rejoin to form another arteriole – the efferent arteriole. • The blood that was not filtered in the glomerulus goes to efferent arteriole.
Vascular Component (cont) • Efferent arteriole gives second set of capillaries, the peritubular capillaries, which supply the renal tissue and are important in exchange with tubular system. (Peritubular means around the tubular system) • Peritubular capillaries rejoin to form venules, that drain in renal vein.
Nephron (Tubular component) • Hollow, fluid-filled tube • single layer of epithelial cells • Components • Bowman’s capsule • Proximal convoluted tubule • Loop of Henle • Descending limb (thin) • Ascending limb (thin and thick part) • Distal convoluted tubule • Collecting duct or tubule • Juxtaglomerular apparatus
Tubular component (cont) • Bowman’s capsule – expanded double walled invagination that cups around the glomerulus to collect fluid from the glomerular capillaries. • From bowman’s capsule, filtered fluid passes into PCT. PCT lies entirely in the cortex. • From PCT, fluid passes into loop of Henle [LH].
Tubular component (cont) • Loop of Henle – form U-shaped or hair pin loop, LH dips into renal medulla. • LH – descending limb of LH goes from cortex to medulla, and ascending limb of LH passes from medulla to cortex. • Ascending limb of LH, forms distal convoluted tubule (DCT), DCT lies in cortex.
Tubular component (cont) • DCT empties into the collecting tubule or duct. • Collecting tubule gets fluid from about 8 nephron (DCT). • Collecting tubule passes from cortex to medulla and empties into renal pelvis.
Juxtaglomerular apparatus • Distal convoluted tubule, afferent and efferent arterioles are specialized to form Juxtaglomerular apparatus (JGA). • Cells in the DCT are called Macula densa and the cells in the afferent (mainly) and efferent arteriole which contain secretary cells are called Juxtaglomerular cells. • The combination of Macula Densa and JGC is called JGA. • JGA secretes Renin in blood.
Nephron • Two types of nephrons • Distinguished by location and length of their structures • Juxtamedullary nephrons • Cortical nephrons
Regional differences in nephron structure • Cortical nephrons • - About 80% nephron • have glomeruli located in the outer cortex. • have short loops of Henle that penetrate only a short distance into the medulla, • In cortical nephron, peritubular capillaries do not form vasarecta, but go around the short loop of Henle • Involved in solute reabsorption.
Regional differences in nephron structure (cont.) • Juxtamedullary nephrons. • About 20 per cent of the nephrons • have glomeruli that lie in the inner layer of the renal cortex near the medulla • have long loops of Henle that dip deeply into the medulla. • - have peritubular capillaries called vasarecta (straight vessels) which run with the loop of Henle • - Play important role in concentration mechanism of urine
URINE FORMATIONBasic Renal Processes • Glomerular filtration • Tubular reabsorption • Tubular secretion Urine results from these three processes. Excretion = Filtration – Reabsorption + Secretion
IMPORTANT • All nephron originate in the cortex. • Glomeruli of cortical nephron lie in the outer layer of cortex and glomeruli of Juxta medullary nephron lie in the inner layer of the cortex near the medulla • The kidney cannot regenerate new nephrons. • Aging causes a gradual decrease in nephron number.
What You Should Know • list the general functions of the kidney • Identify the different parts of nephron • Describe in sequence the tubular segments through which ultrafiltration flows after it is formed at Bowman’s capsule • Identify each structure as being located in the renal cortex or renal medulla. • Compare Cortical with Juxtamedullary nephron • Correlate the structure of nephron with functions • Describe in sequence the blood vessels through which blood flows when passing from the renal artery to the renal vein • Describe juxtamedullary apparatus and its function