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THE URINARY SYSTEM STRUCTURE AND
FUNCTIONS OF NEPHRON
LECTURE - 1Dr. Zahoor
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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
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Urinary System
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Kidney FunctionsUrine formationMaintain H2O balance in the bodyMaintain osmolarity of body fluids, primarily
through regulating H2O balanceRegulate the quantity and concentration of most
ECF ions e.g. Na+, Cl-, K+, Ca2+, H+, HCO3-, PO4
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Maintain plasma volume
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Kidney Functions (cont)
Help maintain acid-base balance in the body
Excreting (eliminating) the end products (wastes) of bodily metabolism e.g. urea, uric acid, creatinine, bilirubin
Excreting - drugs, food additiveProducing erythropoietinProducing ReninConverting vitamin D into its active form
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Excretion of 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
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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)
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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.
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Regulation of Erythrocyte Production
O2 Delivery Kidney
Erythropoietin
Erythrocyte Production in Bone Marrow
Hypoxia
Kidneys produce Erythropoietin: a hormone that stimulates bone marrow to produce red cells.
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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.
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Metabolism of Hormones
Most peptide hormones are metabolized and excreted by the kidney (e.g., insulin, angiotensin II, etc.)
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Urinary System
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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
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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 kidneyUrine is drained into ureters, and stored
temporarily in the urinary bladder before it’s emptied through the urethra to outside the body.
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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
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Urinary Bladder Temporarily stores urine
Hollow, distensible, smooth muscle-walled sac
Periodically empties to the outside of the body through the urethra
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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
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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.
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Nephron Functional and structural unit of the kidney Approximately 1 million nephrons/kidney Each nephron has two components
○ Vascular component○ Tubular component
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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
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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.
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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.
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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.
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Nephron (Tubular component)Hollow, fluid-filled tube
○ single layer of epithelial cellsComponents
○ 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
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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].
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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.
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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.
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Nephron
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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.
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Juxtaglomerular apparatus
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Nephron Two types of nephrons Distinguished by location and length of their
structuresJuxtamedullary nephronsCortical
nephrons
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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
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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
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URINE FORMATIONBasic Renal Processes Glomerular filtration Tubular reabsorption Tubular secretion
Urine results from these three processes.
Excretion = Filtration – Reabsorption + Secretion
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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.
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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
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