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BLOCK: URIN 313
PHYSIOLOGY OF THE URINARY SYSTEM
LECTURE 1
Dr. Amel Eassawi
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STRUCTURE AND FUNCTIONS
OF NEPHRON
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OBJECTIVES
The student should be able to:
List and understand the major functions of the kidneys.
State the role of erythropoietin and the stimulus for its secretion
Describe the location of the kidneys and their gross anatomical features.
Describe the different parts of the nephron and their location within the
kidney.
List the individual nephron segments, in order and identify each structure
as being in the cortex or medulla.
List the structures of the renal circulation and their association with
different aspects of the nephron.
Distinguish between superficial and juxtamedullary nephrons.
Describe the essential elements of the juxtaglomerular apparatus and its
function in the nephron.
Describe the basic renal process of urine formation.
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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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KIDNEY FUNCTIONS
– Urine formation
– Maintain H2O balance in the body
– Maintain osmolarity of 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
– Help maintain acid-base balance in the body: 1. Excrete acids
(kidneys are the only means of excreting non-volatile acids, such
as sulfuric acid and phosphoric acid. 2. Regulate body fluid
buffers ( e.g. Bicarbonate)
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KIDNEY FUNCTIONS
- Excreting (eliminating) the end products (wastes) of bodily
metabolism e.g. urea (from protein metabolism), uric acid (from
nucleic acid metabolism), creatinine (from muscle metabolism),
bilirubin (from hemoglobin metabolism).
- Excreting metabolites of various hormones.
- Excreting - drugs, food additive.
- Produce erythropoietin.
- Produce Renin.
- Convert vitamin D into its active form.
1,25 dihydroxycholecalciferol
In vivo vitamin D synthesis involves a series of biochemical
transformations, the last of which occurs in the kidneys. The
active form of vitamin D (1,25-dihydroxyvitamin D3) ( also known
as calcitriol), is activated in the kidneys,
and its rate of synthesis is regulated by
hormones that control calcium and phosphate
balance.
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Regulation of Erythrocyte Production
Start
Normal blood oxygen levels Stimulus: Hypoxia due
to decreased RBC
count, decreased
availability of O2 to
blood, or increased
tissue demands for O2
Increases
O2-carrying
ability of blood
Reduces O2
levels in
blood
Enhanced
erythropoiesis
increases
RBC count
Erythropoietin
stimulates red
bone marrow
Kidney (and liver to
a smaller extent)
releases
erythropoietin
Kidneys produce Erythropoietin: a hormone that
stimulates bone marrow to produce red cells.
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Renin-Angiotensin-Aldosterone System
Renin 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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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.
• 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.
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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
•
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•
•
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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
– Has dual function
• Provides route for eliminating urine from bladder.
• Passageway for semen from reproductive organs.
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
• Arrangement of the nephrons within the 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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VASCULAR COMPONENT OF THE NEPHRON
• The renal corpuscle consists of a compact tuft of interconnected capillary
loops, the glomerulus (pl. glomeruli) or glomerular capillaries, surrounded by a
balloon-like hollow capsule: Bowman’s capsule. Blood enters and leaves
Bowman’s capsule through arterioles that penetrate the surface of the capsule
at the vascular pole.
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VASCULAR COMPONENT OF THE NEPHRON
• 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.
• 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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TUBULAR COMPONENT OF THE NEPHRON
– 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
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TUBULAR COMPONENT OF THE NEPHRON
• 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].
• Loop of Henle – form U-shaped or hair pin loop, LH dips into renal
medulla.
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TUBULAR COMPONENT OF THE NEPHRON
• Loop of Henle – 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.
• Distal convoluted tubule 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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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 Juxtaglomerular cells is called
Juxtaglomerular apparatus.
• Juxtaglomerular apparatus 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 structures
– Juxtamedullary nephrons
– Cortical 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.
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JUXTAMEDULLARY NEPHRONS
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About 20 per cent of the nephrons
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have glomeruli that lie in the inner layer of the renal cortex near the
medulla
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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 FORMATION
Basic 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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REFERENCES
Human physiology by Lauralee Sherwood, seventh edition
Text book physiology by Guyton &Hall,11th edition
Text book of physiology by Linda .s contanzo, third edition
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