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THE URINARY SYSTEM STRUCTURE AND

FUNCTIONS OF NEPHRON

LECTURE - 1Dr. Zahoor

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Urinary System

Consists ofUrine forming organs

○ kidneysStructures 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 medullaRenal 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 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 (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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