Urinary System L 1 Functional Structures of the kidney

Prof. Madaya Dr Than Kyaw1 October 2012

Waste products of metabolism – toxic (CO2, ammonia, etc)Removal from tissues – blood and lymph

- respiratory system - skin – sweat & sebaceous gland

Urinary system –

Not only important for - removal of metabolic wastes - e.g. nitrogenous wasteBut also maintenance of body fluid and electrolyte balance (fluid homeostasis) - also has endocrine functions

Urinary System

Urinary System

The urinary system consists of- paired kidneys and ureters, - a urinary bladder, - sphincter muscles- and a urethra

Location of kidneys in animals

Location of kidneys in animals

Small ruminant

Kidneys

Paired organs suspended from the dorsal abdominal wall by a peritoneal fold and blood vessels that serve them

Located slightly cranial to the mid lumber region Retroperitoneal structure – separated from the abdominal

cavity by their envelopment of peritoneum Renal artery and veins carry blood to and from the kidneys Renal artery – arises directly from the aorta Renal vein – empties directly into the caudal vena cava

Ventral view (dog) of kidneys showing renal arteries, veins and ureters and their portions relative to the aorta, vena cava and adrenal glands.

Renal hilus – concave edge of the kidney – ureter, blood vessels, nerves and lymphatics enter or leave

Neural innervation -- Sympathetic (adrenergic) division of autonomic nervous system;

postganglionic renal nerves enter the hilus of kidney in association with the renal vessels

-- Innervation to - renal vasculature, - all segments of the nephron, and - juxtaglomerular glandular cells

Kidneys

A. Heart-shaped - Horse

B. Lobulated - Cattle

C. Bean-shaped - most of the domestic animals (dog, cat, sheep)

Types of kidney

Ventral views of right kidney1. Renal artery2. Renal vein3. ureter

A. Unilobar kidney - carnivores, equine and small ruminantsB. Multilobar - large ruminants, smooth surface, clear demarcationC. Multilobar - pig, cortical portion of the lobes fused

Types of kidney (sagittal section)

Medulla and cortex Each kidney - composed of outer cortex and inner medulla

Greater detail gross structures

Pelvis

Hilus

Bisected preserved sheep kidney

Red arrow = renal arteryBlue arrow = renal vein

3 = Ureter4 = Major calyx5 = Minor calyxP = renal pyramidB = renal column

Medulla and cortex

Medulla - striations - Loops of Henle - Collecting tubules

• 90% of blood entering the kidney supplies the cortical tissues where the bulk of nephrons are located

• The veins trace the same pathways in reverse

• At hilus – renal artery branches into smaller segmental arteries to supply sectors or segments of the mass of kidney tissues

• Each segmental artery – divides to create lobar arteries which divide to yield interlober arteries that pass between the pyramids of the medulla

• These interlobar artries branch into the arcuate arteries that arch over the base of renal pyramids.

• Small interlobular arteries radiate outwards to supply the tissues of the cortex

Renal blood flow

Renal blood flow

Aorta and renal artery

INA = Interlobular arteryAR = arcuate arteries IA = interlobar SA = Segmental artries

Renal blood flow

The nephron(Model)

Renal glomeruli

- yellow arrows

Proximal convoluted tubule

Descending loop of Henle

Ascending loop of Henle

CD = collecting duct

Distal convoluted tubules

Note: thin portion of ascending and descending loops

Functional unit of the kidney Understanding the function of nephron – essential

for understanding kidney function Nephron number – vary considerably among species Within species – nephron number relatively constant

Dogs - kidneys of large breed contain approximately similar numbers of nephrons as in small breed- compensation by having larger nephrons rather than more nephrons

The nephron

The approximate number of nephrons in domestic animals and man

Species Nephrons/kidney

Cattle 4,000,000

Pig 1,250,000

Dog 415,000

Cat 190,000

Man 1,000,000

Types of nephron

Identified by - location of glomeruli and - depth of penetration of loop of Henle into the medulla

2 types

1. Cortical or corticomedullary nephrons - glomeruli in the outer and middle cortices - loop of Henle extend to the junction of the cortex and medulla

or into the outer zone of the medulla

2. Juxtamedullary nephrons - glomeruli in the cortex close to the medulla - loop of Henle extend more deeply into the medulla - some extend as deep as the renal pelvis

Types of nepharon

Justamedullary nephrons - develop and maintain osmotic gradient from low to

high in the outer medulla to the inner medulla respectively

- 3% in pigs - 100% in cat - 14% in man

Tubular fluid from both types of nephrons - enter collecting tubules and collecting ducts

Types of nepharon

Component parts of juxtamedullary nephron

Glomerulus - tuft of capillaries - filtration function - afferent arteriole – conduct blood to the glomerulus - efferent arteriole – conduct blood away from

glomerulus - efferent arterioles – peritubular capillaries – to vasa

recti – - - to pelvis (see renal blood flow slide)

Nephron components

Nephron tubules and ducts (filtrate/fluid flow)

Filtrate from glomerulus

Bowman capsule

Proximal tubule

Loop of Henle

Distal tubule

Cortical collecting tubule

Collecting Duct

Ureter Pelvis

urethra

Bladder(store)

1. Bowman capsule2. Proximal tubule3. Descending limb of LOH4. Thin ascending limb of

LOH5. Thick ascending limb of

LOH6. Distal tubule7. Connecting tubule8. Cortical collecting tubule9. Outer medullary

collecting duct10. Inner medullary

collecting duct11. Afferent arteriole12. Glomeruls13. Efferent arteriole14. Periyubular capillaries15. Vasa recta16. To renal vein

Functional nephron with blood supply

Loop of Henle

Composed of 3 segments- Thin descending limb- Thin ascendind limb- Thick ascending limb - Lumen diameter does not change

- Descending limb of cortical nephrons - only go as deep as the outer space of the outer

medulla- Descending limb of juxtamedullary nephrons

- may extend up to the pelvis

Juxtaglomerular appatatus (JG apparatus)

JG apparatus- The junction of distal tubules and glomerulus- JG cells + macula densa + extraglomerula mesangial cells- regulate amount of blood flowing to the kidney- secretion of enzyme renin – important for the formation of angiotensin II

Macula densa - collectively named tubular cells involved in the JG apparatusJuxtaglomerular (JG) cells – cells enclosing afferent and efferent arterioles in the junction

of distal tubule and glomerulusMesangial cells – cells between macula densa and arterioles

Formation of Urine

Terminology

Renal Blood Flow (RBF) - the rate at which blood flows to the kidney (in ml

per minute)Renal Plasma Flow (RPF) - refers to part of RBF that is plasmaGlomerular Filtration Rate (GFR) - the rate at which filtrate is formed (in ml per min)Filtrate fraction (FF) - the ratio of GFR to RPF (GFR:RPF)

Some approximate value of renal function variables(11.35 kg dog in a normal state of hydration)

Variable value

Cardiac output (ml/min) 1500

Blood flow to kidney(% of cardiac output)

20

Renal blood flow (ml/min) 300

Renal plasma flow (ml/min) 180

GFR (ml/min) 45

Filtration fraction 0.25

Urine volume (24 h) 681

Glomerular filtrate in 24 h (ml) 64,800 (64.8 liters)

Volume of urine as % of filtrate 1.05

Filtrate reabsorbed (%) 98.95

Formation of Urine

GFR – normally about 100 times that of RBF - 3-5 ml/kg BW/minHigh GFR - allows a continuous filtration of plasma - rapid removal of toxic substances from the body - if they can readily pass through the glomerular

filtration barrier and not reabsorbed from the renal tubules

Formation of Urine

3 processes involved in the urine formation

1. Glomerular filtration

2. Tubular reabsorption (selective)

3. Tubular secretion (selective)

Glomerular filtration

Kidneys have functional counter part of 2 capillary beds- glomerular capillaries (filter)- peritubular capillaries (reabsorption and/or secretion)

Glomeruli- Have high pressure system (high hydrostatic pressure)- favour filtration

Peritubular- Low pressure system- favour reabsorption

High pressure system in the glomerulus favour net formation of filtrate (fluid) in the capsular space

As filtrate flows away from the capsule – colloidal osmotic pressure may be negligible

COP = colloidal osmotic pressure

HP = hydrostatic pressure

Net filtration pressure = 60 – (18+32) = 10 mm Hg

Blood flows through glomeruli – large quantity of filtrate formed

k/s glomerular filtrate

Physical barriers for filtration1. capillary endothelium of the glomerulus2. inner layer of Bowman’s capsule

3. basement membrane between endothelium and glomerulus

Glomerular filtration

Glomerular endothelium- fenestrated – porous- highly permeable

Glomerular filtration

E = endothelial poreEP = endothelial poresP = podocytes

PP = pedicels of podocyteFS = filtration slitUS = filtrate or urinary space

GBM = glomerular basement membreane

Glomerular filtration

Podocytes- Cells of the inner layer of Bowmen’s capsule

- Have cellular extensions that rest on the glomerular

basement membrane

- Slit-like pores between the extensions permit the passage

of the filtrate

Glomerular filtration

Glomerular filtration barrier

- acts like a sieve

- substances up to a molecular weight of 65,000 pass

through the barrier

- small percentage of plasma proteins pass through it

- glucose, amino acids, urea, creatinine, Na, K, chlorine,

and bicarbonate ions – readily cross the barrier

- concentration in the initial filtrate – about same as

plasma

Glomerular filtration

Pressure maintained by vasoconstriction of efferent vessels

Bow

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Porous walls + high pressure

Water and solutes <10kDa out

Water, sugars, salts, amino acids, Urea (sometimes assisted by active transport)

Large things (e.g. proteins) remain behind

Primary Urine: Dilute, no proteins etc.

Glomerular filtration

Proteinuria

- presence of abnormal amounts of protein voided in the urine

- kidney diseases that localize in or primarily affect glomeruli

- associated with proteinuria or hematuria

Factors controlling Glomerular filtration Rate

Forces determining rate of movement of fluid across the

glomerular filtration barrier are generally similar as those that

determine fluid movement out of capillaries throughout the

body.

Diameter of afferent

Dilatation of arterioles

- increase blood flow to glomeruli

- increase HP and potential for filtration

Constriction of efferent arterioles

- increase glomerular HP

Factors controlling Glomerular filtration Rate

For a given molecular size

- positively charges molecules are more readily filtered than

negatively charged molecules

- this is because of negatively charged (anionic) sites in the

glomerular basement membrane

- normally most of plasma proteins are restricted from

filtration

- kidney diseases - change of electrostatic charge on the

glomerular membrane – allow filtration

Factors controlling Glomerular filtration Rate

Effective filtration pressure

- the pressure tending to force fluid out of the capillaries

- It is the difference between the blood hydrostatic pressure

in the capillary and osmotic pressure generated by plasma

protein of the blood in the capillary

- also hydrostatic pressure and osmotic pressure of urinary

space of Bowman’s capsule - important, especially in

diseased state (blockage of urinary tract or renal tubules)

Autoregulation

Renin-angiotensin system

Circulating blood globulin, angiotensinogen to form

Angiotensin I

Renin - secreted by juxtaglomerula

cells of the kidney Angiotensin II

Aldosteronesecretion

(Zona glomerulosa)

Promote Na reabsorptionAnd retention of water

Low blood pressure

- Systemic arteriolar vasoconstriction- Increase systemic blood pressure

Angiotensin converting enzyme

(Vascular endothelium)