URINARY SYSTEM

Physiology 2

Presented by: Dr. Shaimaa Nasr Amin

Lecturer of Medical Physiology

General Education Program

Introduction and physiologic

anatomy of the kidney

1-Overview of Renal Function

Kidneys have both excretory and regulatory functions

that participates in homeostasis

*Excreting excess solutes ,water and rid waste

products

*Regulate volume and composition of body fluids

within very narrow range

Major regulatory functions of the kidneys

1-Regulation of water and electrolyte balance.

2-Excretion of metabolic waste products.

3-Excretion of foreign chemicals.

4-Endocrine functions .

5-Regulation of arterial blood pressure.

6-Regulation of acid base balance.

7-Gluconeogenesis.

8-Secretion of prostaglandins and bradykinin.

Physiologic anatomy of the kidneys

-Site: on the posterior wall of the abdomen ,outside the

peritoneal cavity.

-Weight= 150 gm /each kidney.

-Dimensions: Length <12cm.

Width <8cm.

-Surrounded by thin but tough capsule.

-The renal artery, vein, lymphatics, nerve supply and

ureter enter the kidney at he hilum on the medial side

Renal mass is

divided into

Outer cortex

Granular and

deeper in colour

Inner medulla

Striated and

lighter in colour

Pelvis

Major

calyces

Minor calyces

Renal papilla

Multiple renal

Pyramids

Medulla

1 . Renal Vein 2 . Renal Artery 3 . Renal Calyx 4 . Medullary

Pyramid 5 . Renal Cortex 6 . Segmental Artery 7 . Interlobar Artery 8 . Arcuate Artery 9 . Arcuate Vein 10 . Interlobar Vein 11 . Segmental Vein 12 . Renal Column 13 . Renal Papillae 14 . Renal Pelvis 15 . Ureter

Nephron as a functional unit -There are about 1.3 million

nephrons in each human kidney.

-Each is capable of forming urine

(functional unit)

Parts of the nephron

1- Glomerulus

2-Proximal convoluted tubule

3-Loop of Henle

4-Distal convoluted tubule

5-Collecting ducts

1-Glomerulus *Tuft of capillaries contained within the dilated blind

end of renal tubule (Bowman’s Capsule).

*The capillaries are supplied by afferent arteriole

and drained by efferent arteriole.

*It is high pressure capillary bed (,the hydrostatic

pressure is 60 mmHg)

2-Proximal convoluted tubules *15mm ,lie in the cortex

*Wall made of single layer of cells with apical tight

junctions.

*The luminal edges have brush borders due to

presence of many microvilli.

3- Loop of Henle U-shaped extension of PCT that dips in renal medulla.

Consists of :

a) Descending limb .

b) Ascending limb.

*The wall of the descending +lower½ of the ascending

limb= Thin Segment of loop of Henle

*Upper ½of the ascending limb = Thick segment of loop

of Henle

4-Distal Convoluted Tubule 5mm long lies in the cortex.

No distinct brush border but there are few microvilli.

5-Collecting Ducts 20mm long pass through renal cortex and medulla to

empty into the pelvis of the kidney

Collecting ducts lined by 2 types of cells:

2-Intercalated cells 1-Principal cells

2-Intercalated cells

(I cells)

1-Principal cells

(P cells)

*Found in smaller number .

*Have more microvilli,

mitochondria and

cytoplasmic vesicles.

Involved in:

Acid secretion.

Bicarbonate reabsorption.

Predominant ,relatively tall

Involved in :

Na+ reabsorption.

H2O reabsorption.

Types of Nephrons Juxtamedullary

Nephrons

Cortical Nephrons

15% 85% % of total

number

Deep in renal cortex near the

medulla

In the outer portion of renal

cortex

Glomeruli

Long &dips deeply into the

medullary pyramids

Short &,penetrate a short

distance into the medulla

Loop of Henle

Vasa recta + Peritubular

capillary

The tubule is surrounded by

peritubular capillary

Vascular

structures

supplying tubule

Play important role in urine

concentration

Special Functions

Juxta Glomerular apparatus

-Specialized tubular and vascular cells located at the

vascular pole where afferent and efferent arterioles

enter and leave the glomerulus.

-Plays important role in auto regulation of renal blood

flow and GFR during change in arterial pressure .

JGA Consists of:

1-Macula densa.

2-Juxtaglomerular

cells.

3-Lacis cells.

1-Macula densa *Modified tubular cells in the initial portion of distal

tubule that comes in contact with afferent and efferent

arterioles .

*The macula densa is in close proximity to JG cells.

*They monitor the composition of fluid in tubular lumen

at this point ( chemoreceptors that is stimulated by ↓

Nacl load ) .

2-Juxtaglomerular cells -Epithelioid granular cells located at the media of

afferent arterioles and to lesser extent the efferent arterioles as they enter the glomeruli.

*They secret renin.

*Act as baroreceptors and respond to change in perfusion pressure

*Stimulated by ↓ renal perfusion pressure or hypovolemia

3-Lacis cells

-These agranular cells located at the

junction between afferent and efferent

arterioles.

-They contain renin .

Renal blood flow The kidneys receive 21% of cardiac output

Renal Vascular arrangement:

*Aorta → Renal artery → interlobar arteries & Arcute & intralobular arteries

*Interlobular arteries → Afferent arterioles glomerular capillary → efferent arterioles→Peritubular capillaries → interlobular veins → arcute veins → interlobar veins → renal vien

capillary beds 2

associated with

each nephron

Glomerular

capillary bed

Peritubular

capillary bed

1-The glomerular capillary bed

(High pressure bed) *Receive its blood from afferent arterioles .

*The hydrostatic pressure is about 60mmHg (causes

rapid filtration of fluid).

What are the causes of this high

pressure?

*This high pressure is due to: 1-The renal arteries are direct branches of abdominal

aorta.

2-The afferent arterioles are short straight branches of

interlobular arteries .

3-The efferent arterioles have high resistance than the

afferent arterioles.

2-Peritubular capillary bed

(Low pressure bed) *The hydrostatic pressure is about 13mmHg.

*Behave like venous end of the capillaries.

*The low pressure permits fluid reabsorption

from the interstitium into the blood.

Regional blood flow *Renal cortex receives most of renal blood flow 98%

for filtration of large volume of plasma through the

glomeruli.

*while renal medulla receives 2% of renal blood flow

to form concentrated urine.

Autoregulation of renal blood flow -When the kidney is perfused at a moderate

pressure (90-220 mmHg) the renal blood flow is

relatively kept constant by change of renal

vascular resistance .

-Renal autoregulation is present in denervated and

isolated kidney (independent of nerves or

hormones)

Mechanisms of autoregulation of renal

blood flow 1- With rise of pressure:

-Stretch cause contraction of smooth muscles of

afferent arterioles ,this prevent excessive ↑ in renal

blood flow.

-Stretch of the vascular wall ↑ Ca++ influx from ECF

into the muscle fibers causing them to contract.

2-At low pressure:

Angiotensin II plays an important role by

constricting the efferent arteriole thus

maintaining the GFR.

Aim of autoregulation: To maintain constant GFR and to allow control

of renal excretion of water and solutes

despite marked changes in arterial blood

pressure.

Innervation of renal vessels and renal

tubules Sympathetic fibers supply: stimulation causes

1)Renal vessels:

V.C with ↓ in renal blood flow and GFR.

2)JGA:

↑ renin secretion by juxtaglomerular cells (mediated by B1

adrenergic receptor).

3)Renal tubule: ↑Na+ reabsorption by tubular cells by direct

action of norepinephrine on renal tubular cells.

Thank you