Renal Function pdf - Dr. Abdullah

بسم هللا الرحمن الرحيم

Renal function

it is important to recognize that the kidneys serve

multiplefunctions, including the following:



Excretion of metabolic waste

products and foreign

chemicals

 Regulation of

water and electrolyte

balances

 Regulation of body fluid

osmolality

and electrolyte

concentrations

Regulation of

arterial pressure

 Regulation of

acid-base balance



Secretion

metabolism

, and

excretion of hormones



Gluconeogenesis

Introduction

• In the kidneys, a fluid that resembles plasma is filtered

through the glomerular capillaries into the renal

tubules (

glomerular filtration

• As this glomerular filtrate passes down the tubules, its

volume is reduced and its composition altered by the

processes of

tubular reabsorption

(removal of water

and solutes from the tubular fluid) and

tubular

secretion

(secretion of solutes into the tubular fluid) to

form the urine that enters the renal pelvis.

•

A comparison

of the composition of the

plasma

and an

average

urine

specimen illustrates the magnitude of

some of these changes and emphasizes the manner in

which wastes are eliminated while water and

important electrolytes and metabolites are conserved.

Urinary and plasma concentrations of some physiologically

important substances

Introduction

• Furthermore, the composition of the urine can be

varied

, and many

homeostatic regulatory

mechanisms

minimize or prevent changes in the

composition of the ECF by changing the amount

of water and various specific solutes in the urine.

• From the renal pelvis, the urine passes to the

bladder and is expelled to the exterior by the

process of urination, or

micturition

• The kidneys are also endocrine organs, making

kinins

and

1,25-dihydroxy-cholecalciferol

and

making and secreting

renin

Functional anatomy

FUNCTIONAL ANATOMY

• The Nephron

Each individual renal tubule and its

glomerulus is a unit (

nephron

• There are approximately

1.3 million nephrons

in each

human kidney.

• The glomerulus, is formed by the invagination of a tuft of

capillaries into the dilated, blind end of the nephron

(

Bowman's capsule

• The capillaries are supplied by an

afferent arteriole

and

drained by a slightly smaller

efferent arteriole

• There are two cellular layers separating the blood from the

glomerular filtrate in Bowman's capsule: the capillary

endothelium

and the specialized epithelium of the capsule

that is made up of

podocytes

overlying the glomerular

capillaries .

• These layers are separated by

a basal lamina

FUNCTIONAL ANATOMY

• Stellate cells called

mesangial cells

are located

between the basal lamina and the endothelium.

• Mesangial cells are especially common

between two

neighboring capillaries

, and in these locations the

basal membrane forms a sheath shared by both
capillaries

• The

mesangial cells

are

contractile

and play a role in

the regulation of glomerular filtration .

• They also secrete various substances, take up

immune complexes, and are involved in the
production of glomerular disease.

FUNCTIONAL ANATOMY

• The

endothelium

of the glomerular capillaries is

fenestrated

, with pores that are

70-90 nm

in diameter.

• The cells of the epithelium (

podocytes

) have

numerous pseudopodia that interdigitate to form

filtration slits

along the capillary wall.

• The slits are approximately

25 nm wide

, and each is

closed by a thin membrane. The basal lamina does not

contain visible gaps or pores.

• Functionally

, the glomerular membrane permits the

free passage of neutral substances up to

4 nm

diameter and almost totally excludes those with

diameters greater than

8 nm

• However, the

charges on molecules

as well as their

diameters

affect their passage into Bowman's capsule .

FUNCTIONAL ANATOMY

• The human

proximal convoluted tubule

• Its wall is made up of a

single layer of cells

that

interdigitate with one another and are united by

apical tight junctions.

• Between the bases of the cells, there are

extensions of the extracellular space called the

lateral intercellular spaces

• The luminal edges of the cells have a striate

brush border due to the presence of innumerable

microvilli

Functional anatomy

• The convoluted portion of the proximal tubule (

pars

convoluta

) drains into the straight portion (

pars

recta

), which forms the first part of the

loop of Henle

• The proximal tubule terminates in the thin segment

of the descending limb of the loop of Henle, which
has an

epithelium made up of attenuated, flat cells

• It ends in the

thick segment of the ascending

limb.

• The cells of the thick ascending limb are

cuboid

• They have numerous

mitochondria

, and the basilar

portions of their cell membranes are extensively

invaginated.

Functional anatomy

• The

thick ascending limb

of the loop of Henle

reaches the glomerulus of the nephron from which

the tubule arose and to its afferent arteriole and

efferent arteriole.

•

The walls of the afferent arterioles contain the

renin-secreting

juxtaglomerular cells

• At this point, the tubular epithelium is modified

histologically to form the

macula densa

•

The juxtaglomerular cells, the macula densa,

and the lacis cells near them are known

collectively as the

juxtaglomerular apparatus

Functional anatomy

• The

distal convoluted tubule

. the epithelium is lower than

that of the proximal tubule, and although there are a few
microvilli, there is no distinct brush border.

• The distal tubules coalesce to form

collecting ducts

and pass

through the renal cortex and medulla to empty into the pelvis
of the kidney at the apexes of the

medullary pyramids

• The epithelium of the collecting ducts is made up of

principal

cells

(

P cells

) and

intercalated cells

(

I cells

• The

P cells

, which predominate, are relatively tall and have

few organelles. They are involved in

reabsorption and

vasopressin-stimulated water

reabsorption.

• The

I cells

, which are present in smaller numbers and are also

found in the distal tubules, have more microvilli, cytoplasmic
vesicles, and mitochondria. They are concerned with

acid

secretion and HCO

transport

Blood Vessels

• The

efferent arteriole

from each glomerulus

breaks up into capillaries that supply a number of
different nephrons.

•

In humans, the total surface of the renal

capillaries is approximately equal to the total
surface area of the tubules, both being about 12
m

• The volume of blood in the renal capillaries at any

given time is 30-40 mL.

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