NEPHROLOGY:
THE MAKING OF
URINE
DR. NAEEM
Outline
Introduction
Function
Cross Section of the kidney
Blood supply
Nephron
Nephron Physiology
INTRODUCTION
• The kidneys are a pair of
bean shaped organs
found along the posterior
wall of the abdominal
cavity.
• The left kidney is located
slightly more superior
than the right kidney due
to the larger size of the
liver on the right side of
the body.
Function of the kidneys
Homeostatic functions
Regulation of electrolytes
Maintenance of acid-base
Regulation of blood pressure
Filtration of blood
Formation of URINE
Production of hormones - calcitriol, erythropoietin and
the enzyme renin.
Blood Supply
The renal arteries branch directly from the abdominal aorta and enter the
kidneys.
Once in the kidney, the renal arteries diverge into smaller arterioles.
Afferent arterioles carry blood into the renal cortex, where it separated into
a bundle of capillaries known as a GLOMERULUS.
From the glomerulus, the blood recollects into smaller efferent arterioles
that descend into the renal medulla.
The efferent arterioles separate into the peritubular capillaries that
surround the renal tubules.
Next, the peritubular capillaries merge to form veins that merge again to
form the large renal vein.
From the renal vein, the blood joins the IVC which carries blood back to
the heart.
Blood Supply cont…
Nephrons
The actual removal of wastes occurs in tiny units
inside the kidneys called nephrons. Each kidney
has about a million nephrons.
In the nephron, a glomerulus—which is a tiny
blood vessel, or capillary—intertwines with a tiny
urine-collecting tube called a tubule.
The glomerulus acts as a filtering unit, or sieve,
and keeps normal proteins and cells in the
bloodstream, allowing extra fluid and wastes to
pass through.
Renal Corpuscle
•
The renal
corpuscle is the
initial bloodfiltering component
of a nephron.
•
It consists of 2
structures: the
glomerulus and a
Bowmans capsule.
Renal Corpuscle Continued…
The structures of the layers of
the Glomerulus determines the
permeability.
Factors that influence the
selectivity are the negative
charges on the Basement
Membrane , the epithelium and
the pore size of the glomeruli.
As a result, small ions like Na &
K will pass freely while large
molecules like proteins, albumin
and hemoglobin have no
permeability.
Renal Corpuscle Continued
Basic Filtration unit of the kidney.
GFR:
Glomerular Filtration Unit
Rate at which blood is filtered through all of the
golmeruli, and thus the measure of the overall renal
function.
Proximal Tubule
Also known as the
Proximal Convoluted
Tubule (PCT).
A duct system
transporting urine from
the Bowmans Capsule
to the Loop of Henle.
65-80% of the filtrate is
reabsorbed in the PCT.
Promixal Tubule Continued…
Substance
Reabsorbed
% of Filtrate
Na & H2O
~66%
Glucose& amino acids
~100%
Potassium
~65%
Urea
~50%
Phosphate
~80%
A summary of the reabsorption along
the proximal tubule:
Na: Diffusion through ion channels
followed by water and Cl or Cotransported with glucose or amino acids.
K: Potassium is absorbed mainly by the
paracellular route with water via osmosis.
Na/K ATPase is pumping 2 K back into
the epithelial cells & K is then cleared
from the cells using a co-transporter with
chlorine.
Urea: 50% of filtered urea is reabsorbed
in the proximal tubule. However the
concentration of urea actually increases
thanks to the reabsorption of 70% of the
filtered water in the same portion of the
nephron.
Glucose: Reabsorption of glucose can
only occur in the proximal tubule and
occurs regardless of the concentration
gradient as it is completed via secondary
active transport. It is reabsorbed using a
co-transported with sodium.secondary
active transport. It is reabsorbed using Na.
H Secretion/HCO3 Reabsorption:
• The secretion of H+ in this section of
the nephron is mainly a result of the
Na+/H+ exchanger. Energy for it
provided by Na/K ATPase.
Loop of Henle
Is the part of the nephron
that connects the proximal
convoluted tubule (PCT) to
the distal convoluted tubule
(DCT).
The main function of the
Loop of Henle is to
maintain a concentration
gradient.
Loop of Henle Continued…
Is divided into 4 parts.
Thin descending limb of the loop of Henle.
Thin ascending limb of loop of Henle
Thick ascending limb
Cortical thick ascending limb
Loop of Henle Continued…
Loop of Henle Continued…
Overall the loop of Henle reabsorbs around 25% of filtered
ions and 20% of the filtered water in a normal kidney.
These ions are mostly
Na
Cl
K
Ca
HCO3.
Distal Convoluted Tubule
The distal convoluted tubule (DCT) is a portion of kidney nephron
between the loop of Henle and the collecting duct system.
Functions:
Reabsorbs Na+ ions through coupled secretion of H+ or K+ ions
into the tubular fluid via aldosterone. By acidifying the urine the
distal convoluted tubule plays an important role in acid-base
balance.
Normally is relatively impermeable to water. However in the
presence of antidiuretic hormone (ADH) its permeability to water
increases permitting concentration of the urine.
Secretes ammonium ions and some drugs.
Forms part of the juxtaglomerular apparatus.
Hemodynamic regulation via macula densa and renin
Electrolyte homeostasis via Calcium reabsorption
Distal Convoluted Tubule
cont…
On the apical surface
(lumen side), the DCT
cells have a Na/CL
cotransporter and are
permeable to Ca.
On the basolateral surface
(blood), there are several
other pumps for Ca, Na
& K.
Distal Convoluted Tubule
cont…
Sodium and potassium levels are controlled by secreting K+ and
absorbing Na+.
Na absorption by the distal tubule is mediated by the hormone
aldosterone. Aldosterone increases Na reabsorption.
The DCT also participates in Calcium regulation by reabsorbing
Ca in response to the Parathyroid hormone (PTH).
Collecting Duct
Is the final portion in the nephron.
The filtrate travels from the Distal Convoluted Tubule,
through the collecting duct and into the ureter.
It participates in electrolyte and fluid balance through
reabsorption and excretion. These processes are regulated by
the hormones aldosterone and antidiuretic hormone
(ADH).
In humans, the system accounts for 4–5% of the kidney's
reabsorption of sodium and 5% of the kidneys reabsorption
of water.
Collecting duct cont…
The wide variation in water
reabsorption levels for the collecting
duct system reflects its dependence on
hormonal activation. The collecting
ducts are largely impermeable to water
without the presence of antidiuretic
hormone (ADH or vasopressin).
In the absence of ADH, water in the
renal filtrate is left alone to enter the
urine, promoting diuresis.
When ADH is present, aquaporis allow
for the absorption of water, thereby
inhibiting diuresis.
URINE
Urine – is the sterile liquid of the body secreted by the
kidneys through a process called urination and
excreted through the urethra.
Color, odor, pH, presence/absence of blood, turbidity
& volume are all important aspects of the urine.
THE END!
QUESTIONS??