Download The Kidney

The Kidney
11.3
Kidney functions
• Kidneys filter blood in the body to remove
metabolic waste products (such as urea),
toxins, and excess water
– Excretion is the removal from the body of the
waste products of metabolism (Note: defecation is
NOT considered excretion because feces is not a
waste product of metabolic reactions – it is simply
undigested food)
• Kidneys also function to reabsorb essential
nutrients (water, proteins, glucose, ions) back
into the bloodstream
• The kidney is part of the urinary (excretory)
system, which functions to filter/ balance the
blood and produce urine
‘Clean’ blood out
(renal vein, vena cava)
‘Dirty’ blood in
(aorta, renal artery)
Gross Anatomy of the Kidney
Structure
Function
Renal Cortex (“outer portion”)
Ultrafiltration and selective reabsorption of blood
contents (glucose, salt, water)
Renal Medulla (“middle”)
Reabsorption of water (osmoregulation)
Renal Pelvis
Urine is delivered here by the collecting ducts and is
passed on from the pelvis to the ureter
Ureter
Carries urine to the urinary bladder
(Bowman’s capsule)
Nephron Structure and Function
1.
2.
4.
5.
3.
1. Ultrafiltration (cortex): the process by which various substances are
filtered through the glomerulus under extremely high pressure
(also called “Bowman’s capsule”)
Step
Description
1
Blood enters the capsule
through the afferent arteriole
(which is larger than the
efferent (draining) arteriole –
creating high pressure inside
the capsule)
2
Blood passes into the
glomerulus (capillary bed
inside the capsule)
3
High pressure forces water
and blood contents (except
large proteins, platelets and
blood cells) through
fenestrations (small slits/
pores) in the glomerulus
4
Filtered contents of the blood
(glomerular filtrate – water,
glucose, amino acids, urea)
pass through the basement
membrane of the capsule into
the proximal convoluted
tubule
5
Blood cells, proteins, platelets
etc. that did not become part
of the filtrate exit the capsule
through the efferent arteriole
2. Selective Reabsorption
• Filtrate from the glomerulus contains many important
molecules (glucose, water, amino acids) and ions (salt ions
such as Na+, K+, and Cl-) that the body cannot afford to lose
(your entire blood volume is filtered by your kidneys ~25
times per day!)
• The proximal convoluted tubule (of the nephron) acts to
reabsorb these important substances into the bloodstream
The Proximal Convoluted Tubule
• Filtrate flows inside the PCT within its lumen (the interior
portion of the tube)
• Microvilli surround the lumen (to increase surface area
for reabsorption)
• The wall of the PCT surrounds the microvilli and is only
ONE CELL THICK (cells contain mitochondria for active
transport)
• A network of capillaries (called the peritubular capillary
bed) surrounds the PCT for reabsorption
Peritubular capillary bed
PCT wall (one cell thick)
Lumen
•
•
Selective Reabsorption
Salt ions are actively transported from the PCT wall cells into the peritubular capillaries
Decreased salt ions in the PCT cells creates a concentration gradient between the lumen
(high salt ions in the filtrate) and the wall cells (low salt ions), causing salt ions to move
passively into the wall cells of the PCT (from the lumen) by facilitated diffusion
• Water moves out of the PCT by osmosis (FOLLOWING the salt ions – to balance them
out)
• Due to movement of sodium ions, glucose and amino acids are cotransported from the
PCT lumen into wall cells
• Glucose and amino acids move
from the wall cells into the blood
by facilitated diffusion
Note: Passive processes only move
~50% of glucose back into the blood.
The rest of the glucose in the filtrate is
ACTIVELY transported back into the
blood so that ALL of it (100%) is
reabsorbed – there should NEVER be
glucose (or amino acids) in the urine.
Glucose in urine can be an indicator of
diabetes!
Osmoregulation
• Osmoregulation is the control of water balance
(blood, tissue, cytoplasm) in an organism
• In the kidney, the loop of henle and the collecting
duct work together to maintain water balance
The loop of Henle
• The loop of henle consists of two parts: a descending
portion (which dives down into the medulla of the kidney)
and an ascending portion
• Filtrate from the PCT passes into the descending loop
(which is permeable to water)
– Water passes out of the tubule by osmosis into the hypertonic
environment of the fluid in the medulla
• Filtrate then passes into the ascending loop (which is
permeable to salt ions)
– Sodium ions are actively pumped out of the filtrate and into the
medulla (creating a hypertonic environment in the fluid)
• Overall, the loop of henle causes the filtrate volume to
decrease (less water), and a large amount of salt ions are
removed (creating more dilute filtrate)
The Collecting Duct
• From the ascending portion of the loop of henle, filtrate moves into a
tube (called the distal convoluted tubule), where it is “fine tuned”
(with regard to solute concentration) before moving into a tube called
the collecting duct
• The collecting duct (in the medulla of the kidney) acts to reabsorb
water into the blood (under the hormonal control of ADH)
• In the collecting duct, water moves back into capillaries
(reabsorption) through osmosis
• IF a person is dehydrated, ADH acts on the walls of the collecting
duct- producing aquaporins (water channels), making the duct
MORE permeable to water (so more water flows out of it and is
reabsorbed back into the bloodstream)
– This makes the urine MORE concentrated (hypertonic), which passes to the
renal pelvis, then the ureter, and is collected in the bladder for excretion
Kidney Health and Diagnostics
• In a healthy individual, the filtration of blood through the kidney
looks similar to the following:
Molecule
Amount in blood plasma
Amount in glomerular Amount in urine (fluid
(blood entering the kidney
filtrate (fluid in the
in the renal pelvis,
in the renal artery)/ mg per PCT)/ mg per 100ml
which flows to the
100ml
bladder)/ mg per
100ml
Proteins
>700
0
0
Glucose
>90
0
0
Urea
30
30
>1800
Why?
a. Proteins are too large to fit through the basement membrane (in the
glomerulus) – they stay in the blood and are NEVER part of the filtrate/ urine
b. Glucose is 100% reabsorbed into the bloodstream through mechanisms in the
PCT
c. Urea (metabolic waste) becomes more concentrated in the filtrate as more
water is removed/ reabsorbed
Urine Analysis
WHY would this happen?
Active transport of
glucose (in the PCT)
reaches a maximum
rate – the maximum
threshold
concentration of
glucose that can be
transported this way is
exceeded in diabetics,
causing some glucose
to remain in the filtrate/
urine