Download The Kidney

The Kidney Excretion
and
Homeostasis
HL
Paper 1 and 2
Assessment Statement
11.3.1
11.3.2
11.3.3
11.3.4
11.3.5
11.3.6
11.3.7
11.3.8
11.3.9
Define excretion
Draw and label a diagram of the kidney
Annotate a diagram of a glomerulus and associated nephron to show the
function of each part
Explain the process of ultrafiltration, including blood pressure,
fenestrated blood capillaries and basement membrane
Define osmoregulation
Explain the reabsorption of glucose, water and salts in the proximal
convoluted tubule, including the roles of microvilli, osmosis and active
transport
Explain the roles of the loop of Henle, medulla, collecting duct and ADH
(vasopressin) in maintaining the water balance of the blood
Explain the differences in the concentration of proteins, glucose and urea
between blood plasma, glomerular filtrate and urine
Explain the presence of glucose in the urine of untreated diabetic patients
Answer the following:
What is excretion? What are the main products we excrete?
Why do we need to eliminate waste?
This is done by the Lungs, digestive tract and Kidneys.
The Kidney
The kidneys are a pair of reddish organs shaped like kidney beans. They are located just
above the waist, close to the posterior wall of the abdominal cavity. The right kidney is
slightly lower than the left, because of the space taken up by the liver.
Each kidney has a renal artery leading to it and a renal vein and a ureter leading away
from it. The renal vein takes the ‘clean’ blood away from the kidney while the ureter
leads the urine to the bladder.
Draw a diagram of the Human Urinary System
The kidney itself has several parts:
1. Cortex
2. Medulla
3. Pyramid
4. Pelvis
The cortex and the renal pyramid make up the functional unit of the kidney. The
functional unit, or the parenchyma, consist of about 1 million microscopic structures
called nephrons.
Draw a diagram of the Kidney
Functions of the Kidney
Find the three main functions of the kidney.
What is urea? (NH2)2CO
- Urea is a toxic substance!
- If urea is allowed to build up in your body, it can cause uremia (build up of
ammonia in blood – can be fatal)
- urea is a nitrogen based compound
- when compound like amino acids are broken down, ammonia is created (NH3)
- ammonia is a toxic substance with a basic pH
- ammonia flows through the blood to the liver, where it is converted into a
compound, urea
- urea is then transported through the blood to the kidneys, where it is filtered out
and eliminated
Mammals
- Mammals excrete urea
- Various mammals concentrate urea to a differing level,
depending on the environment
- I.e. – those that live in drier environments will pull more
water out of the urea. Desert animals, for example, will
have a longer loop of Henle, which we will discuss next.
Nephron and Filtration
The products of excretion differ from organism to organism and from excretory
system to excretory system. For example, oxygen is a waste product in plants,
excreted via an organ called the stomata. In humans, carbon dioxide is a waste
product, but is excreted, via our lungs and respiratory system.
The functional unit of excretion is the nephron.
There activity of the nephron is based upon three principles:
1.
2.
3.
Ultra-filtration – fluid part of the blood under high pressure is forced into the
nephron
Reabsorption – substances that your body wants are diffused back into the
blood, for example, glucose.
Secretion – All the “bad” stuff (urea) are secreted in the filtrate, and eventually
urinated out.
Only plasma and small particles can be filtered. Large proteins and cells remain in the
blood.
The Glomerulus and the Nephron Diagram
The Process of Ultrafiltration
Blood carries the particles left to be absorbed. It is from the blood that passes through the
kidney, that the last of the nutrients the body needs is taken.
1.
2.
3.
4.
The renal artery supplies the kidney with blood.
It splits into many smaller blood vessels and each nephron has an afferent
vessel, which carries the blood to the glomerulus, which is a bundle of
specialized capillaries.
The blood eventually returns to the efferent vessel, which carries the blood
around to the other parts of the nephron.
Blood, that has been filtered passes out into larger vessels and becomes the renal
vein.
Connected to the glomerulus is the Bowman’s Capsule. The blood in the glomerulus is
under high pressure and this is where ultrafiltration takes place.
Reason – The efferent vessel (e = exit), is narrower than the afferent vessel going in, and
therefore, the blood pressure is much higher. Particles and fluid are pushed into the
Bowman’s Capsule.
****In the kidney, all of the blood in the body passes through very 5 minutes!!!
Approximately 15 –20% of the fluid in the blood will pass into the Bowman’s
Capsule, which is equivalent to almost 200 L of water!!! (Bathroom break anyone?)
Bowman’s Capsule
In order for the filtrate to pass into the capsule from the glomerulus, is has to pass a
barrier.
There are three layers which the blood fluid must pass through to enter the nephron:
1. Inner wall of the glomerulus – which contains small pores that allows plasma
to mover through. It is said to be fenestrated. There are small slits along with
the pores to filter the fluid.
2. Basement membrane of the glomerulus – protein membrane outside the cells;
it contains no pores and serves as a filter during ultrafiltration. Its main job is to
stop the blood cells and large proteins. (Acts like a dialysis membrane, made of
negatively charged glycoproteins).
3. Outer wall of the Glomeruls – made of specialized cells called podocytes.
They have many folds that surround the blood vessels and a network of filtration
slits that hold back the blood cells.
This process is passive and unselective.
Podocytes – cells with feet. See the diagram.
Content of Filtrate per dm-3 of blood plasma
Solutes
Na+ ions (mol)
Cl- ions (mol)
Glucose (mol)
Urea (mol)
Proteins (mol)
Plasma
151
110
5
5
740
Filtrate
144
114
5
5
3.5
Ultrafiltration at the glomerulus and Bowman’s Capsule
Osmoregulation and reabsorption
Define Osmoregulation
We said earlier that almost 200 L of water goes into the Bowman’s capsule. 198 L is
reabsorbed along the way. Therefore the net loss of water in the urine is usually 1.5
to 2 L.
The Proximal Convoluted Tubule
The Bowman’s capsule is the first part of the nephron. This is where the filtrate goes.
From the Bowman’s capsule, the filtrate goes to the proximal convoluted tubule. Most
of the re-absorption takes place here. The fluid is similar to plasma, as it contains
glucose, amino acids, vitamins, hormones, urea, salt, ions and water.
All of the glucose, amino acids, vitamins, hormones and most of the sodium chloride and
water are reabsorbed into the peritubular capillaries. Osmosis drives the re-absorption of
the water. Sodium, glucose, and salts are absorbed by active transport. Chloride follows
the actively transported sodium, based upon an electrochemical gradient. The water
follows the salts, because in the proximal tubule, the salts create a concentrated
environment. Water flows out of the tubule to the vessels to dilute the salts.
To facilitate the movement of across the wall of the PCT, the lumen of the tube is
covered with microvilli, similar to the small intestine. In the microvilli, are
mitochondria to provide energy (ATP) for the active transport.
Draw diagram of Proximal Convoluted Tubule
The Loop of Henle
The Loop of Henle is an extension of the proximal tubule, but it extends into the medulla
of the kidney. There are two sections, the descending loop and the ascending loop. It
creates a hypertonic environment in the medulla.
In the descending loop, water leaves by osmosis due to the increasing concentration of
salt. The water immediately passes into the blood and is removed from the area. Some
salt diffuses out as well.
The ascending loop in impermeable to water and salt is lost from the filtrate by active
transport. The amount of salt lost here is greater than in the descending loop. The salt
remains near the loop, in the tissue, to maintain a concentration gradient in the medulla.
The fluid that leaves the Henle’s Loop is less concentrated than the tissue fluid around it.
The concentration gradient is maintained by the vasa recta, the blood vessels that run
along the loop of Henle. There is no direct exchange to the blood and substances must
pass through the tissue of the medulla. The tissue maintains a concentration gradient,
called the vasa recta countercurrent exchange.
Therefore, as the blood, from the afferent vessel enters the medulla, and from the
descending capillary, it will lose water by osmosis and gain salt and urea by diffusion.
In the ascending capillary, the reverse happens. The concentration of the blood does not
change and go from tissue to capillary. All the salts and urea are recycled in the medulla.
As the filtrate leaves the loop of Henle, there is water, very little sodium and salts, and
large amounts of urea.
From Nephron to Bladder and Beyond
The wall of the distal convoluted tubule is permeable to water and the water can pass
from the ultrafiltrate into the blood vessels, to be carried away. The same happens in the
collecting duct.
But this only happens in the presence of ADH.
ADH increases the permeability of the walls of the distal convoluted tubule and the
collecting duct.
When there is a lack of water, as we saw before, ADH is released. The dilute filtrate
coming from the Henle’s Loop can then lose water from the distal convoluted tubule and
the collecting duct. The water is reabsorbed by the blood.
When ADH is absent, the walls are impermeable, and the urine is dilute (ie. Lots of
Water)
Extension – Kidney Stones
Occasionally, the crystals of the salts present in urine may solidify into insoluable stones
called renal calculi (Kidney Stones).
They may be formed in any portion of the urinary tract.
Ingestion of too much calcium, a decrease in the amount of water, abnormally alkaline or
acidic urine, or overactivity of the parathyroid glands have been attributed to the
development of kidney stones.
When a stone gets lodged in a narrow opening, like a ureter, the pain is immense. The
only way to relieve the pain is to get rid of the stone.
Now back to the main character – The Kidney
Questions
1. Review your notes and determine what is in the following sections around the
kidney.
Blood in the Renal Artery vs. Blood in the Renal Vein.
a) High O2 content, more urea, salt and possibly more water than the set value
b) More CO2 than before, correct amounts of water and salt
Glomular Filtrate vs. Urine
a) Less water, less salt, no glucose, no proteins or amino acids but a lot more
concentrated urea
b) No large proteins – otherwise the same as blood plasma
2. Explain why the following concentrations are observed.
Molecule
Proteins
Glucose
Urea
Amount in blood
plasma in mg 100
ml-1
> 700
> 90
30
Amount in
glomerular filtrate
in mg 100 ml-1
0
> 90
30
3. Why can diabetes lead to the presence of glucose in the urine?
Amount in urine
in mg 100 ml-1
0
0
>1800