11.3 - The Kidney

Uploaded by luizgmello on 05.11.2012

Hey everybody and welcome to topic 11.3, the Kidney!
Many people wonder why the kidney is not studied together with the digestive system. There’s
a big misconception that urine is the body’s way of getting rid of the liquid you drink
and don’t need. After your studies of the digestive system,
you’re probably aware that the digestive tract starts at the mouth and ends at the
anus, forming a tube that goes through your body. Food in this ‘tube’ never really
enters your body, except for the nutrients that are broken down by enzymes and absorbed
in the small intestines. Water is taken up in the large intestines. Anything left over
leaves the body as feces, including different amounts of water and molecules for which we
don’t have enzymes. This includes cellulose in plant materials that are part of our regular
diet. It’s a different story with the kidney.
The role of the urinary system is to filter the blood and get rid of nitrogen waste material
produced by the body, in particular during the degradation of aminoacids (so nothing
to do with what you drink). A buildup of nitrogenous waste can damage cells, hence the importance
to remove them from the body. This function of the kidney is known as ‘excretion’.
The other function of the kidneys is known as osmorregulation. This concept is nothing
new. You remember from your study of cell membranes that cells need to maintain water
balance to keep them from shriveling or bursting. This was even more important for plant cells
who need to maintain turgor to stand up against gravity. The movement of water across membranes
was called osmosis. Well, osmosis happens in order to promote osmorregulation: the ideal
water balance within a cell or organism. If the blood contains too much water, the kidney
will filter some of it out; if the blood contains too little water, your kidneys will reduce
the production of urine. These levels are monitored by the kidneys, which signal the
brain to induce the feeling of thirst or craving for salt.
- The kidney is shaped like a bean, with one
renal artery delivering blood to each kidney. The renal vein removes the blood that has
been filtrated back to the vena cava inferior and the heart. The ureter removes urine produced
by the kidneys and delivers it to the bladder. If the kidney were a bean, these three structures
would enter and leave through the area of the micropyle.
- The medulla and the cortex is where filtration
takes place. The blood that comes in through the renal artery is taken to individual structures
called nephrons, responsible for the ultrafiltration that occurs in the kidney. If you look at
the structure of a single nephron, you’ll see which parts are in the cortex and which
parts are on the medulla. The arterioles bringing blood to the nephron will form a capillary
network called the glomerulus. The glomerulus sits inside of a cup-like structure called
the Bowman’s capsule. -
The structure and function of each of these parts of the nephron are, not surprisingly,
connected. The glomerulus is made up of arterioles with fenestrated walls that allow large molecules
to pass. The Bowman’s capsule will absorb the fluid leaked out of the glomerulus. This
fluid then moves from the capsule to the proximal convoluted tubule, then to the loop of Henle,
and then to the distal convoluted tubule. From there, the urine drips down the collecting
duct and is directed to the bladder for temporary storage. Check out the image of the structure
of a nephron on Allott page 101 for a visual. -
Ultrafiltration is the process of filtration that occurs in the nephron. Many processes
and structures make this process possible. Firstly, the high pressure caused by the narrow
capillaries forces molecules against the walls of the glomerulus and out towards the Bowman’s
capsule. Remember that the walls of these capillaries are fenestrated, which allows
large molecules to go through. You’re probably thinking that if any molecule,
of any size, is able to pass through, that the body would probably lose important molecules
in this process. And you’re right. To prevent this, a basement membrane (made of glycoproteins)
between the capillary bed and the Bowman’s capsule prevents molecules larger than 68,000
amu to go through. These molecules have nowhere to go except back into the capillary.
- We defined osmorregulation earlier as the
control of water balance within a cell or organism. This typically involves the movement
of minerals through diffusion, facilitated diffusion or active transport – water will
then follow the concentration gradient through osmosis).
- The Bowman’s capsules in the kidneys remove
about 6 liters of fluid per hour from the capillaries. Consider the fact that an average
adult has approximately the same amount of blood circulating in their body! This would
certainly result in the death of the organism, if not for the fact that the proximal convoluted
tubule reabsorbs 80% of the fluid back into the blood stream.
- The process of reabsorption is outlined in
this slide. The fluid collected at the Bowman’s capsule drips down into the proximal convoluted
tubule. This fluid contains glucose and minerals (no proteins, because they’re larger than
68,000 amu!). 100% of the glucose and 80% of minerals are reabsorbed by the cells lining
the tubule. The large mitochondrial density within these cells provides energy when the
concentration gradient is no longer favorable for diffusion. As the concentration increases,
water follows by osmosis. Because structure relates to function, in this case, absorption,
these cells also have microvilli to increase surface area.
- In this picture, you can see the water moving
out of the proximal convoluted tube and the descending limb of the loop of Henle, which
was described in the previous slide. In the descending limb, sodium is pumped in via active
transport and water follows by osmosis. The walls of the descending limb are permeable
to water, but not sodium (hence the need for active transport). The ascending limb is permeable
to sodium, but not water. Overall, the loop of Henle increases the absorption of minerals
into the medulla, to make sure they’re not all removed by the body. Low fluid intake
and diets rich in salt can cause renal calculi (or kidney stones); their formation is the
result of the inability of the loop of Henle to deal with the excess salt concentration.
- This slide shows the concentration of glucose,
urea (the molecule used by the body to remove nitrogen) and proteins. You can see their
concentrations in four different parts of the process of ultrafiltration. First, compare
the contents of the blood delivered by the renal artery and the blood taken away by the
renal vein. Only the urea concentration is lowered, suggesting that this is the main
objective of the kidneys – remove nitrogen waste. Then, compare the concentration of
glucose in the blood in the renal artery and the venal artery, and the glomerular filtrate.
This indicates that all of the glucose passes through the basement membrane in the Bowman’s
capsule into the proximal convoluted tubule, but is reabsorbed into the blood stream. Lastly,
note that no proteins make their way to the glomerular filtrate because of their inability
to cross the basement membrane. -
In patients with diabetes, the blood concentration of glucose is abnormally high, and most of
it would pass to the filtrate. Because of the excess glucose, however, active transport
is unable to pump 100% of the glucose back into the blood stream. This leads to an increase
in the glucose concentration in the urine. -
With this information in mind, deduce what Table 2 should look like in comparison to
Table 1. Pause the video now. The urea and protein concentrations would
remain unchanged, while the glucose concentration would be abnormally high in the blood stream
and the filtrate. The nephron’s inability to reabsorb all of the glucose would lead
to some glucose present in the urine. -
And this concludes topic 11.3, the Kidney. Keep checking Moodle for related assignments
and make sure that you are taking thorough notes on all of the video lectures – as
you would in a regular class! I’ll see you guys in class.