Lesson 2: Filtration

Question 1

The Nephron

Answer 1

Within the cortex and medulla of
each kidney are about one million
tiny filters called nephrons.

Each nephron consists of five
parts;

1. Bowman’s Capsule
2. Proximal Tubule
3. Loop of Henle
4. Distal Tubule
5. Collecting Duct

Only the loop of Henle extends into the renal
medulla, the majority of a nephron is in the
cortex.

The tubes of the nephrons are surrounded by
cells and a network of blood vessels that spreads
throughout the tissue. Any material that leaves
the nephron enters the surrounding cells and
eventually returns to the bloodstream through
the network of blood vessels.

By controlling what leaves and what remains in
the nephron, the kidneys keep the levels of
water, ions, and other materials nearly constant
and within limits necessary to maintain
homeostasis.

Question 2

Filtering Through the
Nephron

Answer 2

Blood enters the Bowman’s capsule
through a tiny artery that branches to
form a network of porous thin –
walled capillaries called the
glomerulus.
Under the influence of blood pressure, some blood plasma and small
particles are forced out of the capillaries into the surrounding capsule.
Larger blood components such as blood cells and proteins remain in the
capillaries.

The fluid in the Bowman’s capsule is called nephritic filtrate. Nephritic
filtrate is pushed out of the capsule into the proximal tubule where
re-absorption takes place.

Question 3

Proximal Tubule

Answer 3

Re-absorption is the process by which
materials required by the body are removed
from the filtrate and returned to the body.

Osmosis, diffusion and active transport draw
water, glucose, amino acids and ions from the
filtrate into the surrounding cells – from here
these materials are returned to the
bloodstream.

When the filtrate reaches the end of the
proximal tubule the fluid is isotonic with the
surrounding cells.

Question 4

The Loop of Henle

Answer 4

From the proximal tubule, the filtrate moves
to the loop of Henle. The primary function of
the loop of Henle is to remove water from
the filtrate by the process of osmosis.

The loop of Henle descends into the inner
renal medulla (descending loop) and returns
to the cortex (ascending loop).

The cells of the medulla have an increased
concentration of sodium ions (Na+). These
ions increase the concentration gradient.

This increasing concentration gradient acts
to draw water from the filtrate down the
length of the descending loop.

The high levels of Na+ in the medulla are a
result of the active transport of Na+ ions out
of the ascending loop.
The amount of water removed from
the filtrate by the time it reaches the
bottom of the loop of Henle results in
an increased concentration of all
materials dissolved in the remaining
tissues.

Thus, as the filtrate moves up the
ascending loop, Na+ is actively pulled
from the filtrate into the surrounding
tissue.

Water cannot re – enter because this
loop is impermeable to water.

Question 5

Distal Tubule

Answer 5

From the loop of Henle the filtrate enters the distal tubule.

The distal tubule is responsible for a process called tubular secretion.

Tubular secretion involves active transport to pull substances such as
hydrogen ions, creatine and drugs out of the blood into the filtrate.

The fluid from a number of nephrons moves from the distal tubules into a
common collecting duct, which carries what can now be called urine into the
renal pelvis.