Organs of The Urinary System

Organs of The Urinary System

The urinary bladder stores urine prior to micturition, the urethra expels urine

from the bladder, the ureters bring urine to the bladder. But the function of the kidneys is NOT to make urine, it is to maintain homeostasis of the blood: excreting wastes, keeping nutrients, maintaining electrolytes, acid base balance, and other things.

Structure of the Kidney

The kidney is composed of several layers and is covered with a fibrous

capsule, the
renal capsule
. The outer layer of the kidney is the
cortex
. It

contains the major (upper) portion of the
nephrons.
The middle layer of the

kidney is the medulla. It is composed of the triangular shaped pyramids and

the renal columns. The pyramids contain the collecting tubules and loops of

Henle, the lower portion of the nephrons.

Kidney Vascularization

The blood supply of the kidney is paramount in its function. The two kidneys

receive between 15 and 20% of the body's systemic blood flow at rest. The

renal artery
branches into
lobar
and then interlobar arteries.

Nephrons: Functional Units of the Kidneys

The Nephron
: - The nephrons are the functional units of the kidney, i.e.

individually and collectively they perform the functions of the kidney.

Each nephron is served with blood by the
afferent arteriole
. This vessel

brings blood into a capillary tuft called the
glomerulus
. Blood leaving the

glomerulus flows into the
efferent arteriole
.

The Bowman's capsule opens into the
proximal convoluted tubule
which

leads to the
loop of Henle
. The loop of Henle has a
descending limb

which passes into the medulla, recurves, and becomes the
ascending limb

which leads back up to the
distal convoluted tubule
in the cortex. Most

human nephrons are termed
cortical
nephrons because their corpuscles are

located in the mid to outer cortex and their loops of Henle are very short and

pass only into the outer medulla. But a small portion are called

juxtamedullary nephrons
and their loops travel deep into the inner

medulla.

Step 1 in urine formation,
Filtration
- Fluid pressure forces water and

dissolved substances out of the blood into Bowman's capsule. Filtration

averages 125 ml/min for your two kidneys. This amounts to about 180 Liters

per day. Since we urinate an average of 1500 ml per day, more than 99%

must be returned to the blood. Filtration involves the small molecules: water,

electrolytes, urea, glucose, amino acids. It does
not
involve the blood

proteins or cells.

1. Filtration

Filtration - Hydrostatic pressure (blood pressure) forces

water and dissolved substances out of the glomerular blood

into Bowman’s capsule.

H2O, glucose, amino acids, electrolytes, wastes

Averages 125 ml/min for both kidneys 􀃆 180 liters/day

The Glomerulus and Bowman’s Capsule

The capillaries of the glomerulus are surrounded by specialized cells which

form the inner (visceral) layer of Bowman's capsule.

These specialized cells are called
podocytes
(foot cells) because they

have processes called
pedicels
which interdigitate or interlace producing

openings called
filtration slits
. The capillaries are fenestrated in order to

allow a large amount of filtration. The outer (parietal) layer of Bowman's

capsule consists of epithelial cells with tight junctions and serves to contain

the filtrate in the
capsular space
.

Step 2: Reabsorption

Reabsorption - the return of substances from

filtrate in the nephron tubule to the blood or

interstitial fluid.

The return of substances from the filtrate to the blood and

interstitial fluid. The major substances reabsorbed are water, NaCl, glucose, and

amino acids. Some of the urea, together with other salts are also reabsorbed.

Reabsorption occurs in each of these areas for various substances and to

various degrees: Most reabsorption occurs in the
PCT
(
Proximal

Convoluted Tubule
), but reabsorption of water also occurs from the

descending limb
of the
Loop of Henle
, reabsorption of salt from the

ascending limb
and the
DCT
(
Distal Convoluted Tubule
), and more water

from the
Collecting Duct
.

The Counter-Current MechanismsThis mechanism works in the loop of Henle to increase water reabsorbed from the descending limb as a result of salt

reabsorbed from the ascending limb. The term countercurrent comes from

the fact that fluid is moving in opposite directions in the two limbs of the loop.

This magnifies the effect of transport from one limb on transport from the

other limb.

The Counter-Current Mechanisms

1) The counter-current multiplier - increases the

amount of H2O reabsorbed because of opposite

movement in the two limbs of the loop of Henle.

2) The countercurrent exchange of salt -

increases the reabsorption of H2O by retaining

NaCl in the medulla.

Step 3: Secretion

Secretion is the active release of substances into the

nephron tubule by the tubular lining cells. Secretion is the release by active transport of substances into the filtrate. It is accomplished by the tubular lining cells. The substances released are usually derived from the blood in the peritubular capillaries. Actually secretion has already been going on but it is the third process we consider. It

begins in the proximal convoluted tubule and continues in the distal

convoluted tubule and the collecting tube. Secretion occurs as an active transport by the cells of the nephron tubule in

which they transport substances

obtained from the peritubular blood or interstitial fluid into the nephron

tubule. It occurs in the proximal

convoluted tubule, distal convoluted tubule, and collecting tubule.

Normal Constituents of Urine

Water (sp. Gravity 1.001 to 1.035),

urea,

uric acid,

creatinine,

Na+, K+, PO4

-3, SO4

-2, Ca+2, Mg+2

Abnormal Constituents of Urine
(Table 26.2)

Glucose - Recent intake of sugary foods, diabetes m.

Protein - Physical exertion, high protein;

hypertension, glomerulonephritis.

Ketone bodies - Starvation, untreated diabetes mellitus

Hemoglobin - Hemolytic anemia, severe burns

Bile pigments - Hepatitis, cirrhosis, bile obstruction

Erythrocytes - Bleeding due to trauma, kidney stones,

infection, cancer.

Leucocytes - Urinary tract infection