Download Kidney Reading- ADVANCED - Hatboro

Diagnostic Activity: The Kidney
Name: _____________________________________________
In this class you will be expected to be able to read and appropriately annotate complex information
whether it be text or online research. The following selection contains many terms with which you probably
are unfamiliar. Annotate the information and try your best to get a basic grasp on the concept – kidney
structure and function. You will be asked to complete an individual task tomorrow (using the reading) that
will assess your understanding. The results will provide information we can use to determine your initial
skill level.
Kidney Structure
The kidney has a bean-shaped structure having a convex and
a concave border. A recessed area on the concave border is
the renal hilum, where the renal artery enters the kidney and
the renal vein and ureter leave. The kidney is surrounded by
tough fibrous tissue, the renal capsule.
The substance, or parenchyma, of the kidney is divided into two
major structures: the outer renal cortex and the inner renal
medulla. Grossly, these structures take the shape of 8 to 18 coneshaped renal lobes, each containing renal cortex surrounding a
portion of medulla called a renal pyramid. Between the renal
pyramids are projections of cortex called renal columns.
The tip, or papilla, of each pyramid empties urine into a minor
calyx; minor calyces empty into major calyces, and major calyces
empty into the renal pelvis which is connected to the ureter.
Kidney Function
The kidneys are bean-shaped organs that serve several essential
regulatory roles in vertebrates.
The nephron is the basic
structural and functional unit of the kidney. In humans, a normal
kidney contains 800,000 to 1.5 million nephrons. Each nephron
spans the cortex (where the initial filtering portion is located) and
medulla. The nephron’s chief function is to regulate the
concentration of water and soluble substances like sodium
salts by filtering the blood, reabsorbing what is needed and
excreting the rest as urine. Nephrons also eliminate wastes from
the body, regulate blood volume and blood pressure, and
regulate blood pH.
Structure of the Nephron
Blood enters the nephron from a ball of intertwined capillaries
called the glomerulus. The glomerulus is enclosed in a cupshaped structure called the Bowman’s capsule. The Bowman’s
capsule connects to a long tube called the renal tubule. The tube
has 4 segments: the proximal convoluted tubule, the loop of
Henle, the distal convoluted tubule and the collecting duct which
opens to a minor calyx.
Function of the Nephron
Properties of the cells that line the nephron change dramatically
along its length; consequently, each segment of the nephron has
highly specialized functions.
Blood enters the kidney from the afferent arteriole which ends in
a capillary tuft called a glomerulus. The glomerular blood
pressure provides the driving force for water and solutes to be
filtered out of the blood and into the space made by Bowman's
capsule. The pressure is created because the diameter of the
efferent arterioles is smaller than that of afferent arterioles. The
glomerular filtration membrane allows only elements that are
small enough to pass through. Normal filtrate consists of water,
glucose, amino acids, urea, creatinine, and solutes such as
sodium chloride, potassium ions and bicarbonate ions. Toxins
and drugs may also be present. Proteins or red blood cells are
not present in the filtrate because they are too large to pass
through the glomerular filtration membrane.
The remainder of the blood (only approximately 1/5 of all plasma
passing through the kidney is filtered through the glomerular wall
into the Bowman's capsule) passes into the efferent arteriole.
Blood from the efferent arteriole, containing everything that was
not filtered out in the glomerulus, moves into the peritubular
capillaries which eventually rejoin the main bloodstream via the
renal vein.
The glomerular filtrate continues to the renal tubule. As the
filtrate flows through the renal tubule, its composition is
modified by the exchange of materials among the renal tubule,
the capillaries, and the extracellular fluid. The movement of
substances from the nephron back into the blood is known as
reabsorption while the movement of substances from the blood
into the nephron is known as secretion. The long and winding
course of both the renal tubule and the surrounding capillaries
provides a large surface area for the efficient exchange of
materials.
All nutrients (glucose and amino acids) and 70-80% of the ions
(potassium, sodium chloride, bicarbonate) and water are
reabsorbed from the filtrate in the proximal convoluted tubule.
Hydrogen ions, uric acid and drugs are secreted from the
peritubular capillaries directly into the proximal convoluted
tubule (they are not filtered out by the glomerulus). The filtrate
then passes into the Loop of Henle.
Reabsorption in this segment is minimum. However, the loop of
Henle plays a significant role in controlling the concentration of
urine by adjusting the concentration of salt in the interstitium,
the tissue surrounding the loop. The descending limb of the loop
is permeable to water but almost impermeable to other
substances. Water leaves and thereby concentrates the filtrate.
The ascending limb is impermeable to water but allows the
transport of other substances. Cells in the ascending wall of the
loop transport chloride ions from the filtrate to the fluid between
the loops and the collecting duct. Positively charged sodium ions
follow the chloride ions into the fluid. This process ensures that
the sodium chloride concentration of the fluid between the loops
and the collecting duct remains high and thus promotes the
reabsorption of water from the last part of the renal tubule - the
collecting duct.
Filtrate in the loop of Henle has a high concentration of metabolic
waste products such as urea, uric acid and creatinine. By the time
the filtrate reaches the loop of Henle, all the nutrients and
substances that the body needs would have already been
reabsorbed. The filtrate then passes into the distal convoluted
tubule.
The main function of the distal convoluted tubule is to regulate
sodium, potassium and pH levels. Sodium chloride and water are
reabsorbed from the distal convoluted tubule into the blood
stream as needed. Bicarbonate ions can also be absorbed.
Hydrogen and potassium ions are secreted from the blood into
the distal convoluted tubule. The pH of the blood is adjusted by
the amount of hydrogen ions that are secreted from the blood
into the filtrate and the amount of bicarbonate ions that are
reabsorbed from the filtrate into the blood. Hydrogen ions
increase acidity of the blood and bicarbonate ions decrease
acidity of the blood. The filtrate then travels to the final portion
of the renal tubule – the collecting duct.
In the collecting duct, sodium chloride and water are reabsorbed
back into the blood stream. Water is reabsorbed because of the
high sodium concentration around the tube (which was created
by the loop of Henle). The collecting duct is also permeable to
urea, allowing some of it to leave the collecting duct. This also
causes water to move out of the collecting duct and be
reabsorbed by the blood.
The plasma sodium concentration directly impacts blood pressure.
When sodium chloride reabsorption from the collecting duct is
increased, water reabsorption increases as well. This expands the
extracellular fluid compartment which raises blood pressure.
Water, sodium chloride, potassium, bicarbonate, hydrogen ions,
creatinine and urea are the components of urine which leaves
the collecting duct through the renal papillae, emptying into
the renal calyces, the renal pelvis, and finally into the urinary
bladder via the ureter.