Unformatted text preview: 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology Boundless Anatomy and Physiology
Urinary System Physiology of the Kidneys 1/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology Overview of Urine Formation
Urine is formed in three steps: ltration, reabsorption, and secretion. LEARNING OBJECTIVES Summarize the steps in urine formation KEY TAKEAWAYS Key Points Filtration involves the transfer of soluble components,
such as water and waste, from the blood into the
glomerulus.
Reabsorption involves the absorption of molecules,
ions, and water that are necessary for the body to
maintain homeostasis from the glomerular ltrate back
into the blood.
Secretion involves the transfer of hydrogen ions,
creatinine, drugs, and urea from the blood into the
collecting duct, and is primarily made of water.
Blood and glucose are not normally found in urine.
Key Terms urine: A liquid excrement consisting of water, salts, and urea, which is made in the kidneys then released
through the urethra.
glomerulus: A small, intertwined group of capillaries within nephrons of the kidney that lter the blood to
make urine. 2/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology Urine is a waste byproduct formed from excess water and metabolic
waste molecules during the process of renal system ltration. The
primary function of the renal system is to regulate blood volume and
plasma osmolarity, and waste removal via urine is essentially a
convenient way that the body performs many functions using one
process.
Urine formation occurs during three processes:
1. Filtration
2. Reabsorption
3. Secretion Filtration
During ltration, blood enters the a erent arteriole and ows into the
glomerulus where lterable blood components, such as water and
nitrogenous waste, will move towards the inside of the glomerulus, and
non lterable components, such as cells and serum albumins, will exit via
the e erent arteriole. These lterable components accumulate in the
glomerulus to form the glomerular ltrate.
Normally, about 20% of the total blood pumped by the heart each minute
will enter the kidneys to undergo ltration; this is called the ltration
fraction. The remaining 80% of the blood ows through the rest of the
body to facilitate tissue perfusion and gas exchange. Reabsorption
The next step is reabsorption, during which molecules and ions will be
reabsorbed into the circulatory system. The uid passes through the
components of the nephron (the proximal/distal convoluted tubules, loop
of Henle, the collecting duct) as water and ions are removed as the uid
osmolarity (ion concentration) changes. In the collecting duct, secretion
will occur before the uid leaves the ureter in the form of urine. 3/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology Secretion
During secretion some substances±such as hydrogen ions, creatinine,
and drugs—will be removed from the blood through the peritubular
capillary network into the collecting duct. The end product of all these
processes is urine, which is essentially a collection of substances that
has not been reabsorbed during glomerular ltration or tubular
reabsorbtion.
Urine is mainly composed of water that has not been reabsorbed, which
is the way in which the body lowers blood volume, by increasing the
amount of water that becomes urine instead of becoming reabsorbed.
The other main component of urine is urea, a highly soluble molecule
composed of ammonia and carbon dioxide, and provides a way for
nitrogen (found in ammonia) to be removed from the body. Urine also
contains many salts and other waste components. Red blood cells and
sugar are not normally found in urine but may indicate glomerulus injury
and diabetes mellitus respectively. Normal kidney physiology: This illustration demonstrates the normal kidney
physiology, showing where some types of diuretics act, and what they do. Glomerular Filtration
Glomerular ltration is the renal process whereby uid in the blood is 4/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology ltered across the capillaries of the glomerulus. LEARNING OBJECTIVES Explain the process of glomerular ltration in the kidneys KEY TAKEAWAYS Key Points The formation of urine begins with the process of
ltration. Fluid and small solutes are forced under
pressure to ow from the glomerulus into the capsular
space of the glomerular capsule.
The Bowman’s capsule is the ltration unit of the
glomerulus and has tiny slits in which ltrate may pass
through into the nephron. Blood entering the
glomerulus has lterable and non- lterable
components.
Filterable blood components include water, nitrogenous
waste, and nutrients that will be transferred into the
glomerulus to form the glomerular ltrate.
Non- lterable blood components include blood cells,
albumins, and platelets, that will leave the glomerulus
through the e erent arteriole.
Glomerular ltration is caused by the force of the
di erence between hydrostatic and osmotic pressure
(though the glomerular ltration rate includes other
variables as well).
Key Terms glomerulus: A small, intertwined group of capillaries within nephrons of the kidney that lter the blood to 5/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology make urine.
hydrostatic pressure: The pushing force exerted by the pressure in a blood vessel. It is the primary force that
drives glomerular ltration. Glomerular ltration is the rst step in urine formation and constitutes the
basic physiologic function of the kidneys. It describes the process of
blood ltration in the kidney, in which uid, ions, glucose, and waste
products are removed from the glomerular capillaries.
Many of these materials are reabsorbed by the body as the uid travels
through the various parts of the nephron, but those that are not
reabsorbed leave the body in the form of urine. Glomerulus Structure
Blood plasma enters the a erent
arteriole and ows into the
glomerulus, a cluster of
intertwined capillaries. The
Bowman’s capsule (also called
the glomerular capsule)
surrounds the glomerulus and is
composed of visceral (simple
squamous epithelial cells—inner)
and parietal (simple squamous
epithelial cells—outer) layers.
The visceral layer lies just
beneath the thickened
glomerular basement membrane
and is made of podocytes that
form small slits in which the uid
passes through into the nephron.
The size of the ltration slits
restricts the passage of large Glomerulus structure: A diagram
showing the a erent and e erent
arterioles bringing blood in and out
of the Bowman’s capsule, a cup-like
sac at the beginning of the tubular
component of a nephron. molecules (such as albumin) and 6/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology cells (such as red blood cells and platelets) that are the non- lterable
components of blood.
These then leave the glomerulus through the e erent arteriole, which
becomes capillaries meant for kidney–oxygen exchange and
reabsorption before becoming venous circulation. The positively
charged podocytes will impede the ltration of negatively charged
particles as well (such as albumins). The Mechanisms of Filtration
The process by which glomerular ltration occurs is called renal
ultra ltration. The force of hydrostatic pressure in the glomerulus (the
force of pressure exerted from the pressure of the blood vessel itself) is
the driving force that pushes ltrate out of the capillaries and into the
slits in the nephron.
Osmotic pressure (the pulling force exerted by the albumins) works
against the greater force of hydrostatic pressure, and the di erence
between the two determines the e ective pressure of the glomerulus
that determines the force by which molecules are ltered. These factors
will in uence the glomeruluar ltration rate, along with a few other
factors. Regulation of Glomerular Filtration Rate
Regulation of GFR requires both a mechanism of detecting an
inappropriate GFR as well as an e ector mechanism that corrects it. LEARNING OBJECTIVES List the conditions that can a ect the glomerular ltration rate
(GFR) in kidneys and the manner of its regulation 7/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology KEY TAKEAWAYS Key Points Glomerular ltration is occurs due to the pressure
gradient in the glomerulus.
Increased blood volume and increased blood pressure
will increase GFR.
Constriction in the a erent arterioles going into the
glomerulus and dilation of the e erent arterioles
coming out of the glomerulus will decrease GFR.
Hydrostatic pressure in the Bowman’s capsule will work
to decrease GFR.
Normally, the osmotic pressure in the Bowman’s space
is zero, but it will become present and decrease GFR if
the glomerulus becomes leaky.
Low GFR will activate the renin–angiotensin feedback
system that will address the low GFR by increasing
blood volume.
Key Terms Bowman’s capsule: A cup-like sac at the beginning of the tubular component of a nephron in the mammalian
kidney.
osmotic pressure: The pressure exerted by proteins that attracts water. Water tends to follow proteins based
on an osmotic pressure gradient. Glomerular Filtration Rate
Glomerular ltration rate (GFR) is the measure that describes the total
amount of ltrate formed by all the renal corpuscles in both kidneys per
minute. The glomerular ltration rate is directly proportional to the 8/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology pressure gradient in the glomerulus, so changes in pressure will change
GFR.
GFR is also an indicator of urine production, increased GFR will increase
urine production, and vice versa.
The Starling equation for GFR is:
GFR=Filtration Constant × (Hydrostatic Glomerulus Pressure–Hydrostatic
Bowman’s Capsule Pressure)–(Osmotic Glomerulus Pressure+Osmotic
Bowman’s Capsule Pressure)
The ltration constant is based on the surface area of the glomerular
capillaries, and the hydrostatic pressure is a pushing force exerted from
the ow of a uid itself; osmotic pressure is the pulling force exerted by
proteins. Changes in either the hydrostatic or osmotic pressure in the
glomerulus or Bowman’s capsule will change GFR. Hydrostatic Pressure Changes
Many factors can change GFR through changes in hydrostatic pressure,
in terms of the ow of blood to the glomerulus. GFR is most sensitive to
hydrostatic pressure changes within the glomerulus. A notable bodywide example is blood volume.
Due to Starling’s law of the heart, increased blood volume will increase
blood pressure throughout the body. The increased blood volume with
its higher blood pressure will go into the a erent arteriole and into the
glomerulus, resulting in increased GFR. Conversely, those with low blood
volume due to dehydration will have a decreased GFR.
Pressure changes within the a erent and e erent arterioles that go into
and out of the glomerulus itself will also impact GFR. Vasodilation in the
a erent arteriole and vasconstriction in the e erent arteriole will
increase blood ow (and hydrostatic pressure) in the glomerulus and will
increase GFR. Conversely, vasoconstriction in the a erent arteriole and
vasodilation in the e erent arteriole will decrease GFR. 9/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology The Bowman’s capsule space exerts hydrostatic pressure of its own that
pushes against the glomerulus. Increased Bowman’s capsule hydrostatic
pressure will decrease GFR, while decreased Bowman’s capsule
hydrostatic pressure will increase GFR.
An example of this is a ureter obstruction to the ow of urine that
gradually causes a uid buildup within the nephrons. An obstruction will
increase the Bowman’s capsule hydrostatic pressure and will
consequently decrease GFR. Osmotic Pressure Changes
Osmotic pressure is the force exerted by proteins and works against
ltration because the proteins draw water in. Increased osmotic pressure
in the glomerulus is due to increased serum albumin in the bloodstream
and decreases GFR, and vice versa.
Under normal conditions, albumins cannot be ltered into the Bowman’s
capsule, so the osmotic pressure in the Bowman’s space is generally not
present, and is removed from the GFR equation. In certain kidney
diseases, the basement membrane may be damaged (becoming leaky to
proteins), which results in decreased GFR due to the increased
Bowman’s capsule osmotic pressure. 10/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology Glomeruluar ltration: The glomerulus (red) lters uid into the Bowman’s
capsule (blue) that sends uid through the nephron (yellow). GFR is the rate
at which is this ltration occurs. GFR Feedback
GFR is one of the many ways in which homeostasis of blood volume and
blood pressure may occur. In particular, low GFR is one of the variables
that will activate the renin–angiotensin feedback system, a complex
process that will increase blood volume, blood pressure, and GFR. This
system is also activated by low blood pressure itself, and sympathetic
nervous stimulation, in addition to low GFR. Tubular Reabsorption
Tubular reabsorption is the process by which solutes and water are
removed from the tubular uid and transported into the blood. LEARNING OBJECTIVES Describe the process of tubular reabsorption in kidney 11/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology physiology KEY TAKEAWAYS Key Points Proper function of the kidney requires that it receives
and adequately lters blood.
Reabsorption includes passive di usion, active
transport, and cotransport.
Water is mostly reabsorbed by the cotransport of
glucose and sodium.
Filtrate osmolarity changes drastically throughout the
nephron as varying amounts of the components of
ltrate are reabsorbed in the di erent parts of the
nephron.
The normal osmolarity of plasma is 300 mOsm/L, which
is the same osmolarity within the proximal convoluted
tubule.
Key Terms NA+/K+ ATPase: An ATPase pump that consumes ATP to facilitate the active transport of ions in ltrate of the
nephron.
peri-tubular capillaries: The capillaries through which components of ltrate are reabsorbed from the lumen
of the nephron. Filtrate
The uid ltered from blood, called ltrate, passes through the nephron,
much of the ltrate and its contents are reabsorbed into the body.
Reabsorption is a nely tuned process that is altered to maintain 12/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology homeostasis of blood volume, blood pressure, plasma osmolarity, and
blood pH. Reabsorbed uids, ions, and molecules are returned to the
bloodstream through the peri-tubular capillaries, and are not excreted as
urine. Mechanisms of Reabsorption
Reabsorption in the nephron may
be either a passive or active
process, and the speci c
permeability of the each part of
the nephron varies considerably
in terms of the amount and type
of substance reabsorbed. The
mechanisms of reabsorption into
the peri-tubular capillaries
include:
Passive di usion—passing
through plasma
membranes of the kidney
epithelial cells by
concentration gradients.
Active transport— Tubular secretion: Diagram showing
the basic physiologic mechanisms
of the kidney and the three steps
involved in urine formation. Namely
ltration, reabsorption, secretion,
and excretion. membrane-bound ATPase
pumps (such as NA+/K+ ATPase pumps) with carrier proteins that
carry substances across the plasma membranes of the kidney
epithelial cells by consuming ATP.
Cotransport—this process is particularly important for the
reabsorption of water. Water can follow other molecules that are
actively transported, particularly glucose and sodium ions in the
nephron.
These processes involve the substance passing though the luminal
barrier and the basolateral membrane, two plasma membranes of the
kidney epithelial cells, and into the peri-tubular capillaries on the other 13/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology side. Some substances can also pass through tiny spaces in between
the renal epithelial cells, called tight junctions. Osmolarity Changes
As ltrate passes through the nephron, its osmolarity (ion concentration)
changes as ions and water are reabsorbed. The ltrate entering the
proximal convoluted tubule is 300 mOsm/L, which is the same
osmolarity as normal plasma osmolarity.
In the proximal convoluted tubules, all the glucose in the ltrate is
reabsorbed, along with an equal concentration of ions and water
(through cotransport), so that the ltrate is still 300 mOsm/L as it leaves
the tubule. The ltrate osmolarity drops to 1200 mOsm/L as water leaves
through the descending loop of Henle, which is impermeable to ions. In
the ascending loop of Henle, which is permeable to ions but not water,
osmolarity falls to 100–200 mOsm/L.
Finally, in the distal convoluted tubule and collecting duct, a variable
amount of ions and water are reabsorbed depending on hormonal
stimulus. The nal osmolarity of urine is therefore dependent on whether
or not the nal collecting tubules and ducts are permeable to water or
not, which is regulated by homeostasis. 14/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology Reabsorption throughout the nephron: A diagram of the nephron that
shows the mechanisms of reabsorption. Tubular Secretion
Hydrogen, creatinine, and drugs are removed from the blood and into
the collecting duct through the peritubular capillary network. LEARNING OBJECTIVES Describe the purpose of tubular secretion in kidney physiology KEY TAKEAWAYS Key Points The substance that remains in the collecting duct of the
kidneys following reabsorption is better known as urine.
Secreted substances largely include hydrogen,
creatinine, ions, and other types of waste products,
such as drugs. Tubular secretion is the transfer of
materials from peritubular capillaries to the renal tubular
lumen and occurs mainly by active transport and
passive di usion.
It is the tubular secretion of H+ and NH4+ from the
blood into the tubular uid that helps to keep blood pH
at its normal level—this is also a respiratory process.
Urine leaves the kidney though the ureter following
secretion.
Key Terms 15/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology collecting duct: A system of the kidneys that consists of a series of tubules and ducts that connect the nephrons
to the ureter.
peritubular capillaries: Tiny blood vessels that travel alongside nephrons, allowing reabsorption and
secretion between blood and the inner lumen of the
nephron.
lumen: The inside space of a tubular structure, such as an artery or intestine. Tubular secretion is the transfer of materials from peritubular capillaries
to the renal tubular lumen; it is the opposite process of reabsorption.
This secretion is caused mainly by active transport and passive di usion.
Usually only a few substances are secreted, and are typically waste
products. Urine is the substance leftover in the collecting duct following
reabsorption and secretion. Mechanisms of Secretion
The mechanisms by which secretion occurs are similar to those of
reabsorption, however these processes occur in the opposite direction.
Passive di usion—the movement of molecules from the peritubular
capillaries to the intersitial uid within the nephron.
Active transport—the movement of molecules via ATPase pumps
that transport the substance through the renal epithelial cell into
the lumen of the nephron.
Renal secretion is di erent from reabsorption because it deals with
ltering and cleaning substances from the blood, rather than retaining
them. The substances that are secreted into the tubular uid for removal
from the body include:
Potassium ions (K+) 16/18 8/7/2019 Physiology of the Kidneys | Boundless Anatomy and Physiology Hydrogen ions (H+)
Ammonium ions (NH4+)
Creatinine
Urea
Some hormones
Some drugs (e.g., penicillin)
Many pharmaceutical drugs are
protein-bound molecules
thatDiagram showing the basic
physiologic mechanisms of the
kidney an...
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