Download NEPHRON 1 The nephron – the functional unit of the kidney

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The nephron – the functional unit of the kidney
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The nephron – the functional unit of the kidney
The kidney is an important excretory organ in the body as it is responsible for the
evacuation of urine and retention and excretion of different electrolytes (Netter, 2014). In
essence, the different physiological processes are helpful in the maintenance of homeostasis.
The absence of kidney function may lead to electrolyte imbalances and accumulation of
toxins creating an environment that is incompatible to life. Physiologically, humans have two
kidneys and the nephrons are the functional units of these anatomical structures. The paper
will illustrate the anatomical structures that make up the nephron and their physiological roles
in the function of the kidney (Netter, 2014).
As mentioned above, the nephron is the functional unit of the kidney. Each of the
kidneys is comprised of approximately 1.5 million nephrons (Netter, 2014). The kidney
nephron is comprised of the renal tubule and renal corpuscle. The renal corpuscle is
responsible for filtration of the blood plasma. The corpuscle of the kidney has two main parts
that include the glomerulus and the Bowman’s capsule (Netter, 2014).
The glomerulus is a network of capillaries that make up the first part of the nephron.
The blood enters the glomerular system through the afferent arteriole, which then enters the
glomerular capillaries and leaves through the efferent arteriole (Moore et al., 2013). The
afferent arteriole is a branch of the interlobular artery. Many physiological processes affect
the passage of blood through these arterioles. Constriction through prostaglandin inhibitors
may lead to constriction of the afferent arteriole and reduce the amount of blood and
therefore urine excretion. In addition, sympathetic nervous systems and other hormones also
affect the afferent arteriole and influence glomerular filtration rate (Moore et al., 2013).
The capillaries in the glomerulus are lined by an endothelium which is a layer of
fenestrated endothelial cells which enable the passage of blood components. The fenestrae
are approximately 80nm in diameter (Nielsen et al., 2012). The pores are large enough to
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allow proteins and plasma solutes. However, they are not large enough for red blood cells to
pass through the capillaries. The second layer is comprised of the type IV collagen, laminins,
nidogen and agrin which makes up the basement membrane (Moore et al., 2013). The
basement membrane is approximately 300nm thickness. The basement membrane has a key
role in the selectively and permeability to proteins such as albumin which is a larger
molecule. Another layer lining the capillary is comprised of podocytes (Moore et al., 2013).
There are also spaces between the cells which consist of intraglomerular mesangial cells. As
mentioned above, the efferent arteriole is responsible for carrying blood away from the
glomerulus. This is a physiological aspect that allows better blood control. Arterioles contract
and dilate more readily than venules. The filtrate that is a passed through the capillaries enters
the Bowman’s capsule. From the Bowman’s corpuscle it enters the tubular part of the
nephron (Moore et al., 2013).
The physiological functions of the glomerulus have been briefly stated above.
However, the main function of the glomeruli is to filter plasma and produce glomerular
filtrate (Netter, 2014). There are several factors that help regulate the filtration process and
these have also been previously discussed. After the blood is filtered through the capillaries it
enters the lumen of the tubule and becomes glomerular filtrate. The structures of the
glomeruli determine the level of permeability. The different factors that have an impact on
the permeability are the negative charge of the podocyte epithelium and basement membrane
(Netter, 2014). In addition, the pore size also determines the particles that can pass through
the capillaries. Due to the charge, negative and large particles pass less readily across the
pores whilst the small and positively charged particles pass easily through the barriers. Small
ions such as potassium and sodium pass easily through the pores. On the other hand, larger
particles such as albumin and hemoglobin cannot penetrate the barrier (Netter, 2014).
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Other physiological processes that are regulated by the nephron are blood pressure.
The regulation is carried out through the renin angiotensin system. Renin is produced by the
smooth muscle cells in the afferent arterioles. When there is a reduction in the volume of
blood passing through the afferent arteriole renin is produced. Renin is an enzyme that
converts angiotensinogen into angiotensin I in the liver. The angiotensin I is converted into
angiotensin II in the lungs through angiotensin converting enzyme. Angiotensin II stimulates
the release of aldosterone which acts on the collecting duct. The hormone allows the retention
of water and Na ions which further increases water retention. There are several issues that are
associated with this problem especially in patients with congestive heart failure. The low
volume of blood to the afferent arteriole and the activation of the renin angiotensin system
lead to a vicious circle. There is excessive fluid retention which further leads to an increase of
the work load exerted on the heart (Leipziger, 2014).
The other part of the nephron is comprised of the renal tubule. The renal tubule is
comprised of proximal convoluted tubule, thin descending limb, loop of Henle, thick
ascending limb, distal convoluted tubule and the collecting duct. In the proximal convoluted
tubule, glucose and protein are reabsorbed (Bourne, 2012). Most of the ions such as Na, Cl,
K, HCO3 and PO4 are reabsorbed. The absorption is isotonic and there is secretion of NH3
which acts as a buffer for H+ (Bourne, 2012). PTH acts on this part of the tubule and blocks
the Na and PO3 co transport. ATII also acts on this part by stimulating exchange between Na
and H+ ions. The medulla surrounding the descending loop is hypertonic; hence, there is
passive reabsorption. The thick ascending loop actively absorbs K, Na, and Cl. There is also
indirect reabsorption of Ca and Mg (Bourne, 2012). The main function of this part of the
tubule is making the urine less concentrated. The early distal convoluted tubule reabsorbs the
Cl, Na and makes the urine hypotonic. PTH also acts on this part and stimulates the
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reabsorption of calcium. Finally, the collecting duct is responsible for the Na reabsorption for
the exchange of H and K. The process is regulated aldosterone (Bourne, 2012).
In summation, the nephron is the functional unit of the kidney and takes part in the
filtration process and formation of urine. The process is responsible for detoxification and the
regulation of fluid in the body (Bankir, 2013). The anatomical structure of the nephron makes
it possible to carry out different activities competently. In addition, there are different
physiological processes that act on the nephron and these influence the rate at which the
processes of filtration and reabsorption take place. Different medications act on different
structures of the nephron and help in the regulation of blood pressure and the evacuation of
excess fluids.
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