The Basic Structure And Function Of The Normal Kidney

Of the many organs in the human body, the kidney could have been considered one of the most important of them all. The kidneys basically functions to remove water and waste products (Hawley, Margeruite). They are both located on each side of the spinal column at the back of the abdomen at about the level of the lower ribs. The kidneys are responsible for the secretion of essential hormones like renin, erithropoietin and Vitamin D.

A normal adult kidney normally weighs one-fourth pound. Its normal size should be close to 4 inches long, 2.5 inches wide and 1.5 inches thick. When in normal condition, the kidney receives twenty percent of the blood directly coming from the heart (per beat). ²The normal blood flow rate should run to 1.2 liters per minute. Kidney function is measured by glomerular filtration rate (GFR) which varies according to age, sex and body size factors. The normal GFR of young adults should be approximately 20-130ml/min/1.73 m2 but this standard declines as a person ages. When a person’s GFR runs below 60 ml/min/1.73 m2, there will be an indication of the risk of kidney disease including cardiovascular diseases. The figure below shows the parts of the human kidney.

There are six general functions of the normal kidney: balances solute and water transport, excretes metabolic wastes, conserves essential nutrients, regulates acid-base balance, secrets hormones and forms urine. It is the control of the body fluid concentration that makes kidneys a vital part of the human system by excreting excess amount of body fluid as our urine in case where the body fluid is too dilute. The normal kidney knows if our body fluid is too concentrated and in that case will excrete excess solutes. With regards to acid-base balance, the kidneys are able to excrete hydrogen ions (when blood is too acidic) and bicarbonate ions (when the blood is too alkaline). The figure below shows the normal process  undegone by the kidney when in works.

To be able to do all of these functions, the kidney units called nephrons, should be in the right condition. Nephrons are responsible for the processing of the blood in order to form urine. Each kidney contains one million nephrons but health experts say that a kidney can still normally function even if only one third of the total nephrons are functioning. With less than one third of these nephrons, the kidney can no longer excrete waste products at the normal level. The result would be that more waste products like urea and creatinine are retained in the kidney.

In each nephron are tuft of capillaries called glomerulus which allow the large amounts of solute-rich fluids to pass from the capillaries into the capsule through its porous structure. Surrounding the glomerulus is the Bowman’s capsule (epithelium) to which the fluids, which is actually the raw material of urine are carried through. This raw material flows from the capsule through the proximal convoluted tubule (PCT) of the nephron. It is in the PCT that sodium ion transport accounts for 80% of the process of sodium reabsorption. Simultaneously, soduim chrloride are also reabsorbed including the amino acids and glucose. ¹This primary process of reabsorption also occurs in other tubules of the nephron except in the descending loop of the Henle.

Specifically, the PCT alows the process called iso-osmotic water re-absorption both for eletrolytes and non-electrolites. In the capillaries lining the PCT almost all potassium, uric acid, glucose and amino acids are filtered and completely re-absorbed. Most of the calcium and phosphate are re-absorbed in this process including  90% of bicarbonate and two-thirds of filtered sodium. Water, chroride and urea on the other hand are reabsorbed through the process of passive transport. In these tubules, hydrogen ion and creatinine are secreted.

The glomerulus is the round less than 0.2 mm in diameter, blind beginning of the nephron (Slomianka, Lutz 2006). Small foot-like processes callled pedicles form a fenestrated epithelium around the fenestrated capillaries of the glomerulus.Filtration slits are the openings found between the pedicles spanned by the thin filtration slit membrane. The basal lamina and the slit membranes form the glomerular filtration barrier, which prevents some large molecules from entering the capsular space between the outer and inner epithelial layers of Bowman’s capsule. The figure below illustrates the tubular system of the human kidney.

            The tubules of the nephron are divided as proximal, intermediate and distal tubules. The proximal tubule is the longest section of the nehpron which measures about 14mm, diameter of which measures less than 65 µm.  Its convulated part coils to the glomerulus in the cortex.On the other hand, the straight portion decends towards the medulla. In the proximal tubules the volume of the glomerular filtrate is reduced by about 75%.

            Connecting the proximal and distal tubules is the intermediate tubule which run from the cortex down to the medulla then ascends in coil to the cortex forming the loop of Henle.The thin segment of Henle’s loop leads into the straight part of the distal tubule, which is formed by low cuboidal cells without a brush border. The epithelian cells in these tubules transport active chloride and passive sodium ions out of the tubular lumen into the peritubular space.

            The straight portion of the distal tubule contacts the glomerulus forming the macula densa. The cells in the distal tubules are highly sensitve to the hormone aldosterone produced by the adrenal glands. Aldosterone stimulates the active resorption of sodium ions and the excretion of potassium ions. The convoluted distal tubule merges, via connecting tubules, with the collecting ducts. Working together with antidiuretic hormone (ADH), osmotic forces move water out of the lumen of the tubules as they pass through the medulla. It in this area that high concentrations of sodium were established. From here, the collecting ducts then merge with the papillary ducts f the Bellini and then would empty into the minor calyx, a funnel shaped part of the ureter. From the ureter, the urine then makes its way to the bladder.

            The kidneys are also important in the conversion of Vitamin D into active metabolite in order that calcium maybe absorbed from the the intestine. It also produces the hormone called erythropoietin which is responsible for the sitimulation of the production of red blood cells in the bone marrow. Renin is another kidney-produced hormone that is important in sodium and blood pressure control.

REFERENCES

Hawley, Marguerite A. Normal and Abnormal Kidney Function. 26 December 2007 <http://msl1.mit.edu/ESD10/kidneys/HndbkHTML/ch1.htm>

2GFR. 26 December 2007 <http://kidneydiseases.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=kidneydiseases&cdn=health&tm=33&gps=45_638_1020_610&f=00&su=p726.2.152.ip_p284.8.150.ip_&tt=2&bt=1&bts=1&zu=http%3A//www.kidney.org/professionals/KLS/gfr.cfm%2313>

Hawley, Marguerite A. Normal and Abnormal Kidney Function. 26 December 2007 <http://msl1.mit.edu/ESD10/kidneys/HndbkHTML/ch1.htm>

Slomianka, Lutz. The Kidneys. 26 December 2007 <http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Urinary/urinary.htm>

The Kidney. 26 December 2007 <http://www.cancerhelp.org.uk/help/default.asp?page=4031>

1The Normal Renal Function. 26 December 2007 <http://classes.kumc.edu/cahe/respcared/cybercas/dialysis/portkid.html>