Kidneys

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The kidneys are a vital organ of the renal system and are primarily responsible for removing waste products from the body. However, there are also numerous other functions carried out by the kidneys, unknown to many and therefore the kidneys are essential for body function. They are a pair of organs that are situated at the back of the abdomen. The shape of the kidneys resembles a bean shape and they are in line with the 12th rib. Each kidney is approximately 4 inches in length.

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Anatomy of the Kidneys

Unlike other abdominal organs, the kidneys are not enclosed in a thin membrane called the peritoneum but lie between this membrane and the abdominal cavity. For protection, they are surrounded by a layer of fat and areolar tissue. Both kidneys sit in similar positions in the abdomen although the right kidney sits slightly lower than the left kidney due to the positioning of the liver. They weigh approximately a maximum of 170 grams but receive 20% of the cardiac output, making them a vital organ.

The sides of the kidneys are flattened and the organs are larger at the top of the organ and smaller and narrower towards the bottom. A cross section through the kidney shows that the kidney has two main regions. These are named the cortex and the medulla. The cortex is the outer layer and has a reddish brown appearance whereas the medullary region is much darker and appears striated. The medulla can be divided further into renal pyramids and each pyramid is surrounded by a cortical arch which is part of the cortical region. the pyramids are made up of subunits called nephrons and these are responsible for filtering the blood and producing urine. Within the nephrons is a coiled tube called the renal tubule and it forms a hairpin shape within the nephron. Fluid is passed through this tube and is modified with the help of interstitial fluid. At the pyramids tip is the papilla and it is here that a number of tubules called collecting ducts reside. The fluid from the nephrons flows into the collecting ducts and the collecting ducts drain their products into minor calyces which are pathways throughout the kidney. These will converge to form major calyces and these drain into the renal pelvis which is the first part of the uretar. The fluid produced at the end of this pathway is urine.

Microscopic Anatomy

The nephrons themselves are highly complex structures that are essential for the function of the kidneys. The nephrons can be divided into two main components; the renal corpuscle and the renal tubules.

The renal corpuscle's major role is to filter the blood and it is also the site of tubular fluid. It is made up of the Bowman's capsule, which is a spherical structure situated at the inflow end of the tubule, and the glomerulus which is a band of capillaries. The blood enters the renal corpuscle into the glomerular capillaries via the afferent arteriole. A process called glomerular filtration then proceeds to occur. During the filtering, protein free plasma flows through the capillaries and into the Bowman's capsule and the rest of the blood leaves through the efferent arteriole. This process can be regulated by the arterioles at either end of the system for they can contract or relax depending on certain stimuli.

The renal tubule receives the filtrate from the Bowman's capsule and it enters the proximal convoluted tubule which has a highly folded structure. Following this, the filtrate enters the proximal straight tubule. These tubules then empty into the loop of Henle which is the hairloop bend described earlier. The loop can be divided into three different regions including the descending limb, the thin ascending limb and the thick ascending limb. The descending limb is a thin tube that extends into the renal medulla. It reverses on itself and becomes the thin ascending limb which eventually becomes the thick ascending limb. Once the fluid has passed through this region, it enters the distal convoluted tubule and further into the connecting tubule. The connecting tubule connects the nephron to the collecting duct which then empty into the minor calyces.

Nephrons

Depending on the location, there are two different types of nephron. The first is the cortical nephrons and these make up the majority of the kidneys. They can be found in the renal cortex and in the tip of the loop of Henle. The other type is called the juxtmedullary nephrons and these make up approximately 20% of the nephrons found in the kidneys. Although similar, there are important differences between the two nephron types. Both contribute to the formation of urine but the juxtamedullary nephrons also maintain an osmotic gradient within the renal medulla. This is essential to urine production and control of water.


Blood Supply

A good supply of blood is crucial to kidney function and therefore the kidneys are well supplied. The main artery is called the renal artery and this branches into several others called segmented arteries. These further branch into interlobar arteries which in turn branch into the arculate arteries. Further branching leads to the interlobular arteries so that blood can be carried to individual nephrons by the afferent arterioles. These supply the capillary beds which are essential for filtration. Once the blood is filtered, the blood leaves via the efferent arteriole. This leads to other types of capillary beds such as peritubular capillaries and Vasa Recta. Both of these drain away into the interlobular veins and blood leaves via th arcurate veins into the interlobar veins and finally away from the kidneys through the renal vein.

Function of the Kidneys

There are a number of primary and secondary functions carried out by the kidneys which are critical for survival. The primar functions involve:

  1. Removal of metabolic waste - The kidneys function to remove metabolic by products such as urea and uric acid from the blood and into the urine so that they can be excreted. This is a highly important role for these products are toxic to the body. The kidney also removes other unwanted products such as drugs or pesticides.
  2. Regulation of plasma volume - The kidneys can control the rate of water excretion and therefore they can control the the volume of plasma which can affect blood pressure.
  3. Regulation of ion composition in plasma - The kidneys can regulate a number of different ions such as calcium, magnesium, sodium, potassium and bicarbonate. It does so by increasing or decreasing the rate of excretion of these ions.
  4. Regulating pH - The kidneys can control the level of hydrogen and bicarbonate in the blood and therefore they can regulate the pH level of the blood.

Aside from this major functions, the kidney also carries out a series of other tasks. They are an endocrine organ and therefore secrete hormones. The hormone produced by the kidneys is called erythropoietin which can activate the production or red blood cells (known as erythrocytes) in bone marrow. The kidney also secretes Renin which is an enzyme key to producing angiotensin III. The role of angiotensin III is to control blood pressure by regulating water and salt in the body. The kidneys also play a role in vitamin D3 activation which is essential for the regulation of calcium and phosphate in the blood. The kidneys also play a key role during periods of fasting by providing the plasma with glucose. It does so by releasing the major components of gluconeogenesis - the process of making glucose - into the blood stream.

Renal Exchange

In order for the kidneys to remove waste products from the body, solutes need to be exchanged. This occurs in the nephrons and there are three main steps that make up the exchange process.

The first step is glomerular filtration. This is driven by Starling forces which are the hydrostatic and osmotic pressure gradients. These forces are applied in other capillary filtration systems throughout the body. The plasma that filters through does not contain any proteins. In order to move through into the Bowman's capsule, the filtrate has three barriers which it must cross. These are referred to as the glomerular membrane and include the endothelial cell layer, epithelial cell layer and the basement layer. The epithelial cell layer is adapted to serve the purpose of filtration and has finger like extensions called podocytes. In between these podocytes are small pores called slit pores which can be regulated. They allow the flow of filtrate into the Bowman's capsule. Approximately 180 litres of fluid is filtered through the tubules per day but only 1.5 litres of urine is actually excreted in the same time frame. This is because the next step of the process is reabsorption and the process is tightly controlled by both intrinsic and extrinsic factors. These factors include changes in arterial pressure.

The next stage of the renal exchange process is reabsorption and this puts solutes from the tubules back into the plasma. This process is essential for body function as without it, too much water would be lost in a short space of time. Reabsorption usually occurs in the proximal and distal convoluted tubes. These tubes have large surface areas due to their highly folded structure. In order for reabsorption to occur, the solutes must cross over the epithelial cells in the tubules and this sometimes requires energy. This is carried out by exchange pumps which use ATP in order to drive the solutes across the epithelial cells.

The final step in the renal exchange process is secretion. During this process, molecules move into the renal tubule and become the filtrate. Again, sometimes energy is required for this movement as secretion faces the same barrier as reabsorption. After secretion has occurred, there is an increase in the amount of solute present in the urine.

Excretion

This process is the removal of the solutes and water into the urine from the kidneys and as a rule anything that enters the kidneys that has not been reabsorbed will be excreted.

Kidney disorders

There are a number of problems associated with kidneys and they are potentially life threatening. It is possible for an individual to live with only one kidney as long as the kidney is fully functional. The kidneys are a pair of organs so that one can act as a back up if a kidney becomes damaged or dyfunctional simply because the processes that the kidneys carry out are key to survival.

Acute renal failure is an extremely serious condition that usually results in death. The kidneys stop working suddenly and there is a serious deterioration in the patients health. This could be caused by dehydration or a blockage in the tubules of the kidneys but it can be reversible. Sometimes the damage is so great that a person will require a kidney transplant. Dialysis can be used to buy some time for the patient. Chronic renal failure is the progressive break down in function of the kidneys and can be caused by diabetes amongst other things.

Cancer can also affect the kidneys whereby the kidney cells multiply uncontrollably to create a tumour. The main cause of renal carcinoma is smoking. Other conditions include polycystic kidney disease where large cysts grow on the kidney and impair its function.

Kidney stones are also a painful problem. These occur when the minerals in the kidneys become too concentrated and from crystals which go on to form large stones. These can get stuck anywhere in the kidney or ureter causing a great deal of pain. Diet can have a major impact on the chance of forming kidney stones and the patient is usually hospitalised in order to deal with the condition.

Dialysis

Kidney dialysis has saved thousands of lives since its invention. It is not a cure for kidney disease but it can prolong a patients life long enough for a donor organ to be found. The blood of the patient is pumped through an artificial kidney which is made up of two components. A membrane seperates two compartments and the blood flows in one side whilst dialysis fluid flows in the other. Molecules can be exchanged through this membrane and a series of gradients ensure that the waste products are taken out of the blood and essential solutes are reabsorbed.

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