Kidneys are located on the posterior wall of the abdomen and protected by lower ribs.
They are supplied by blood, which is transported from aorta through renal arteries, and filtered blood will be transported back through renal veins (Bradley & Calvert 2011). A kidney consists of outer cortex, inner medulla and renal pelvis, where urine is hold and passed to ureter and then to the bladder (Bradley & Calvert 2011). Osmoregulation is controlled by kidneys (electrolytes and fluid levels in the body). Kidneys are also responsible for excretion of urea (nitrogenous waste) (Bradley & Calvert 2011).Electrolytes and body fluid levels are very important for functioning cells and tissues. Even slight changes in plasma potassium can cause heart failure and paralysis and kidneys regulate this (Bradley & Calvert 2011).
The fluids and electrolytes from plasma are going through filtration and then appropriate level will be reabsorbed into blood to keep sufficient plasma level. The process of reabsorption is achieved by hormonal control (Bradley & Calvert 2011). Kidneys are also responsible for the removal of the waste nitrogen, which is the product of proteins breakdowns (Bradley & Calvert 2011).Any dysfunctions of kidneys can be fatal as homeostatic state will be interrupted, therefore it is important to understand the mechanism of this important organ (Bradley & Calvert 2011). The kidneys not only processing blood plasma and form the urine but also regulate and maintain production of antidiuretic hormone, erythropoietin, aldosterone and synthesize vit D (Thibodeau & Patton 2007).
The kidneys also regulate blood pressure and blood pH.Blood pressure is maintained by secretion of enzyme renin, which stimulates renin-angiotensin and aldosterone by correcting blood flow that enters and leaves kidneys and by correcting blood volume Thibodeau & Patton 2007). Controlling returning water to the blood or removing it with urine regulates blood volume. Moving sufficient number of hydrogen ions from blood into the urine regulates blood pH (Thibodeau & Patton 2007).They also protect blood bicarbonate ions HCO3-, a buffer for hydrogen ions, which also contribute to maintain blood pH (Thibodeau & Patton 2007). Structure and function of nephrons Kidneys share between each other around 2-4 millions of nephrons, which are the main functional unit (Thibodeau & Patton 2007).
Nephrons have very different shape, which helps processing blood plasma and form urine Thibodeau & Patton 2007). Approximately 85 % nephrons are called cortex nephrons due to their location in the renal cortex, the rest of nephrons are called juxtamedullary as they are position next to cortical and medullary layers (Thibodeau & Patton 2007). Juxtamedullary nephrons are responsible for urine concentration (Thibodeau & Patton 2007).
Each nephron is build of following parts which will be discussed later such as: Bowman’s corpuscle, proximal tubule, loop of Henle, distal convoluted tubule and collecting ducts (Fig1)(Thibodeau & Patton 2007). Fig 1. Nephron structure. American Society of Health and Pharmacists. 1998)[Online] Nephrons have tubular structure with two ends: one closed and one opened (Bradley & Calvert 2011). At the closed end there is Bowman’s capsule containing many capillaries- the glomerulus.Glomeruli can be found in the renal cortex, where filtration of the plasma occurs (Bradley & Calvert 2011).
The proteins in plasma will not be moved through Bowman’s capsule (Bradley & Calvert 2011). Every hour around 6 L of plasma undergo filtration in kidneys, it is iso-osmolar with plasma and consists electrolytes, water, amino acids, urea and glucose (Bradley & Calvert 2011).The main responsibility of the nephrons is to retrieve the sufficient amount of metabolites while reducing filtrate from 6 L to 60 ml/hr of urea (Bradley & Calvert 2011).
That process starts in the proximal tubule where glucose, electrolytes, NaCl, amino acids are reabsorbed (Bradley & Calvert 2011). Osmotic force is reached due to movement of these solutes, which moves water with them (Bradley & Calvert 2011). Glucose and amino acids and approximately 80% of water will be absorbed here. However, sodium also will be reabsorbed here but controlled by hormone angiotensin II (Bradley & Calvert 2011).The filtrate then moves to loop of Henle, which is connected with renal medulla (Bradley & Calvert 2011). The loop of Henle has countercurrent system, which helps to obtain osmolarity gradient between the top and bottom of the loop (Bradley & Calvert 2011).
At the lower part of the loop of Henle, the tubular and interstitial fluids obtain osmolarity of 1200 mosmol, around 4 times higher then the plasma (Bradley & Calvert 2011). The gradient is achieved because ascending limb does not allow water to pass through, but moves sodium to ascending limb, therefore solute concentration increases (Bradley & Calvert 2011).The filtrate then moves from the ascending limb to distal tubule, where sodium and water continues to be removed (Bradley & Calvert 2011). Excretion of sodium is controlled by aldersterone, a hormone which production occurs in adrenal gland, when Na+ions level is low (Bradley & Calvert 2011). If sodium absorption increases, water also increases. The final excretion of water occurs in collecting ducts, which are connected with renal medulla directed to renal pelvis (Bradley & Calvert 2011).
Because they move through the renal medulla, it passes through high osmolarity at the lower part of loop of Henle (Bradley & Calvert 2011).Epithelium cells of colleting ducts consist water channels- aquaporins that speeds up movement of water out of the duct and into interstitial fluid (Bradley & Calvert 2011). Aquaporins are more permeable to water then urea, therefore urea will be more concentrated in the urine (Bradley & Calvert 2011). Aquaporin channels are controlled by antidiuretic hormone ADH, which is formed in the hypothalamus when water level in blood become lower then it supposes to be (plasma osmolarity increases) (Bradley & Calvert 2011). In consequences more aquaporin channels are produced, that helps more water to be reabsorbed (Bradley & Calvert 2011).In a situation when higher amount of water is consumed, plasma volume increases and antidiuretic hormone production is minimized, aquaporin channels reduced and water pass through collecting ducts, then ureter and bladder (Bradley & Calvert 2011).
Bladder is able to store approximately 1 L of urine (Bradley & Calvert 2011). At this point, it is easy to understand that each part of nephron has different function and it is responsible of movement different substances at different stages such as renal corpuscle will filter water, ions, glucose and other solute particles by passive transport (Thibodeau & Patton 2007).Proximal tubes uses active transport for reabsorption of sodium ions and cotransport of amino acids and glucose as well as passive transport for reabsorption of Cl-, PO4, urea and transport of H20 by osmosis (Thibodeau & Patton 2007). Loop of Henle in descending limb uses passive transport for reabsorption of water and secretes urea (Bradley & Calvert 2011). In ascending limb active transport take place moving sodium ions and diffusion of Cl- (Thibodeau & Patton 2007).Distal tube uses active transport for movement of sodium ions and passive transport for diffusion of Cl- as well as transport of water by osmosis but controlled by ADH (Thibodeau & Patton 2007). In distal tube we also have two types of secretions, one is passive and one is active (Thibodeau & Patton 2007). The passive secretion diffuses ammonia and the other one uses active transport for secretion oh K+, H+ and some drugs (Thibodeau & Patton 2007).
Finally in collecting ducts reabsorption occurs by active transport of Na+ and passive reabsorption by diffusion of urea and again osmosis moving water controlled by ADH (Thibodeau & Patton 2007).Renal failure can be caused by many factors, however most likely can occur due to high blood pressure present for long period of time (Bradley & Calvert 2011). The kidney glomerulus’s filtration barrier will be impaired and this will cause removal of protein and glucose in the urine (Bradley & Calvert 2011). If the substances are found in the urine, this will lead to the water absorption being stopped and in consequences urine will be highly diluted (Bradley & Calvert 2011). At some point nephrons will not be able to survive and this will have high impact on ability to remove toxic substances from the blood (Bradley & Calvert 2011).In this case a dialysis machine will support the patient to help and remove unwanted substances from the blood until kidney transplant can be provided (Bradley & Calvert 2011). Summary Kidneys are responsible for removal waste products and additional fluids from the body through the urine Kardiasz 2009 [Online] The urine is obtained by complex processes of excretion and absorption, and this is the very important to maintain homeostasis, by keeping the balance of metabolites of body stable Kardiasz 2009 [Online]An important regulation of K, NaCl takes place in kidneys National Kidney Foundation. (2012) [Online].
Kidneys are also responsible for production of hormones that may have an effect on functions of other systems such as a particular hormone obtained from kidneys will initiate production of erythrocytes, where another hormone obtained from kidneys will maintain blood pressure and calcium levels National Kidney Foundation. (2012) [Online]. As mentioned above homeostasis of water and electrolytes depends on kidney function National Kidney Foundation. 2012) [Online]. If renal filtration would not take place, blood plasma’s range point would change dramatically National Kidney Foundation. (2012) [Online].
However, without cardiovascular system, which is responsible for maintenance of blood pressure, the kidneys would not be able to process blood plasma (Thibodeau & Patton 2007). For that reason both urinary and cardiovascular mechanisms are dependent on each other respectively (Thibodeau & Patton 2007). Urinary system is also interdependent on nervous because nerve reflexes control urinary functions.Also endocrine system interacts with urinary functions as regulation of urinary system take place outside the kidney by endocrine hormone production (Tortora & Derrickson 2007). The kidneys removes nitrogen waste and unwanted metabolic acids from blood plasma, which was formed sue to chemical reactions in almost every cell in the body (Tortora & Derrickson 2007). They can also remove toxins and other substances that can be found in the blood via respiratory or digestive tracts (Tortora & Derrickson 2007).
