Basic Renal Physiology fundamental function of the kidneys is the regulation of extracellular fluid (plasma and liquid interstitial) of the body (3) (4). This is achieved through the formation of urine, which is a modified filtered urine. During this process the kidneys regulate:
- ionic composition of blood (1) (3) (5)
- regulation of blood pH (1) (3) (5)
- plasma volume regulation (1) ( 3)
- blood pressure regulation (1) (4) (5)
- Maintenance of water levels and solute (osmolality) (1) (4) (5)
- hormone production (1) (5 )
- Regulation of glucose (a) (5)
- Excretion waste and foreign substances (1) (3) (5)
The functional unit responsible for the formation of urine is the nephron
(3). Its main parts are the glomerulus and the glomerular capsule or Bowman capsule that surrounds the proximal tubule, loop of Henle, distal convoluted tubule and collecting tubule (3).
Diagram of the kidneys and the nephrons
Taken Biosphere Project, Government of Spain
http://recursos.cnice.mec.es/biosfera/alumno/1bachillerato/animal/contenidos14.htm
nephrons and collecting tubules develop three processes Basic (1) (5):
- Glomerular Filtration: The water and most of the solutes in the blood plasma moving through the wall of the glomerular capillaries into the glomerular capsule. Tubular reabsorption
- : As the filtrate flows through the renal tubule and collecting tubule, 99% of water and solutes are reabsorbed useful.
- Tubular Secretion: As the filtrate flows through the renal tubule and collecting tubule, secreted waste products, drugs and ions in excess.
Glomerular Filtration The
capillary endothelial cells and glomerular podocytes which surround (1) possess fenestrae (pores) that are highly permeable to plasma water and solutes dissolved in it. These pores are considered large compared to common hair, but it still does not allow the passage of red blood cells, leukocytes and platelets from the blood (1) (3).
The glomerular filtration
is the fluid that enters the glomerular capsule is modified as it passes through the different tubules of the nephron and urine becomes the end of the process (3). Passing the glomerular capsule fluid must pass through the membrane filtration
is consists of three layers: the glomerular endothelial cells
the basal lamina
and podocytes
located in the visceral layer of glomerular capsule.
In the diagram below we can see that the podocytes with cytoplasmic-like pedicels thick arms
calls (1) (3). Molecules must pass through the filtration slits
between pedicielos (3) to enter the glomerular filtration rate (a) (3).
membrane filtration
KidneyPathology.com Taken Http://bioweb.wku.edu/courses/Biol131/images/filtrationmembrane.GIF
fluid leaving the glomerulus and enters the proximal tubule urine is known as primitive and consists of water and small solutes in concentrations similar to plasma. The big difference is that it contains no blood cells, proteins or other substances of high molecular weight (5).
The filtration principle is the same in the glomerular capillaries in the rest of the capillaries of the body. It is based on the use of pressure to move liquid and solutes through a membrane (3). However, the volume filtered in the renal corpuscle is greater for the following reasons (3):
- glomerular capillaries have a long and extensive surface
- filtration membrane is thin and porous
- The glomerular capillary blood pressure is higher
The average filtration rate (GFR) in adults is 125 ml / min in men and 105 ml / min in women, while the daily volume round by the 150L to 180L (1). Over 99% of the liquid return to the plasma by reabsorption in the tubules, so that only 1 or 2 liters are excreted in the urine (1) (2). This also means that all the blood (about 5 liters) passes the glomerular filtration in less than an hour.
Some authors suggest that the process is called ultrafiltration
by the small size of the solutes that are able to cross the membrane filtration (5).
The net pressure filtration
(PNF) depends on three main factors: the glomerular blood hydrostatic pressure (PHSG) that promotes the filtration and capsular hydrostatic pressure (PHC) and colloid blood (PCS) that oppose filtration (1).
PNF = PHSG - (PHC + PCS)
The
glomerular blood hydrostatic pressure (PHSG) or pressure capillary hydrostatic (5) is blood pressure in the glomerular capillaries and that forces the output of plasma and solutes through membrane filtration. Its value is usually 45 to 55 mm Hg (1) (5). The
capsular hydrostatic pressure (PHC) or urinary space hydrostatic pressure (5) is the pressure of the liquid is already in the capsular space. PHSG opposes a force of close to 15 mm Hg (1). The
blood colloid osmotic pressure (PCS) or capillary oncotic pressure (5) is the pressure given by the presence of proteins such as albumin, globulin and fibrinogen in blood plasma. It has an average value of 30 mmHg (1).
If we take the average values \u200b\u200bof PHSG = 55 mmHg, PHC = 15 PCS = 30mmHg mmHg and must be net filtration pressure is approximately:
NFP = 55 mm Hg - (15 mm Hg +30 mm Hg) = 10 mm Hg
Diagram Solute reabsorption in the nephron
botanica.cnba.uba.ar Taken
http://www.botanica.cnba.uba .ar/Pakete/6to/membr-casos/Fisiol-Nefron/Aparato-Urinario.htm
Tubular reabsorption
Most of the filtered water and solutes that are filtered at the glomerulus return to the bloodstream for tubular reabsorption (1). About 99% of the water is absorbed like most sugars, amino acids, ketones, ions and urea (a) (5).
The movement of chemicals and water via two mechanisms (1):
- paracellular reabsorption: passive process in which the liquid filters through the transcellular reabsorption
- cells: a process in which the substance crosses the membrane
cell transcellular reabsorption may be active or passive depending on whether you use or energy derived from ATP hydrolysis. If the primary active transport, ATP is used for pumping the substance through the membrane (ie sodium-potassium pump). If secondary active transport, an ion moves with its gradient of concentration along with another substance which is coupled to be moved against its concentration gradient (1). In fact, the reabsorption of sodium, chloride and Glucose "forces" the reabsorption of water through these mechanisms (1). The cells lining the proximal tubule and the descending portion of the loop of Henle are particularly permeable to water due to the existence of an integral protein called aquaporin I (1).
Most reabsorption occurs in proximal tubules
(1) (5). In this first segment returned to the bloodstream throughout glucose and amino acids were ultrafiltrates (1) (5). Lactic acid, water-soluble vitamins and other nutrients (1) also are absorbed as they travel mostly by the proximal tubules (1).
A countertransport mechanism moves the sodium into the interstitial fluid while hydrogen ions are transported into the proximal tubule (1).
When the liquid enters the loop of Henle
their chemical composition has changed from the glomerular filtrate. No glucose or amino acids and other substances have already been re-absorbed (1). Osmolarity (concentration of substances) is still fluid similar to blood.
Here is reabsorbed between 20 and 30% of sodium, potassium and calcium, and chloride 35% and 15% water. However, at this point the reabsorption of water by osmosis is not related to the movement of solute (1). Even it is said that the ascending portion of Henle's loop is completely waterproof, it does not absorb any water. Only solutes present in the tubular fluid return into the blood plasma (1).
is worth mentioning that in the thick ascending loop portion of Henle there cotransporters Sodium, potassium and chloride, however, potassium back into the tubular fluid for its concentration gradient. After the tubular fluid leaving the loop of Henle, is said to hypoosmotic to plasma because it contains a lower concentration of solutes (5).
When the fluid enters the distal convoluted t
úbulos , 80% of the water has already returned to the blood plasma. At this point, are reabsorbed from 10 to 15% water and some amount of sodium and chloride. Also secrete hydrogen ions and potassium (1) (5).
In collecting tubule, where 90 to 95% of water has been reabsorbed and filtered solutes have returned to the bloodstream, is also performed sodium reabsorption and potassium secretion (1). This process is done through channels instead of cotransporters as in the previous sections. The sodium-potassium pump in collecting duct cells are responsible for moving sodium into the interstitial fluid (and then the blood plasma) while moving potassium from the interstitial fluid into the collecting tubule. Hormonal regulation
The regulation of sodium, chloride and water and potassium secretion is controlled by four hormones (1):
- Angiotesina II
-
- Aldosterone Antidiuretic or Vasopressin (ADH) peptide
- Natridiurético Headset
The first two hormones belonging to the renin-Angiotesina
-Adosterona . In this system, when the volume and decrease blood pressure, the walls of afferent arterioles are stretched less and juxtaglomerular cells secrete the enzyme renin (1). The enzyme catalyzes the synthesis of Angiotesina I then becomes Angiotesina II is the active form of the hormone. The angiotesina II affects renal physiology in three main ways (1):
- GFR decreases by vasoconstriction of the afferent arterioles
- enhances reabsorption of sodium, chloride and water
- stimulates the adrenal cortex to release aldosterone
The
to aldosterone, a hormone that in turn stimulates the collecting ducts to reabsorb sodium and chloride while secreting more potassium. Osmotic consequence of this process is the decreased secretion of water which increases blood volume and pressure simultaneously.
The vasopressin, that bookcase from the posterior pituitary regulates water reabsorption by increasing the permeability of the cells in the distal convoluted tubule and along the collecting tubule (1). Permeability increase means that more water goes into the blood plasma and therefore its volume increases. When the hormone is not secreted, the walls become virtually impermeable to water and the reabsorption is minimal. If the absorption is low, more fluid is secreted to the outside as urine and blood volume decreases (1). The
atrial natriuretic peptide is a hormone that, besides being a potent vasodilator, inhibits sodium and water reabsorption in the proximal tubule and collecting tubule. It also suppresses the secretion of aldosterone and vasopressin, and increases secretion sodium in the urine. All these processes decrease the blood volume and therefore blood pressure.
References
(1) Tortora, Gerald. Derrickson, Bryan. 2006. Principles of Anatomy and Physiology. 11 th. Edition. Editorial Médica Panamericana. Mexico DF. Mexico. Cap 26.
(2) Cutillas Arroyo, Blanca. Urinary System: Anatomy. Virtual infermeria. Barcelona, \u200b\u200bSpain.
Available online:
http://www.infermeravirtual.com/ca-es/activitats-de-la-vida-diaria/la-persona/dimensio-biologica/sistema-urinari/pdf/sistema-urinari.pdf
(3) Fox, Stuart. 2008. Human Physiology. 10 th edition. Editorial McGrawHill. Madrid, Spain. Cap 17.
(4) Montalvo Diago, Jane A. Anthology of Human Physiology. American University. San Jose, Costa Rica.
(5) Bustos, Jorge. Presentation of Renal Physiology. Human Physiology course. American University. San Jose, Costa Rica.
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