Reabsorption
Encyclopedia
In physiology
, reabsorption or tubular reabsorption is the flow of glomerular filtrate from the proximal tubule
of the nephron
into the peritubular capillaries
, or from the urine into the blood. It is termed "reabsorption" because this is technically the second time that the nutrients in question are being absorbed into the blood, the first time being from the small intestine into the villi. This happens as a result of sodium
transport from the lumen into the blood
by the Na+/K+ ATPase
in the basolateral membrane of the epithelial cells. Thus, the glomerular filtrate becomes more concentrated
, which is one of the steps in forming urine
. In this way, many useful solutes
(primarily glucose
and amino acid
s), salt
s and water
that have passed in the proximal tubule through the Bowman's capsule
, return in the circulation
. These solutes are reabsorbed isotonically
, in that the osmotic potential of the fluid leaving the proximal tubule is the same as that of the initial glomerular filtrate. However, glucose, amino acids, inorganic phosphate, and some other solutes are reabsorbed via secondary active transport
through cotransport channels
driven by the sodium gradient out of the nephron.
Renin Angiotensin System:
1. The kidneys sense low blood pressure.
2. Release renin into the blood.
3. Renin causes production of Angiotensin I.
4. Angiotensin Converting Enzyme (ACE) converts Angiotensin I to Angiotensin II.
5. Angiotensin II stimulates the release of Aldosterone, ADH, and thirst.
6. Aldosterone causes kidneys to reabsorb sodium. ADH increases the uptake of water.
7. Water follows sodium.
8. Blood volume goes up, therefore blood pressure goes up.
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
, reabsorption or tubular reabsorption is the flow of glomerular filtrate from the proximal tubule
Proximal tubule
The proximal tubule is the portion of the duct system of the nephron of the kidney which leads from Bowman's capsule to the loop of Henle.-Structure and appearance:...
of the nephron
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....
into the peritubular capillaries
Peritubular capillaries
In the renal system, peritubular capillaries are tiny blood vessels that travel alongside nephrons allowing reabsorption and secretion between blood and the inner lumen of the nephron....
, or from the urine into the blood. It is termed "reabsorption" because this is technically the second time that the nutrients in question are being absorbed into the blood, the first time being from the small intestine into the villi. This happens as a result of sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
transport from the lumen into the blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
by the Na+/K+ ATPase
ATPase
ATPases are a class of enzymes that catalyze the decomposition of adenosine triphosphate into adenosine diphosphate and a free phosphate ion. This dephosphorylation reaction releases energy, which the enzyme harnesses to drive other chemical reactions that would not otherwise occur...
in the basolateral membrane of the epithelial cells. Thus, the glomerular filtrate becomes more concentrated
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
, which is one of the steps in forming urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
. In this way, many useful solutes
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
(primarily glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
and amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s), salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
s and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
that have passed in the proximal tubule through the Bowman's capsule
Bowman's capsule
The Bowman's capsule is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac...
, return in the circulation
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
. These solutes are reabsorbed isotonically
Tonicity
Tonicity is a measure of the osmotic pressure gradient of two solutions separated by a semipermeable membrane. It is commonly used when describing the response of cells immersed in an external solution...
, in that the osmotic potential of the fluid leaving the proximal tubule is the same as that of the initial glomerular filtrate. However, glucose, amino acids, inorganic phosphate, and some other solutes are reabsorbed via secondary active transport
Active transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...
through cotransport channels
Symporter
A cotransporter is an integral membrane protein that is involved in secondary active transport. It works by binding to two molecules or ions at a time and using the gradient of one solute's concentration to force the other molecule or ion against its gradient....
driven by the sodium gradient out of the nephron.
Renin Angiotensin System:
1. The kidneys sense low blood pressure.
2. Release renin into the blood.
3. Renin causes production of Angiotensin I.
4. Angiotensin Converting Enzyme (ACE) converts Angiotensin I to Angiotensin II.
5. Angiotensin II stimulates the release of Aldosterone, ADH, and thirst.
6. Aldosterone causes kidneys to reabsorb sodium. ADH increases the uptake of water.
7. Water follows sodium.
8. Blood volume goes up, therefore blood pressure goes up.