So, the correct answer is 'Proximal convoluted tubule'.
Antidiuretic hormone binds to receptors on cells in the collecting ducts of the kidney and promotes reabsorption of water back into the circulation. In the absense of antidiuretic hormone, the collecting ducts are virtually impermiable to water, and it flows out as urine.
Na+ is reabsorbed by active transport using ATP. Most of the energy used for reabsorption is for Na+. Major calyces are: Major calyces are the large branches of the renal pelvis.
Each day, the alimentary canal processes up to 10 liters of food, liquids, and GI secretions, yet less than one liter enters the large intestine. Almost all ingested food, 80 percent of electrolytes, and 90 percent of water are absorbed in the small intestine.
The urea reabsorbed increases the medullary concentration of the solute, which is critical for the reabsorption of water from the thin inner medullary part of the descending limb of the loop of Henle. Here, there is no osmotic gradient to cause water movement in the diluting kidney.
Selective reabsorption is the process whereby certain molecules (e.g. ions, glucose and amino acids), after being filtered out of the capillaries along with nitrogenous waste products (i.e. urea) and water in the glomerulus, are reabsorbed from the filtrate as they pass through the nephron.
This process is called secretion. The secreted ions combine with the remaining filtrate and become urine. The urine flows out of the nephron tubule into a collecting duct. It passes out of the kidney through the renal pelvis, into the ureter, and down to the bladder.
About 67 percent of the water, Na+, and K+ entering the nephron is reabsorbed in the PCT and returned to the circulation.
The alpha-intercalated cell of collecting duct is the main responsible for hydrogen secretion into the urine. The carbon dioxide, which is generated in the cells and enters from the blood, is changed to carbonic acid. The hydrogen ion is secreted into the lumen by the luminal H(+)-ATPase.
Glucose, amino acids and other substances diffuse out of the epithelial cell down their concentration gradients on passive transporters and are then reabsorbed by the blood capillaries.
About 99% of the water-like filtrate , small molecules, and lipid-soluble substances , are reabsorbed downstream in the nephron tubule. This means that the amount of urine eliminated is only about one percent of the amount of fluid filtrated through the glomeruli into the renal tubules.
Learning Objectives
| Table 1. Substances Secreted or Reabsorbed in the Nephron and Their Locations |
|---|
| Substance | PCT | Collecting ducts |
|---|
| Potassium | 65 percent reabsorbed; diffusion | Secretion controlled by aldosterone; active |
| Calcium | Reabsorbed; diffusion | Reabsorbed if parathyroid hormone present; active |
| Magnesium | Reabsorbed; diffusion | |
Physiology. The descending loop of Henle receives isotonic (300 mOsm/L) fluid from the proximal convoluted tubule (PCT). The descending portion of the loop of Henle is extremely permeable to water and is less permeable to ions, therefore water is easily reabsorbed here and solutes are not readily reabsorbed.
Under normal circumstances, up to 180g/day of glucose is filtered by the renal glomerulus and virtually all of it is subsequently reabsorbed in the proximal convoluted tubule. This reabsorption is effected by two sodium-dependent glucose cotransporter (SGLT) proteins.
The principal cells are responsible for sodium reabsorption via the amiloride-sensitive sodium channel ENaC. Principal cells also secrete potassium via the ROMK potassium channel. The ability of the cortical collecting duct to absorb water is controlled by antidiuretic hormone (ADH).
Reabsorption allows many useful solutes (primarily glucose and amino acids), salts and water that have passed through Bowman's capsule, to return to the circulation.
Filtration involves the transfer of soluble components, such as water and waste, from the blood into the glomerulus. Reabsorption involves the absorption of molecules, ions, and water that are necessary for the body to maintain homeostasis from the glomerular filtrate back into the blood.
So a second process, called reabsorption, moves essential materials from the nephron back into the blood. Reabsorption occurs when transport proteins molecules in the walls of the nephron return essential substances such as glucose, amino acids, water, and salt to the capillaries that surround the nephron.
The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries (glomerulus) and a small tube called a renal tubule.
Reabsorption is the process in urine formation which takes place in the PCT of the nephron. Two substances which are selectively reabsorbed are amino acids and glucose. During this process the essential substances are taken back by the blood plasma from the tubule of the nephron.
In the case of the body being dehydrated, the kidneys reabsorb as much water as possible back into the blood to produce highly concentrated urine full of excreted ions and wastes. The changes in excretion of water are controlled by antidiuretic hormone (ADH).
The proximal tubules reabsorb about 65% of water, sodium, potassium and chloride, 100% of glucose, 100% amino acids, and 85-90% of bicarbonate. This reabsorption occurs due to the presence of channels on the basolateral (facing the interstitium) and apical membranes (facing the tubular lumen).
The removal of proximal convoluted tubule PCT from the nephron results in lack of reabsorption of high threshold substances frkm renal tubules and obligatory reabsorption of water is also affected leading to more diluted urine. Main function is to recover water and sodium chloride from urine.
The proximal convoluted tubule is where a majority of reabsorption occurs. About 67 percent of the water, Na+, and K+ entering the nephron is reabsorbed in the proximal convoluted tubule and returned to the circulation.
The Proximal Convoluted Tubule Passively Reabsorbs UreaIt is made predominantly in the liver from ammonia and bicarbonate and is one of the main components of urine. The rate of synthesis varies from 300 to 600 mmol/day depending on the protein intake. All of this urea eventually finds its way into the urine.
Potassium is freely filtered by the glomerulus. The bulk of filtered K+ is reabsorbed in the proximal tubule and loop of Henle, such that less than 10% of the filtered load reaches the distal nephron. In the proximal tubule, K+ absorption is primarily passive and proportional to Na+ and water (Figure 3).
Under normal circumstances, up to 180 g/day of glucose is filtered by the renal glomerulus and virtually all of it is subsequently reabsorbed in the proximal convoluted tubule. This reabsorption is effected by two sodium-dependent glucose cotransporter (SGLT) proteins.
Epithelial cells in the proximal convoluted tubule (PCT) reabsorb components of the glomerular filtrate that have nutritional significance (e.g., glucose, ions and amino acids). To facilitate absorption, these cells have numerous microvilli, Mv, along their apical surface.
They move in the direction from the blood stream into the tubules, which is in the reverse direction of reabsorption. These substances are K+ ions, H+ ions, NH4+ ions, creatinine, urea, some hormones, and some drugs.
