Reducing the glomerular pressure also functions to protect the fragile capillaries of the glomerulus. When blood pressure drops, the same smooth muscle cells relax to lower resistance, increasing blood flow. The vasodilation of the afferent arteriole acts to increase the declining filtrate formation, bringing NFP and GFR back up to normal levels. The tubuloglomerular feedback mechanism involves the juxtaglomerular JG cells, or granular cells, from the juxtaglomerular apparatus JGA and a paracrine signaling mechanism utilizing ATP and adenosine.
These juxtaglomerular cells are modified, smooth muscle cells lining the afferent arteriole that can contract or relax in response to the paracrine secretions released by the macula densa. This mechanism stimulates either contraction or relaxation of afferent arteriolar smooth muscle cells, which regulates blood flow to the glomerulus Table Recall that the DCT is in intimate contact with the afferent and efferent arterioles of the glomerulus.
The increased fluid movement more strongly deflects single nonmotile cilia on macula densa cells. This increased osmolarity of the filtrate, and the greater flow rate within the DCT, activates macula densa cells to respond by releasing ATP and adenosine a metabolite of ATP. ATP and adenosine act locally as paracrine factors to stimulate the myogenic juxtaglomerular cells of the afferent arteriole to constrict, slowing blood flow into the glomerulus.
This vasoconstriction causes less plasma to be filtered, which decreases the glomerular filtration rate GFR , which gives the tubule more time for NaCl reabsorption. Conversely, when GFR decreases, less NaCl is in the filtrate, and most will be reabsorbed before reaching the macula densa, which will result in decreased ATP and adenosine, allowing the afferent arteriole to dilate and increase GFR. This vasodilation causes more plasma to be filtered, which increase the glomerular filtration rate GFR , which gives the tubule less time for NaCl reabsorption increasing the amount of NaCl in the filtrate.
The extrinsic control mechanisms have an effect on GFR, but their primary function is to maintain systemic blood pressure. The kidneys are innervated by the sympathetic neurons of the autonomic nervous system via the celiac plexus and splanchnic nerves.
Reduction of sympathetic stimulation results in vasodilation and increased blood flow through the kidneys during resting conditions. When the frequency of action potentials increases, the arteriolar smooth muscle constricts vasoconstriction , resulting in diminished glomerular flow, so less filtration occurs. Under conditions of stress, sympathetic nervous activity increases, resulting in the direct vasoconstriction of afferent arterioles norepinephrine effect as well as stimulation of the adrenal medulla.
The adrenal medulla, in turn, produces a generalized vasoconstriction through the release of epinephrine. This includes vasoconstriction of the afferent arterioles, further reducing the volume of blood flowing through the kidneys. This process redirects blood to other organs with more immediate needs.
Under severe stress, such as significant blood loss, the sympathetic nervous system kicks into high gear to keep the blood routed to essential organs and keep the body alive. The strong vasoconstriction required to maintain systemic blood pressure under these severe conditions significantly reduces blood flow to the kidneys and can be damaging to the kidney tissues. If blood pressure falls, the sympathetic nerves will also stimulate the release of renin which we will discuss next.
Recall that renin is an enzyme that is produced by the granular cells of the afferent arteriole at the JGA. It enzymatically converts angiotensinogen made by the liver, freely circulating into angiotensin I. Its release is stimulated by paracrine signals from the JGA in response to decreased extracellular fluid volume. Ultrafiltration UF Ultrafiltration is a process that is similar to microfiltration, but with smaller pore sizes ranging from 0.
UF membranes are used in rejecting viruses and polypeptides, and are widely used in protein concentration and wastewater treatment. Nanofiltration NF Nanofiltration membranes are similar to reverse osmosis membranes in that they contain a thin-film composite layer Reverse Osmosis RO Reverse osmosis membranes are even tighter than nanofiltration membranes, and are able to reject all monovalent ions while allowing water molecules to pass through in aqueous solutions. They can also remove viruses and bacteria found in feed solutions.
Common applications for reverse osmosis filtration include seawater desalination and industrial water treatment. Once the membrane has been compacted to a certain level, the permeate flux begins to stabilize and fluctuate less. The flux response to pressure during membrane compaction is illustrated in the drawing below.
The deformation caused by membrane compaction is often irreversible, especially for flat sheet membranes. Secretion involves the transfer of hydrogen ions, creatinine, drugs, and urea from the blood into the collecting duct, and is primarily made of water.
Blood and glucose are not normally found in urine. Key Terms urine : A liquid excrement consisting of water, salts, and urea, which is made in the kidneys then released through the urethra. Glomerular Filtration Glomerular filtration is the renal process whereby fluid in the blood is filtered across the capillaries of the glomerulus.
Learning Objectives Explain the process of glomerular filtration in the kidneys. Key Takeaways Key Points The formation of urine begins with the process of filtration. Fluid and small solutes are forced under pressure to flow from the glomerulus into the capsular space of the glomerular capsule. Blood entering the glomerulus has filterable and non-filterable components. Filterable blood components include water, nitrogenous waste, and nutrients that will be transferred into the glomerulus to form the glomerular filtrate.
Non-filterable blood components include blood cells, albumins, and platelets, that will leave the glomerulus through the efferent arteriole. Glomerular filtration is caused by the force of the difference between hydrostatic and osmotic pressure though the glomerular filtration rate includes other variables as well.
Key Terms glomerulus : A small, intertwined group of capillaries within nephrons of the kidney that filter the blood to make urine. It is the primary force that drives glomerular filtration. Learning Objectives List the conditions that can affect the glomerular filtration rate GFR in kidneys and the manner of its regulation.
Key Takeaways Key Points Glomerular filtration is occurs due to the pressure gradient in the glomerulus. Increased blood volume and increased blood pressure will increase GFR. Constriction in the afferent arterioles going into the glomerulus and dilation of the efferent arterioles coming out of the glomerulus will decrease GFR.
Water tends to follow proteins based on an osmotic pressure gradient. Tubular Reabsorption Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. Learning Objectives Describe the process of tubular reabsorption in kidney physiology.
Key Takeaways Key Points Proper function of the kidney requires that it receives and adequately filters blood. Reabsorption includes passive diffusion, active transport, and cotransport. Water is mostly reabsorbed by the cotransport of glucose and sodium. Filtrate osmolarity changes drastically throughout the nephron as varying amounts of the components of filtrate are reabsorbed in the different parts of the nephron. Osmolarity Changes As filtrate passes through the nephron, its osmolarity ion concentration changes as ions and water are reabsorbed.
Tubular Secretion Hydrogen, creatinine, and drugs are removed from the blood and into the collecting duct through the peritubular capillary network. Learning Objectives Describe the purpose of tubular secretion in kidney physiology. Key Takeaways Key Points The substance that remains in the collecting duct of the kidneys following reabsorption is better known as urine. Secreted substances largely include hydrogen, creatinine, ions, and other types of waste products, such as drugs. Cells must be able to move large polar and charged molecules across the lipid bilayer of the membrane in order to carry out life processes.
To allow these molecules, which are not soluble in the lipid bilayer, to pass across the hydrophobic barrier it is necessary to provide ports, channels or holes through the membrane.
The molecules will still move spontaneously down a concentration gradient from high to low concentration. Some of these channels can remain open at all times, allowing the molecules to move freely according to the concentration gradient.
Others can be gated channels that open and close in response to the needs of the cell. In most cases these channels are very discriminatory and will only allow specific molecules to pass.
The process of moving impermeable molecules across a membrane down their concentration gradients using channels or pores is referred to as facilitated diffusion. Because the molecules are moving down a concentration gradient, the process is driven by simple diffusion and does not require the input of additional energy from the cell.
Cells continually encounter changes in their external environment. Most cells have a similar blend of solutes within them, but interstitial or extracellular fluid can vary.
As you know, molecules will tend to move down their concentration gradients until equilibrium is reached. You might think that solutes will flow into our out of the cell until the solute concentrations are equal across the membrane. However, not all molecules can pass through the cell membrane. The plasma membrane lipid bilayer is significantly less permeable to most solutes than it is to water. Therefore the WATER tends to flow in a way that establishes an equal concentration of solutes on either side of the membrane.
The water flows down its own concentration gradient, with a net movement toward the region that has a higher concentration of solutes. This movement of water across a semipermeable membrane in response to an imbalance of solute is called osmosis. The relationship between the solute concentration and amount of water is an inverse relationship.
The more concentrated a solution is, the less water it contains. The fewer solutes, the more water — i. Water follows gradients and moves from an area of more water to less water but in reality water is moved to the area with the greater number of solutes.
This creates a pressure termed osmotic pressure. Cells cannot actively move water, it must follow osmotic gradients. Solutions that have a greater solute concentration will pull water via osmotic pressure.
This depends on the total number of solutes, not the type. Note that some water can pass through the cell membrane but most water passes through protein membrane channels termed aquaporins. Cells may find themselves in three different sorts of solutions.
The terms isotonic, hypertonic, and hypotonic refer to the concentration of solutes outside the cell relative to the solute concentration inside the cell.
In an isotonic solution, solutes and water are equally concentrated within and outside the cell. The cell is bathed in a solution with a solute concentration that is similar to its own cytoplasm. Many medical preparations saline solutions for nasal sprays, eye drops, and intravenous drugs are designed to be isotonic to our cells. Distilled pure water is the ultimate hypotonic solution. If a cell is placed in a hypotonic solution, it will tend to gain water.
A hypertonic solution has a high solute concentration lower water concentration compared to the cell cytoplasm. Very salty or sugary solutions brines or syrups are hypertonic to living cells. If a cell is placed in such a solution, water tends to flow spontaneously out of the cell. Filtration is another passive process of moving material through a cell membrane. While diffusion and osmosis rely on concentration gradients, filtration uses a pressure gradient.
Molecules will move from an area of higher pressure to an area of lower pressure.
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