Each nephron has a glomerulus , the site of blood filtration. As blood flows through the glomerulus, blood pressure pushes water and solutes from the capillaries into the capsule through a filtration membrane. This glomerular filtration begins the urine formation process. Inside the glomerulus, blood pressure pushes fluid from capillaries into the glomerular capsule through a specialized layer of cells.
This layer, the filtration membrane , allows water and small solutes to pass but blocks blood cells and large proteins.
Those components remain in the bloodstream. The filtrate the fluid that has passed through the membrane flows from the glomerular capsule further into the nephron. The glomerulus filters water and small solutes out of the bloodstream. The resulting filtrate contains waste, but also other substances the body needs: essential ions, glucose, amino acids, and smaller proteins.
When the filtrate exits the glomerulus, it flows into a duct in the nephron called the renal tubule. As it moves, the needed substances and some water are reabsorbed through the tube wall into adjacent capillaries. This reabsorption of vital nutrients from the filtrate is the second step in urine creation. The filtrate absorbed in the glomerulus flows through the renal tubule, where nutrients and water are reabsorbed into capillaries.
In this event, a number of symptoms, signs, or laboratory findings point to the diagnosis. Renin-angiotensin mechanism — triggered by the juxtaglomerular apparatus; when filtration rate decreases, the enzyme renin is released. Renin converts a plasma protein called angiotensinogen into angiotensin I.
Angiotensin I is quickly converted into angiotensin II by another enzyme. Angiotensin II causes 3 changes:. Secretion — waste products such as urea and uric acid, drugs and hydrogen and bicarbonate ions are move out of the peritubular capillaries into the filtrate; this removes unwanted wastes and helps regulate pH.
The volume of filtrate formed by both kidneys per minute is termed the glomerular filtration rate GFR. The heart pumps about 5 L blood per min under resting conditions.
Approximately 20 percent or one liter enters the kidneys to be filtered. Ninety-nine percent of this filtrate is returned to the circulation by reabsorption so that only about 1—2 liters of urine are produced per day.
If a person has a hematocrit of 45, then the renal plasma flow is 55 percent. It is the renal plasma flow times the fraction that enters the renal capsule 19 percent. It is the GFR times the fraction of the filtrate that is not reabsorbed 0.
Recall that filtration occurs as pressure forces fluid and solutes through a semipermeable barrier with the solute movement constrained by particle size. Hydrostatic pressure is the pressure produced by a fluid against a surface. If you have a fluid on both sides of a barrier, both fluids exert a pressure in opposing directions. Net fluid movement will be in the direction of the lower pressure.
Osmosis is the movement of solvent water across a membrane that is impermeable to a solute in the solution. This creates a pressure, osmotic pressure, which will exist until the solute concentration is the same on both sides of a semipermeable membrane. As long as the concentration differs, water will move. There is also an opposing force, the osmotic pressure, which is typically higher in the glomerular capillary. To understand why this is so, look more closely at the microenvironment on either side of the filtration membrane.
Recall that cells and the medium-to-large proteins cannot pass between the podocyte processes or through the fenestrations of the capillary endothelial cells. This means that red and white blood cells, platelets, albumins, and other proteins too large to pass through the filter remain in the capillary, creating an average colloid osmotic pressure of 30 mm Hg within the capillary.
Urine — filtrate after it has passed through the nephron and undergone filtration, reabsorption, and secretion. The urine passes into the collecting duct, which joins with the minor calyx, major calyx, and eventually the renal pelvis. Usually, glucose, ketones, protein, and bilirubin are not detectable in urine.
The following are not normally found in urine: Hemoglobin. It consists of water, urea from amino acid metabolism , inorganic salts, creatinine, ammonia, and pigmented products of blood breakdown, one of which urochrome gives urine its typically yellowish colour. Glomerular filtrate contains a lot of water, but also important molecules like glucose, amino acids, salts and excretory material, urea.
Thus selective reabsorption is necessary along the length of nephron, to reabsorb necessary materials from filtrate which are not meant for excretion. The urinary system includes the kidneys, ureters, bladder and urethra. This system filters your blood, removing waste and excess water.
This waste becomes urine. The most common urinary issues are bladder infections and urinary tract infections UTIs. The fluid filtered from blood, called filtrate, passes through the nephron, much of the filtrate and its contents are reabsorbed into the body. Reabsorption is a finely tuned process that is altered to maintain homeostasis of blood volume, blood pressure, plasma osmolarity, and blood pH. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule.
Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.
There are three main steps of urine formation: glomerular filtration, reabsorption, and secretion. These processes ensure that only waste and excess water are removed from the body. If you have significant symptoms that accompany the white particles in your urine, you may want to see your doctor. Clear urine is a sign of good hydration and potential overhydration. Pale yellow urine is an indicator of good hydration. Dark yellow urine is a sign to drink more fluids.
Amber-colored urine can indicate dehydration.
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