This rate determines how much solute is retained or discarded, how much water is retained or discarded, and ultimately, the osmolarity of blood and the blood pressure of the body. Glomerular filtration has to be carefully and thoroughly controlled because the simple act of filtrate production can have huge impacts on body fluid homeostasis and systemic blood pressure.
Due to these two very distinct physiological needs, the body employs two very different mechanisms to regulate GFR. The kidney can control itself locally through intrinsic controls, also called renal autoregulation. These intrinsic control mechanisms maintain filtrate production so that the body can maintain fluid, electrolyte, and acid-base balance and also remove wastes and toxins from the body.
There are also control mechanisms that originate outside of the kidney, the nervous and endocrine systems, and are called extrinsic controls. The nervous system and hormones released by the endocrine systems function to control systemic blood pressure by increasing or decreasing GFR to change systemic blood pressure by changing the fluid lost from the body.
The kidneys are very effective at regulating the rate of blood flow over a wide range of blood pressures. Your blood pressure will decrease when you are relaxed or sleeping. It will increase when exercising. Yet, despite these changes, the filtration rate through the kidney will change very little.
This is due to two internal autoregulatory mechanisms that operate without outside influence: the myogenic mechanism and the tubuloglomerular feedback mechanism. The myogenic mechanism regulating blood flow within the kidney depends upon a characteristic shared by most smooth muscle cells of the body. When you stretch a smooth muscle cell, it contracts; when you stop, it relaxes, restoring its resting length.
This mechanism works in the afferent arteriole that supplies the glomerulus and can regulate the blood flow into the glomerulus. When blood pressure increases, smooth muscle cells in the wall of the arteriole are stretched and respond by contracting to resist the pressure, resulting in little change in flow. This vasoconstriction of the afferent arteriole acts to reduce excess filtrate formation, maintaining normal NFP and GFR. 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.
The next step is reabsorption , during which molecules and ions will be reabsorbed into the circulatory system. In the collecting duct, secretion will occur before the fluid leaves the ureter in the form of urine. The end product of all these processes is urine, which is essentially a collection of substances that has not been reabsorbed during glomerular filtration or tubular reabsorbtion.
Urine is mainly composed of water that has not been reabsorbed, which is the way in which the body lowers blood volume, by increasing the amount of water that becomes urine instead of becoming reabsorbed.
The other main component of urine is urea, a highly soluble molecule composed of ammonia and carbon dioxide, and provides a way for nitrogen found in ammonia to be removed from the body. Urine also contains many salts and other waste components. Red blood cells and sugar are not normally found in urine but may indicate glomerulus injury and diabetes mellitus respectively.
Normal kidney physiology : This illustration demonstrates the normal kidney physiology, showing where some types of diuretics act, and what they do. Glomerular filtration is the renal process whereby fluid in the blood is filtered across the capillaries of the glomerulus.
Glomerular filtration is the first step in urine formation and constitutes the basic physiologic function of the kidneys. It describes the process of blood filtration in the kidney, in which fluid, ions, glucose, and waste products are removed from the glomerular capillaries. Many of these materials are reabsorbed by the body as the fluid travels through the various parts of the nephron, but those that are not reabsorbed leave the body in the form of urine.
Blood plasma enters the afferent arteriole and flows into the glomerulus, a cluster of intertwined capillaries. The visceral layer lies just beneath the thickened glomerular basement membrane and is made of podocytes that form small slits in which the fluid passes through into the nephron. The size of the filtration slits restricts the passage of large molecules such as albumin and cells such as red blood cells and platelets that are the non-filterable components of blood.
These then leave the glomerulus through the efferent arteriole, which becomes capillaries meant for kidney—oxygen exchange and reabsorption before becoming venous circulation. The positively charged podocytes will impede the filtration of negatively charged particles as well such as albumins. The process by which glomerular filtration occurs is called renal ultrafiltration.
The force of hydrostatic pressure in the glomerulus the force of pressure exerted from the pressure of the blood vessel itself is the driving force that pushes filtrate out of the capillaries and into the slits in the nephron. Osmotic pressure the pulling force exerted by the albumins works against the greater force of hydrostatic pressure, and the difference between the two determines the effective pressure of the glomerulus that determines the force by which molecules are filtered.
These factors will influence the glomeruluar filtration rate, along with a few other factors. Regulation of GFR requires both a mechanism of detecting an inappropriate GFR as well as an effector mechanism that corrects it. List the conditions that can affect the glomerular filtration rate GFR in kidneys and the manner of its regulation.
Glomerular filtration rate GFR is the measure that describes the total amount of filtrate formed by all the renal corpuscles in both kidneys per minute. The glomerular filtration rate is directly proportional to the pressure gradient in the glomerulus, so changes in pressure will change GFR.
GFR is also an indicator of urine production, increased GFR will increase urine production, and vice versa. The filtration constant is based on the surface area of the glomerular capillaries, and the hydrostatic pressure is a pushing force exerted from the flow of a fluid itself; osmotic pressure is the pulling force exerted by proteins.
Many factors can change GFR through changes in hydrostatic pressure, in terms of the flow of blood to the glomerulus. GFR is most sensitive to hydrostatic pressure changes within the glomerulus. What two structures constitute the renal corpuscle?
What arteries branch off the arcuate arteries? Which of the following is the most likely to cause pyelonephritis? Which vessels supply the cortical tissue of the kidney with blood? Which of the following is NOT a cause of anuria? In which kidney region are all renal corpuscles located? Which statement best describes the effect diuretics have? Diuretics increase urinary output. Where does the efferent arteriole of the juxtamedullary nephron carry blood to? The descending limb of the loop of Henle is permeable to both solutes and water.
Which of the following substances is not normally found in filtrate? What is the primary driving force pressure that produces glomerular filtration? Fenestrated vessels that allow passage of all plasma elements but not blood cells. Glomerular capillaries. May form meandering vessels or bundles of long straight vessels.
High pressure vessels that forces fluid and solutes into the glomerular capsule. Afferent arterioles. Low pressure, porous vessels that reabsorb solutes and water from the tubule cells. Which part of the brain controls the micturition reflex? Which of the following is the functional unit of the kidney? Which of the following best describes glomerular filtration rate GFR?
GFR regulation mechanisms primarily affect which of the following? Afferent arteriole diameter would decrease. Where in the nephron does most solute reabsorption occur? The individual exceeded the transport maximum.
Which of the following transporters in the luminal membrane results in secretion? Overall, which of the following pressures is ultimately responsible for glomerular filtration? Which of the following is NOT one of the things that must happen for micturition to occur? The extrusor muscle must relax. Which of the following is NOT a function of the kidneys?
Which of the following is NOT a major urine formation process? Which statement best describes the function of the urethra? The urethra transports urine from the bladder to the outside of the body.
Which of the following is not reabsorbed by the proximal convoluted tubule? What is the most direct function of the juxtaglomerular apparatus? What type of capillaries make up the glomerulus?
What would happen if the capsular hydrostatic pressure were increased above normal? Net filtration would decrease. Bowman's capsule and glomerulus. Site at which most of the tubular absorption occurs. Blood supply that directly receives substances from the tubular cells. Site that drains the distal convoluted tubule. Which of the following does not describe the juxtaglomerular complex?
Its macula densa cells produce aldosterone. What is the function of the macula densa cells of the juxtaglomerular complex JGC? The leading cause of chronic renal disease is hypertension. Which of the choices below is the salt level-monitoring part of the nephron? Which of the following is not associated with the renal corpuscle? What vessel directly feeds into the glomerulus? Select the correct statement about the nephrons. The parietal layer of the glomerular capsule is simple squamous epithelium.
Where does antidiuretic hormone ADH exert its effects to promote water reabsorption? Which of the choices below is not a function of the urinary system?
Which of the following is not true regarding tubular reabsorption? It is a purely passive transport process. Which of the choices below is the least important role of tubular secretion? Which of the following does NOT impact how much sodium is reabsorbed? Which of the following is NOT involved in triggering the thirst mechanism?
What accounts for the route through which most fluid is lost in a day? What is the effect of hyperventilation on pH? What is the most common cation found in the interstitial fluid? Which buffer system is the most abundant in the body?
Where is the majority of water stored in the human body? Which of the following creates the greatest osmotic pressure? What would be the compensation? What solute in body fluids determines most of their chemical and physical reactions? In a given day, what is the typical value for water intake? Which of the following is NOT a hormone involved in water and electrolyte balance? How much water is generated per day from cellular metabolism?
What is the most important trigger for aldosterone release? Which age group most commonly has fluid, electrolyte, and acid-base balance issues? What results from increased levels of aldosterone? Of the three buffering mechanisms in the body, which is the strongest? Carbonic acid is broken down into water and CO2; the CO2 is then exhaled. In a respiratory acidosis the kidney would do which of the following? The kidney would reabsorb bicarbonate and secrete hydrogen ions.
The proximal convoluted tubule is the first segment of renal tubule. It begins at the urinary pole of the glomerulus. The convoluted portion of the tubule leads into a straight segment that descends into the medulla within a medullary ray and becomes the loop of Henle.
The loop of Henle forms a hair-pin structure that dips down into the medulla. It contains four segments: the pars recta the straight descending limb of proximal tubule , the thin descending limb, the thin ascending limb, and the thick ascending limb.
The turn of the loop of Henle usually occurs in the thin segment within the medulla, and the tubule then ascends toward the cortex parallel to the descending limb. The end of the loop of Henle becomes the distal convoluted tubule near its original glomerulus. The loops of Henle run in parallel to capillary loops known as the vasa recta. Recall from Physiology that the loop of Henle serves to create high osmotic pressure in the renal medulla via the counter-current multiplier system.
Such high osmotic pressure is important for the reabsorption of water in the later segments of the renal tubule. The distal convoluted tubule is shorter and less convoluted than the proximal convoluted tubule. Further reabsorption and secretion of ions occur in this segment. The initial segment of the distal convoluted tubule lies right next to the glomerulus and forms the juxtaglomerular apparatus.
The juxtaglomerular apparatus is a specialized structure formed by the distal convoluted tubule and the glomerular afferent arteriole. It is located near the vascular pole of the glomerulus. The main function of the apparatus is the secretion of renin, which regulates systemic blood pressure via the renin-angiotensin-alodosterone system. The juxtaglomerular apparatus is composed of:. The terminal portion of the distal tubule empties through collecting tubules into a straight collecting duct in the medullary ray.
The collecting duct system is under the control of antidiuretic hormone ADH. When ADH is present, the collecting duct becomes permeable to water. Numerous collecting ducts merge into the renal pelvis, which then becomes the ureter.
The ureter is a muscular tube, composed of an inner longitudinal layer and an outer circular layer.
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