Regulation of Renal Blood Flow

the kidneys main job is to filter the blood to remove waste so it shouldn't be surprising that they receive about a quarter of the blood that the heart pumps with each beat on average the heart pumps out almost 5 liters of blood every minute so one quarter of that or 1.25 liters flows into the renal artery every minute blood from the renal artery flows into smaller and smaller arteries eventually reaching the tiniest of arterioles called the afferent arterioles after the afferent arterial blood moves into a tiny capillary bed called the glomerulus the glomerulus is part

of the functional unit of the kidney called the nephron there's about a million nephrons in each kidney and each of them consists of a renal corpuscle made up of the glomerulus and the bowman's capsule surrounding it and a renal tubule interestingly once the blood leaves the glomerulus it doesn't enter into venules instead the glomerulus funnels blood into efferent arterioles which divide into capillaries a second time these capillaries are called peritubular capillaries because they are arranged around the renal tubule now blood filtration starts in the glomerulus where a urine precursor called filtrate is formed the amount

of blood filtered into the nephrons by all of the glomeruli each minute is called the glomerular filtration rate and it's actually just a small fraction of the blood that gets to the kidneys because the glomerulus doesn't allow red blood cells and proteins to pass through and be excreted into urine so right from the start what passes through the glomerulus is mostly plasma which normally makes up about 55 percent of blood what's more the glomerulus only filters about 20 percent of that plasma in one go so when all is said and done of the around 1.25

liters at the heart pumps out every minute the glomerular filtration rate is normally approximately 125 milliliters this filtrate then enters the renal tubule the renal tubule is made up of a proximal convoluted tubule the nephron loop also known as the loop of henle which has an ace ending and a descending limb and finally the distal convoluted tubule as filtrate makes its way through the renal tubule waste and molecules such as ions and water are exchanged between the tubule and the peritubular capillaries until blood is filtered of any excess finally the peritubular capillaries reunite to form

larger and larger venous vessels the veins follow the path of the arteries but in reverse so they keep uniting until they finally form the large renal vein which exits the kidney and drains into the inferior vena cava now renal blood flow is proportional to the pressure gradient which is the difference in pressure between the renal artery and the renal vein divided by the resistance in the renal arterioles so a high systemic blood pressure and a low resistance in the renal arterioles leads to a high renal blood flow and in turn glomerular filtration rate and vice

versa regulation of renal blood flow is mainly accomplished by increasing or decreasing arteriolar resistance there are two key hormones that act to increase arteriolar resistance and in turn reduce renal blood flow adrenaline and angiotensin adrenaline also known as epinephrine is a hormone secreted by the adrenal gland right above the kidneys in response to sympathetic stimulation adrenaline produces a fight-or-flight response by binding to adrenergic receptors on cells all over the body adrenaline binds to the alpha-1 adrenergic receptors along the afferent and efferent arterioles and causes the smooth muscle cells that wrap around those arterioles to contract

making the afferent and efferent arterioles quickly constrict the increased arterial resistance leads to a low renal blood flow so when you're being chased by a kangaroo and the fight-or-flight mode is on blood flow is basically diverted away from the kidneys and towards more important tissues like your leg muscles angiotensin ii on the other hand is synthesized in response to low blood pressure by endothelial cells that line the blood vessels throughout the body angiotensin ii is the final product in a cascade of reactions that start with renin an enzyme produced in the kidneys by specialized smooth

muscle cells called juxtaglomerular cells which can be found in the walls of the afferent arterioles when there's low blood pressure renin is released in the blood where it cleaves angiotensin 1 from angiotensinogen now endothelial cells in general but mostly those lining the vessels in the lungs make an enzyme called angiotensin converting enzyme or ace for short which converts angiotensin 1 to angiotensin 2. angiotensin ii then travels through the blood and when it reaches the kidneys it binds to angiotensin receptors along the afferent and efferent arterioles just like adrenaline it causes those arterioles to constrict and

as before the increased arterial resistance leads to a low renal blood flow however there's a mechanism to ensure that even though less blood gets to the kidneys glomerular filtration rate remains constant the way this is possible is that the efferent arterioles are much more responsive to angiotensin ii than the afferent arterioles so when there are low levels of angiotensin ii only the efferent arterioles constrict and this makes less blood leave the glomerulus or said differently it makes more blood remain in the glomerulus thereby preserving the glomerular filtration rate however when there are high levels of

angiotensin ii both the afferent and efferent arterials constrict and this decreases both renal blood flow and glomerular filtration rate now other hormones come into play when it comes to decreasing arteriolar resistance and increasing renal blood flow first off there's atrial natriuretic peptide or anp secreted by the atria of the heart and brain natriuretic peptide or bnp secreted by the ventricles of the heart despite the name suggesting otherwise fun fact it's only named after the brain because it was first discovered in pig brain extracts both anp and bnp get secreted when there's an increased cardiac workload

and the walls of the atria or ventricles get stretched amp and bnp bind to specific natriuretic peptide receptors expressed by smooth muscle cells and initiate a cascade of intracellular events that result in the dilation of afferent arterioles and the construction of efferent arterials increasing renal blood flow other molecules that lower arteriolar resistance and increase renal blood flow are prostaglandins the kidneys produce prostaglandin e2 and prostaglandin i2 in response to sympathetic stimulation and it makes both the afferent and efferent arterioles dilate a bit to make sure renal blood flow doesn't get too low even during those

fight or flight situations after all the last thing you need after a quick getaway from a kangaroo is kidney damage from too little blood flow finally there's dopamine which is synthesized by cells in the brain and the kidneys in the brain dopamine functions as a neurotransmitter in addition to that in the brain and the rest of the body it binds to specific dopaminergic receptors on smooth muscle cells constricting the capillaries in our skin and muscles and dilating the small vessels around vital organs such as the heart and the kidneys with vasodilation of both the afferent

and efferent arterioles low concentrations of dopamine increase renal blood flow now let's switch gears and look at autoregulation which refers to local mechanisms within the kidney that keep renal blood flow and glomerular filtration rate constant over a range of systemic blood pressures in other words the mechanisms that allow the kidney to adjust their own arterial resistance to keep renal blood flow constant even when blood pressure might range between 80 millimeters of mercury and 200 millimeters of mercury autoregulation can be seen graphically when systolic blood pressure falls below 80 millimeters of mercury renal blood flow is

also low at 80 millimeters of mercury renal blood flow reaches an optimal value and the smooth muscle cells in the arterial wall are completely relaxed between 80 and 200 millimeters of mercury the smooth muscle cells gradually become more constricted as blood pressure rises maintaining a constant renal blood flow above 200 millimeters of mercury renal blood flow increases parallel to renal blood pressure there are two mechanisms of kidney autoregulation first there's an arterial smooth muscle reaction called the myogenic mechanism which is based on a reflex of smooth muscle cells to contract when they are stretched by

blood coming in at high pressures the more they get stretched by the blood which is what happens when pressures are high the more they want to contract which causes vasoconstriction of the afferent and efferent arterioles second there's the tubular glomerular mechanism which involves the distal convoluted tubule and the glomerulus it turns out that a part of the distal convoluted tubule loops around and gets quite close to the afferent arterial this region where they are in close contact is called the juxtaglomerular apparatus with juxta meeting next to the glomerulus now in this region of the distal

convoluted tubule there's a group of cells collectively called the macula densa these macula densa cells can sense when glomerular filtration rate increases based on the quantity of sodium and chloride ions flowing through the tubule here's how it works when blood pressure rises renal blood flow and as a consequence glomerular filtration rate also increases this means that there's more fluid and more dissolved sodium and chloride ions that reach the macula densa in response to the increased fluid and sodium and chloride ions macula densa cells release adenosine which diffuses over to the nearby afferent arteriole acting as

a paracrine signal this increases arteriolar resistance and reduces the glomerular filtration rate in an autoregulatory fashion all right as a quick recap adrenaline and angiotensin ii increase arteriolar resistance and decrease renal blood flow whereas atrial and brain natriuretic peptides decrease arteriolar resistance and increase renal blood flow in autoregulation the kidneys keep blood flow constant over a wide range of systolic blood pressures there's the myogenic mechanism which is when smooth muscle cells contract when stretched and the tubular glomerular mechanism when macula densa cells secrete adenosine which has a paracrine effect on the afferent arteriole making it

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