Summary of Renin-angiotensin aldosterone system
Flashcards on Renin-angiotensin aldosterone system
Renin-angiotensin aldosterone system
Antidiuretic hormone activation of vasopressin 2 receptors on the principal cells of the renal increases water reabsorption and maintenance of body fluid osmolarity.
Transcript for Renin-angiotensin aldosterone system
Renin-angiotensin aldosterone system
The main job of the cardiovascular system is to keep the blood moving, and to help facilitate that - blood pressure is kept under tight control. A major way the body does that is through a set of hormones that make up the renin- angiotensin- aldosterone system.
But, first things first. Everything starts in the kidney. Now, within each kidney, blood from the renal artery flows into smaller and smaller arteries, eventually reaching the tiniest of arterioles called the afferent arterioles. After the afferent arteriole, 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 1 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. The renal corpuscle is where blood filtration begins, and it includes the Bowman’s capsule which surrounds the glomerulus. Interestingly, once the blood leaves the glomerulus, it does not 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, the renal tubule is made up of a proximal convoluted tubule, the nephron loop - also known as the loop of Henle - which has an ascending 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 until, finally, urine is formed. At the same time, 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.
Okay - now if we zoom into the wall of the afferent arterioles, we’ll find a very special kind of smooth muscle cells, called juxtaglomerular cells, because they’re next to or “juxta” the glomerulus. The main job of these cells is to always keep an eye open for signals that the blood pressure needs to rise. These signals can come in three ways. First, juxtaglomerular cells are mechanoreceptors, and they’re designed to mechanically feel low blood pressure in the incoming blood. Second, juxtaglomerular cells are supplied by sympathetic nerve fibers. Sympathetic nerves start firing when there’s something stressful going on, like chasing a bicycle thief. Sympathetic nerves help boost how hard and how fast the heart beats and cause contraction of smooth muscle cells around arterioles causing vasoconstriction throughout the body, as well as in the juxtaglomerular cells - this boosts blood pressure in stressful situations. The third signal for juxtaglomerular cells comes from the wall of the distal convoluted tubule, which is where specialized cells called macula densa cells are found. Macula densa cells are chemoreceptors that 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 consequence, glomerular filtration rate also increase. 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 juxtaglomerular cells in the afferent arteriole, juxtaglomerular cells to tell them that something’s going wrong. So, all three types of signals stimulate the juxtaglomerular cells to secrete renin.
Renin is an enzyme that gets into the plasma, and looks for its primary substrate - angiotensinogen. Angiotensinogen is a large protein made up of over 400 amino acids that’s produced by the liver and is always hanging out in the blood. When they meet up, renin cleaves off a huge chunk of the angiotensinogen protein, leaving behind a tiny fragment called angiotensin I that’s just 10 amino acids long. A bit like a tiny pearl found within a giant oyster. Angiotensin I floats through the blood, until it reaches various capillaries in the body. Now, endothelial cells in general, but mostly those lining the vessels in the lungs, have an enzyme on their surface called angiotensin converting enzyme - or ACE for short, which converts angiotensin I to angiotensin II. Angiotensin converting enzyme grabs angiotensin I and chop off two of its amino-acids, leaving behind an 8- amino-acid chain that’s a really powerful hormone, called angiotensin II.