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Cardiovascular system anatomy and physiology
Action potentials in pacemaker cells
Action potentials in myocytes
Excitability and refractory periods
Cardiac excitation-contraction coupling
Electrical conduction in the heart
ECG rate and rhythm
Stroke volume, ejection fraction, and cardiac output
Blood pressure, blood flow, and resistance
Compliance of blood vessels
Microcirculation and Starling forces
Pressures in the cardiovascular system
Resistance to blood flow
Normal heart sounds
Abnormal heart sounds
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renin-angiotensin-aldosterone system p. 612
renin-angiotensin-aldosterone system p. 612
renin-angiotensin-aldosterone system and p. 612
Antonia Syrnioti, MD
Ursula Florjanczyk, MScBMC
Tanner Marshall, MS
The main job of the cardiovascular system is to keep the blood moving, and to help facilitate that - blood pressure and more importantly 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 starts.
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 like 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 renin-angiotensin-aldosterone system (RAAS) is a hormone system that plays a key role in regulating blood pressure and fluid balance in the body. It is composed of several hormones and enzymes that work together to regulate blood pressure by controlling the amount of fluid in the blood vessels.
Whenever there's a decrease in blood pressure as detected by the baroreceptors of the carotid sinus or aortic arch or the juxtaglomerular cells, the sympathetic nerves getting stimulated, or the macula densa cells sensing less sodium and chloride ions flowing through the tubules, kidneys secrete renin that converts angiotensinogen to angiotensin I, and then angiotensin-converting enzyme converts angiotensin I to angiotensin II.
Angiotensin II causes the efferent arteriole to constrict more than the afferent arteriole, which increases the glomerular filtration rate, it also causes the proximal tubule to reabsorb more sodium ions from the filtrate, increases thirst, and helps increase blood pressure, while it, also, stimulates the adrenal cortex to release aldosterone, which gets the kidneys to retain sodium and water, further raising blood pressure.
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