Where does the cardiac output go?
If referring to the cardiac output of the right ventricle, there is only one place for it to go: the lungs. Said another way, 100% of the cardiac output from the right ventricle is destined for our lungs. However, the blood pumped out of the left ventricle has many destinations. Under resting and comfortable environmental conditions about 13% of the left ventricle’s cardiac output goes to our brain, 4% goes to our heart, 20 to 25% goes to our kidneys, and 10% goes to our skin. The remaining cardiac output from the left ventricle (48 to 53%) will then go to the remaining tissue in our body, such as the digestive tract, liver, and pancreas.
During exercise, a greater proportion of this cardiac output is routed to working skeletal muscle. This requires some redistribution or stealing of blood routed to other less active areas at that time, such as our digestive tract. Contrarily, during a big meal and for a few hours afterward, a greater proportion of this cardiac output is routed to the digestive tract, which steals a portion of the blood directed to areas having no immediate need, such as skeletal muscle.
What is blood pressure?
Whether blood is in the heart or in blood vessels, it has a certain pressure associated with it. In fact, blood moves through circulation from an area of greater blood pressure to an area of lower blood pressure. As mentioned earlier, when the heart contracts the pressure of the blood in the ventricles increases. This establishes a blood pressure gradient that then drives the movement of blood through the blood vessels. This is somewhat like turning on a garden hose. When you turn on a garden hose, the water pressure is greatest close to the faucet (versus toward the open end of the hose). The result is that water moves from the area of greater water pressure toward the area of lesser water pressure and out the end of the hose.
We define pressure as a force exerted upon a surface and can measure it in mm Hg (mercury). If we apply this definition to our blood, we can say that blood pressure is the force exerted by blood upon the walls of a blood vessel. When blood pressure is measured two numbers are provided, for instance 120/80 or “120 over 80”. What this means is that the pressure exerted by the blood is 120 mm Hg during heart contraction and 80 mm Hg when the heart is relaxing between beats. The first number is the systolic or blood pressure when our heart contracts. The second number is the diastolic pressure and it is blood pressure when our heart is relaxing. Blood pressure is typically measured in the large artery of the arm because of its accessibility.
OUR KIDNEYS ARE FILTERING SYSTEMS
What do our kidneys do?
Typically understated in function, our kidneys regulate the composition and volume of the blood. Our two kidneys, along with their corresponding ureters, the bladder, and the urethra, make up our urinary or renal system. Although our kidneys are only about 1% of our total body weight, they receive about 20 to 25% of our left ventricle’s cardiac output. Amazingly, our kidneys will filter and process approximately 47 gallons (180 liters) of blood-derived fluid daily.
Each one of our two kidneys is home to about one million tiny blood processing units called nephrons. Each nephron will engage in two basic operations. First, they filter plasma into a series of tubes; second, they will process the filtered fluid. As you might expect, the filtered plasma-derived fluid not only contains water but also small substances dissolved within, such as electrolytes, amino acids, and glucose. Cells (e.g., red and white blood cells) and most proteins in our blood are too large and are not filtered out of the blood.
There are two possible fates for the components of the filtered fluid. They can either be returned to the blood or not and ultimately become a component of urine. Normally, the reuptake of substances such as glucose and amino acids back to the blood is extremely efficient. Contrarily, the reuptake of water and electrolytes is more regulated. For example, if the concentration of sodium is too high in the blood, then less sodium will be returned to the blood and more will go into urine so that an optimal blood level is achieved. On the other hand, if the level of sodium in the blood is low, then more of the filtered sodium is returned to the blood and less is lost in the urine. As you might expect, the processes engaged in reabsorbing glucose, amino acids, electrolytes, and other desired substances require a lot of energy (ATP). Because of this normal kidney operations make a significant contribution to our total daily energy use.
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