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Our Kidneys Are Filtering Systems


Digestion Makes Nutrients Available to Our Body









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.


What is the composition of urine?

Of the forty-seven gallons of fluid filtered and processed by the nephrons daily, less than 1 percent actually becomes urine. Our urine is generally comprised of things our body has no need for, such as some by-products of cell metabolism, and also excessive quantities of things we normally need such as water and electrolytes. About 95 percent of urine is water, while the remaining 5 percent is substances dissolved within that water.


Do our kidneys do anything else?

Beyond regulating the composition of our blood, the kidneys engage in other operations involved in homeostasis. For instance, our kidneys are very sensitive to the amount of oxygenbeing transported in the blood. If they detect that the level of oxygen in our blood is too low, they will release a substance (hormone) into the blood that tells bones to make more red blood cells. If there are more red blood cells, then logically more oxygen can be transported in the blood. Furthermore, the kidneys are vital in the normal metabolism of vitamin D, which will be discussed later.




What does “digestion” mean and what is it all about?

The term digest means to break down or disintegrate. Therefore, digestion serves to break down the food we eat into smaller substances that are suitable for absorption into our body. All of the activities of digestion take place in our digestive or gastrointestinal tract. The digestive tract is a tube 22 to 28 feet long that actually passes through our body. As food moves through the length of the digestive tract, it is really on the outside of the body. Only when a substance crosses the cell lining of the digestive tract and enters into our circulation is it actually inside our body, which is called absorption.


Digestion requires both physical and chemical operations. The teeth, along with the musculature of the mouth, stomach, and small intestine, work to physically grind, knead, and mix food with digestive juices. At the same time, the muscular lining of our digestive tract serves to propel the digestive mixture forward. Meanwhile, chemical digestion involves the activities of digestive enzymes that will break down large complex food molecules into smaller substances appropriate for absorption. Proteins, carbohydrates, and lipids must be split into simpler molecules for absorption. Also, the vitamins and minerals found in foods must be liberated from other food molecules and complexes in order to be absorbed as well. Bile is also involved in chemical digestion; however, it functions not as an enzyme but more as a detergent. Bile is pivotal in the digestion and absorption of lipid substances.


What happens to food in the mouth?

Once food is in the mouth it is bathed in saliva. Saliva adds moisture to the food that is being chewed. This will improve the ease of swallowing. Each day we will produce about 1 to 1.5 quarts (liters) of saliva. Furthermore, saliva also contains both a carbohydrate and lipid digestive enzyme that begins the chemical digestive process. Once we swallow, food travels through the esophagus and gets deposited in the stomach.


What is the stomach and what does it do?

The stomach, typically a bit less than a foot in length, functions as a food reservoir for swallowed food. The volume of our stomach depends on the quantity of food therein. An empty stomach may have a volume of only 1 to 3 ounces (~ 50 to 75 milliliters) whereas a full stomach can expand to volumes of 2 to 3 quarts (~ 2 to 3 liters).

The stomach is a very muscular organ. It churns food and mixes it with stomach juice. Stomach juice contains hydrochloric acid (HCl), which renders the stomach a very acidic environment (pH 1.5 to 2.5). A protein-digesting enzyme is also found in stomach juices. The presence of this enzyme, along with the acidic environment, will begin protein digestion. On the average, our stomach may produce about 2 to 3 quarts (~ 2 to 3 liters) of stomach juice daily. Beyond protein digestion, the acidic stomach juice also kills most bacteria in foods.


Our stomach is sealed at both ends by tight muscular enclosures called sphincter muscles. This prevents acidic juices from entering the esophagus at one end and also allows separation between the stomach and small intestine at the other end. If stomach juice is able to reflux into our esophagus it can produce a burning sensation commonly referred to as heartburn. This is why chronic heartburn is routinely treated with antacids, as they attempt to neutralize the acid in the stomach. Other drugs may be used that attempt to decrease acid production by the stomach.


What happens to food after it leaves the stomach?

The mixture of partially digested food drenched in acidic stomach juice is slowly sent into the small intestine. This portion of our digestive tract is the location of the majority of digestive enzyme activity and the absorption of nutrients. The wall of the small intestine ­pre­sents a very sophisticated pattern of folds and projections. This design allows the small intestine to have an absorptive surface approximating the size of a tennis court. This allows for very efficient absorption.


When the food mixture is spurted into the small intestine from the stomach, it hardly resembles what we ate. Yet most of the nutrients still need further digestion to reach their absorbable state. First, bicarbonate produced by the pancreas enters the small intestine and neutralizes the acidic food mixture draining from our stomach. Then digestive enzymes that are also produced by our pancreas and bile from the gallbladder and liver make their way to the small intestine as well. These factors, along with digestive enzymes produced by the cells that line the small intestine, will complete digestion.


What is bile?

Bile is made up of several substances, the most outstanding being bile acids (bile salts). During digestion, the small intestine is a watery place to be. Along with the water entering our digestive tract in foods and beverages, water is also the basis of digestive juices. Water-insoluble substances in our diet, such as fats, cholesterol, and fat-soluble vitamins, will clump together into droplets in the small intestine. This would decrease their digestibility and absorption. This is where bile comes in. Bile acts as an emulsifier or detergent interacting with lipid droplets so that many smaller lipid droplets result instead of fewer larger ones. The advantage to creating many smaller lipid droplets is that more contact occurs between lipids and lipid-digesting enzymes. If bile were absent, as in certain disorders, lipids would stay as larger droplets in the small intestine and for the most part remain undigested and unabsorbed and end up in the feces.


Bile is produced by the liver and oozes in the direction of the small intestine twenty-four hours a day, seven days a week. The liver is connected to the small intestine via a series of tubes or ducts. During periods of time in between meals, some of the bile drains into the gallbladder, where it is stored. Then during a meal the gallbladder squeezes the bile out and it heads to the small intestine. This allows for more bile to be present in the small intestine during digestion.


What is the colon?

By the time the digestive mixture reaches the large intestine or colon most of the nutrients have been absorbed. Although some water and electrolytes will be absorbed in the colon, its primary responsibility is to form the feces that will eventually leave the digestive tract. The colon is also home to a rich bacteria colony—as many as 400 different species of bacteria may be found. These bacteria provide some benefit to the body as they make some vitamins and fatty acids that can help nourish the body. Research is underway in an effort to better understand the relationship between the colon’s bacteria and human health.


What is the composition of feces?

Human feces is a combination of water, bacteria, parts of cells that line the digestive tract, and undigested food components, such as fibers. The coloring of feces is attributable to several of the substances that are removed from the body in the feces. For instance, when the body breaks down hemoglobin, coloring pigments are produced. These substances become part of bile, which empties into the digestive tract. These add color to the feces


Kidneys + Digestion ...