Water Flashcards
Water
Water is the essenal nutrient with the highest daily requirement. We need water for the body’s structure but
it is also important as a solvent and in various physiological acvies. Water is so important to the body’s funcon and structure that if we do not consume enough, our bodies tell us almost immediately to consume more. Since we do not store water in the body and we connuously lose it, we need to consume a constant supply. Accordingly, our thirst is very sensive to reducons in the water content of blood and ssues.
Structure of water
A water molecule is composed of two hydrogen atoms aached to an oxygen atom, giving it the chemical for- mula H2O. Geometrically, the molecule has a V-shape, due to the respecve slightly posive (δ+), and slightly negave (δ–) charges of the hydrogen and oxygen atoms (Figure 4.1). The negave charge forms one pole of the molecule, while the posive charge forms the other pole. Water is thus aOpolar molecule. This charge arrangement allows water to be aracted to other water molecules, as well as other molecules that are polar. Polar molecules that are aracted to and dissolve easily in water are hydrophilic, or water loving. Those that are not polar and not aracted to water are hydro- phobic, or water hang. For instance, lipids are nonpolar, and therefore separate themselves from water when the two are combined.
Extra cellular and intercellular
The human body is 60–70% water by weight (Mitchell et al., 1945). Approximately two thirds of this water is found within cells, or intracellularly, and the remainder is found outside of cells, or extracellularly (Figure 4.2).
Water and cell
Cells have lipid walls, but are comprised mostly of cytoplasm, a gel-like substance that is 80% water. Given that the human body has more than 30 trillion cells (Bianconi et al., 2013), the water in cytoplasm therefore accounts for the majority of the body’s water. In healthy people, the water content of cells does not change substanally. However, the extracellular water content is constantly being used for the body’s needs and must be regularly replenished.
Most extracellular water is found within blood. Red blood cells give blood its characterisc colour, however, more than 9E0% of the volume of blood is water. Extracellular water can also be found around the joints, in the lungs, lining certain ssues and in the lymph. It is further found in the extracellular space around cells. Water can be exchanged between this extracellular space and the blood. Certain factors regulate this exchange and the content of water in the blood versus the extracellular space.
Osmosis
Osmosis drives water to move across semi-permeable membranes, like blood capillaries, with the goal of eve- ning out concentraon differences. For instance, the blood carries many charged, polar substances, such as protein. These aract water and draw it in to the blood from the extracellular space (Figure 4.3). That is one funcon of protein: to maintain fluid balance and make sure that water doesn’t build up in the extracellular space. Conversely, blood pres- sure provides a force that pushes water out of blood and into the extracellular space.
Functions of water
Structure solvent hydroliss defense infection protection injury temp regulation ,
Structure
Water helps structures maintain their form. For instance, the water within cells gives them the three-di- mensional shape necessary for cellular organelle to funcon properly. Another example is the water within synovial fluid found in sacs between joints. Not only do these synovial sacs promote joint structure, but they also allow bones to glide by each other more fluidly. The eye also maintains its structure because of the fluid, or huomour, found within it (Figure 4.4).
Solvent
Water is the most important biological solvent because
of the variety of polar substances it can dissolve (Figure 4.5). This is important for moving things around the body via blood vessels or the digesve tract. For instance, blood can transport oxygen, nutrients and other cellular needs to the ssues while also Iremov-
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armful waste products. The water within cells also allows certain material to move around the cell. It also brings the reac- tants of chemical processes together. Water is a fluid that is always in moon and so the substances within water are also in moon. Accordingly, when compable reactants are dissolved in water, there is an increased chance of them colliding and undergoing a chemical reacon.
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Hydroliss
As we learned in Chapter 3, hydorolysis reacons use water to split larger molecules into smaller ones. An exam- ple of a hydrolysis reacon is the breakdown of the double-sugar maltose into two molecules of single-sugar glucose
Defence infection
Within the lymph you will find many immune
cells suspended within a watery environment. If an
infecous agent gets into the body, it can be moved
it for removal. Furthermore, mucus, which is mostly water, helps trap and gather pathogeens together for removal. This is why doctors advise us to drink plenty of fluids when we have certain infecons (Figure 4.7).
Injury protection
Water can reduce the fricon and damage from
movement or trauma that can negavely affect our ssues.
For instance, the water found in synovial fluid helps protect
bones that arculate from scrapping against each other (Fig-
ure 4.8). Also, the mucus found lining body ssues, such as
the digesve tract and respiratory system, can help protect
Ito
these ssues from injuries. Around and within certain parts
of the brain and spinal cord is cerebrospinal fluid. This wa-
ter-based fluid helps to protect the brain from various forces
Temp regulation
The human body’s temperature must be maintained at around 37 ̊C to funcon properly. Even a 0.5 ̊C change in body temperature can negavely affect body physiology. This presents a challenge, as the temperature of our external environments can change dramacally throughout the day. Also, without a way to regulate temperature, body tempera- ture can increase for other reasons such as increased physical acvity (Figure 4.9). Our bodies accordingly use two main strategies to maintain internal temperature; both involve water.
Sweating
Sweang involves the release of watery sweat from our sweat glands. When this sweat evaporates, it cools down our skin and bodies. Sweang in a humid environment compromises this process, as the air’s high water content does not let sweat evaporate. This leaves us feeling hot, scky and uncomfortable. During the hot summer months in many Canadian cies you may accordingly hear the phrase, “It’s not the heat that gets you, it’s the humidity.”
When body temperature increases, as is the case during strenuous exercise, the face oen becomes redder. This is because the body tries to maintain its temperature by opening blood vessels close to the skin. This allows blood to shunt some of the heat from the body’s core to the skin’s surface. As the skin heats up, this then triggers the sweat response, which can further cool the body.
Heat illness
When the body is unable to regulate temperature, heat illness can occur. Symptoms of heat illness can range from minor heat cramping to heatstroke, which can be life-threatening. Heat stroke occurs when the body temperature rises above 40 ̊C (Bouchama & Knochel, 2002). It is accompanied by neurological symptoms such as delirium, confusion and convulsions and can lead to coma or death. This is more likely to occur in elderly individuals, or those with disease who have compromised temperature regulaon physiology (Bouchama et al., 2007). As global temperatures connue to rise, the incidence of heat illness and heat injury may also increase. Indeed, in the United States, heat waves result in more fatalies than any other extreme weather event (Naonal Weather Service, n.d.).
Sources of water
Every day, we lose water through urine, feces and evaporaon. Since the body does not store water, we must constantly replenish this water. Beverages account for most of the water we take in, though foods also provide water. The plants and animals we eat are made up of cells. When we eat them, we break down their cells and release the water from their cytoplasm. Water is also gained during certain metabolic processes. Recall from Chapter 3 that water is a by- product of cellular respiraon. All three of these contribute to our daily water needs (Figure 4.10).
Maintaining water balance
Water homeostasis is one of the body’s main priories. To this end, there is a ghtly regulated feedback process in place to make sure that water is maintained at desirable levels. When blood volume decreases, two main mechanisms are employed to help maintain water levels. First, our thirst increases. This is due to several body sensors that indicate to the brain that the concentraon of dissolved parcles in the blood is high, meaning that the concentraon of water
is low. Thirst is the body’s way of telling us it is deficient in water. The kidneys also play a key role in regulang water levels. They decide what stays in the blood and what is excreted in urine (Figure 4.11). When blood volume is high, excess water is excreted at the kidneys. If there are a lot of waste products for the kidneys to remove, this also contrib- utes to water loss, since water is needed to help these materials pass outward. When blood volume and pressure is low, the kidneys decrease the producon of urine. Individuals that are deficient in water will noce that their urine is more darkly coloured, as it contains less water to dilute waste products.
Dehydration
Most of us have experienced a degree of dehydraon in our lives. Dehydraon can be caused by not consuming enough water or by an excessive loss of water, potenally due to sweang, diarrhea or voming. Mild dehydraon can be uncomfortable; extreme dehydraon can be deadly. Dehy- draon is especially dangerous for young children and older adults and measures should be taken to prevent it.
Symptoms of dehydraon include increased thirst, dry mouth, headaches, fague, dizziness, irritability and dark urine (Figure 4.12). Chronic dehydraon can lead to more severe complicaons, including kidney damage, seizures and hypovolemic shock.
Kidney disease
Dehydraon puts extra stress on the kidneys, as they do not have enough water to help secrete waste products. This increases the risk of urinary tract infecons, kidney stones, and in extreme cases can lead to chronic kidney disease and even death (Roncal-Jimenez et al., 2015).
Seizure
Excessive sweang not only promotes water loss, but important electrolytes can also be lost in this process. Elec- trolyte imbalance can compromise the body’s electrical acvity, potenally promong seizures. Those with sodium and other electrolyte disorders are more suscepble to these dehydraon-induced seizures (Nardone et al., 2016).
Hypovolemic shock
ellular fluid. Excessively low blood volume can promote a significant drop in blood pressure. Low blood volume and
pressure can compromise oxygen and nutrient delivery to the ssues. Symptoms of hypovolemic shock include increased heartrate, low blood pressure, blue skin colour, cool and clammy skin and mental status changes. The symptoms and severity depend on the health of the person and the length of me the person stays in hypovolemic shock. If untreated, it can be fatal.
Water intoxication
Water intoxicaon, also known as water poisoning, is a potenally fatal condion where the content of water in the body is too high with respect to the level of electrolytes. Water intoxicaon can occur when an individual consumes excessive amounts of water in a short period of me and does not excrete it through urinaon. However, most cases of water intoxicaon occur when a significant amount of water is lost due to excessive sweang, diarrhea or voming. Both water and electrolytes are lost in sweat, diarrhea and vomit. If these losses are replaced by only drinking water, it dilutes the electrolytes in the body (Figure 4.13). This can result in a condion called hyponatremia, or low sodium in the blood. decrease in the concentraon of sodium and other dissolved parcles negavely impacts many body func- ons. The brain is parcularly suscepble to hyponatremia, where it can lead to an increase in intracranial pressure. Most symptoms of water intoxicaon are accordingly neurological and include headache, confusion, personality changes, irritability and drowsiness. In extreme cases this can lead to malfuncon of the central nervous system and an increased risk for seizures, brain damage, coma and even death (Box 4.1). Extra electrolytes may be necessary in cases of extreme fluid loss. Endurance athletes such as marathon runners, for instance, oen consume electrolyte mixtures in addion to replenishing water.
Water intoxication and fatal radio
In 2007, a radio staon in Sacramento, California held a contest called “Hold your Wee for a Wii” (ABC News, 2009). The premise: the person who could drink the most water without urinang would win a Ninten- do Wii video game console, a hot commodity at the me. One of the contestants, Jennifer Strange, report- edly drank almost eight litres of water in three hours. She died of water intoxicaon. The disc jockeys had apparently been made aware before and during the compeon about the potenally fatal outcome, but they were unphased and even joked about it on air. In 2009, the radio staon’s parent company was ordered to pay US$16,577,118 in damages to Jennifer’s family.
Diuretics
Diurecs are substances that promote water losses through urinaon. Certain medicaons act as diurecs, as do
certain psychoacve drugs like caffeine and alcohol.
Caffeine
Caffeine is a psychoacve drug found in certain foods and beverages. It is a compound found naturally in many seeds, nuts and leaves. Its most well-known source is coffee (Figure 4.14). While caffeine acts as a diuret-
ic, it is typically consumed for its effects on the central nervous system, as it can promote alertness, while reduc- ing fague and drowsiness., Caffeine has been studied for its potenal disease-reducing effects. A review of meta-analyses found that coffee
intake probably decreases risk of cardiovascular disease, Parkinson’s disease, type 2 diabetes and certain types of can-
cers (Grosso et al., 2017). Cauon should be taken in interpreng these results, as this review looked at coffee and not
caffeine specifically. Coffee is believed to have anoxidant and an-inflammatory properes that perhaps contribute to
the observed effects. While is it generally recognized as safe, caffeine has some potenal side effects. The authors of the
above study found an increased risk of miscarriage with coffee consumpon. The diurec effects of caffeine may also
promote kidney problems. Indeed, a large prospecve epidemiolocal study of more than 16 million people found that
caffeine consumpon is associated with a higher risk of kidney stones, especially in women (Sun et al., 2019).
