Chapter Two: Anatomy and Physiology
SECTION ONE – The Digestive System
The digestive system is comprised of two components: the digestive or gastrointestinal (GI) tract and the associated glandular organs (often referred to as visceral organs) (see Figure 2-1).
The gastrointestinal tract is the passage of the body through which food passes as it is digested and absorbed into the circulatory system. It consists of the mouth, pharynx (throat), esophagus, stomach, small intestine and large intestine. In humans, the GI tract is approximately nine metres in length.
The glandular organs associated with the GI tract include the salivary glands, which produce saliva (spit), the pancreas, which secretes digestive enzymes and important hormones, the gallbladder and the liver, which has many important functions related to digestion.
Hormone
A chemical released by a cell, a gland or an organ in one part of the body that circulates at low concentrations in the blood and affects cells in other parts of the body.
Function
The principal function of the digestive system is digestion – the process of breaking down food into nutrient particles small enough to be absorbed by the body.
Digestion begins in the mouth, where food is chewed and swallowed, entering the esophagus. Waves of muscle contractions (peristalsis) move the food along until it enters the stomach.
The stomach continues to break down food both physically (through muscular movement) and chemically by secreting gastric juices, primarily gastric acid (hydrochloric acid).
Exiting the stomach, the partially digested food – now called chyme – moves into the small intestine, the main site of chemical digestion. In the duodenum, the first part of the small intestine, a number of different substances (bile fluid from the liver and gallbladder and many different secretions from the pancreas, including about 15 enzymes) break down lipids (fats), proteins and carbohydrates into simpler molecules that are more easily absorbed by the body.
The jejunum and ileum form the middle and final parts of the small intestine where digestion is completed and basic nutrients are absorbed into the blood.
By the time food leaves the small intestine, most of the important nutrients have been absorbed. The remaining liquefied, partially digested food moves from the small intestine into the large intestine through the ileocecal valve.
After passing through the cecum, the pouch-shaped entryway to the large intestine, partially digested food moves slowly through the colon. Sodium, chloride and water are absorbed through the lining of the colon, gradually converting the liquefied food into semi-solid feces.
Two or three times a day, strong muscle contractions in the transverse colon move feces into the rectum, the final part of the large intestine. This triggers defecation. Muscular contractions push the feces through the anal sphincters, which relax, allowing the feces to exit the body.
Chyme
A semi-solid mixture of food and gastric secretions that is delivered from the stomach to the small intestine.Bile
A fluid secreted by the liver and stored in the gallbladder. When released, bile aids in digestion of fats within the intestine.Enzyme
One of many different complex proteins produced by living cells that catalyze (trigger) biochemical reactions at body temperature.
Nutrients
Nutrients are substances (usually found in food) that are used by the body for energy, maintenance and growth. Nutrients are divided into macronutrients and micronutrients.
Macronutrients
Macronutrients (so called because the body requires them in large amounts) include carbohydrates, lipids and proteins.
Carbohydrates (sugars and starches) are mostly found in fruits, vegetables and grains. During digestion, more complex carbohydrates (disaccharides and polysaccharides) are broken down into simple carbohydrates, also known as monosaccharides (see Figure 2-2). The process starts with digestive enzymes in the mouth and continues in the small intestine. Eventually all the carbohydrates in the food are broken into simple monosaccharides (such as glucose) that are easily absorbed into the bloodstream via the small intestine.
Macronutrient
A substance required in large amounts for the growth and normal functioning of an organism.Carbohydrate
An organic compound containing carbon, hydrogen and oxygen. Carbohydrates are the body’s primary source of energy.
Macronutrients (continued)
A third type of carbohydrate is known as dietary fibre. Unlike other carbohydrates, dietary fibre is not processed by digestive enzymes. Instead, it remains intact as it passes through the digestive tract. Instead of providing nutrition, fibre adds bulk to the contents of the intestine and acts as a natural laxative by stimulating muscle contractions that help to move food through the digestive tract. Fibre slows the absorption of molecules such as sugars so that their uptake occurs gradually rather than in a sudden “rush.” Fibre also binds with some fatty substances (such as cholesterol) and prevents them from being absorbed, which may explain why a fibre-rich diet may lower cholesterol and protect against heart disease.
Glucose
A sugar (a form of carbohydrate) that serves as an energy source for living cells. Also known as blood sugar.Glycogen
A polysaccharide (complex carbohydrate) found mainly in the liver and muscles that is the primary form of storage for glucose in animals. Glycogen can be converted to glucose when energy is required.Did You Know?
According to current guidelines, healthy adults should include about 25-38 g of fibre in their daily diet, but most Canadians consume much less – about 4.5-11 g a day on average.
Macronutrients (continued)
Lipids (often called fats), compounds that are insoluble (do not dissolve) in water, are present in varying amounts in many different foods, including meat, poultry, dairy, fish and plant foods such as vegetable oils as well as processed foods. Digestive enzymes split lipids into fatty acids and monoglycerides, which are then absorbed into the bloodstream from the small intestine.
Lipids play a number of important roles in the body. The different types of lipids include triglycerides, phospholipids, steroids and prostaglandins.
Triglycerides are the type of lipid most abundant in the body and (depending on the type of fatty acid they contain) may be either saturated or unsaturated (see Figure 2-3). They are stored as fat in adipose cells, either under the skin (subcutaneous fat) or surrounding the body’s internal organs (visceral fat). (See also Figure 2-4, “Digestion and absorption of fats.”)
Lipid
Any one of a group of fats and fat-like substances that are insoluble in water. Lipids are a source of fuel and also constitute an important component of cells.Did You Know?
Lipids are often referred to as fats. Strictly speaking, fats are fatty acids, one of the types of molecules that make up lipids.Did You Know?
The energy yield of 1 g of fatty acids is approximately 9 kcal (37 kJ), compared to just 4 kcal (17 kJ) for most carbohydrates.
Macronutrients (continued)
Did You Know?
Triglycerides (also known as fats) are the body’s most concentrated source of energy. During times of scarcity when carbohydrates are in short supply, triglycerides are converted into glucose through a process known as gluconeogenesis.Gluconeogenesis
The formation of glucose from molecules that are not carbohydrates (for example, amino acids [the building blocks of proteins] or the glycerol portion of lipids).
Macronutrients (continued)
Phospholipids, compounds similar to triglycerides, are a major component of cell membranes.
Steroids are a group of compounds that play both structural and functional roles in the body. One of the most important steroids, cholesterol, is present in the plasma membrane of every cell in the body, where it helps to provide stability. As well, cholesterol forms hormones such as corticosteroids, estrogen and testosterone, and fat-soluble vitamins such as Vitamin D.
Lipase
An enzyme that catalyzes the breakdown of fats and lipoproteins, usually into fatty acids and glycerols.Emulsion
A mixture of liquids that do not dissolve in each other; instead, droplets of one liquid are scattered throughout the other.Cholesterol
An important lipid that plays many roles in the body, both structural (a component of cell membranes) and functional (used in the creation of some hormones).Did You Know?
Saturated fats and cholesterol are associated with damage to the blood vessels leading to hardening of the arteries (known as atherosclerosis).
Macronutrients (continued)
As lipids are relatively insoluble in blood, they are packaged into spherical, soluble particles called lipoproteins. Each lipoprotein particle consists of a core of insoluble lipids (for example, triglycerides and cholesterol) surrounded by a layer of soluble phospholipids with a specialized protein (an apoprotein) attached (see Figure 2-5). Some lipoproteins are known as high-density lipoproteins (HDLs) because they have a high density of protein (more protein than lipid) while others, known as low-density lipoproteins (LDLs), contain less protein and more lipid.
Lipoprotein
A complex of protein and lipid that allows lipids to be transported through the bloodstream.
Macronutrients (continued)
Proteins are found in meat, fish, poultry, dairy products and some plant sources such as whole grains, legumes and beans, including soybeans. In the stomach and small intestine, hydrochloric acid and enzymes break proteins down into their simplest constituents: amino acids, the building blocks that the body uses to manufacture its own proteins (see Figure 2-6).
Proteins play both structural and functional roles in the body. Structural proteins are the body’s building blocks and are found in every cell, from muscle and bone to blood and tissue. In contrast, functional proteins accelerate processes and reactions in the body. Enzymes are functional proteins that catalyze chemical reactions. Hormones (such as insulin) are proteins that induce chemical changes in cells (such as glucose uptake). Antibodies, a key component of the body’s immune system, are Y-shaped proteins, each with a unique amino acid sequence that allows it to identify (and attack) a specific foreign invader such as a bacterium or virus.
Although the body’s primary sources of energy are carbohydrates and lipids, proteins can also be converted into glucose and used as an energy source in the absence of sufficient amounts of carbohydrates and lipids.
Protein
A complex substance made up of connected amino acids that constitutes an essential component of living cells and serves many important biological functions.Amino acid
The basic structural unit of all proteins. There are 20 basic amino acids.
Macronutrients (continued)
Micronutrients
Micronutrients, which the body requires only in small quantities, include vitamins and minerals.
Vitamins are organic substances that help to regulate a variety of different bodily functions. Often, they become part of coenzymes, molecules that work together with enzymes to help control metabolic processes. Regular vitamin intake is essential, as the body can synthesize only a few of the vitamins necessary to life and health. Vitamins are absorbed through both the small and large intestine and are obtained from many different foods. Some vitamins are added to foods (for example, in Canada Vitamin D is added to milk), a process known as fortification.
Some important vitamins include:
- Vitamin D, found in foods such as fish, eggs, liver and mushrooms, helps the body absorb the minerals calcium and phosphate. A lack of Vitamin D is associated with osteomalacia, a softening of the bones.
- Vitamin C, found in many fruits and vegetables as well as liver, plays an essential role in many different enzymatic reactions in the human body, including synthesis of collagen, a type of protein that is the main component of the body’s connective tissue. Vitamin C deficiency produces scurvy, a disease that was historically common among sailors, who were away from land (and a supply of fresh fruits and vegetables) for extended periods of time.
- Vitamin K, found in leafy green vegetables such as spinach as well as liver and egg yolks, plays an important role in blood coagulation (clotting) and bone formation. Low levels of Vitamin K are associated with a higher risk of bleeding.
- The main sources of Vitamin B12 are meat and other animal products. Vitamin B12 is important to the normal functioning of the brain and nervous system and the formation of blood. Vitamin B12 deficiency can cause nerve damage and anemia.
Minerals are inorganic substances (for example, calcium, iron and iodine) that play many different roles in the body. Calcium, the most abundant mineral in the human body, is an important constituent of bones and teeth, while iron is an essential component of the oxygen-transporting hemoglobin in red blood cells. Like vitamins, minerals can be found in many different foods and are available as supplements. Most minerals are absorbed from the small intestine and colon.
Micronutrient
A substance required in small amounts for the growth and normal functioning of an organism.Hemoglobin
An iron-containing protein found in red blood cells that is the primary means by which the body transports oxygen to its cells.
SECTION TWO – The Endocrine System
Structure and Function
Most people are somewhat familiar with the nervous system, the most important communication network in the body. Less well known is the endocrine system (see Figure 2-7), which is made up of glands that secrete hormones (chemical messengers) directly into the blood, where they circulate at very low concentrations. Endocrine glands (for example, the thyroid), send out chemical signals in much the same way as the nervous system sends out electrical impulses, but at a much slower speed: nerve impulses act in a fraction of a second, while hormones may take effect over days, weeks, months or even years.
This section will provide a brief overview of the endocrine system, focusing on the pancreas and a few other endocrine glands relevant to metabolic syndrome and diabetes.
Endocrine
A gland that releases a secretion directly into the blood. May also refer to the secretion produced by the gland.Gland
A specialized cell, group of cells or organ that synthesizes a substance for release into the body or to its outer surface. Glands may be endocrine (secreting into the bloodstream) or exocrine (secreting substances via a duct into a cavity or outwardly to the body’s surface).
Components
Pituitary
The pituitary (known as the master gland because it influences so many other glands in the endocrine system) is located just below the hypothalamus, a cluster of nerve cells that links the central nervous system with the endocrine system by communicating with the pituitary.
The pituitary comprises two glands. The anterior (front) pituitary produces several different protein-based hormones and releases them into the bloodstream, while the posterior (back) pituitary secretes two hormones that are actually synthesized in the hypothalamus and transported to the posterior pituitary through nerve fibres.
Adrenal Glands
Located just above the kidneys, the adrenal glands (see Figure 2-8) produce several different lipid-based (steroid) hormones. The outer layer (the cortex) of the adrenals produces steroid hormones (including aldosterone and cortisol) that aid in regulating the body’s sodium and potassium concentration and help to maintain fluid balance and blood pressure. The inner layer (the medulla) secretes catecholamines (adrenaline and noradrenaline), the so-called fight-or-flight hormones that ready the body for action.
Pancreas
The pancreas, which is found behind the stomach, has two functions. As part of the endocrine system, it secretes hormones into the bloodstream that, among other functions, help to regulate levels of glucose in the blood. As part of the exocrine system (glands that secrete into parts of the body other than the bloodstream), the pancreas secretes enzymes through ducts into the digestive tract to assist in the chemical digestion of food in the intestine.
Scattered throughout the pancreas are microscopic structures known as the islets of Langerhans. Within these structures are four different types of cells that secrete different hormones, each with its own purpose (see Table 2-1 and Figure 2-9).
Did You Know?
The islets of Langerhans are named for their discoverer, Paul Langerhans, a German pathologist, physiologist and biologist. He discovered the islets of Langerhans in 1869 when studying for his doctorate at the Berlin Pathological Institute.Exocrine
A gland that releases a secretion through a duct. May also refer to the secretion produced by the gland.Islets of Langerhans
Microscopic patches of tissue in the pancreas that secrete insulin, which helps to regulate blood glucose, and glucagon, a hormone that promotes breakdown of glycogen into glucose.
Pancreas (continued)
Insulin
While glucose is the primary source of energy for the body’s cells, the human body does not store glucose. Instead, glucose is converted to glycogen, which is stored in the liver and in muscle cells. Later it is converted back to glucose and released into the bloodstream as needed.
Insulin plays a key role in regulation of blood glucose. Its function is to lower glucose levels, preventing hyperglycemia (abnormally high levels of glucose in the blood).
Insulin is a protein made of two chains of amino acids linked by disulfide bonds (see Figure 2-10).
Did You Know?
Glycogen acts as an energy reserve. When the body has a sudden need for energy, glycogen stored in the liver and in muscle cells is quickly converted back into glucose through the action of glucagon. Glycogen reserves are limited, however. During endurance exercise such as long-distance running, athletes may experience glycogen depletion after prolonged exertion. This is known as “hitting the wall.”Insulin
A hormone secreted by the beta cells of the pancreas that regulates carbohydrate and fat metabolism in the body by causing cells in the liver, muscle and fat tissue to take up glucose from the blood.Disulfide bond
Also known as a sulfur bond, this bond is formed when a cysteine sulfur atom on one polypeptide chain links with another cysteine sulfur atom on a nearby polypeptide chain.
Insulin Binding and Signalling
Insulin removes glucose from the blood by binding to specific receptors on the surface of target cells, which results in a cascade of events culminating in the transport of glucose into the cell (see Figure 2-11). Although insulin receptors are found in all types of tissues, they are abundant in highly insulin-sensitive tissues such as the muscles, liver and adipose tissue – all areas where glucose is stored.
Metabolic Effects of Insulin
Insulin plays a crucial role in carbohydrate, lipid and protein metabolism. In addition to regulating blood glucose, insulin facilitates storage of fat and promotes synthesis of proteins from amino acids (see Figure 2-12).
Glucagon
Glucagon is another hormone that plays an important role in regulating glucose in the blood. Its function is to raise glucose levels when they are low in order to prevent hypoglycemia (abnormally low levels of glucose in the blood).
When the body needs energy (for example, between meals), low blood glucose levels cause pancreatic alpha cells to produce a hormone that promotes breakdown of glycogen into glucose. Glucose molecules enter the bloodstream and travel to tissues where they are absorbed into cells and are broken down, releasing energy in a process known as glycolysis.
Did You Know?
While most tissues require insulin in order to absorb glucose efficiently, the brain, eyes and kidneys do not. Glucose rapidly enters the cells of these organs without insulin.Glucagon
A hormone that promotes breakdown of glycogen into glucose, thereby increasing levels of glucose in the bloodstream.
Incretins and the Incretin Effect
Incretins are gastrointestinal hormones that are released during a meal in response to the presence of glucose or nutrients in the gut. This phenomenon, the incretin effect, was discovered when scientists noticed that a dose of glucose given orally promoted insulin secretion to a greater extent than the same dose given intravenously. Incretins have multiple effects on energy metabolism (see Figure 2-13):
- Glucose-dependent insulin release. Incretins stimulate the pancreas to produce insulin only when glucose is present in the bloodstream.
- Glucagon inhibition. At the same time, incretins inhibit the release of glucagon by alpha cells in the pancreas, which helps to lower blood sugar (since glucagon raises blood sugar).
- Slowed nutrient absorption. Incretins reduce gastric emptying, slowing the rate of absorption of nutrients into the blood.
- Appetite suppression. Reduced gastric emptying also suppresses appetite.
Incretins only spring into action when glucose concentrations rise above a certain minimum level, and they are active for a brief period – just a few minutes. The two primary incretins are glucose-dependent insulinotropic polypeptide (GIP) and glucagonlike peptide 1 (GLP-1).
Incretin
One of a group of hormones released from the gastrointestinal tract that has beneficial effects on insulin secretion and carbohydrate metabolism.Glucagonlike peptide 1 (GLP-1)
An incretin hormone, released in the gastrointestinal tract in response to the presence of food, that stimulates insulin secretion, inhibits release of glucagon and suppresses appetite while reducing gastric emptying.
SECTION THREE – The Cardiovascular System
Structure and Function
The cardiovascular system consists of the heart, its connected network of blood vessels and the blood that flows through them. This system carries oxygen and nutrients to cells throughout the body and takes away carbon dioxide and other waste products (see Figure 2-14).
Components
Heart
The heart is the muscular pump that powers the flow of blood through the circulatory system.
Inside the heart are four chambers. The two lower chambers (ventricles) are strong and muscular, providing the power required to propel blood through the circulation. The two upper chambers (atria) play a less active role.
To keep blood moving through the body, the heart pumps continuously. On average, a heartbeat lasts less than one second; during stress or intense activity, it will pump even faster.
During the first part of each heartbeat, called diastole, the heart relaxes and fills with blood. In the second part, known as systole, it contracts, causing blood to move out of the heart and into the bloodstream.
Blood
Blood is a fluid made up of specialized cells that bring oxygen and nutrients to cells and carry away carbon dioxide and other waste products. Blood also carries hormones to different parts of the body, helps to control temperature by distributing heat and plays a role in battling infection and healing injuries.
More than half of blood is composed of plasma, a pale yellow fluid that is mostly water containing dissolved substances including glucose, enzymes, hormones and waste products. Plasma also contains proteins such as albumins (which help to transport other substances, including fatty acids and some hormones), fibrinogen (a protein that plays an important role in clotting) and globulins such as antibodies that attack foreign substances as part of the body’s immune response.
The other portion of blood is made up of specialized cells. Erythrocytes (red blood cells) carry oxygen; leucocytes (white blood cells) are part of the immune system; and platelets (also called thrombocytes) play a role in blood clotting.
Diastole
The portion of the heartbeat during which the heart relaxes, filling with blood.Systole
The portion of the heartbeat during which the heart contracts, pumping blood through the circulatory system.Did You Know?
Red blood cells (erythrocytes) live about 120 days. During this short lifespan, they travel approximately 1500 kilometres as they circulate through the body.Did You Know?
Although they are called red blood cells, erythrocytes are not considered true cells because they lack a nucleus and other cellular structures and do not divide to reproduce. Instead, they are manufactured in bone marrow.
Blood Vessels
There are three types of blood vessels: arteries, veins and capillaries.
Arteries are large blood vessels that transport oxygen-rich blood from the heart to nourish organs and tissues. Arteries branch into smaller vessels, arterioles, that connect to capillaries.
Veins carry blood back to the heart after it has delivered oxygen and nutrients to tissues and organs. Smaller blood vessels (venules) collect blood from capillaries and connect with veins.
Capillaries, the smallest blood vessels, carry blood between arteries and veins. Capillaries are extremely thin – only slightly wider than a single red blood cell – with walls that act as semi-permeable membranes, allowing oxygen and nutrients to enter cells and carbon dioxide and other waste products to exit so that they can be carried out of tissue. In many places, a mass of capillaries forms a capillary bed, the location where the exchange between cells and capillaries takes place.
Circulation
Blood flows through the body via two key circulatory systems: systemic circulation and pulmonary circulation. Two other systems, the hepatic and hypophyseal portal systems, also play important roles.
Systemic circulation is the movement of oxygen-rich blood pumped by the left side of the heart, through the aorta and smaller arteries, arterioles and capillaries to the body’s tissues and organs (with the exception of the lungs). Once depleted of oxygen, the blood travels back to the heart through two large veins (the superior vena cava and the inferior vena cava).
Pulmonary circulation is the movement of oxygen-depleted blood pumped by the right side of the heart through the pulmonary artery to the lungs, where it is re-oxygenated and returns to the heart through the pulmonary veins. After this, blood begins to travel once again through the systemic circulation.
In the hepatic portal circulation, oxygen-depleted but nutrient-rich blood from several parts of the digestive tract (among them the intestines, pancreas, stomach and spleen) travels to the liver through the hepatic portal vein. After passing through the liver, where toxins absorbed in the intestines are filtered out, blood returns to the heart through the inferior vena cava. Another portal system is the hypophyseal portal system, where blood vessels connect the hypothalamus with the anterior pituitary. This circulatory link allows hormones to travel quickly between the two glands for rapid communication.
High Blood Pressure (Hypertension)
Blood pressure is the pressure exerted by the blood against the inner walls of the arteries through the pumping action of the heart. This pressure is greater when the heart is contracting (systole) because it is pumping blood out of the heart and into the arteries, and lower when it is relaxed (diastole).
Blood pressure is written as two numbers: the higher number, or systolic blood pressure (measured during systole), followed by the lower number, or diastolic blood pressure (measured during diastole). Blood pressure is measured in millimetres of mercury (written as mm Hg). Millimetres of mercury is the pressure measured using a sphygmomanometer, or blood pressure meter. If an individual’s blood pressure is 120 mm Hg systolic and 80 mm Hg diastolic, it will be written as 120/80 mm Hg and described as “one-twenty over eighty.” Generally, a blood pressure consistently greater than 140/90 mm Hg is considered high blood pressure, or hypertension. The higher a person’s blood pressure, the greater the risk of stroke, heart disease or kidney failure.
SECTION FOUR – The Renal System
Structure and Function
The renal system, also known as the urinary system, comprises the kidneys, the ureters, the bladder and the urethra (the tube that carries urine from the bladder to the exterior of the body during urination).
The kidney’s primary role is filtration of blood and formation of urine. Both functions are critical to maintaining the body’s homeostatic balance. Homeostasis refers to the body’s tendency to maintain a stable internal environment. The kidneys contribute to maintenance of homeostasis by controlling the amounts of water and electrolytes in the blood.
Homeostasis
The tendency of the body to maintain stability under varying environmental conditions. Examples of homeostasis include body temperature, blood glucose levels and blood volume.Did You Know?
Diabetes can be detected by the presence of glucose in the urine.
Anatomy of the Kidney
The main body of the kidney consists of three layers. The renal cortex is an outer region filled with knots of capillaries (known as glomeruli) and their capsules. Next is the renal medulla, which contains capillaries and tubules where urine is formed. Finally, the renal pelvis is a space at the centre of the kidney where urine collects before exiting the kidney (see Figure 2-15).
Did You Know?
In an adult, each kidney weighs about 175 g and is about the size of a fist.
The Nephron
The nephron is the basic functional unit of the kidney (see Figure 2-16). Its role is to eliminate waste from the body and regulate the concentration of water and soluble substances by filtering the blood, reabsorbing whatever is needed and excreting the rest as urine. Each kidney contains more than a million nephrons.
The Kidney in Action
After entering the kidney through the renal artery, blood travels through smaller arterioles and eventually enters the nephron through the afferent arteriole. The nephron then filters waste products, reabsorbs some substances and concentrates urine. Eventually, filtered blood leaves the nephron to join the renal vein, while urine accumulated in the urine-collecting duct of the nephron leaves the nephron and travels to the renal pelvis, finally exiting the kidney through the ureter. (For a detailed account of how the nephron filters blood, see Figure 2-16).
Kidneys and Glucose Homeostasis
The kidneys play an important role in glucose homeostasis through plasma filtration and resorption. Normally, all the glucose that is filtered through the glomeruli in the kidney is resorbed back into the circulation in the proximal tubules through two types of special channels, sodium-glucose cotransporter 2 (SGLT2) and SGLT1. When the kidney is working properly, SGLT2 mediates the resorption of 90% of filtered glucose, and SGLT1 mediates resorption of the remaining 10%.
Did You Know?
In 1 minute, approximately one-quarter of the body’s total blood supply passes through and is cleaned by the kidneys.Did You Know?
Although the kidneys filter 150-180 L of blood plasma each 24 hours, they produce less than two litres of urine.Sodium-glucose cotransporter 2 (SGLT2)
Glucose transporter found in the proximal tubules of the kidneys; mediates 90% of glucose resorption.
The Renin-Angiotensin-Aldosterone System
The renin-angiotensin-aldosterone system (RAAS) is a negative feedback mechanism that helps to maintain homeostasis (constancy) of blood pressure. (The mechanism of the RAAS is described in Figure 2-17.)
Renin-angiotensin-aldosterone system (RAAS)
A hormone system, controlled by secretions from the kidney, that regulates sodium balance, blood pressure and fluid balance. The RAAS raises blood pressure when it becomes too low.Negative feedback
Feedback that works to stabilize a process by reducing the rate or output of the process when the effects of that process are too great.
Post-Test
Congratulations! You have completed Chapter Two: Anatomy and Physiology. Test your knowledge of the topics covered in this chapter by completing the Post-test. Use this test to identify information you have mastered and gaps in your knowledge. Reinforce your learning as necessary by reviewing those sections of the chapter where further study is required.
It should take you approximately 10 minutes to complete the Post-test. Once you have finished, compare your answers with the Post-test Answer Key that follows this test.
Please circle the one correct response for each question:
1. Which of the following terms refers to the synthesis of glucose in the liver?
a. Gluconeogenesis
b. Glycogenesis
c. Glycogenolysis
d. Glycolysis
2. Glucagon is synthesized in:
a. The alpha cells of the islets of Langerhans
b. The alpha cells of the small intestine
c. The beta cells of the islets of Langerhans
d. The beta cells of the small intestine
3. Glucose is stored primarily as glycogen in:
a. Muscle and liver tissue
b. Muscle and renal tissue
c. Pancreatic and cardiac tissue
d. Renal and brain tissue
4. Which of the following is true about lipids?
a. Animal fats are generally unsaturated while plant fats are generally saturated
b. Lipids are a large group of fatty compounds that are insoluble in the bloodstream
c. Lipids are fats that have been broken down into the smallest fat molecules
d. Lipids move freely in the bloodstream and they are broken down in the liver
5. Which of the following is true about the metabolic effects of insulin?
a. Insulin promotes the breakdown of glycogen
b. Insulin promotes the breakdown of triglycerides
c. Insulin promotes the storage of glucagon
d. Insulin promotes the storage of nutrients
6. The key feature of incretins is that they stimulate pancreatic insulin secretion in a:
a. Glucose-dependent fashion
b. Glucose-independent fashion
c. Glucose-inhibitory fashion
d. Glucose-resistant fashion
7. Which of the following best describes the functions of blood circulating in the body?
a. Nutrient and oxygen transportation, waste removal, heat distribution, fighting infection, clotting
b. Nutrient and oxygen transportation, waste removal, temperature regulation, fighting infection, clotting
c. Nutrient transportation, waste removal, heat distribution, clotting
d. Nutrient transportation, waste removal, temperature regulation, fighting infection, clotting
8. Which of the following is true about systemic circulation?
a. Blood flows to and from all cells, tissues and organs except the liver
b. Blood flows to and from all cells, tissues and organs except the lungs
c. Blood flows to and from the lungs, which drains blood from subcutaneous fat
d. Blood flows to and from the lungs, which drains blood from visceral tissues to the liver
9. Which of the following describes portal circulation?
a. Blood circulates through the capillaries of one organ to the capillaries of another organ before returning to the heart
b. Blood circulates through the extremities
c. Blood circulates through the liver prior to returning to the kidneys
d. Blood circulates through pulmonary veins prior to returning to the heart
10. Blood pressure is regulated by:
a. Hormonal and cardiovascular control
b. Hormonal, liver and cardiovascular control
c. Hormonal, liver and nervous system control
d. Hormonal, renal and nervous system control
11. The pituitary is known as the master gland because:
a. It controls glands in both the endocrine and exocrine systems
b. It controls the nervous system via the hypothalamus
c. It influences many other glands in the endocrine system
d. It ultimately controls all other glands
12. Which of the following is NOT a function of the kidney?
a. Blood filtration
b. Blood oxygenation
c. Blood pressure control
d. Electrolyte regulation
13. Which of the following is NOT true about the renin-angiotensin-aldosterone system (RAAS)?
a. Angiotensin I is converted to angiotensin II by the action of angiotensin converting enzyme (ACE)
b. Angiotensin II acts as a vasodilator and stimulates the liver to produce aldosterone
c. It provides a means to control blood pressure
d. Specialized cells in the kidneys produce renin
14. Homeostasis refers to:
a. Regulation of glucose by the pancreas
b. The cessation of insulin production following a meal
c. The tendency of the body to maintain stability under stable environmental conditions
d. The tendency of the body to maintain stability under varying environmental conditions
15. Which of the following is true regarding adrenal glands?
a. They are positioned above the kidneys and help to maintain fluid balance and blood pressure
b. They are positioned above the kidneys and influence insulin production
c. They are positioned below the kidneys and secrete catecholamines (fight-or-flight hormones)
d. They are positioned below the kidneys and secrete hormones that help regulate sodium and potassium concentration
Post-Test Anwser Key
The correct answers are highlighted in bold.
1. Which of the following terms refers to the synthesis of glucose in the liver?
a. Gluconeogenesis
b. Glycogenesis
c. Glycogenolysis
d. Glycolysis
2. Glucagon is synthesized in:
a. The alpha cells of the islets of Langerhans
b. The alpha cells of the small intestine
c. The beta cells of the islets of Langerhans
d. The beta cells of the small intestine
3. Glucose is stored primarily as glycogen in:
a. Muscle and liver tissue
b. Muscle and renal tissue
c. Pancreatic and cardiac tissue
d. Renal and brain tissue
4. Which of the following is true about lipids?
a. Animal fats are generally unsaturated while plant fats are generally saturated
b. Lipids are a large group of fatty compounds that are insoluble in the bloodstream
c. Lipids are fats that have been broken down into the smallest fat molecules
d. Lipids move freely in the bloodstream and they are broken down in the liver
5. Which of the following is true about the metabolic effects of insulin?
a. Insulin promotes the breakdown of glycogen
b. Insulin promotes the breakdown of triglycerides
c. Insulin promotes the storage of glucagon
d. Insulin promotes the storage of nutrients
6. The key feature of incretins is that they stimulate pancreatic insulin secretion in a:
a. Glucose-dependent fashion
b. Glucose-independent fashion
c. Glucose-inhibitory fashion
d. Glucose-resistant fashion
7. Which of the following best describes the functions of blood circulating in the body?
a. Nutrient and oxygen transportation, waste removal, heat distribution, fighting infection, clotting
b. Nutrient and oxygen transportation, waste removal, temperature regulation, fighting infection, clotting
c. Nutrient transportation, waste removal, heat distribution, clotting
d. Nutrient transportation, waste removal, temperature regulation, fighting infection, clotting
8. Which of the following is true about systemic circulation?
a. Blood flows to and from all cells, tissues and organs except the liver
b. Blood flows to and from all cells, tissues and organs except the lungs
c. Blood flows to and from the lungs, which drains blood from subcutaneous fat
d. Blood flows to and from the lungs, which drains blood from visceral tissues to the liver
9. Which of the following describes portal circulation?
a. Blood circulates through the capillaries of one organ to the capillaries of another organ before returning to the heart
b. Blood circulates through the extremities
c. Blood circulates through the liver prior to returning to the kidneys
d. Blood circulates through pulmonary veins prior to returning to the heart
10. Blood pressure is regulated by:
a. Hormonal and cardiovascular control
b. Hormonal, liver and cardiovascular control
c. Hormonal, liver and nervous system control
d. Hormonal, renal and nervous system control
11. The pituitary is known as the master gland because:
a. It controls glands in both the endocrine and exocrine systems
b. It controls the nervous system via the hypothalamus
c. It influences many other glands in the endocrine system
d. It ultimately controls all other glands
12. Which of the following is NOT a function of the kidney?
a. Blood filtration
b. Blood oxygenation
c. Blood pressure control
d. Electrolyte regulation
13. Which of the following is NOT true about the renin-angiotensin-aldosterone system (RAAS)?
a. Angiotensin I is converted to angiotensin II by the action of angiotensin converting enzyme (ACE)
b. Angiotensin II acts as a vasodilator and stimulates the liver to produce aldosterone
c. It provides a means to control blood pressure
d. Specialized cells in the kidneys produce renin
14. Homeostasis refers to:
a. Regulation of glucose by the pancreas
b. The cessation of insulin production following a meal
c. The tendency of the body to maintain stability under stable environmental conditions
d. The tendency of the body to maintain stability under varying environmental conditions
15. Which of the following is true regarding adrenal glands?
a. They are positioned above the kidneys and help to maintain fluid balance and blood pressure
b. They are positioned above the kidneys and influence insulin production
c. They are positioned below the kidneys and secrete catecholamines (fight-or-flight hormones)
d. They are positioned below the kidneys and secrete hormones that help regulate sodium and potassium concentration
Further Reading
Amori RE, Lau J, Pittas AG. Efficacy and safety of incretin therapy in type 2 diabetes: systematic review and meta -analysis. JAMA. 2007 Jul 11;298(2):194-206.
Barter P, Gotto AM, LaRosa JC, Maroni J, Szarek M, Grundy SM, et al. HDL cholesterol, very low levels of LDL cholesterol, and cardiovascular events. N Engl J Med. 2007 Sep 27;357(13):1301-10.
Batterham RL, Le Roux CW, Cohen MA, Park AJ, Ellis SM, Patterson M, et al. Pancreatic polypeptide reduces appetite and food intake in humans. J Clin Endocrinol Metab. 2003 Aug;88(8):3989-92.
Canadian Diabetes Association [Internet]. Toronto: Canadian Diabetes Association; c2012 [cited 2013 Jun 21]. The benefits of eating fibre; [about 3 screens]. Available from: http://www.diabetes.ca/diabetes-and-you/nutrition/fibre/
Chaudhri OB, Wynne K, Bloom SR. Can gut hormones control appetite and prevent obesity? Diabetes Care. 2008 Feb;31 Suppl 2:S284-9.
Drucker DJ, Nauck MA. The incretin system: glucagon-like peptide-1 receptor agonists and dipeptidyl peptidase-4 inhibitors in type 2 diabetes. Lancet. 2006 Nov 11;368(9548):1696-705.
Drucker DJ, Philippe J, Mojsov S, Chick WL, Habener JF. Glucagon-like peptide I stimulates insulin gene expression and increases cyclic AMP levels in a rat islet cell line. Proc Natl Acad Sci U S A. 1987 May;84(10):3434-8.
Elrick H, Stimmler L, Hlad CJ Jr, Arai Y. Plasma insulin response to oral and intravenous glucose administration. J Clin Endocrinol Metab. 1964 Oct;24:1076-82.
Ghezzi C, Loo DDF, Wright EM. Physiology of renal glucose handling via SGLT1, SGLT2 and GLUT2. Diabetologia. 2018 Oct;61(10):2087-2097.
Habener J. Insulinotropic glucagon-like peptides. In: LeRoith D, Taylor SI, Olefsky JM, editors. Diabetes mellitus: a fundamental and clinical text. 2nd ed. Philadelphia: Lippincott Williams & Wilkins; 2000. p. 604-15.
Health Canada [Internet]. Ottawa: Health Canada; c2012 [updated 2012 Mar 15; cited 2013 Jun 21]. Do Canadian adults meet their nutrient requirements through food intake alone?; [about 12 screens]. Available from: http://www.hc-sc.gc.ca/fn-an/surveill/nutrition/commun/art-nutr-adult-eng.php
Health Canada [Internet]. Ottawa: Health Canada; c2012 [updated 2010 Nov 16; cited 2013 May 27]. Vitamins and minerals; [about 2 screens]. Available from: http://www.hc-sc.gc.ca/fn-an/nutrition/vitamin/index-eng.php
Parker S. The concise human body book: an illustrated guide to its structure, function and disorders [Internet]. New York: Dorling Kindersley; 2009. 320 p.
Patton KT, Thibodeau GA. Anatomy & physiology. 7th ed. St. Louis: Mosby Elsevier; 2010.
Toft-Nielsen MB, Damholt MB, Madsbad S, Hilsted LM, Hughes TE, Michelsen BK, et al. Determinants of the impaired secretion of glucagon-like peptide-1 in type 2 diabetic patients. J Clin Endocrinol Metab. 2001 Aug;86(8):3717-23.
Chapter Glossary