What are systems and organs?

Structure and Functions

There are nine main systems in the human body: the nervous, cardiovascular, respiratory, gastrointestinal, renal, endocrine, reproductive, thermoregulatory, and musculoskeletal systems. All these systems are essential to sustain life, and many work together to perform their functions efficiently. All the other systems need the nervous system to operate or to coordinate their functions. The first six of these systems will be discussed in this article.

The nervous system is composed of the central nervous system (the brain) and the peripheral nervous system (the spinal cord and nerves extending to every part of the body). The brain receives information from the body by way of the sensory nerves. It then evaluates all the information and sends out the appropriate signals to respond. For example, the ears send information to the brain that there are noises coming from behind; the brain tells the head to turn in the direction of the sounds. The eyes send the signals that the noises are coming from, for example, a gorilla. The brain must decide to run, fight, or stand and try to reason with the gorilla. Meanwhile, the brain tells the heart to beat faster and harder. It also tells the stomach and intestines to stop digestion and reduce its
blood flow because blood may be needed by the muscles for running. This is called the “fight or flight” response to stress, which the nervous system controls.

Sensory information can come from any of the five senses—sight, smell, hearing, touch, or taste—but it can also come from other sensors. Sensory nerves send the brain information on pain, temperature, blood pressure, and what is going on in the stomach and intestines (hunger or a full feeling). The brain receives millions of signals each second from every part of the body and must constantly decide how to respond. Humans can choose not to respond instinctively, as animals do. For example, humans often eat when they are not hungry.

Different areas of the brain are dedicated to specific functions. The upper portion of the spinal cord and lower portion of the brain (the brain stem) are dedicated to controlling involuntary functions such as breathing, the maintenance of blood pressure and heart rate, and the responses to heat and cold. The middle portion of the brain coordinates movement. The middle brain also coordinates information from upper portions of the brain and generates emotions. The uppermost and outermost portions of the brain (the cerebrum) process the information from the senses and generate responses, such as telling the body to move. The cerebrum also performs such intellectual functions as reasoning.

The cardiovascular system is composed of the heart and blood vessels. Its job is to pump blood containing oxygen and foodstuffs (sugars, proteins, and fat) to every part of the body. Blood is composed of red and white blood cells suspended in plasma, a pale yellow fluid which flows through the cardiovascular system. Red blood cells are the carriers of oxygen, the main source of energy for the body. White blood cells help fight disease and are delivered to parts of the body that are hurt or diseased. The plasma contains platelets that help blood to clot when necessary. Blood also transports wastes produced by the body from tissues to organs that can dispose of them. For example, carbon dioxide is produced by the tissues when oxygen is used for energy. Blood carries carbon dioxide back to the lungs to be removed from the body in exhaled air.

Blood is pumped by the heart in a circuit in the cardiovascular system (also called the circulatory system). The heart has four chambers, two atria and two ventricles. Blood enters the heart through the left atrium, a small pocket of muscles that help pump blood into the left ventricle. The ventricle is a larger chamber with a thick wall of muscle that can pump very hard; it pushes blood into the arteries. The left ventricle pumps blood into the main artery of the body, the aorta. The aorta branches many times into smaller arteries, which in turn branch into capillaries. Every part of the body has millions of tiny capillaries just big enough for a blood cell to pass through them; in fact, blood cells must fold to get through some capillaries. In capillaries, oxygen and foodstuffs leave the blood, and then carbon dioxide and other waste products enter the blood to be taken away. Blood flows from the capillaries into small veins, which join to make larger and larger
veins. The largest veins, the venae cavae, empty into the heart, in the right atrium. The blood is pumped from the right atrium to the right ventricle. Blood is then pumped by the right ventricle through the lung and back into the left atrium to start its journey again.

The lungs are the major organ of the respiratory system. The function of the respiratory system is to bring air into the lungs in order to supply the blood with oxygen. Air enters the respiratory system through the nose and mouth, which connect to the main windpipe, the trachea. The trachea branches into smaller airways called bronchi. Bronchi in turn branch into smaller airways, bronchioles. The ends of bronchioles form many rounded sacs (alveoli) that resemble bunches of grapes. These sacs of air have very thin walls that are shared with the walls of the lung’s capillaries. This close arrangement of air and blood provides a minimal distance for oxygen to travel into the blood and for carbon dioxide to leave the blood.

Air is moved into the lungs when the muscles of respiration contract and expand the lungs. The diaphragm is a large sheet of muscle which separates the chest from the abdomen. When the diaphragm contracts, it pulls the lungs down. At the same time, muscles on the chest wall contract, pulling the lungs up and out. This expansion of the lungs causes air to be sucked into and fill the alveoli. During exhalation, the respiratory muscles are relaxed, the lung collapses, and air rushes out, carrying carbon dioxide with it.

Blood, specifically red blood cells, is specialized to carry large amounts of oxygen and carbon dioxide. Red blood cells contain hemoglobin, a special substance that attaches to these gases. When the amounts of oxygen and carbon dioxide in the plasma increase, they tend to leak back into the alveoli or tissues, respectively. These gases are effectively removed from the fluid by hemoglobin, allowing more gases to enter the blood without leaking back out. Hemoglobin also coordinates the release of these gases—oxygen to the tissues and carbon dioxide to the lungs—at the correct time.

The gastrointestinal system is a multiorgan system that breaks down food to be absorbed into the blood. Initially, food is broken down by chewing and by mixing with saliva. Then it is swallowed into the esophagus, a tube that travels through the chest and empties into the stomach. The stomach adds acid and other chemicals to the food, which breaks it down even more. The food, now called chyme, passes into the upper small intestine (the duodenum). The pancreas adds enzymes; a chemical called bicarbonate, which neutralizes the acid added by the stomach; and the hormones insulin and glucagon. Insulin is the major hormone secreted by the pancreas. It is absorbed into the blood by the intestine and signals the body to get ready to receive the products of digestion, primarily sugar (glucose). Bile is also added to the contents of the upper duodenum by the liver. Bile helps to break down fat to be absorbed into the blood. As the intestinal contents move along the small intestine, from the jejunum to the ileum, water and mucus are added to help move it along, and carbohydrates are absorbed in their smallest form, glucose. Enzymes attached to the wall of the intestine break up proteins to be absorbed as their component parts, amino acids. Water in chyme is constantly being reabsorbed. Finally, the colon, or large intestine, absorbs most of the remaining water. The remaining solids are excreted through the rectum.

Food is moved through the gastrointestinal system by a special type of muscle called smooth muscle. The walls of the gastrointestinal tract are composed of muscle arranged in circular fashion around the tube and along its length. Initially, a circular group of muscles contracts, narrowing a short segment of intestine. This process is called segmentation. The contraction spreads down the muscles arranged lengthwise, squeezing the contents down the length of intestine. Movement of chyme is aided by relaxation of the muscles ahead of the contraction. This motion is called peristalsis. Peristalsis is coordinated by the nervous system, but the intestines have their own set of nerves. These intestinal nerves can control the motion of the intestine without help from the central nervous system.

The kidneys are the primary organs of the renal system. It is the function of the kidneys to regulate both the amount and the composition of the fluid in the body, in spite of wide variations in the human environment and in an organism’s intake of food and water. Since blood circulates everywhere in the body, the kidneys can change the composition and amount of plasma, and the other fluids of the body then equalize with it. Therefore, the kidneys can regulate all body fluids. The body has sensors for both the amount of fluid in the blood and the concentration of the important elements in the blood, such as sodium, hydrogen, and potassium.

The kidneys regulate plasma volume and composition by filtering the plasma and returning only the appropriate amounts of fluid and substances back to the blood. Arteries entering the kidneys rapidly branch into capillaries. Approximately 20 percent of all plasma flowing into the kidneys leaves the capillaries and is collected in the capsules that surround them. This fluid is funneled into specialized tubes.

Substances are taken out and put into the fluid in the tubes in order to regulate fluid volume and composition. In the beginning of the tube (the proximal tubule), most of the salt, water, glucose, and amino acids are taken back into the blood. The next sections (the loop of Henle, distal tubule, and collecting ducts) help to regulate the final amount of water excreted in urine. The collecting ducts join to form the ureter, which carries the remaining fluid, urine, to the bladder, where it is stored. From the bladder, the urine is expelled through the urethra.

The endocrine system is another multiorgan system that helps to control and modify the function of almost all other systems. Endocrine glands produce chemicals and release them into the blood to direct the functions of cells and tissues elsewhere in the body. There are three classes of hormones: amines, peptides and proteins, and steroids. Adrenaline is an example of the amine group, insulin is a protein hormone, and estrogen is a steroid hormone. Each of these is produced by a different gland.

The adrenal glands, small glands located near the kidneys, make several hormones. The outer portion, the cortex, produces three types of steroid hormones referred to as corticosteroids: glucocorticoids, mineralocorticoids, and small amounts of androgenic hormones. The major glucocorticoid, cortisol, regulates the production and use of glucose, fats, and amino acids by many cells and tissues. It also plays a helper role for other hormonal actions, such as making them more potent during stress, and it helps prevent inflammation and swelling. The major mineralocorticoid, aldosterone, can modify the kidneys’ excretion of sodium, potassium, and hydrogen. A person unable to produce mineralocorticoids will die in a few days without treatment but can be saved by aldosterone therapy. Therefore, these
steroids are said to be lifesaving. The androgenic steroids can cause the development of adult male sexual characteristics, the same effect as the male sex hormone, testosterone.

The interior portion, or medulla, of the adrenal glands makes catecholamines, such as adrenaline (epinephrine). Adrenaline helps the cardiovascular system during exercise and stress. It is the hormone that stimulates much of the “fight or flight” response, making the heart beat faster and harder. Adrenaline also helps to increase blood flow to muscles, in case flight is the action of choice.

The pituitary gland, also known as the hypophysis, is located at the base of the brain and produces many hormones with a variety of functions. The pituitary is divided into two areas: the anterior lobe (also known as the adenohypophysis) and the posterior lobe (also known as the neurohypophysis). The anterior pituitary produces growth hormone
, a protein that has a major influence on all metabolic activity. It causes the body to store carbohydrates, to make proteins for growth, and to use fat for energy. The other anterior pituitary hormones cause other glands to increase their production of hormones. The glands stimulated by distinct pituitary hormones are the thyroid, adrenal cortex, ovaries, testicles, and mammary glands. The posterior pituitary produces the peptide hormones antidiuretic hormone (also known as vasopressin) and oxytocin. Antidiuretic hormone (ADH) decreases the amount of water that the kidneys can excrete, which keeps the body from dehydrating. ADH can also cause the blood pressure to rise, which helps if fluid is lost as a result of bleeding. Oxytocin causes the uterus to contract during the birthing process, and it also stimulates the production of milk in new mothers. The pituitary has direct regulating control over many glands and tissues, and it regulates nearly all tissues and organs indirectly by way of its stimulating hormones. This gland is regulated in a similar fashion by the hypothalamus, a small part of the brain just above the pituitary.

The thyroid gland is located in the neck around the voice box, or larynx. The parathyroid glands are located next to the thyroid gland. Thyroid hormones (peptides) cause almost all tissues in the body to increase the use of foodstuffs for the production of proteins, aiding in growth. In addition, the thyroid gland produces calcitonin. Calcitonin and the parathyroid hormones regulate the amount of calcium in the blood. Parathyroid hormone acts by freeing calcium from bone when more calcium is needed in the blood, while calcitonin causes the opposite action. Therefore, when the level of one of these hormones goes up, the other must go down.

The sex organs are also endocrine glands. The ovaries make estrogen and progesterone, while the testes produce testosterone. These steroid hormones cause the body to develop primary and secondary sexual characteristics. When a woman is pregnant, the placenta (the part of a woman’s uterus that nourishes the fetus) produces hormones that prepare her body for childbirth and breast-feeding.

Disorders and Diseases

The body has control mechanisms to ensure that its systems function properly. Many systems use what is called negative feedback to fine-tune their functioning. An example of negative feedback is the control of blood pressure. Sensors in the arteries allow the brain to monitor the body’s blood pressure level. When pressure is too high, the brain tells the cardiovascular system to decrease pressure by slowing the heart and opening the blood vessels and tells the kidneys to excrete fluid. Thus, when blood pressure is high, the feedback that the brain provides is negative, because it causes a response that is opposite to the unwanted change from the normal state.

The endocrine system uses negative feedback to regulate many hormones. The simplest endocrine feedback system involves insulin and glucose. When blood glucose increases, insulin secretion increases, which in turn decreases blood glucose. A decrease in blood glucose tells the pancreas to slow down the secretion of insulin. The failure of this system results in diabetes mellitus, a disease in which the ability to regulate blood sugar is lost. When this control is lost, other systems are damaged as a result, such as the renal and cardiovascular systems.

There are also much more complex feedback control systems. The regulation of the adrenal hormone cortisol serves as an example of such a system. Cortisol secretion is controlled by secretion of the pituitary hormone adrenocorticotropic hormone (ACTH)
, also called corticotropin. The secretion of ACTH is controlled by a hypothalamic hormone called corticotropin-releasing factor (CRF). Stress causes CRF to be released, which causes the release of ACTH and in turn stimulates the secretion of cortisol. In general, when the level of any of these hormones becomes too high, the release of one of the others can be shut down. High levels of cortisol turn off CRF and ACTH secretion. High levels of ACTH can turn off the secretion of the hormone that triggers its secretion, CRF. It is also thought that CRF can provide feedback to its organ of origin, the hypothalamus, and halt its own secretion. Thus, if one of the control systems fails to function, a backup system guards against total malfunction.

The systems and organs of the body are dedicated to specific functions, but each needs the others to function properly. In addition, each system requires the coordination from the central nervous system to perform efficiently. The lungs bring vital oxygen to the cardiovascular system, and all systems need the nutrients brought to them by the cardiovascular system. Some organs of the endocrine system, such as the adrenal glands, are essential to life. The kidneys keep the blood clean and maintain the body’s fluid volume. The reproductive system is essential to maintaining the existence of the species. All these systems must perform their functions for the body to work well. If one system malfunctions, other systems are affected and may malfunction as well.

An example of one system malfunction that causes the failure of many others is renal failure. Kidney failure can be caused by a malfunction of the cardiovascular system such as clogging of the capillaries, which prohibits the kidneys from doing their job. As a result, hydrogen and potassium ions will accumulate. The effects of high levels of hydrogen and potassium ions on the cardiovascular system are a weakened heart and lower blood pressure. The nervous system can sense the increase in hydrogen ions and will tell the respiratory system to breathe faster and deeper to rid the body of carbon dioxide and its hydrogen ions. The increase in breathing helps to lower the hydrogen ion levels in the blood but cannot completely compensate for the kidney malfunction; in fact, the increase in work by the respiratory muscles can produce more hydrogen ions. When levels of hydrogen become too high, the brain begins to malfunction. The patient may experience dizziness, have seizures, or lose consciousness. If these symptoms are not reversed, the malfunction of each system will aid in the deterioration of other systems. The brain will be irreversibly damaged, and the heart will stop.

There are several ways to treat renal failure to avoid multiple-organ sickness. Treatment of infections with antibiotics before they become severe can help to avoid early and mild kidney failure. In severe, long-term kidney failure, kidney transplantation may become necessary. The damaged kidney (or kidneys) is removed and replaced with an organ from a deceased or a living donor (a person can live a normal life with only one functioning kidney). Kidneys, even from deceased donors, are rare, and many people in need of a transplant must wait for years to receive one that will not be rejected by the body’s immune system. For such patients, dialysis is necessary for survival. Dialysis is the use of a machine to perform some of the functions of the kidneys. Patients with total kidney failure must be hooked up to an artificial kidney machine for several hours several times per week. Even with this treatment, they will still be very sick, because the machine cannot perform all functions of the kidneys.

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