Life Science
Introduction to basic functional aspects of mammalian systems- digestive, circulatory, respiratory, nervous, excretory, endocrine and reproductive
1. The Mammalian Digestive System
The mammalian digestive system consists of the alimentary canal ( complete digestive tract) and various accessory glands that secret digestive juices into the canal through the ducts. The food is moved along the tract by the contraction of smooth muscles in the walls of the canal. These rhythmic contraction waves are called peristalsis. The regulation of passage of material from one chamber to another within the canal is controlled by ring-like valves called sphincters.
The accessory glands of the mammalian digestive tract are three pairs of salivary glands, the pancreas, the liver, and its storage organ the gall bladder.
Now lets follow a meal through the human digestive canal.
The Oral Cavity – Mouth
Physical and chemical digestion of food begin in the mouth. During chewing the food is made easier to swallow and the food’s surface area is increased. The presence of food in the mouth triggers a nervous reflect that causes the salivary glands to secret saliva into the mouth. Often saliva is secreted due to a smell or sight. In humans , almost a liter of saliva is secreted into the mouth daily. That may sound like a lot, but for horses it is gallons! Human saliva contains mucin which is a slippery glycol-protein which protects the mouth from abrasion and lubricated the food for swallowing. Saliva contains buffers that help prevent tooth decay by neutralizing acids in the mouth. There are also antibacterial agents in the saliva. The digestion of carbohydrates begins in the mouth. Saliva contains salivary amylase, a digestive enzyme that hydrolyses starch (a glucose polymer from plants and glycogen (a glucose polymer from animals). This enzyme breaks down the carbohydrates into smaller polysaccharides and the disaccharide maltose. The tongue located in the oral cavity helps to manipulate the food during chewing and shapes the food into a ball called the bolus which it pushes to the back of the mouth and into the pharynx.
The Pharynx
Out throat is the pharynx which leads to both the esophagus and the windpipe (trachea).
When a human swallows the to of the windpipe moves up so that its opening , the glottis, is blocked by a flap of cartilage called the epiglottis. This helps to ensure that the bolus enters the esophagus.
The Esophagus
The esophagus channels food from the pharynx to the stomach. The muscles in the walls move the food. The first part of swallowing is a voluntary act but then the involuntary waves of contraction of the smooth muscles take over. Salivary amylase continues to hydrolyze starch as the bolus passes through the esophagus.
The Stomach
The stomach is located on the left side of the abdomen , just below the diaphragm. Since its walls are elastic and it has accordion-like folds, the stomach can hold up to 2 liters of food and water. The walls of the stomach secret gastric juices, a digestive fluid that mixes with the food. Since this fluid has a high percentage of HCl , its pH is about 2 which is acidic enough to dissolve iron nails. The functions of this acid include : 1. disrupt the extra-cellular matrix that binds cells together 2. kill most bacteria in the food.
Also present in this gastric juice is pepsin, an enzyme that begins the hydrolysis of proteins by breaking peptide bonds. Cells of the stomach wall are protected from pepsin by a coating of mucus. The epithelial cells which generate this mucus are eroded by the acid and therefore the stomach lining must be replaced by mitosis every three days.
Much of the time the stomach is closed off at both ends. The opening from the esophagus to the stomach is called the cardiac orifice. This opening will open when a bolus is ready to pass. Occasionally there will be backflow of acid chime from the stomach into the lower ed of the esophagus, causing heartburn. If heartburn persists, An ulcer could develop in the esophagus. The opening from the stomach to the small intestine is the pyloric sphincter. This opening regulates the passage of chime into the small intestine. This happens a squirt at a time, taking about 2-6 hours to empty the stomach after a meal.
Small Intestine
The small intestine is the major organ of digestion and absorption. The small intestine is the longest section of the digestive tract at more than 6 meters in length. It is referred to as the small intestine because its diameter is smaller than the large intestine. Most digestion and absorption happens in this organ. The pancreas, liver and gall bladder participate in digestion.
The first part of the small intestine is called the duodenum ( about 25 cm in length). This is where the chime from the stomach mixes with digestive juices from the pancreas, liver gall bladder and gland cells from the intestinal wall. The pancreas produces bicarbonate which helps to offset the acidity of the stomach. The liver produces bile which is stored in the gall bladder. Bile does not contain digestive enzymes. It contains bile salts which act as detergents and aid in the digestion and absorption of fats. Bile also carries wastes from the liver (where old red blood cells are destroyed).
Carbohydrate digestion
Pancreatic amylases hydrolyze starch, glycogen, and smaller polysaccharides into disaccharides including maltose. The enzyme maltase completes the digestion of maltose by splitting it into two molecules of glucose. Sucrase hydrolyzes sucrose ( table sugar). Lactase digest lactose ( sugar found in milk). As people get older, they have less lactase in their system. These disaccharidases are in the membranes of the intestinal epithelial where the final monomers are absorbed by the blood.
Protein digestion
Enzymes in the duodenum break the polypeptide chains down into amino acids. These enzymes are supplies by the pancreas.
Nucleic acid digestion
Enzymes called nucleases hydrolyze DNA and RNA in food into their nucleic acids. Other enzymes break the nucleotides down.
Fat digestion
Nearly all the fat in a meal reaches the small intestine completely undigested. Fat molecules are insoluble in water. Bile salts coat the tiny fat droplets to keep them from coalescing in a process called emulsification. Since the droplets are small, a large surface area is exposed to lipase which is an enzyme which hydrolyzes fat. Most of this digestion happens in the duodenum. The remaining regions of the small intestine, jejunum and ileum, function mainly in the absorption of nutrients and water.
Absorption of nutrients
Most of the absorption of nutrients takes place in the small intestine while there is some absorption in the stomach and small intestine. The lining of the small intestine has a surface area about the size of a tennis court. Large circular folds in the lining have villi and each of the cells of the villi have microvilli. These villi absorb nutrients which are then transported across the capillary membranes.
Large Intestine
The large intestine or colon is connected to the small intestine at a T-shaped junction where a sphincter regulates the movement of materials. One arm of the T structure is a sac called the cecum which has a fingerlike extension called the appendix. The main part of the colon is an upside down U about 1.5 meters in length. Connected to the cecum is the right or ascending colon which is connected to the transverse colon which is connected to the descending colon which is connected to the sigmoid colon which is connected to the rectum.
The main function of the colon is to reabsorb water although most re-absorption of water happens in the small intestine with the absorption of nutrients. The small and large intestines absorb about 90% of the water that enters the digestive tract. The wastes of the digestive tract, feces, become more solid as they move along the colon.
Many harmless bacteria live in the colon. E. coli is in the colon. Intestinal bacteria live on organic bacteria that would otherwise be included in feces. By-products of colon bacteria metabolism include gases (like methane and hydrogen sulfide) and some vitamins. Bacteria in the colon generate Vitamin K which is used in blood clotting. The terminal portion of the colon is called the rectum which is where feces are stored until they can be eliminated. Between the rectum and the anus are two sphincters, one involuntary and one voluntary. Once or more a day, strong contractions of the colon create an urge to deficate.
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2. Animal Respiratory System
Every cell in an animal requires oxygen to perform cellular respiration which gives off carbon dioxide and water as waste products. Respiration is the process by which animals exchange these gases with their environment. Animals have specialized systems of structures that help them to do this successfully and efficiently. Even a fish will drown if it cannot successfully breathe underwater.
Gas Exchange
The actual exchanging of the gases is dependent upon important structures such as lungs or gills, and the principle of diffusion. Diffusion says that the molecules or particles will move from an area where they are very concentrated into an area where they are less concentrated.
Mammals Respiratory System
The chief organ in mammalian respiration is the lungs. The lungs are actively ventilated via a suction-pump mechanism of inhalation and exhalation. Breathing is dependent upon the rib muscles and the diaphragm, which is a structure located just beneath the lungs like a dome-shaped floor (or a dome-shaped roof for the intestinal cavity.
Inhalation happens when the rib cage opens up and the diaphragm flattens and moves downward. The lungs can then expand into the larger space that causes the air pressure inside them to decrease, and the drop in air pressure inside the lung makes the outside air rush inside.
Exhalation is the opposite process. The diaphragm and the rib muscles relax to their neutral state that causes the lungs to contract. The squashing of the lungs increases their air pressure and forces the air to flow out.
A diagram of ventilation in most mammals. The left image shows inhalation with a flattened diaphragm. The right side shows the dome shaped diaphragm forcing the air out during exhalation.
In most mammals, the first place that air enters upon inhalation is the nose. It gets warmed, moistened, and filtered by cilia and mucus membranes which can trap dust and pathogens. Air then reaches the epiglottis, which is the tiny leaf shaped flap at the back of the throat. The epiglottis regulates air going into the windpipe and closes upon swallowing to prevent food from being inhaled. It is the gatekeeper to the lungs. If the epiglottis is the gatekeeper, who's the key master?
The trachea is a long structure of soft tissue surrounded by c-shaped rings of cartilage. In humans the trachea splits into two bronchi branches that lead to each lung. Each bronchi divides into increasingly smaller branches, until they form a massive tree of tubes. The smallest branches are called the bronchioles, and each bronchiole ends with a tiny air sac (no larger than a grain of sand) called an alveolus.
The tiny alveoli (alveoli is the plural of alveolus) are crucial because they increase the surface area that can be used for gas exchange. If the lungs were just empty sacs the only area available for gas exchange would be the walls of the lungs, which in humans is approximately 0.01 meters squared. In contrast, the alveoli structures provide 75 square meters of surface area where oxygen absorption can take place. That is the size of half a volleyball court.
Diagram of an alveolus near a capillary and the gas exchange process in the lungs
As discussed above, gas exchange takes place in the capillaries, so the alveoli are closely aligned with the network of capillaries. This brings the blood carrying waste products into close enough proximity with fresh air for diffusion to take place. The waste is removed and the oxygen is taken up by the blood.
The blood is able to carry the fresh oxygen in red blood cells because of the hemoglobin protein, which can attach oxygen molecules. Think of hemoglobin like a bus that carries oxygen passengers. Each hemoglobin protein can carry four passengers of oxygen at one time.
When red blood cells are oxygen rich they are bright red, and when they are deoxygenated they are a deep purple. When the blood reaches the systemic capillaries near the cells, the carbon dioxide and oxygen diffuse in opposite directions.
After circulating through the heart, the blood arrives at the capillaries near the lungs. Water vapor and carbon dioxide are exhaled, and the process begins again with inhalation.
Just as the heart beats on its own, following sinoatrial node signals, breathing is done without conscious effort. There are sections of the brain, called the medulla and pons, that regulate respiration. They decide how fast respiration needs to take place by monitoring the level of carbon dioxide in the blood. In times of excitement or during exercise, the cells require more oxygen than normal. Respiration speeds up. Additionally, the heartbeat increases because the circulatory system is required for the respiration system to function.
Tidal volume is the amount of air breathed in or out during a respiratory cycle. The tidal volume and respiratory frequency vary amongst species and can also be affected by age, pregnancy, exercise, excitement, temperature, and body size. Horses have an average respiration of 12 times per minute, but pigs breathe an average of 40 times per minute.
Horses are obligate nasal breathers, which means that they must breathe through their noses. Humans and many other mammals can breathe through either their mouths or their nasal passages. A horse cannot breathe through its mouth. It is thought that this modification allows horses to graze with their heads down while separate nasal passages breath in air and sniff for potential predators.
Marine mammals breathe oxygen with lungs just like their terrestrial brethren, but with a few differences. First of all, to prevent water from getting into their airway they have adapted muscles or cartilaginous flaps to seal their tracheas when under the water. Additionally, they exchange up to 90% of their gases in a single breath, which helps them gather as much oxygen as possible. A sperm whale can last for 138 minutes on a single breath.
Lastly, it can be dangerous for diving mammals to have air in their lungs when they dive to great depths. For this reason, many marine mammals will prepare for a deep dive by taking a breath, exchanging gases in the blood, and exhaling to empty their lungs.
Diagram of structures of the lungs
Reptiles and Amphibians
Reptiles and amphibians both have lungs and exchange gases in the capillaries like mammals, but there are some differences in how they ventilate their respiratory systems. Reptiles do not typically breathe the same way as mammals since many reptiles lack a diaphragm. Without it they rely on muscles used in locomotion to ventilate their lungs.
Amphibians are capable of buccal pumping to push air into the lungs. This begins by muscles pulling air through the mouth or nose into a buccal cavity. Throat muscles then pump and move the floor of the mouth up in a way that Is visible from the outside. This forces air out of the mouth and into the lungs. Have you seen at frog's throat that is constantly moving.
Apart from their capillaries, amphibians can also perform gas exchange directly through their highly vascularized skin. This means that their skin has lots of blood vessels going through it. Since the blood vessels are close to their permeable skin surface, diffusion can take place right through the skin. In fact, some salamanders have no lungs at all, and they get all of their oxygen through their skin. The take home message is never get in a breath holding contest with a salamander. We wouldn't recommend a staring contest, either.
Birds
The respiratory system of birds is similar to that of mammals. Air is pulled in using a suction-type pull. Gases are exchanged in the capillaries. The major difference is the route of airflow through the bird. Birds have air sacs that collect air. They then force the air through their lungs like bellows stoking a fire.
When a bird inhales, air is brought into the posterior air sacs, which expand. Upon exhalation, the air is forced from the posterior air sacs into the lungs. This is where gas exchange takes place. A second inhalation will move the air from the lungs to the anterior air sac. A second exhalation will push the air out of the body.
This progression of air through the bird means that the lungs are compressed during inhalation and expand during exhalation. It also takes two full inhalations and exhalations to move one gulp of air through the bird. That's a lot of gulps.
Aquatic Respiration
In fish, respiration takes place in their gills. Gills can collect dissolved oxygen from the water and release carbon dioxide. Gills are much more complex than just a slit in the cheek of a fish.
Gills are comprised of gill arches with hundreds of gill filaments extending from them. Each filament is lined with rows of lamellae, and the gas exchange takes place as water flows through them. The frills and flaps increase the surface area to allow more gas exchange to take place, just as the alveoli do in the lungs.
Fish utilize a countercurrent exchange pathway (except for cartilaginous fish), which means that their arteries are arranged so that blood flows in the opposite direction of water movement against the gills. By having their respiration pathway in this orientation, maximum gas exchange can take place.
If the blood and the water were moving in the same direction, the blood would always be next to the same bit of water which would soon be depleted of oxygen. By setting up a countercurrent pathway, the blood is always passing water that still has oxygen. This allows the blood to gather as much oxygen as it can hold.
Since water must be flowing over the gills to provide a continual source of oxygen, fish have developed several ways to keep them ventilated. Some fish swim with their mouths open almost all of the time. Other fish have a special flap called an operculum, which is used to force water across the gills.
The exception to all fish having gills is the lungfish, which has working lungs. It can survive when its water habitat dries up from seasonal drought. Aptly named fish. Similarly, there are also certain land crabs that use gills to breathe outside of the water.
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3.Human Circulatory system
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4.Human Excretory system
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5.Human Nervous system
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6.Human Endocrine system
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7.Human Reproductive system
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