Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

The feces are stored in the rectum and passed out through the anus to complete the digestion process.

 

Liver

 

The liver has an important function in processing the products of human digestion. For example, cells of the liver remove excess glucose from the bloodstream and convert the glucose to a polymer called glycogen for storage.

 

The liver also functions in amino acid metabolism. In a process called deamination, it converts some amino acids to compounds that can be used in energy metabolism. In doing so, the liver removes the amino groups from amino acids and uses the amino groups to produce urea. Urea is removed from the body in the urine (see Chapter 26). Fats are processed into two-carbon units that can enter the Krebs cycle for energy metabolism. The liver also stores vitamins and minerals, forms many blood proteins, synthesizes cholesterol, and produces bile for fat digestion.

 

Nutrition in Animals

 

The nutritional requirements of most animals are relatively extensive and complex compared with the simple requirements of plants. The nutrients used by animals include carbohydrates, lipids, nucleic acids, proteins, minerals, and vitamins.

 

   Carbohydrates are the basic source of energy for all animals. Animals obtain their carbohydrates from the external environment (compared with plants, which synthesize carbohydrates by photosynthesis). About one-half to two-thirds of the total calories every animal consumes daily are from carbohydrates. Glucose is the carbohydrate most often used as an energy source. This monosaccharide is metabolized during cellular respiration (see Chapter 6), and part of the energy is used to synthesize adenosine triphosphate (ATP). Other useful carbohydrates are maltose, lactose, sucrose, and starch.

 

   Lipids are used to form cellular and organelle membranes, the sheaths surrounding nerve fibers, and certain hormones. One type of lipid, fats, are extremely useful energy sources.

 

   Nucleic acids are used for the construction of deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and ATP. Animals obtain their nucleic acids from plant and animal tissues, especially from cells that contain nuclei. During digestion, the nucleic acids are broken down into nucleotides, which are absorbed into the cells.

 

   Proteins form the framework of the animal body. Proteins are essential components of the cytoplasm, membranes, and organelles. They are also the major components of muscles, ligaments, and tendons, and they are the essential substances of enzymes. Proteins are composed of 20 kinds of amino acids. Although many amino acids can be synthesized, many others must be supplied in the diet. During digestion, proteins are broken down into their constituent amino acids, which are absorbed into the body.

 

   Among the minerals required by animals are phosphorus, sulfur, potassium, magnesium, and zinc. Animals usually obtain these minerals when they consume plants. Vitamins are organic compounds essential in trace amounts to the health of animals. Vitamins can be water soluble or fat soluble. Water-soluble vitamins must be consumed frequently, while fat-soluble vitamins are stored in the liver in fat droplets. Among the many essential vitamins are vitamin A for good vision, vitamin B for substances used in cellular respiration (FAD, NAD, and coenzyme A), and vitamin D to assist calcium absorption in the body.

 

 Animals obtain their nutrients through a broad variety of feeding patterns. Sponges, for example, feed on small particles of food that enter their pores. Other aquatic organisms, such as sea cucumbers, wave their tentacles about and trap food on their sticky surfaces. Mollusks, such as clams and oysters, feed by filtering materials through a layer of mucus in their gills. Certain arthropods feed exclusively on fluids.

 

Some animals feed on food masses, and they usually have organs for seizing, chewing, and consuming food. Herbivores are animals that eat only plants, while carnivores are animals that eat only other animals. Omnivores, which consume both plants and animals, are typified by humans.

Chapter 23: Gas Exchange

  Human Respiratory System

 

The human respiratory system consists of a complex set of organs and tissues that capture oxygen from the environment and transport the oxygen into the lungs. The organs and tissues that comprise the human respiratory system include the nose, pharynx, trachea, and lungs.

 

Nose

 

The respiratory system of humans begins with the nose, where air is conditioned by warming and moistening. Bone partitions separate the nasal cavity into chambers, where air swirls about in currents. Hairs and hairlike cilia trap dust particles and purify the air.

 

Pharynx

 

The nasal chambers open into a cavity at the rear of the mouth called the pharynx (throat). From the pharynx, two tubes called Eustachian tubes open to the middle ear to equalize air pressure there. The pharynx also contains tonsils and adenoids, which are pockets of lymphatic tissue used to trap and filter microorganisms.

 

Trachea

 

After passing through the pharynx, air passes into the windpipe, or trachea. The trachea has a framework of smooth muscle with about 16 to 20 open rings of cartilage shaped like a C. These rings give rigidity to the trachea and ensure that it remains open.

 

The opening to the trachea is a slitlike structure called the glottis. A thin flap of tissue called the epiglottis folds over the opening during swallowing and prevents food from entering the trachea. At the upper end of the trachea, several folds of cartilage form the larynx, or voice box. In the larynx, flaplike pairs of tissues called vocal cords vibrate when a person exhales and produce sounds.

 

At its lower end, the trachea branches into two large bronchi (singular, bronchus). These tubes also have smooth muscle and cartilage rings. The bronchi branch into smaller bronchioles, forming a bronchial “tree.” The bronchioles terminate in the alveoli.

 

Lungs

 

Human lungs are composed of approximately 300 million alveoli. Red blood cells pass through the capillaries in single file, and oxygen from each alveolus enters the red blood cells and binds to the hemoglobin. In addition, carbon dioxide contained in the plasma and red blood cells leaves the capillaries and enters the alveoli when a breath is taken. Most carbon dioxide reaches the alveoli as bicarbonate ions, and about 25 percent of it is bound loosely to hemoglobin.

 

When a person inhales, the rib muscles and diaphragm contract, thereby increasing the volume of the chest cavity. This increase leads to reduced air pressure in the chest cavity, and air rushes into the alveoli, forcing them to expand and fill. The lungs passively obtain air from the environment by this process. During exhalation, the rib muscles and diaphragm relax, the chest cavity volume diminishes, and the internal air pressure increases. The compressed air forces the alveoli to close, and air flows out.

 

The nerve activity that controls breathing arises from impulses transported by nerve fibers passing into the chest cavity and terminating at the rib muscles and diaphragm. These impulses are regulated by the amount of carbon dioxide in the blood: A high carbon dioxide concentration leads to an increased number of nerve impulses and a more rapid breathing rate.

 

 

 

Mechanisms for Gas Exchange

 

All living things obtain the energy they need by metabolizing energy-rich compounds, such as carbohydrates and fats. In the majority of organisms, this metabolism takes place by respiration, a process that requires oxygen. In the process, carbon dioxide gas is produced and must be removed from the body.

 

In plant cells, carbon dioxide may appear to be a waste product of respiration, too, but because it is used in photosynthesis , carbon dioxide may be considered a by-product. Carbon dioxide must be available to plant cells, and oxygen gas must be removed. Gas exchange is thus an essential process in energy metabolism, and gas exchange is an essential prerequisite to life, because where energy is lacking, life cannot continue.

 

The basic mechanism of gas exchange is diffusion across a moist membrane. Diffusion is the movement of molecules from a region of greater concentration to a region of lesser concentration, in the direction following the concentration gradient. In living systems, the molecules move across cell membranes, which are continuously moistened by fluid.

 

Simple organisms

 

Single-celled organisms, such as bacteria and protozoa, are in constant contact with their external environment. Gas exchange occurs by diffusion across their membranes. Even in simple multicellular organisms, such as green algae, their cells may be close to the environment, and gas exchange can occur easily.

In larger organisms, adaptations bring the environment closer to the cells. Liverworts, for instance, have numerous air chambers in the internal environment. Sponges and hydras have water-filled central cavities, and planaria have branches of their gastrovascular cavity that connect with all parts of the body.

 

Plants

 

Although plants are complex organisms, they exchange their gases with the environment in a rather straightforward way. In aquatic plants, water passes among the tissues and provides the medium for gas exchange. In terrestrial plants, air enters the tissues, and the gases diffuse into the moisture bathing the internal cells.

 

In the leaf of the plant, an abundant supply of carbon dioxide must be present, and oxygen from photosynthesis must be removed. Gases do not pass through the cuticle of the leaf; they pass through pores called stomata in the cuticle and epidermis. Stomata are abundant on the lower surface of the leaf, and they normally open during the day when the rate of photosynthesis is highest. Physiological changes in the surrounding guard cells account for the opening and closing of the stomata .

 

Animals

 

In animals, gas exchange follows the same general pattern as in plants. Oxygen and carbon dioxide move by diffusion across moist membranes. In simple animals, the exchange occurs directly with the environment. But with complex animals, such as mammals, the exchange occurs between the environment and the blood. The blood then carries oxygen to deeply embedded cells and transports carbon dioxide out to where it can be removed from the body.

 

Earthworms exchange oxygen and carbon dioxide directly through their skin. The oxygen diffuses into tiny blood vessels in the skin surface, where it combines with the red pigment hemoglobin. Hemoglobin binds loosely to oxygen and carries it through the animal’s bloodstream. Carbon dioxide is transported back to the skin by the hemoglobin.

 

Terrestrial arthropods have a series of openings called spiracles at the body surface. Spiracles open into tiny air tubes called tracheae, which expand into fine branches that extend into all parts of the arthropod body.

Fishes use outward extensions of their body surface called gills for gas exchange. Gills are flaps of tissue richly supplied with blood vessels. As a fish swims, it draws water into its mouth and across the gills. Oxygen diffuses out of the water into the blood vessels of the gill, while carbon dioxide leaves the blood vessels and enters the water passing by the gills.

 

Terrestrial vertebrates such as amphibians, reptiles, birds, and mammals have well-developed respiratory systems with lungs. Frogs swallow air into their lungs, where oxygen diffuses into the blood to join with hemoglobin in the red blood cells. Amphibians can also exchange gases through their skin. Reptiles have folded lungs to provide increased surface area for gas exchange. Rib muscles assist lung expansion and protect the lungs from injury.

 

Birds have large air spaces called air sacs in their lungs. When a bird inhales, its rib cage spreads apart and a partial vacuum is created in the lungs. Air rushes into the lungs and then into the air sacs, where most of the gas exchange occurs. This system is birds’ adaptation to the rigors of flight and their extensive metabolic demands.

 

The lungs of mammals are divided into millions of