Ensayos de híbrido simple en levadura

In addition to controlling the amount of water that is reabsorbed from the filtrate, which has an effect on blood volume and blood pressure, the kidneys release an enzyme, renin, into the blood.

Renin sets off a series of reactions in the blood that results in the production of another enzyme, angiotensin II. Angiotensin II constricts blood vessels, causing a rise in blood pressure. It also causes the adrenal cortex to release more aldosterone, which raises blood volume and blood pressure.

Support and Locomotion

One of the biggest differences between animals and plants is also one of the simplest: animals move, plants don’t. While the simplest animals are propelled by cilia on their cell surfaces, most animals are too large for such tiny structures to have a significant effect on their locomotion. Many cnidarians have primitive contractile fibers that allow them to propel themselves through water. A number of invertebrates, such as earthworms, possess what is known as a hydrostatic skeleton, in which muscles are arranged longitudinally down the length of the body and in circular rings around the body. When either of these types of muscle contract, an incompressible fluid maintains the body at a constant volume but allows the worm to change shape. Contraction of the circular muscles lengthens the body, while longitudinal muscle contraction shortens the body. Earthworms are segmented and can control the muscles within each segment independently. By contracting the muscles in waves along its body, the earthworm can propel itself through the soil. Tiny hairs called setae on the worm’s surface provide traction against the soil.

Arthropod muscles connect to a rigid exoskeleton that encloses the body and is made of chitin. When arthropod muscles contract, they pull on inward extensions of the exoskeleton, causing it to move. Range of motion is provided by joints connecting different sections of the exoskeleton. While the exoskeleton works well for animals as small as insects, it would be too heavy and impractical for larger animals.

The Vertebrate Skeletal System

In direct contrast to arthropods, which live inside an exoskeleton, vertebrates have evolved a hard internal skeleton, or endoskeleton. The skeleton is made of two tissues: bone and cartilage. Bones are rigid structures composed of living cells rooted in a matrix of calcium, phosphate salts, and collagen fibers. Blood vessels and nerves pass through a central canal in the bone; blood makes its way to embedded cells through tiny pores. Bones form the majority of the endoskeleton in higher vertebrates, including humans, and provide structural support to all the other tissues in the body. In addition, bones:

• Protect the soft, delicate organs and structures within the body. The skull and rib cage are examples of hard bone protecting the vital organs in the head and chest.

• Store minerals such as calcium. When the calcium supply in the blood is high, it is stored in bones. When the supply is low, bones give off calcium.

• Have marrow, found in cavities at the centers of bones, that produces blood cells. Bones meet each other at joints that are held together by ligaments and are often bathed in a lubricating and cushioning fluid called synovial fluid. Joints allow bones to meet and bind together without actually grinding together. In this way, joints allow for smooth skeletal movement.

Cartilage is firm but somewhat flexible. It will bend under strain and spring back to its original shape when the force is removed. The skeletons of sharks and rays are composed entirely of cartilage, as are the skeletons of developing embryos. In higher vertebrates, cartilage is retained in portions of the skeleton that need to remain flexible, such as in the rib cage, which needs to expand during inhalation, the tip of the nose and ears, at the end of bones, and in joints. Cartilage contains no blood vessels or nerves, and it takes a very long time to heal when damaged.

The Muscular System

Joints allow a skeleton to move. Muscles actually make it move. Bones interface with muscles by way of tendons. Movement is achieved when muscles contract, pulling on the bones to which

they are attached, bending the joints. An extensor muscle straightens the bones in a joint. An example is the triceps muscle in your upper arm, which straightens out the elbow joint. A flexor muscle bends a joint. The bicep muscle, which bends your elbow, is a flexor. (Note that both extensors and flexors perform their functions by contracting; when an extensor contracts, it straightens a joint, and when a flexor contracts, it bends a joint.) Muscles also help the skeleton support and protect the body. Vertebrates have three classes of muscles:

• Skeletal muscles, also called striated muscles, are associated with the skeletal system and are primarily involved in voluntary movement. A vertebrate generally has conscious control over its skeletal muscles. Each skeletal muscle cell contains many nuclei.

• Smooth muscle is found in the walls of the internal organs such as the stomach, intestines, and urinary bladder and is an involuntary muscle and not under voluntary control.

• Cardiac muscle makes up the heart. Cardiac muscles are involuntary and can contract without stimulation from the nervous system.

Muscles can be thought of as the enactors of the nervous system. Through voluntary impulse or involuntary instinct, nerves send messages to muscles. Muscles turn these messages into movement and action by contracting or relaxing.