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Human Body Systems

Objective

In this lesson, you will review systems of the human body. Scientists, regardless of their specialization, need to possess a basic level of science literacy across all disciplines. This lesson provides an overview of the major concepts pertaining to human body systems that all students of the sciences should know. The chapters that follow will focus in greater detail on these ideas.

Previously Covered:

  • Genetic variation in living things is introduced via mutation and during sexual reproduction.
  • The diversity of living things can be attributed to changes in the genetic code, acted upon by natural selection, that have occurred over billions of years.

Systems Analyses

Humans are complex organisms, well adapted to almost any environment on earth because of our well developed brains. Human bodies have multiple levels of organization. The brain is an organ composed of nervous tissue and part of a larger organ system, the nervous system. The human body has multiple organ systems, all coordinated to maintain homeostasis and to allow us to acquire energy and reproduce.

Nervous system diagram

The nervous system is responsible for integration and control of all other organ systems. It receives sensory input, processes it, stores it, and coordinates motor output. That control may be voluntary, such as in movement of skeletal muscles, or it may be involuntary (autonomic), such as in digestion. The brain and spinal cord make up the central nervous system, so named because of its location and its function as a processing center. The peripheral nervous system is composed of the sensory and motor neurons of the body.

Endocrine system diagram

The endocrine system is also important in controlling other systems via production of hormones. Hormones are chemical signals that are usually produced by cells of one tissue but function to interact with receptors in a different target tissue. Glands, such as the pituitary or thyroid, secrete hormones, releasing them into the bloodstream. Hormones are active only when they encounter their specific receptors. Hormonal signaling is called a signal transduction pathway. Many physiological functions involve complex hormonal interactions; in fact, hormone activity influences virtually all body tissues.

The cardiovascular system consists of the heart and blood vessels. It is responsible for the transport of oxygen and carbon dioxide, sugars and nutrients, hormones, white blood cells and antibodies, wastes, and other materials around the body. Hemoglobin in erythrocytes (red blood cells) carries oxygen, and blood plasma transports sugars to cells to use in the process of cellular respiration. No cell in the human body is more than a few millimeters from a capillary, the site of exchange of materials between blood and tissues. Fluid lost from blood in tissues is returned to circulation via lymphatic vessels.

Cardiovascular system
Respiratory system
The respiratory system is responsible for getting oxygen into the cardiovascular transport system and removing carbon dioxide from it; the process of gas exchange. Gas exchange occurs both in the alveoli of the lungs and in all body tissues. Exchange of oxygen and carbon dioxide also helps maintain blood pH. Differences in partial pressure of the two gases at exchange sites permits passive exchange.

Urinary system
The urinary or excretory system has several functions important to homeostasis. Kidneys filter the blood and rid the body of nitrogenous wastes such as urea, creatinine, and uric acid. By selectively reabsorbing materials, the urinary system maintains the body’s osmotic (water) balance. The urinary system is also important in maintenance of blood pH. The functional filtering unit of the kidney are called nephrons, and each kidney houses approximately one million nephrons.

The digestive system breaks down food and absorbs needed nutrients from it. The digestive tract serves as a passage for food into our bodies, a site for digestion, and a passage for expulsion of undigested wastes. Accessory organs to digestion, such as the liver, gall bladder, and pancreas, produce or store digestive enzymes and fluids that help process materials during digestion. Most digestion and absorption occurs in the small intestine. The liver processes all absorbed materials and stores energy as glycogen.

Digestive system
The immune system protects us against pathogens and foreign substances entering the body. Our skin and mucous membranes provide the first line of defense against disease. Secondary defenses, such as non-specific immune functions like the inflammatory response, allow our bodies to fight many pathogens with phagocytic white blood cells and special proteins. Specific immune responses, such as production of special proteins called antibodies, give us long-term immunity to disease. Immune system cells such as T lymphocytes and B lymphocytes can recognize foreign antigens and destroy cells infected with them or produce specific antibodies to neutralize them. Once the body’s immune system learns a foreign antigen, it remembers how to produce the correct antibody quickly. This is the basis of immunization. Lymphoid tissues such as tonsils, the thymus, and the spleen are sites of a lot of immune system activity.

The muscular and skeletal systems work together to allow complex movement. The skeleton is a scaffold that supports the body and protects vital organs. It is also a storage area for calcium and phosphorous. All red and white blood cells are produced in bone marrow. Muscles contract to keep our heart beating, to aid in digestion, and to allow us to move. Muscle activity also generates heat to help maintain body temperature. The skeleton provides an anchoring point for muscles, and when they contract, they pull on bones, allowing bending at joints. Muscles contract when actin and myosin filaments interact.

Muscular and skeletal systems
The integumentary system, the skin, hair, nails and associated glands, offers the first line of defense against infections and protects all of our vital organs. It helps regulate body temperature and disposes of wastes by producing sweat. The skin helps us maintain our water balance by secreting oils to prevent dehydration. Some materials are absorbed through the skin and some vitamins are produced there. Many sensory receptors are housed in the integumentary system.

Integumentary system
The reproductive systems of human males and females are designed to produce gametes and allow production of offspring. Production of gametes in the gonads of both sexes allows genetic information of the parents to be passed along. Fertilization of the egg and development of the fetus occurs in the body of the female. Hormones play an important role in all stages of reproduction, from gamete production to the production of breast milk.

Male and female reproductive systems
All of the organ systems of a human must be coordinated, integrated, and functional to maintain a living organism capable of producing offspring. Human organ systems coordinate flows of matter and energy, just as cellular organelles do. In ecosystems, those flows are regulated by natural laws and processes, and sometimes by humans!

Question

Which system produces erythrocytes and phagocytes?

  1. Cardiovascular system
  2. Urinary system
  3. Immune system
  4. Endocrine system

Reveal Answer

C is the correct answer. Erythrocytes and phagocytes are cells produced by the immune system. The cardiovascular, urinary, and endocrine systems do not produce these cells.

Summary

  • Complex living things like humans have multiple levels of organization, from cells to organ systems.
  • Organ systems function together, allowing the human body to acquire energy, maintain homeostasis, and reproduce.
  • Organ systems are integrated and often have overlapping structures and functions.

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