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Unit 7 - Body Systems & Development

Blue Flowers

Physiology & animal development

Homeostasis and body structure overview


Key terms

Homeostasis: The body’s maintenance of a stable, constant internal environment

Negative feedback loop: It acts to oppose the triggering stimulus. EX: Glucagon and insulin are two counteracting hormones that help maintain blood sugar at steady levels

Positive feedback loop: It amplifies the initial signal/stimulus. EX: the process of childbirth, involving oxytocin and the uterus walls

Tissue: It is made of a group of similar cells that work together on a specific task

Organ: Made up of two or more tissues, organized to carry out a specific function


  • The body maintains homeostasis for many factors, including body temperature, blood glucose, and various pH levels

  • Homeostasis is maintained at many levels. For instance, the stomach maintains a pH that's different from that of surrounding organs, and each individual cell maintains ion concentrations different from those of the surrounding fluid


Feedback loops

  • Homeostasis typically involves negative feedback loops that counteract changes of various properties from their target values.

  • Body temperature regulation is an example of a negative feedback loop

  • Example of positive feedback loop: fruits produce ethylene, a ripening hormone; it induces ripening in other fruits that also then release ethylene


Body structure

  • The body has levels of organization that build on each other; cells make up tissues, tissues make up organs, and organs make up organ systems

  • EX: Muscle cell, muscle tissue, bladder (organ), excretory system (bladder, urethra, kidneys, ureter)

Review of the organ systems

Circulatory system

  • The circulatory system consists of the heart and the blood vessels. The circulatory system is vital to the survival of the organism as it transports nutrients and removes waste throughout the body

  • There are three main types of blood vessels:

  • arteries, carry blood from the heart

  • capillaries connect arteries to veins

  • veins, return the blood back to the heart

  • In humans and other animals with two circuits, oxygenated blood does not mix with deoxygenated blood. The blood is pumped through the right side of the heart, where it is sent to the pulmonary artery

  • At the pulmonary artery, the blood is oxygenated and then pumped through to the left side of the heart and then on to the rest of the body through the main artery, the aorta

  • After the blood has distributed oxygen to the rest of the body, it will be sent back to the heart through the venous system

Excretory system

  • This organ system removes the nitrogenous wastes produced by NH3. Humans excrete a form of nitrogen called urea (a major component of urine)

  • The liver detoxifies the ammonia by creating urea

  • Then the liver will then send the urea to the kidneys, which creates urine

Nervous system

  • The function of the nervous system is to create coordination between the cells of the body. This organ system is divided into the peripheral nervous system and the central nervous system

  • Central nervous system = made up of solely the brain and the spinal cord

  • Peripheral nervous system = made up of all of the nerves that are found throughout the body

  • The brain is made up of neurons that transfer a signal from the axon to the dendrites. The dendrites release the signal, and the signal is taken up by a neighboring neuron or muscle cell

  • Neurotransmitters are small chemicals that transmit the signals from one neuron to the next. Important neurotransmitters include: GABA, dopamine, histamine

Endocrine system

  • The endocrine system is closely connected to the nervous system and they work together

  • The endocrine system is made up of glands that make the hormone

  • Hormones are the body's chemical messengers. They transmit information and instructions between cells

  • Hormones control moods, growth and development, metabolism, organs, and reproduction; they often have longer-lasting, more gradual effects

  • The system can send hormones into the bloodstream (“endocrine” means within the blood) so they can travel to other body parts

The following are integral parts of the endocrine system:

  • Hypothalamus

  • Pineal body

  • Pituitary

  • Thyroid and parathyroid

  • Thymus

  • Adrenal gland

  • Pancreas

  • Ovary

Fruit market

Immune System

Key terms:

Pathogen: Any bacteria, virus, one cell organism that can cause disease in humans

Antigen: Any molecule that stimulates an immune response

Innate immune system: Non-specific immune system

Adaptive immune system: Antigen-specific immune system

Antibody: Specialized Y-shaped protein that tags antigens for destruction

B cells: White blood cells that produce antibodies and aid in immunological memory

T cells: White blood cells specialized in assisting B cells (helper T) and others directly kills infected cells (killer T)

Humoral immunity: Adaptive immune defense depending on the action of antibodies

Cell-mediated immunity: Adaptive immune defense in which foreign cells are destroyed by T cells

Virus: Nonliving particle-containing protein and DNA/RNA that can infect a living cell

Vaccine: A killed or weakened form of a pathogen that produces immunity when injected into the body

What is an infectious disease?

  • Infectious diseases are caused by viruses, bacteria, fungi, protists, and other pathogens

The human defense mechanism

  1. Non-specific defense: the innate immune system

  • The human body has a series of non-specific defenses that make up the innate immune system. These defenses provide a general barrier against pathogens

  • The first line of defense: the body's most important non-specific defense is the skin, which acts as a physical barrier to keep pathogens out. Even openings in the skin (such as the mouth and eyes) are protected by saliva, mucus, and tears, which contain an enzyme that breaks down bacterial cell walls.

  • The second line of defense: If a pathogen does make it into the body, there are secondary non-specific defenses that take place.

  • An inflammatory response begins when a pathogen stimulates an increase in blood flow to the infected area. Blood vessels in that area expand, and white blood cells leak from the vessels to invade the infected tissue. These white blood cells, called phagocytes, engulf and destroy bacteria. The area often becomes red, swollen, and painful during an inflammatory response.

  • When a pathogen has invaded, the immune system may also release chemicals that increase body temperature, producing a fever. Increased body temperature may slow or stop pathogens from growing and helps speed up the immune response.


  1. Specific defense: the adaptive immune system

  • When pathogens are able to bypass innate immune defenses, the adaptive immune system is activated

  • Cells that belong in the body carry specific markers that identify them as "self" so that our immune system won’t attack them

  • After the immune system identifies the pathogen that it differs from itself it applies both a cellular and chemical defense to destroy it. After an encounter with a new pathogen, the adaptive immune system often "remembers" the pathogen, allowing for a faster response if the pathogen ever attacks again

Digestive system


The human digestive system breaks food down into small molecules that can be used by cells in the body.

The main parts of the digestive system from top to bottom are:

  • oral cavity

  • esophagus

  • stomach

  • liver

  • gallbladder

  • pancreas

  • small intestines

  • cecum

  • large intestine

  • anus

Key terms:

Digestive system: The body system that converts food into energy and nutrients to fuel the body. It performs 2 types of digestion:

  • Mechanical digestion: The breaking down of food by physical means, such as chewing

  • Chemical digestion: The breaking down of food using chemical agents, such as enzymes and bile

Absorption: The process by which nutrients pass through the walls of the digestive system into the blood

Excretory system: The body system that removes metabolic wastes from the body. Involves the kidneys, which help with blood filtration and production of urine.


Steps of digestion:

  • The first step of digestion starts in the oral cavity, mouth. Here the teeth grind the food into smaller particles which is mechanical digestion. The enzyme called amylase in saliva begins to break down carbohydrates into sugar, which is chemical digestion. That is the reason why chewing a piece of bread will taste sweet after a couple of minutes. The tongue plays an important role, as it creates a bite easy enough to swallow.

  • After swallowing, the piece of food is called a bolus. and it moves down the esophagus. This piece of the digestion system connects the mouth with the stomach,. No digestion happens in the esophagus.

  • At the end of the esophagus, the bolus enters the stomach. Mechanical and chemical digestion also occurs here. The stomach muscles form walls and stir the food around, while digestive enzymes such as pepsin also break it down. This process converts the bolus into a liquid called chyme. The environment of the stomach is heavily acidic to help digestion and the activation of enzymes

  • From the stomach, the chyme is slowly moved into the small intestine, where most of the chemical digestion happens. The liver produces bile, which is stored in the gallbladder and helps digest fats when it is needed. Moreover, enzymes of the pancreas and intestinal walls will start the final part of digestion.

  • Most nutrient absorption occurs in the small intestine. These are absorbed through the walls of the small intestine, ending up in the circulatory system. After the chyme exits the small intestine, water, and indigestible substances abandoned

  • As the last part of digestion, the chyme enters the large intestine. Water is extracted, and the gut bacteria break down part of the materials. Gut flora plays a very important role in human health; they produce vitamins such as vitamin K, and specific hormones as well. The waste material now is very concentrated which is called feces. This is transported into the rectum and leaves the body through the anus.

Animal development

Stages in early animal development

There are four general stages in early animal development:

  • Fertilization: the process of a single sperm cell combining with a single egg cell to form a zygote (diploid)

  • Cleavage: rapid, multiple rounds of mitosis where the overall size of the embryo does not increase. The developing embryo is called a blastula

  • Gastrulation: the reorganization of cells in the blastula to produce the embryonic, multilayered tissues. These will continue to produce the tissues and organs of a developed animal

  • Organogenesis: the process of organ and tissue formation via cell division and differentiation

  • Gastrulation and organogenesis together cooperate with morphogenesis. These are biological processes that create the organism's shape and body structure

  • At the end of cleavage, the typical blastula is a ball of cells with a hollow cavity in the middle (the blastocoel). The next stage of embryonic development is gastrulation. In this phase, the blastula rearranges into three layers of cells. The embryo in this stage is called a gastrula

  • The formation of the embryonic tissues is called germ layers.

  • The germ layers include:

  • endoderm

  • ectoderm

  • mesoderm

Each germ layer will later differentiate into different tissues and organ systems

In addition to knowing the concepts and terminology, it's also crucial to apply them by doing plenty of practice problems.  If you're struggling to find good problems to work through, check out our popular Practice Portal!  

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