To continue our discussion of the lymphatic system, we learned how lymph is formed and how it flows through out our body.  Most of the components of blood plasma filter freely through the capillary walls to form interstitial fluid.  However, more fluid filters out of the blood capillaries than returns to them by the process of reabsorption.  The excess filtered fluid drains into the lymphatic vessels and becomes lymph.  The interstitial fluid only contains a small amount of protein because most plasma proteins are too large to leave blood vessels.  However, the protein that do leave blood plasma cannot return to the blood by diffusion because of the concentration gradient opposing such movement.  The high level of proteins inside the blood capillaries and the low levels outside do not allow this.  The proteins are able to, however, move readily through the more permeable lymphatic capillaries into the lymph.  So because of all this, an important function of the lymphatic system is to return lost plasma protein back to the bloodstream.

The lymphatic vessels are similar to veins, as the contain valves which ensure the one-way movement of the lymph.  They do however, have much thinner walls than veins.  In the lymphatic system, the lymph drains into venous blood through the right duct and the thoracic duct at the junction of the internal jugular and the subclavian veins. 

The actual sequence of fluid flow is goes something like this:  blood capillaries (blood) to interstitial spaces (interstitial fluid) to the lymphatic capillaries (lymph) which lead to the lymphatic vessels, to the lymph nodes (lymph) back to lymphatic vessels (lymph), then to lymphatic ducts (lymph), to the junction of the internal jugular and subclavian veins (blood).  As mentioned before, the lymphatic system and the cardiovascular system work closely together.  

There is also the matter of how the lymph is moved along this system.  As a lymphatic vessel distends, the smooth muscle in its wall contracts, and this helps in moving the lymph from one segment of the vessel to the next segment.  There is also the same two pumps that aid the return of venous blood to the heart that maintains the flow of lymph, and these are the skeletal muscle pump and the respiratory pump.  So the one way flow of lymph is caused by skeletal muscle contractions and respiratory movements.  

The skeletal muscle pump works in the milking action of the skeletal muscle contractions compresses the lymphatic vessels and veins.  this forces lymph toward the junction of the internal jugular and the subclavian.  The Respiratory pump works by having the lymph flow maintained by pressure changes that occur during respiration.  When you breath in (inspire) lymph flows from the abdominal region where the pressure is higher, toward the thoracic region, where the pressure is lower.  When you breath out, however, the valves help to prevent back flow of lymph keeping it a one way pump.  

So its pretty cool to see how the two systems work so closely together to maintain a balance in fluids in our bodies.  Although extremely different from veins, lymphatic vessels carry out fluid to the different regions of the body like the cardiovascular system carries blood to the different regions of the body.  This way we are able to ward off infections that happen in our extremities or abdomen.  

Lymphatic System

So last time we talked about how the body is able to fight off different bacteria and viruses.  This is all part of an immune defense that our body has called the lymphatic system.  The lymphatic system itself is made up of a fluid called lymph.  The lymphatic system is made up of many lymphatic vessels that transport this lymph, a number of organs and structures containing lymphatic tissue, and red bone marrow.  The red bone marrow is where the stem cells develop into the various types of blood cells, including the lymphocytes.  It is the lymphatic systems job to assist in circulating body fluids that helps to defend the body against many disease causing agents.  The majority of the blood plasma filters out of blood capillary walls to form interstitial fluid, and this is the fluid that surrounds the cells of body tissues.  After this fluid passes into the lymphatic tissues, however, it is then called lymph and is a clear fluid.  The only major difference between the interstitial fluid and lymph is its location.  Interstitial fluid is found between the cells, and lymph is found within lymphatic vessels and lymphatic tissue.  

We learned that lymphatic tissue is a specialized form of reticular connective tissue that contains large numbers of lymphocytes.  We also learned previously that lymphocytes are agranular white blood cells.  The two types of lymphocytes that participate in the immune system are the B cells and the T cells. 

 There are three main functions that the lymphatic system plays in the body.  The first function is that it drains excess interstitial fluid.  The lymphatic vessels drain the excess fluid from the tissue spaces and return it to the blood.  The second main function of the lymphatic system is that it transports dietary lipids.  Lymphatic vessels transport lipids and lipid-soluble vitamins that are absorbed by the GI tract back to the blood.  These vitamins would include A, D, E, and K.  The third primary function of the lymphatic system is that it carries out our immune responses.  Lymphatic tissue initiates highly specific responses directed against particular microbes or abnormal cells.      

The Lymphatic System and Immunity

Our bodies need to have a defense against all the harmful things that we subject them to.  We need a defense against just the sun alone and all the harmful rays that beat down on us.  In order to maintain a physically healthy body, our bodies require a continuous combat of the harmful agents in our internal and external environments.  

We are constantly being exposed to a variety of pathogens, such as disease-producing microbes that are bacteria and viruses.  Even though our body encounters these everyday, for the most part our bodies remain healthy.  The surface of our body also endures a lot such as cuts and bumps, the exposure to ultraviolet rays, chemical toxins, and minor burns.  Somehow, our bodies seem to be able to fight of these disasters with an array of defenses.  Our body has what is called resistance, and this is the ability to ward off damages or disease through our defenses.  When our bodies don't have resistance, it called susceptibility.  

Our bodies have two general types of resistance, and this is a nonspecific resistance or innate defenses and specific resistance or immunity.  Nonspecific resistance (innate defenses) are present from birth and includes defense mechanisms that provide immediate but general protection against invasion by a wide range of pathogens.  There are mechanical and chemical barriers that help protect our bodies.  These include the barriers of the skin and mucous membranes as the first line of defense in nonspecific resistance.  Another example of this is the acidity if gastric juice kills many bacteria that are present in the food we eat.  

The second line of defense in nonspecific resistance consists of antimicrobial proteins.  these are interferons, complement, and transferrins.  Some other defenses of the nonspecific resistance includes phagocytes, natural killer cells, inflammation, and fever.  For the phagocytes it is mostly the macrophages and neutrophils that are activated.  

Specific resistance, which is immunity, develops in response to to contact with a particular invader.  This means that it only attacks a specific bacteria or virus that is causing harm to our bodies.  It thus occurs more slowly than nonspecific resistance mechanisms and involves activation of specific lymphocytes that are able to fight against that specific invader.  The organ system that is responsible for specific resistance, as well as some aspects of the nonspecific resistance, is called the lymphatic system.  In a sense it works closely with the cardiovascular system, and it also functions with the digestive system in the absorption of fatty foods.  

Even with all these defense systems in place in our bodies, there is still a chance that our bodies are going to come to harm.  It might be able to protect us against some of the harmful pathogens that we encounter, but others no matter how hard our bodies will try and fight it, will eventually get the best of us.  There is also the possibility that our immune system itself will become infected and not function properly.  There are diseases like cancer that will spread to the lymphatic as well, and then it makes it even more difficult for our bodies to fight of any other diseases or infections.    

Process of Sensation

As discussed in the last post, sensations begin at the sensory receptors in our bodies.  These receptors can either be a specialized cell or the dendrites of a sensory neuron.  An example of a specialized cell would be one of the special senses.  The dendrites associated with a sensory neuron would be more common with the general senses.  There are four events that typically occur when a sensation arises, and these begin with the stimulation of the sensory receptor.  Then the transduction of the stimulus occurs, there is a generation of impulses, and then there is the integration of sensory input. 

In order for the stimulation of a sensory receptor to occur, it has to happen within the sensory receptor's receptive field.  The body region where the stimulation activates a receptor and can elicit a response.  So this would include something like a pin touching the tip of your finger.  This is in an area of the body that can perceive the pin touching the finger, and thus the sensory receptors in that area can react to that stimulus.  

The transduction of the stimulus happens when a sensory receptor transduces energy from the stimulus into a graded potential.  Graded potentials vary in amplitude depending on the strength of of the stimulus that evokes them, and they are not propagated.  An example of this would be odorant molecules in the air stimulate olfactory receptors in the nose.  These then transduce the molecule's chemical energy into electrical energy in the form of a graded potential.

After transduction happens, there is a generation of impulses.  When a graded potential in a sensory neuron reaches its threshold, it then triggers one or more action potentials.  Impulses are then propagated toward the central nervous system. The sensory neurons that conduct impulses from the central nervous system to the peripheral nervous system are called first order neurons.

There is a particular region of the central nervous system that receives and integrates the sensory nerve impulses.   This is where the conscious sensations or perceptions are integrated in the cerebral cortex .  How this works is that you see with your eyes, you hear with your ears, and feel pain in the injured part of the body.  This happens because sensory impulses from each part of the body arrive in a specific region of the cerebral cortex, which then interprets the sensation as coming from the stimulated sensory receptors.  

General and Special Senses

The process of sensation is actually complex, yet we never really stop to think about what is actually happening in our bodies that make us realize we are smelling something good to eat or seeing the sun set.  The way our bodies perceive a smell, a touch, sight, hearing something, and even maintain equilibrium or balance.  Each of these sensations are called a sensory modality.  So what happens in our body as we perceive these sensations is starts when a given neuron carries information for only one sensory modality and its sensory receptor and responds vigorously to only one particular kind of stimulus.  This is a change in the environment that activates sensory receptors.  Some of these receptors are said to have selectivity, meaning that a given receptor may not respond at all or respond weakly.  The receptor selects one modality over all the others.  So what this all means is that nerve impulses from the eyes are perceived as sight, and those from the ears are perceived as sound.  The different types of sensory modalities can be grouped into two classes.  The two classes are general senses and special senses.  

The general senses include the somatic senses and the visceral senses.  The somatic sensory modalities include the tactile sensations, thermal sensations, pain sensations, and proprioceptive sensations.  The tactile sensations include touch, pressure, and vibration.  The thermal sensations include warm and cold.  The Proprioceptive sensations allow perception of both the static (nonmoving) positions of the limbs and body parts, and the movements of the limbs and head.  The visceral sensations provide information about the conditions within internal organs and are mediated by the autonomic nervous system.  The special senses include the sensory modalities of smell, taste, vision, hearing, and equilibrium or balance.

What we learned about how our body perceives physical stimuli is truly amazing.  Not everyones body reacts to the stimuli properly, for example, not everyones sight works properly.  Some people are near sighted, others far sighted.  They also may not be able to see color or only see a few certain colors.  Some people have accidents and some of their nerve receptors were severed and they can no longer feel anything in that area of their body.  It can be easily concluded that life just wouldn't be as fun if we weren't able to process the environment around us, or if our body failed us in certain areas of our sensory capabilities.    

     

   

Anatomy Week Ten

Continuing with the endocrine system and hormones, we learned how they are transported in the blood.  We also learned the mechanism of hormone action.  

The majority of the water-soluble hormone molecules circulate in the watery blood plasma and are in a sort of free state not bound to any other molecule or protein.  The lipid-soluble hormones in contrast have to travel in the bloodstream bound to what are called transport proteins.  These transport proteins are synthesized in the liver and have three main functions.  One function is that they provide a ready reserve of hormone in the bloodstream.  A second function of transport proteins is that they make the lipid- soluble hormones temporarily water-soluble increasing their solubility in the blood.  The last function of the transport protein is that they retard passage of small hormone molecules through the filtering mechanism in the kidneys, and they in turn slow the rate of the hormone that is lost in the urine.  If these hormones are not bound to transport hormones they are in a free state and thus diffuse out of capillaries and bind to receptors.  This triggers the hormonal responses ordered by the body.  What happens as these free hormones are released from the blood and are bound to the receptor cells, the transport hormones release new ones to replenish the free state.  

Lipid soluble hormones and water soluble hormones have different mechanisms of action.  The response to a hormone depends on not only the hormone but also the target cell.  Different target cells respond differently to the same hormone.  The synthesis of new molecules is not always the response to a hormone.  Changing the permeability of the plasma membrane, stimulating transport of a substance into or out of the target cells, altering the rate of specific metabolic reactions, or causing contraction of smooth muscle and cardiac muscle are all hormonal effects.  Because there are so many varied effects of hormones, it is possible a single hormone can set in motion several different cellular responses to that hormone.  

The first mechanism of a hormone is to announce its arrival to a target cell.  The hormone does this by binding to the receptors of this target cell.  The difference in the lipid soluble hormones and those of the water soluble hormones are that the receptors for the lipid soluble hormones are located inside the target cells and the receptors for the water soluble hormones are located in the plasma membrane of the target cells.  How these mechanisms of hormone action actually work are complex.

Anatomy Week Nine

Furthering what we learned about the endocrine systems and hormones, we learned that there are chemical classes of these hormones.  The hormones can be further divided into two main classes which are those that are soluble in lipids, and those which are soluble in water.  These two categories of hormones exert their effects on their target cells differently.  

Lipid soluble hormones include steroid hormones, thyroid hormones, and nitric oxide.  The steroid hormones are derived from cholesterol, and the two thyroid hormones are synthesized by attaching iodine to an amino acid called tyrosine.  Nitric oxide, which is a gas, is both a hormone and a neurotransmitter.  

The majority of the water soluble hormones are amine, peptide, and protein hormones.  Amine hormones are synthesized by modifying certain amino acids, and they are called this because they retain an amino group.  Peptide hormones consist of amino acids in chains of three to forty-nine.  The protein hormones are much larger and include fifty to two hundred amino acids.  

So we learned that there are several different types of hormones that all work together to maintain homeostasis in the body.  They are made from different sources, however, and thus are synthesized and processed differently from one another.  Each hormone has a different action on the body and has different effector cells that it activates.  Some people have deficiencies in hormone levels, and because of this different functions in the body are effected depending on the hormone deficiency.  An example of this would be hypothyroidism, which is an under-functioning of the hormones produced by the thyroid gland.