• Bladder: A hollow organ that stores urine.
• Bowman’s capsule: A cup-shaped structure around the glomerulus that collects the filtered substances; part of the nephron.
• Dialysis: A medical procedure in which blood is filtered with the help of a machine.
• Excretion:The process of removing wastes and excess water from the body.
• Filtration: The process of filtering substances from blood in the glomerulus.
• Glomerulus: Part of the nephron; a cluster of arteries that filters substances out of the blood.
• Homeostasis: The ability to maintain a stable internal environment despite external changes.
• Kidney: Organ that filters the blood and forms urine.
• Kidney stones: Crystals of dissolved minerals that form in urine inside the kidneys.
• Nephrons: The structural and functional units of the kidneys; includes the glomerulus, Bowman’s capsule, and renal tubule.
• Reabsorption: The return of needed substances in the filtrate back to the bloodstream.
• Renal tubule: A long, narrow tube surrounded by capillaries that reabsorbs many of the filtered substances and secretes other substances; part of the nephron.
• Urea: The main waste nitrogen in the urine of mammals. It is produced in the metabolism of proteins.
• Ureter: Tube-shaped structure that brings urine from the kidneys to the urinary bladder.
• Urethra: A muscular tube that carries urine out of the body.
• Urinary system: The organ system that makes, stores, and gets rid of urine.
• Urine: The liquid waste product of the body that is excreted by the urinary system.

Formation of Urine

In the last post about the nephron function we studied how urine is produced, but in this new post we are going to explain very carefully all the process. I hope you'll be able to understand it.
The process of urine formation is as follows:
1. Blood flows into the kidney through the renal artery. The renal artery branches into capillaries inside the kidney. Capillaries and the nephrons lie very close to each other in the kidney.
2. In the nephron, the blood pressure within the capillaries causes water and solutes (small soluble molecules) such as salts, sugars, and urea to leave the capillaries and move into the Bowman’s capsule. (GLOMERULAR FILTRATION).
3. The water and solutes move along through the tubules of nephrons. At this point most of the water and solutes are returned to the capillaries that surround the nephron (TUBULAR REABSORPTION). Some chemicals are secreted in the las part of the tubules (TUBULAR SECRETION).
4. The fluid that remains in the nephron at this point is called urine.
5. The blood that leaves the kidney in the renal vein has much less waste than the blood that entered the kidney.
6. The urine is collected in the ureters and is moved to the urinary bladder where it is stored.
Nephrons filter 125 ml (about ¼ cup) of body fluid per minute. In a 24-hour period nephrons produce about 180 liters of filtrate, of which 178.5 liters are reabsorbed. The remaining 1.5 liters of fluid forms urine.
Urine enters the bladder through the ureters. Similar to a balloon, the walls of the bladder are stretchy. The stretchy walls allow the bladder to hold a large amount of urine. The bladder can hold about 400 to 620 ml of urine, but may also hold more if the urine cannot be released immediately. Urination is the process of releasing urine from the body. Urine leaves the body through the urethra.
Nerves in the bladder tell you when it is time to urinate. As the bladder first fills with urine, you may notice a feeling that you need to urinate. The urge to urinate becomes stronger as the bladder continues to fill up.
In this table it is possible to compare plasma and urine composition.

	plasma g/100ml	urine g/100ml	concentration in urine
water	90-93	95
protein	7 - 8.5	0
urea	0.03	2	x60
uric acid	0.002	0.03	x15
glucose	0.1	0

Nephrons

A single kidney may have more than a million nephrons. Nephrons are the structural and functional units of the kidneys. The diagram represents an individual nephron and shows its main structures and functions. The structures include the glomerulus, Bowman’s capsule, and renal tubule.
Nephron structures.

• The renal corpuscle that consists of two structures:

• The glomerulus is a cluster of capillaries that filters substances out of the blood.
• Bowman’s capsule is a cup-shaped structure around the glomerulus that collects the filtered substances.

• The renal tubule is a long, narrow tube surrounded by capillaries that reabsorbs many of the filtered substances and secretes other substances. It is divided into three parts: the proximal tubule, the Loop of Henle, and the distal tubule.

Tubules of several nephrons join to form a single collecting tubule.
Nephron function.
The nephron function is to produce urine. In this process we can consider two stages
FILTRATION
Filtration is the process of filtering substances from blood in the glomerulus. The renal arteries, which carry blood into the kidneys, branch into the capillaries of the glomerulus of each nephron. The pressure of blood moving through these capillaries forces some of the water and dissolved substances in the blood through the capillary walls and into Bowman’s capsule.
The fluid that collects in Bowman’s space is called filtrate. It is composed of water, salts, glucose, amino acids, and urea. Larger structures in the blood—including protein molecules and blood cells—do not pass into Bowman’s space. Instead, they return to the main circulation.
REABSORPTION AND SECRETION.
From the space inside the Bowman’s capsule the filtrate passes into the renal tubule. The main function of the renal tubule is reabsorption. Reabsorption is the return of needed substances in the filtrate back to the bloodstream.
At the beginning, in the proximal tubule, salts, glucose and amino acids are picked up from the filtrate. In the rest of the tubule the main reabsorbed substance is water. Before the tubule arrives to the collecting tubule, some substances can be secreted in the distal tubule.
The collecting tubule reabsorbs water from tubular fluid and return it to the blood. The remaining fluid, called urine, has a smaller volume and a greater concentration than tubular fluid. From the collecting ducts, urine enters a ureter and is eventually excreted from the body.

The Urinary system

The urinary system is the organ system that makes, stores, and gets rid of urine. It includes two kidneys, two ureters, the bladder, and the urethra. The main function of the urinary system is to filter waste products and excess water from the blood and remove them from the body. Urine is the liquid waste product of the body that is excreted by the urinary system. Recall that in the excretion process, other organs as skin and lungs take part in it.Organs of the urinary system
Kidneys
The kidneys are a pair of bean-shaped, reddish brown organs about the size of a fist. They are located just above the waist at the back of the abdominal cavity, on either side of the spine. Located on top of each kidney is an adrenal gland.
The kidney has three layers, the outer layer is the renal cortex, and the middle layer is the renal medulla. The inner layer, the renal pelvis is urine is collected and is funnelled into the ureter
From the aorta, the renal arteries carry blood to the kidneys to be filtered, then the renal veins carry the filtered blood away from the kidneys to the inferior vena cava.
Ureters
From the kidneys, urine enters the ureters, which carry it to the bladder. Each ureter is a muscular tube about 25 centimeters long.
Bladder
The bladder is a hollow organ that stores urine. It can stretch to hold up to 500 milliliters. When the bladder is about half full, the stretching of the bladder sends a nerve impulse to the sphincter that controls the opening to the urethra. In response to the impulse, the sphincter relaxes and lets urine flow into the urethra.
The urethra is a muscular tube that carries urine out of the body. Urine leaves the body through another sphincter in the process of urination. This sphincter and the process of urination are normally under conscious control.
As in other organ systems here you have a video for kids but it is very clear.

The lymphatic system

The lymphatic system consists of a fluid (lymph), vessels that transport the lymph, and organs that contain lymphoid tissue. It is often called the secondary circulatory system. The lymphatic system has three primary functions:
• The removal of excess intresticial fluids from body tissues.
• The absorption of fats from the small intestine and transport of those fats to the cardiovascular system.
• The third and probably most well known function of the lymphatic system is defense against invading microorganisms and disease. Lymph nodes and other lymphatic organs filter the lymph to remove microorganisms and other foreign particles.
Lymph
Lymph originates as blood plasma that leaks from the capillaries of the cardiovascular system. This blood plasma fills the space between individual
cells of tissue where it becomes part of the interstitial fluid. Most of the interstitial fluid is returned to the capillaries. The excess interstitial fluid is collected by the lymphatic system into lymph capillaries, and is processed by lymph nodes before to being returned to the circulatory system. Once within the lymphatic system the fluid is called lymph, and has almost the same composition as the original interstitial fluid. Returning the fluid to the blood prevents edema and helps to maintain normal blood volume and pressure.
Lymphatic vessels
Lymphatic vessels, unlike blood vessels, only carry fluid away from the tissues. The smallest lymphatic vessels are the lymph capillaries, which begin in the tissue spaces as blind-ended sacs. There isn’t a central pump, lymph movement occurs slowly with low pressure due to the squeezing action of skeletal muscles. Lymph travels through lymp vessels that are similar to capillaries and veins. Lymph moves in one direction only, due to valves in lymph vessels that are similar to the valves found in veins, shown in Figure above. The lymph is transported to progressively larger lymphatic vessels that drain into the circulatory system at the right and left subclavian veins.
Lymphatic organs
Lymphatic organs are characterized by clusters of leukocyte. When the body is exposed to microorganisms and other foreign substances, the leukocytes proliferate within the lymphatic organs and are sent in the blood to the site of the invasion. This is part of the immune response that attempts to destroy the invading agent.
The four types of lymphatic organs :
.Lymph Nodes
Lymph nodes are small bean-shaped structures that are usually less than 2.5 cm in length. They are widely distributed throughout the body along the lymphatic vessels where they filter the lymph before it is returned to the blood.
Tonsils
Tonsils are clusters of lymphatic tissue just under the mucous membranes that line the nose, mouth, and throat (pharynx).
Spleen
The spleen is located in the upper left abdominal cavity, just beneath the diaphragm, and posterior to the stomach. The spleen filters blood in much the way that the lymph nodes filter lymph. The spleen, along with the liver, removes old and damaged erythrocytes from the circulating blood. Like other lymphatic tissue, it produces leukoocytes, especially in response to invading pathogens
Thymus
The thymus is located anterior to the ascending aorta and posterior to the sternum. It is relatively large in infants and children but after puberty it begins to decrease in size so that in older adults it is quite small. Thymus is where the lymphocytes are processed.
In this link you can complete the description of the lymphatic organs.

Vocabulary of circulatory system

Anemia: The condition of not having enough hemoglobin in the blood to carry oxygen to body cells.
Antibodies: Proteins that identify pathogens or other substances as being harmful; flow in blood; can destroy pathogens by attaching to the cell membrane of the pathogen.
Arteries: Blood vessels that carry blood away from the heart.
Atrioventricular (AV) valves: Valves that stop blood from moving from the ventricles back into the atria.
Atrium: One of the two small, thin-walled chambers on the top of the heart that blood first enters.
Blood: A body fluid that is a type of connective tissue; moves oxygen and other compounds throughout the body.
Blood clotting: The complex process by which blood forms solid clots.
Blood pressure: The force exerted by circulating blood on the walls of blood vessels.
Capillaries: The smallest and narrowest blood vessels in the body.
Cardiovascular system: The organ system that is made up of the heart, the blood vessels, and the blood.
Heart attack: Event that occurs when the blood supply to a part of the heart is blocked.
Hemophilia: A group of hereditary diseases that affect the body's ability to control blood clotting.
Hypertension: Also called high blood pressure; a condition in which a person’s blood pressure is always high;
Leukemia: Cancer of the blood or bone marrow; characterized by an abnormal production of blood cells, usually white blood cells.
Lymphatic system: A network of vessels and tissues that carry a clear fluid called lymph; includes lymph nodes, lymph ducts, and lymph vessels.
Plasma: The golden-yellow liquid part of the blood.
Platelets: Fragments of larger cells that are important in blood clotting.
Pulmonary circulation: The part of the cardiovascular system which carries oxygen-poor blood away from the heart to the lungs, and returns oxygen-rich blood back to the heart.
Red blood cells (RBCs) : Flattened disk-shaped cells that carry oxygen, the most common blood cell in the blood. Mature red blood cells do not have a nucleus.
Semilunar (SL) valves: Found in the arteries leaving the heart; prevents blood flowing back from the arteries into the ventricles.
Sickle cell disease: A blood disease that is caused by abnormally-shaped blood protein hemoglobin.
Stroke: A loss of brain function due to a blockage of the blood supply to the brain.
Systemic circulation: The portion of the cardiovascular system which carries oxygen-rich blood away from the heart to the body, and returns oxygen-poor blood back to the heart.
Veins: Blood vessels that carry blood back to the heart.
Ventricle: One of the two muscular V-shaped chambers that pump blood out of the heart.
White blood cells (WBCs): Nucleated blood cells that are usually larger than red blood cells; defend the body against infection by bacteria, viruses, and other pathogens.

Pulmonary and Systemic Circulations

The double circulatory system of blood flow refers to the separate systems of pulmonary circulation and the systemic circulation in amphibians, birds and mammals, including humans. Pulmonary Circulation
The pulmonary circulation is the portion of the cardiovascular system which carries oxygen-poor (deoxygenated) blood away from the heart, to the lungs, and returns oxygenated blood back to the heart. As shown in Figure, deoxygenated blood from the body leaves the right ventricle through the pulmonary arteries, which carry the blood to each lung. The pulmonary arteries are the only arteries that carry deoxygenated blood. In the lungs, red blood cells release carbon dioxide and pick up oxygen during respiration. The oxygenated blood then leaves the lungs through the pulmonary veins, which return it to the left side of the heart, and complete the pulmonary cycle. In the following scheme of the general body circulation, pulmonary circulation is in the right side and the systemic circulation is on the left side.

Systemic Circulation
The systemic circulation is the portion of the cardiovascular system which carries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart. Oxygenated blood from the lungs leaves the left ventricle through the aorta, from where it is distributed to the body's organs and tissues, which absorb the oxygen, through a complex network of arteries and capillaries. The deoxygenated blood is then collected by veins and then into the inferior and superior venae cavae, which return it to the right heart, completing the systemic cycle. The blood is then re-oxygenated through the pulmonary circulation before returning again to the systemic circulation.

Pulmonary Circulation	Systemic Circulation
It is a shorter circulation. The circulation is between heart and lungs. Blood is pumped by right part of the heart and received by the left part. It pumps deoxygenated blood into lungs It brings back oxygenated blood to the heart.	It is a larger circulation. The circulation is between heart and remaining parts of the body except lungs. Blood is pumped by left part of the heart and received by the right part. It pumps oxygenated blood to different parts of the body. It brings back deoxygenated blood to the heart.

Just like every other organ in the body, the heart needs its own blood supply, which it gets through the coronary arteries that branch directly from the aorta, just above the heart. They deliver oxygen-rich blood to the heart

The Heartbeat

1. As we saw in the last post, the heart is a four-chambered organ consisting of right and left halves. Two of the chambers, the left and right atria, are entry-points into the heart, while the other two chambers, the left and right ventricles, are responsible for contractions that send the blood through the circulation.
   The circulation is split into the pulmonary and systemic circulation. The right ventricle's role is to pump deoxygenated blood into the pulmonary circulation through the pulmonary artery. The left ventricle's role is to pump now oxygenated blood into the systemic circulation through the aorta.
   The average human heart, beating at 72 beats per minute, will beat approximately 2.5 billion times during an average 66 year lifespan. Sometime the heart can beat fast, this is called Tachycardia. It happen when you make exercise or are in danger.
   The heartbeat is made up of two parts;.
   Systole is the contraction of the heart chambers, which drives blood out of the chambers.
   Diastole is the period of time when the heart relaxes after contraction.
   In the figura you have the cardiac cycle :

• Atrial diastole. The atria were in diastole and blood from the superior and inferior vena cava (rigth side) and pulmonary veins (left side) flows into the atria slowly to fill them and begin the cycle.
• Atrial systole. This phase involves the contraction of the 2 atria, pushing the blood into the respective ventricles. There is no back flow of blood due to the presence of the atrioventricular (AV) valves ( bicuspid valve – left and tricuspid valve -right) . The bicuspid valve is supported by tendons which look rather like the strings of a parachute.
• Ventricular systole. The thick muscular walls of the ventricles contract.. This begins alongside the end of auricular diastole. The pressure on the blood in the ventricles increases. The atrioventricular valves close rapidly to prevent the backward flow of blood into the auricles.
  As the pressure in the ventricle increases, the semilunar valves are opened and blood enters the arteries. From the right ventricle, the deoxygenated blood enters the pulmonary artery. From the left ventricle, the oxygenated blood enters the aorta, to be taken to all body parts.
• Ventricular diastole.Ventricular systole is followed by ventricular diastole. The atria are already in diastole, so all the chambers of the heart are in diastole. As the pressure in the ventricles decreases to prevent the backward flow of blood, the semilunar valves close rapidly.

The sound of the heart valves shutting causes the heart sounds, or a heartbeat. The closing of the mitral and tricuspid valves (known together as the atrioventricular valves) at the beginning of ventricular systole cause the first part of the "lub-dub" sound made by the heart as it beats. The second part of the "lub-dub" is caused by the closure of the aortic and pulmonic valves at the end of ventricular systole. As the left ventricle empties, its pressure falls below the pressure in the aorta, and the aortic valve closes. Similarly, as the pressure in the right ventricle falls below the pressure in the pulmonary artery, the pulmonic valve closes.
Here you have some videos to get a better knowledge of the heartbeat.

The heart

The heart is usually found in the left to middle of the chest with the largest part of the heart slightly to the left. It is about the size of a fist. The heart is surrounded by the lungs.
It is divided into four chambers, the two upper atria and the two lower ventricles. Atria (singular, atrium) are the thin-walled blood collection chambers of the heart. Atria pump the blood into the ventricles. Ventricles are the heart chambers which collect blood from the atria and pump it out of the heart. The four chambers of the heart are shown in Figure. Each of the four chambers of the heart have a specific job, these are:
• The right atrium receives oxygen-poor (deoxygenated) blood from the body this blood enters from the superior vena cava and the inferior vena cava
• The right ventricle pumps oxygen-poor blood through the pulmonary arteries and toward the lungs. In the lungs, carbon dioxide is released from the blood and oxygen is picked up.
• The left atrium receives oxygen-rich (oxygenated) blood from the lungs through the pulmonary veins.
• The left ventricle pumps oxygen-rich blood out of the heart to the rest of the body through the aorta.
On both sides, the lower ventricles are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is thicker and stronger than the wall surrounding the right ventricle because the left ventricle needs to exert enough force to pump the blood through the body. The right ventricle only needs to pump the blood as far as the lungs, which does not require as much contractile force.
Valves in the heart maintain the flow of blood by opening and closing in one direction only. Blood can move only forward through the heart, and is prevented from flowing backward by the valves. Such movement of the blood is called unidirectional flow. There are four valves of the heart:
• The two atrioventricular (AV) valves ensure blood flows from the atria to the ventricles, and not the other way. The AV valve on the right side of the heart is called the tricuspid valve, and the one on the left of the heart is called the mitral, or bicuspid valve.
• The two semilunar (SL) valves are present in the arteries leaving the heart, and they prevent blood flowing back from the arteries into the ventricles.

The circulatory system

After seeing digestive and respiratory system, the next one is the cardiovascular system. It has a pretty important function in the nutrition role. As you already know, every cell in your body depends on your cardiovascular system. It keeps all of your cells supplied with nutrients from the intestine (digestive system) and oxygen from the lungs (respiratory system). It also removes their waste products, carbon dioxide to the lungs and the nitrogenous wastes to the kidneys.

Actually the circulatory system has many jobs, but we can cosider three main functions:

• Transport of nutrients, oxygen, and hormones to cells throughout the body and removal of metabolic wastes (carbon dioxide, nitrogenous wastes, and heat).
• Protection of the body by white blood cells and antibodies that circulate in the blood and defend the body against foreign microbes and toxins. Clotting mechanisms are also present that protect the body from blood loss after injuries.
• Regulation of body temperature and fluid pH...

This video can be a good help to begin with the circulatory system.


The cardiovascular system shown in Figure is made up of: The heart
It pushes the blood around your body through the blood vessels. The heart is made of cardiac muscle. Blood is collected in the heart and pumped out to the lungs, where it releases carbon dioxide and picks up oxygen before it is pumped to the rest of the body.
The blood vessels
Their job is to channel the blood around the body. There are three main types of blood vessels in the body; arteries, veins, and capillaries
Arteries are blood vessels that carry blood away from the heart. Further from the heart, arteries form smaller arteries. These smaller arteries branch into smaller vessels. The smaller blood vessels help to bring nutrients and oxygen and take away waste from body tissues.
Capillaries are the tiniest blood vessels in the body located within the tissues of the body. They transport blood from the arteries to the veins. The walls of capillaries are only a single layer of cells thick. Oxygen, carbon dioxide, nutrients, and wastes are exchanged through their thin walls. Capillaries are so narrow that blood cells must move in single file through them.
Veins are blood vessels that carry back the blood from the different regions of the body to the heart.
The blood
Blood is a body fluid that is a type of connective tissue. Blood is made of blood cells, and a fluid called plasma. The main types of cells found in blood are red blood cells and white blood cells.
Finally remember you something you studied last course. The cardiovascular system of humans is closed. That means the blood never leaves the large loop of blood vessels in which it travels. Other animals such as invertebrates have open circulatory systems, in which their blood can leave the blood vessels.

Respiratory system vocabulary

alveoli:Little "sacs" at the end of the bronchioles where most of the gas exchange occurs.
asthma:A chronic illness in which the bronchioles are inflamed and become narrow.
bronchitis:An inflammation of the bronchi.
diaphragm:A sheet of muscle that extends across the bottom of the rib cage. When the diaphragm contracts the chest volume gets larger and the lungs take in air; when the diaphragm relaxes, the chest volume gets smaller and air is pushed out of the lungs.
emphysema: A chronic lung disease caused by loss of elasticity of the lung tissue.
epiglottis:A flap of connective tissue that closes over the trachea when food is swallowed to prevent choking or inhaling food.
exhalation:Pushing air out of the body through the nose or mouth.
external respiration:The movement of oxygen into the body and carbon dioxide out of the body.
inhalation:Taking air into the body through the nose and mouth.
internal respiration:The exchange of gases between the blood and the cells of the body.
larynx:Found just below the point at which the pharynx splits into the trachea and the esophagus. Your voice comes from your larynx; air from the lungs passes across thin membranes in the larynx and produces sound; also called the voicebox.
lung cancer:A disease where the cells that line the lungs grow out of control; the growing mass of cells pushes into nearby tissues and can affect how these tissues work.
pathogen: An organism that causes disease in another organism; certain bacteria, viruses, and fungi are pathogens of the respiratory system.
pharynx:A long tube that is shared with the digestive system; both food and air pass through the pharynx.
pneumonia:An illness in which the alveoli become inflamed and flooded with fluid.
respiratory disease
trachea:A long tube that leads down to the chest where it divides into the right and left bronchi in the lungs; also called the windpipe.
tuberculosis (TB): A common and often deadly infectious disease caused by a type of bacterium called mycobacterium.

The Journey of a Breath of Air

When you breath in, oxygen is drawn in through the nose or mouth and down into the lungs. The oxygen then passes across the thin lining of the capillaries and into the blood. The oxygen molecules are carried to the body cells by the blood. Carbon dioxide from the body cells is carried by the blood to the lungs where it is released into the air.
Breathing is only part of the process of delivering oxygen to where it is needed in the body. Gas exchange occurs in the alveoli by passive diffusion of gases between the alveoli and the blood in the capillaries of the lungs.

Diffusion is the movement of substances from an area of higher concentration to an area of lower concentration. The concentration of O2 in the alveoli is at a higher level than in the blood and the concentration of CO2 in the alveoli is at a lower lever than in the blood. O2 molecules diffuse across the thin walls of the alveoli and capillaries and into the blood. Carbon dioxide (CO2) moves out of the blood and into the alveoli in a similar way
The pulmonary artery carries the blood traveling to the lungs and alveoli. Upon reaching the alveoli the blood picks up the inhaled oxygen and at the same time releases the carbon dioxide that needs to be expelled. The pulmonary veins then carry the oxygenated blood to the heart to be pumped through the aorta and around the body. The oxygenated blood travels to the smaller arteries and finally to the capillaries where gas exchange occurs.
The oxygen molecules move out of the capillaries and into the body cells. It is used by the cells to Cellular respiration, the process of breaking down glucose to release energy .
C6H12O6 + 6O2 → 6H2O + 6 CO2 + ENERGY
The waste products of cellular respiration include carbon dioxide and water. The carbon dioxide molecules move out of the cells and into the capillaries that surround the cells. The carbon dioxide is removed from the body by the lungs.

How we breathe

Most of the time, you breathe without thinking of it. Breathing is mostly an involuntary action that is controlled by a part of your brain that also controls your heart beat. If you swim or sing, by instance, you know you can also control your breathing.
The taking in and expelling out of air is done by two movements:Inspiration and expiration. Inspiration: breathing in (a.k.a., inhalation)
Taking air into the body through the nose and mouth. It’s always an active process
caused by muscular contraction, mainly of the diaphragm with the help of intercostal muscles. During inhalation, the diaphragm contracts and moves downward. The rib muscles contract and cause the ribs to move outward. This causes the chest volume to increase. Because the chest volume is larger, the air pressure inside the lungs is less than the air pressure outside. This difference in air pressures causes air to be sucked into the lungs.
Expiration: breathing out (a.k.a., exhalation)
Pushing air out of the body through the nose or mouth It’s typically a passive process caused by elastic recoil of the lungs and relaxation of diaphragm and intercostal muscles. When the diaphragm and rib muscles relax, the chest volume is smaller, the air pressure inside the lungs is bigger than the air pressure outside. This difference in air pressures causes air to be pushed out of the lungs. Exhalation is similar to letting the air out of a balloon.
The lungs cannot move by themselves. As mentioned above, air moves into and out of the lungs by the movement of muscles. The diaphragm and rib muscles contract and relax to move air in to and out of the lungs.
After ventilation, the second stage of breathing is the gas exchange. It takes place in the alveoli. The walls of the alveoli are very thin and are permeable to gases. The alveoli are lined with capillaries, the walls of which are also thin enough to allow gas exchange.
Oxygen diffuses from the alveoli to the blood in the capillaries that surround the alveoli. At the same time, carbon dioxide diffuses in the opposite direction, from capillary blood to the alveoli. At this point, the pulmonary blood is oxygen-rich, and the lungs are holding carbon dioxide. Exhalation follows, thereby ridding the body of the carbon dioxide and completing the cycle of respiration.

Respiratory system.

Hello everybody, here we are in a new year. I hope you rested in Christmas holidays after all the work of the first quarter. I also assume that you had a good time with your family and friends. But we are in a new year and we must return to work. We have to keep improving our knowing about the human body. In the first quarter we finished with the digestive system. we studied its anatomy and fisiology. Now we continue with the nutrition function and we will study:
The respiratory system:
The main function of the respiratory system is to bring oxygen into the body and releases carbon dioxide into the atmosphere. The respiratory system is made up of the organs that take part in this process. These structures include your nose, mouth, larynx, pharynx, lungs, and diaphragm. These structures are shown in Figure


The nose and nasal cavity filters, warms, and moistens the inhaled air. The nose hairs and mucus produced by the cells that line the nose catch airborne particles and prevent them from reaching the lungs.
Behind the nasal cavity, air next passes through the pharynx, a tube that is shared with the digestive system. Both food and air pass through the pharynx. A flap of connective tissue called the epiglottis closes over the trachea when food is swallowed to prevent choking or inhaling food.
The larynx is found just below the point at which the pharynx splits into the trachea and the esophagus. Your voice comes from your larynx. Air from the lungs passes across thin membranes (vocal cords) in the larynx and produces sound.
The trachea, or wind pipe, is a long tube that leads down to the chest where it divides into the right and left bronchi in the lungs. The bronchi branch out into smaller bronchioles in each lung.
The bronchioles lead to the alveoli.
Alveoli are the little sacs at the end of the bronchioles. They look like little bunches of grapes at the end of the bronchioles, as shown in Figure. Gas exchange occurs in the alveoli, oxygen move across a membrane and into the blood and carbon dioxide move out of the blood. The alveoli are the tiny grape-like structures in the lungs and the sites of gas exchange.
The diaphragm is a sheet of muscle that extends across the bottom of the rib cage. It performs an important function in respiration. When the diaphragm contracts the chest volume gets larger and the lungs take in air. When the diaphragm relaxes, the chest volume gets smaller and air is pushed out of the lungs.

Absorption: large intestine

Once food has passed through the small intestine, it is mostly undigestible material and water. It enters the colon (large intestine), named for its wide diameter. The large intestine has three parts: the cecum, the colon ( ascending colon, transverse colon and descending colon) and rectum. The large intestine is about 1.5 metres
The large pouch-shaped cecum marks the beginning of the colon. Attached near the cecum bottom is the vermiform (worm-like) appendix. The appendix contains lymphoid tissue and intercepts pathogenic microorganisms that enter the digestive tract. Sometimes, fecal matter may become trapped in the appendix, resulting in appendicitis (infection and inflammation).
The three parts of the colon absorb water and minerals from the undigested food and compact the remaining material into feces.

Gut flora or intestinal flora consists of microorganisms that live in the intestine, mostly in the colon. The intestinal bacteria of the gut flora prevent the growth of harmful bacteria and synthesize some vitamins and other functions. The composition of the gut flora differs from person to person and depends on age, diet, environment and use of antibiotics.

Defecation is the digestive process final stage: feces (undigested waste products) are carried to the rectum through peristalsis and eliminated through the anus. It has internal and external sphincters.

A first review

A second review

Absorption in the Small Intestine

The small intestine, mainly the jejunum, is where most nutrients are absorbed into the blood.
As shown in Figure, the mucous membrane lining the small intestine is covered with very small, fingerlike projections called villi (singular: villus). Epithelial cells of each villus has thousands of microscopic projections called microvilli (singular: microvillus). Because there are millions of these tiny projections, they greatly increase the surface area for absorption. In fact, villi increase the absorptive surface of the small intestine to the size of a tennis court! This increases the amount of surface area available for the absorption of nutrients.

Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries (amino acids and carbohydrates) and lacteals (lipids). The absorbed substances are transported via the blood vessels to the liver and different organs of the body. The food that remains undigested and unabsorbed passes into the large intestine.

Intestinal digestion

The small intestine is narrow tube that starts at the stomach and ends at the large intestine. In adults, the small intestine is about 7 meters long. It is called “small” because it is smaller in diameter (2.5 cm) than the large intestine. Like the rest of the gastrointestinal tract, the small intestine pushes food along with peristalsis.
The small intestine finishes the process of digestion, absorbs the nutrients, and passes the residue on to the large intestine.
The small intestine is made up of three parts: the duodenum, jejunum, and ileum. Each part has different functions.
The duodenum is the first part of the small intestine. It is about 25 cm long. This is where most chemical digestion takes place. In this part the liver and gall bladder release bile and the pancreas secretes pancreatic juice.
The jejunum is the second part of the small intestine. This is where most nutrients are absorbed into the blood. The jejunum is lined with tiny “fingers” called villi. Each one is only about 1 mm long.
The ileum is the third part of the small intestine. Like the jejunum, the ileum is covered with villi. A few remaining nutrients are absorbed in the ileum. From the ileum, any remaining food waste passes into the large intestine.
Chyme emerging from the stomach into the duodenum is very acidic. The gall bladder release alkaline bile and the pancreas a large amount of sodium bicarbonate which neutralize the acidity of the chyme. This is important for digestion, because digestive enzymes in the duodenum require a neutral environment in order to work. The duodenum is protected from acid by a thick layer of mucus
Inside the duodenal tube, chyme is mixed with the bile and pancreatic juice. Bile breaks down fat particles into smaller droplets thus provide a largely increased surface area for the action of the enzyme pancreatic lipase that breaks down fats. Pancreatic juice conteins many enzymes ( lipase, protease,..) that finalize the food digestion process.

Following the digestion, esophagus and stomach

From the pharynx, the bolus moves into the esophagus. The esophagus is a narrow tube about 20 centimeters long in adults. It begins at the pharynx, passes through the chest, and ends at the opening to the stomach. The function of the esophagus is to pass food from the mouth to the stomach. This takes only a few seconds. The esophagus does not produce digestive enzymes and does not have any other digestive functions.
Food moves through the esophagus due to peristalsis. At the end of the esophagus, a muscle called a sphincter controls the entrance to the stomach. The sphincter opens to let food into the stomach and then closes again to prevent the food from passing back into the esophagus.

The stomach is a saclike organ located between the end of the esophagus and the beginning of the small intestine. In the stomach, food is further digested both mechanically and chemically. Churning movements of the stomach’s thick muscular walls break down food mechanically. The churning movements also mix the food with gastric juice, a fluids secreted by the stomach. These fluids include hydrochloric acid, digestive enzymes and mucus.
• Hydrochloric acid is a strong acid (pH – 2) and gives the stomach a very acidic environment. This helps destroy any bacteria that have entered the stomach in foods or beverages. An acidic environment is also needed for the stomach’s digestive enzymes to work.
• The main digestive enzyme secreted in the stomach is pepsin that breaks down proteins into smaller molecules .
• Mucus secreted by the gastric glands helps protect the stomach lining from the action of gastric juice.
Small molecules like water, alcohol and salts can be absorbed through the lining of the stomach. Most other substances need further digestion in the small intestine before they can be absorbed. The stomach stores the food until the small intestine is ready to receive it. When the small intestine is empty, a sphincter (Pyloric sphincter) opens between the stomach and small intestine. This allows the partially digested food, now called chyme, to enter the small intestine.

The Mouth

The mouth is the first organ in the digestive tract. The foods are going to enter into the mouth in the ingestion process.
The salivary glands, inside the mouth, produce saliva that moistens the food and makes it easier to chew. The salivary enzyme is amylase that breaks down complex starch molecules into simpler sugar molecules (chemical digestion).The mouth also plays an important role in mechanical digestion. The teeth help to digest food mechanically by breaking it into smaller pieces.
There are different teeth types with different shapes and functions:
Incisors: These are your front teeth and are used for biting into your food. They are chisel or wedge-shaped. A human adult has 4 incisors in each jaw
Canines: These are pointed and are used for tearing. An adult human has 2 canines in each jaw.
Premolars and molars at the back of the mouth are larger and broader. They grind food into smaller pieces as you chew. An adult human has 4 pre-molars and 6 molars in each jaw. Third molars are often called wisdom teeth; (they developed thousands of years ago when human diets consisted of mostly raw and unprocessed foods that required the extra chewing and grinding power of a third set of molars. Today wisdom teeth are not needed for chewing and, because they can crowd other teeth, are often removed).
A human baby has 20 baby or milk teeth. These start to develop at about 6 months old and last until you are about 5 - 6 years old. A human adult has 32 teeth called permanent teeth. These start to grow when you are about 5 -6 years old. They replace your milk teeth.
Each tooth consists of a crown and one or more roots.
The crown is the functional part that is visible above the gum. The root is the unseen portion that supports and fastens the tooth in the jawbone. The clear outer layer of the crown is the Enamel, the hardest substance in the human body. The outer layer of the roor is cementum, a bonelike substance that anchors the tooth to the jawbone. Directly beneath the outer layer is dentin, a hard, mineral material that is similar to human bone, only stronger. Dentin surrounds and protects the pulp, or core of the tooth. Pulp contains blood vessels, which carry oxygen and nutrients to the tooth, and nerves, which transmit pain and temperature sensations to the brain.
The muscular tongue helps mix the food with saliva and the enzymes it contains.
When you swallow, the lump of chewed food, now called a bolus, passes into the pharynx.
The pharynx serves both the respiratory system and the digestive system. It connects the mouth to the rest of the digestive tract and, also connects the mouth and nose to the rest of the respiratory system. To prevent food or liquid from entering the trachea (windpipe), the epiglottis (a small flap of tissue) closes over the opening of the larynx (voice box) during swalowing (deglutition).

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What is the digestive process?

The digestive system has three main functions: digestion of food, absorption of nutrients, and elimination of solid waste.
In the digestive process we can consider four stages:
Stage 1: Ingestion.
The intake of food into the body trhough the mouth
Stage 2: Digestion .
The transformation of food into nutrients which the body can absorb. There are two types of digestion:
Mechanical digestion is the physical breakdown of chunks of food into smaller pieces. It takes place mainly in the mouth and stomach.
Chemical digestion: is the chemical breakdown of large, complex food molecules into smaller, simpler nutrient molecules that can be absorbed by the blood. It takes place mainly in the small intestine and trhough the action of enzymes.
Enzymes are substances that speed up chemical reactions. Digestive enzymes speed up the reactions of chemical digestion. Digestive enzymes are secreted by glands in the mucous membranes of salivary glands, stomach, small intestine, and pancreas.
The name of a digestive enzyme typically ends with the suffix -ase, which means “enzyme”. The rest of the name refers to the type of food molecules the enzyme helps digest. For example
proteases split proteins into their monomers, the amino acids
lipases split fat into three fatty acids and a glycerol molecule.
amylases split starch into sugars.
In this video you can learn a little more about enzymes It’s in english but you have in the same page the transcrip of the video.
Stage 3: Absorption:
Absorption is the process in which nutrients pass from the digestive system into the blood stream, where they can circulate throughout the body and carry the nutrients to the cells. Absorption occurs mainly in the small intestine
Stage 4: Egestion:
Egestion is the elimination of undigested food that cannot be absorbed and waste products from the digestive system. These products are transformed into faeces, and expelled from the organism through the anus.
It should not be confused with excretion, which is getting rid of waste formed from the chemical reaction of the body, such as in urine, sweat, etc.