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There are also oxygen and carbon dioxide sensors in the lungs, but they primarily determine the diameters of the bronchioles and pulmonary capillaries , and are therefore responsible for directing the flow of air and blood to different parts of the lungs. If more carbon dioxide than usual has been lost by a short period of hyperventilation , respiration will be slowed down or halted until the alveolar partial pressure of carbon dioxide has returned to 5.

If these homeostats are compromised, then a respiratory acidosis , or a respiratory alkalosis will occur. Oxygen has a very low solubility in water, and is therefore carried in the blood loosely combined with hemoglobin. The oxygen is held on the hemoglobin by four ferrous iron -containing heme groups per hemoglobin molecule.

The reaction is therefore catalyzed by carbonic anhydrase , an enzyme inside the red blood cells. The total concentration of carbon dioxide in the form of bicarbonate ions, dissolved CO 2 , and carbamino groups in arterial blood i. Ventilation of the lungs in mammals occurs via the respiratory centers in the medulla oblongata and the pons of the brainstem.

This information determines the average rate of ventilation of the alveoli of the lungs , to keep these pressures constant. The respiratory center does so via motor nerves which activate the diaphragm and other muscles of respiration. The breathing rate increases when the partial pressure of carbon dioxide in the blood increases.

This is detected by central blood gas chemoreceptors on the anterior surface of the medulla oblongata. Exercise increases the breathing rate due to the extra carbon dioxide produced by the enhanced metabolism of the exercising muscles. Information received from stretch receptors in the lungs limits tidal volume the depth of inhalation and exhalation.

The alveoli are open via the airways to the atmosphere, with the result that alveolar air pressure is exactly the same as the ambient air pressure at sea level, at altitude, or in any artificial atmosphere e.

With expansion of the lungs through lowering of the diaphragm and expansion of the thoracic cage the alveolar air now occupies a larger volume, and its pressure falls proportionally , causing air to flow in from the surroundings, through the airways, till the pressure in the alveoli is once again at the ambient air pressure.

The reverse obviously happens during exhalation. This process of inhalation and exhalation is exactly the same at sea level, as on top of Mt. Everest , or in a diving chamber or decompression chamber.

However, as one rises above sea level the density of the air decreases exponentially see Fig. This is achieved by breathing deeper and faster i.

There is, however, a complication that increases the volume of air that needs to be inhaled per minute respiratory minute volume to provide the same amount of oxygen to the lungs at altitude as at sea level. During inhalation the air is warmed and saturated with water vapor during its passage through the nose passages and pharynx. Saturated water vapor pressure is dependent only on temperature.

In dry air the partial pressure of O 2 at sea level is At the summit of Mt. This reduces the partial pressure of oxygen entering the alveoli to 5. The reduction in the partial pressure of oxygen in the inhaled air is therefore substantially greater than the reduction of the total atmospheric pressure at altitude would suggest on Mt Everest: A further minor complication exists at altitude.

If the volume of the lungs were to be instantaneously doubled at the beginning of inhalation, the air pressure inside the lungs would be halved. This happens regardless of altitude. The driving pressure forcing air into the lungs during inhalation is therefore halved at this altitude.

However, in reality, inhalation and exhalation occur far more gently and less abruptly than in the example given. All of the above influences of low atmospheric pressures on breathing are accommodated primarily by breathing deeper and faster hyperpnea.

The exact degree of hyperpnea is determined by the blood gas homeostat , which regulates the partial pressures of oxygen and carbon dioxide in the arterial blood. This homeostat prioritizes the regulation of the arterial partial pressure of carbon dioxide over that of oxygen at sea level.

If this switch occurs relatively abruptly, the hyperpnea at high altitude will cause a severe fall in the arterial partial pressure of carbon dioxide, with a consequent rise in the pH of the arterial plasma. This is one contributor to high altitude sickness. On the other hand, if the switch to oxygen homeostasis is incomplete, then hypoxia may complicate the clinical picture with potentially fatal results. There are oxygen sensors in the smaller bronchi and bronchioles.

In response to low partial pressures of oxygen in the inhaled air these sensors reflexly cause the pulmonary arterioles to constrict. At altitude this causes the pulmonary arterial pressure to rise resulting in a much more even distribution of blood flow to the lungs than occurs at sea level.

At sea level the pulmonary arterial pressure is very low, with the result that the tops of the lungs receive far less blood than the bases , which are relatively over-perfused with blood.

It is only in middle of the lungs that the blood and air flow to the alveoli are ideally matched. This is a further important contributor to the acclimatatization to high altitudes and low oxygen pressures.

When the oxygen content of the blood is chronically low, as at high altitude, the oxygen-sensitive kidney cells secrete erythropoietin often known only by its abbreviated form as EPO [28] into the blood. In other words, at the same arterial partial pressure of O 2 , a person with a high hematocrit carries more oxygen per liter of blood than a person with a lower hematocrit does. High altitude dwellers therefore have higher hematocrits than sea-level residents. Irritation of nerve endings within the nasal passages or airways , can induce a cough reflex and sneezing.

These responses cause air to be expelled forcefully from the trachea or nose , respectively. In this manner, irritants caught in the mucus which lines the respiratory tract are expelled or moved to the mouth where they can be swallowed. This increases the expired airflow rate to dislodge and remove any irritant particle or mucus. Respiratory epithelium can secrete a variety of molecules that aid in the defense of the lungs. These include secretory immunoglobulins IgA , collectins , defensins and other peptides and proteases , reactive oxygen species , and reactive nitrogen species.

These secretions can act directly as antimicrobials to help keep the airway free of infection. A variety of chemokines and cytokines are also secreted that recruit the traditional immune cells and others to site of infections.

Surfactant immune function is primarily attributed to two proteins: These proteins can bind to sugars on the surface of pathogens and thereby opsonize them for uptake by phagocytes. It also regulates inflammatory responses and interacts with the adaptive immune response. Surfactant degradation or inactivation may contribute to enhanced susceptibility to lung inflammation and infection. Most of the respiratory system is lined with mucous membranes that contain mucosa-associated lymphoid tissue , which produces white blood cells such as lymphocytes.

The lungs make a surfactant , a surface-active lipoprotein complex phospholipoprotein formed by type II alveolar cells. It floats on the surface of the thin watery layer which lines the insides of the alveoli, reducing the water's surface tension.

The surface tension of a watery surface the water-air interface tends to make that surface shrink. The more acute the curvature of the water-air interface the greater the tendency for the alveolus to collapse.

Firstly the surface tension inside the alveoli resists expansion of the alveoli during inhalation i. Surfactant reduces the surface tension and therefore makes the lungs more compliant , or less stiff, than if it were not there. Secondly, the diameters of the alveoli increase and decrease during the breathing cycle. This means that the alveoli have a greater tendency to collapse i. Since surfactant floats on the watery surface, its molecules are more tightly packed together when the alveoli shrink during exhalation.

The tendency for the alveoli to collapse is therefore almost the same at the end of exhalation as at the end of inhalation. Thirdly, the surface tension of the curved watery layer lining the alveoli tends to draw water from the lung tissues into the alveoli. Surfactant reduces this danger to negligible levels, and keeps the alveoli dry.

Pre-term babies who are unable to manufacture surfactant have lungs that tend to collapse each time they breathe out. Unless treated, this condition, called respiratory distress syndrome , is fatal. Basic scientific experiments, carried out using cells from chicken lungs, support the potential for using steroids as a means of furthering development of type II alveolar cells.

The lung vessels contain a fibrinolytic system that dissolves clots that may have arrived in the pulmonary circulation by embolism , often from the deep veins in the legs. They also release a variety of substances that enter the systemic arterial blood, and they remove other substances from the systemic venous blood that reach them via the pulmonary artery. Some prostaglandins are removed from the circulation, while others are synthesized in the lungs and released into the blood when lung tissue is stretched.

The lungs activate one hormone. The physiologically inactive decapeptide angiotensin I is converted to the aldosterone -releasing octapeptide, angiotensin II , in the pulmonary circulation. The reaction occurs in other tissues as well, but it is particularly prominent in the lungs. Angiotensin II also has a direct effect on arteriolar walls , causing arteriolar vasoconstriction , and consequently a rise in arterial blood pressure.

The converting enzyme also inactivates bradykinin. Four other peptidases have been identified on the surface of the pulmonary endothelial cells. The movement of gas through the larynx , pharynx and mouth allows humans to speak , or phonate. Vocalization, or singing, in birds occurs via the syrinx , an organ located at the base of the trachea. The vibration of air flowing across the larynx vocal cords , in humans, and the syrinx, in birds, results in sound.

Because of this, gas movement is vital for communication purposes. Panting in dogs, cats, birds and some other animals provides a means of reducing body temperature, by evaporating saliva in the mouth instead of evaporating sweat on the skin.

Disorders of the respiratory system can be classified into several general groups:. Disorders of the respiratory system are usually treated by a pulmonologist and respiratory therapist.

Where there is an inability to breathe or an insufficiency in breathing a medical ventilator may be used. Horses are obligate nasal breathers which means that they are different from many other mammals because they do not have the option of breathing through their mouths and must take in air through their noses. The elephant is the only mammal known to have no pleural space.

Rather, the parietal and visceral pleura are both composed of dense connective tissue and joined to each other via loose connective tissue. In the elephant the lungs are attached to the diaphragm and breathing relies mainly on the diaphragm rather than the expansion of the ribcage.

The respiratory system of birds differs significantly from that found in mammals. Firstly, they have rigid lungs which do not expand and contract during the breathing cycle. Instead an extensive system of air sacs Fig. Inhalation and exhalation are brought about by alternately increasing and decreasing the volume of the entire thoraco-abdominal cavity or coelom using both their abdominal and costal muscles.

This pushes the sternal ribs, to which they are attached at almost right angles, downwards and forwards, taking the sternum with its prominent keel in the same direction Fig. This increases both the vertical and transverse diameters of thoracic portion of the trunk. The forward and downward movement of, particularly, the posterior end of the sternum pulls the abdominal wall downwards, increasing the volume of that region of the trunk as well. During exhalation the external oblique muscle which is attached to the sternum and vertebral ribs anteriorly , and to the pelvis pubis and ilium in Fig.

Air is therefore expelled from the respiratory system in the act of exhalation. During inhalation air enters the trachea via the nostrils and mouth, and continues to just beyond the syrinx at which point the trachea branches into two primary bronchi , going to the two lungs Fig. The primary bronchi enter the lungs to become the intrapulmonary bronchi, which give off a set of parallel branches called ventrobronchi and, a little further on, an equivalent set of dorsobronchi Fig.

Each pair of dorso-ventrobronchi is connected by a large number of parallel microscopic air capillaries or parabronchi where gas exchange occurs Fig. This is due to the bronchial architecture which directs the inhaled air away from the openings of the ventrobronchi, into the continuation of the intrapulmonary bronchus towards the dorsobronchi and posterior air sacs.

So, during inhalation, both the posterior and anterior air sacs expand, [41] the posterior air sacs filling with fresh inhaled air, while the anterior air sacs fill with "spent" oxygen-poor air that has just passed through the lungs.

During exhalation the pressure in the posterior air sacs which were filled with fresh air during inhalation increases due to the contraction of the oblique muscle described above. The aerodynamics of the interconnecting openings from the posterior air sacs to the dorsobronchi and intrapulmonary bronchi ensures that the air leaves these sacs in the direction of the lungs via the dorsobronchi , rather than returning down the intrapulmonary bronchi Fig.

The air passages connecting the ventrobronchi and anterior air sacs to the intrapulmonary bronchi direct the "spent", oxygen poor air from these two organs to the trachea from where it escapes to the exterior. The blood flow through the bird lung is at right angles to the flow of air through the parabronchi, forming a cross-current flow exchange system Fig. The blood capillaries leaving the exchanger near the entrance of airflow take up more O 2 than do the capillaries leaving near the exit end of the parabronchi.

When the contents of all capillaries mix, the final partial pressure of oxygen of the mixed pulmonary venous blood is higher than that of the exhaled air, [41] [44] but is nevertheless less than half that of the inhaled air, [41] thus achieving roughly the same systemic arterial blood partial pressure of oxygen as mammals do with their bellows-type lungs.

The trachea is an area of dead space: In comparison to the mammalian respiratory tract , the dead space volume in a bird is, on average, 4. In some birds e. The anatomical structure of the lungs is less complex in reptiles than in mammals , with reptiles lacking the very extensive airway tree structure found in mammalian lungs. Gas exchange in reptiles still occurs in alveoli however. Thus, breathing occurs via a change in the volume of the body cavity which is controlled by contraction of intercostal muscles in all reptiles except turtles.

In turtles, contraction of specific pairs of flank muscles governs inhalation and exhalation. Both the lungs and the skin serve as respiratory organs in amphibians. The ventilation of the lungs in amphibians relies on positive pressure ventilation. Muscles lower the floor of the oral cavity, enlarging it and drawing in air through the nostrils into the oral cavity. With the nostrils and mouth closed, the floor of the oral cavity is then pushed up, which forces air down the trachea into the lungs.

The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of mucus from specialised cells, and is involved in cutaneous respiration.

While the lungs are of primary organs for gas exchange between the blood and the environmental air when out of the water , the skin's unique properties aid rapid gas exchange when amphibians are submerged in oxygen-rich water. Oxygen is poorly soluble in water. Fish have developed gills deal with these problems. Gills are specialized organs containing filaments , which further divide into lamellae. The lamellae contain a dense thin walled capillary network that exposes a large gas exchange surface area to the very large volumes of water passing over them.

Gills use a countercurrent exchange system that increases the efficiency of oxygen-uptake from the water. Water is drawn in through the mouth by closing the operculum gill cover , and enlarging the mouth cavity Fig. Simultaneously the gill chambers enlarge, producing a lower pressure there than in the mouth causing water to flow over the gills. Back-flow into the gill chamber during the inhalatory phase is prevented by a membrane along the ventroposterior border of the operculum diagram on the left in Fig.

Thus the mouth cavity and gill chambers act alternately as suction pump and pressure pump to maintain a steady flow of water over the gills in one direction.

Oxygen is therefore able to continually diffuse down its gradient into the blood, and the carbon dioxide down its gradient into the water. In certain active pelagic sharks, water passes through the mouth and over the gills while they are moving, in a process known as "ram ventilation". But a small number of species have lost the ability to pump water through their gills and must swim without rest. These species are obligate ram ventilators and would presumably asphyxiate if unable to move.

Obligate ram ventilation is also true of some pelagic bony fish species. There are a few fish that can obtain oxygen for brief periods of time from air swallowed from above the surface of the water.

Thus Lungfish possess one or two lungs, and the labyrinth fish have developed a special "labyrinth organ", which characterizes this suborder of fish. The labyrinth organ is a much-folded supra branchial accessory breathing organ. It is formed by a vascularized expansion of the epibranchial bone of the first gill arch, and is used for respiration in air.

This organ allows labyrinth fish to take in oxygen directly from the air, instead of taking it from the water in which they reside through use of gills. The labyrinth organ helps the oxygen in the inhaled air to be absorbed into the bloodstream.

As a result, labyrinth fish can survive for a short period of time out of water, as they can inhale the air around them, provided they stay moist. Labyrinth fish are not born with functional labyrinth organs. The development of the organ is gradual and most juvenile labyrinth fish breathe entirely with their gills and develop the labyrinth organs when they grow older.

Some species of crab use a respiratory organ called a branchiostegal lung. Some of the smallest spiders and mites can breathe simply by exchanging gas through the surface of the body. Larger spiders, scorpions and other arthropods use a primitive book lung. Most insects breath passively through their spiracles special openings in the exoskeleton and the air reaches every part of the body by means of a series of smaller and smaller tubes called 'trachaea' when their diameters are relatively large, and ' tracheoles ' when their diameters are very small.

The tracheoles make contact with individual cells throughout the body. Diffusion of gases is effective over small distances but not over larger ones, this is one of the reasons insects are all relatively small. Insects which do not have spiracles and trachaea, such as some Collembola, breathe directly through their skins, also by diffusion of gases. The number of spiracles an insect has is variable between species, however they always come in pairs, one on each side of the body, and usually one pair per segment.

Some of the Diplura have eleven, with four pairs on the thorax, but in most of the ancient forms of insects, such as Dragonflies and Grasshoppers there are two thoracic and eight abdominal spiracles. However, in most of the remaining insects there are fewer. It is at the level of the tracheoles that oxygen is delivered to the cells for respiration. Insects were once believed to exchange gases with the environment continuously by the simple diffusion of gases into the tracheal system.

More recently, however, large variation in insect ventilatory patterns have been documented and insect respiration appears to be highly variable. Some small insects do not demonstrate continuous respiratory movements and may lack muscular control of the spiracles.

Others, however, utilize muscular contraction of the abdomen along with coordinated spiracle contraction and relaxation to generate cyclical gas exchange patterns and to reduce water loss into the atmosphere. The most extreme form of these patterns is termed discontinuous gas exchange cycles.

Molluscs generally possess gills that allow gas exchange between the aqueous environment and their circulatory systems. These animals also possess a heart that pumps blood containing hemocyanin as its oxygen-capturing molecule. The respiratory system of gastropods can include either gills or a lung. Plants use carbon dioxide gas in the process of photosynthesis , and exhale oxygen gas as waste.

The chemical equation of photosynthesis is 6 CO 2 carbon dioxide and 6 H 2 O water , which in the presence of sunlight makes C 6 H 12 O 6 glucose and 6 O 2 oxygen. Photosynthesis uses electrons on the carbon atoms as the repository for the energy obtained from sunlight.

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