Gastrovascular Cavity and Open and Closed Circulatory Systems

 Longitudinal section of Hydra showing gastrovascular cavity
Longitudinal section of Hydra showing gastrovascular cavity

Gastrovascular Cavities

Gastrovascular Cavities are found in organisms under two major phyla: Cnidaria and Platyhelminthes. The gastro vascular cavity is a digestive cavity that has one opening that serves as a mouth and secretion of waste. In cnidarians, the cavity aids in digestion and the distribution of nutrients throughout the body.

Cnidarians have a sac-like body, which two layers; the epidermis and the gastrodermis. In between these two layers, there is a jelly like layer called the mesoglea, which houses extra cellular digestion.Hydra have branches of the gastro vascular cavity extending to their tentacles, which can serve to capture prey.

Open Circulatory System

Open circulatory system of a crayfish

An open circulatory system is common in most molluscs and anthropods. In this type of ciruclatory system, a fluid called hemolymph ( a mix of blood and lymph), is bathed on organs and body tissues in a cavity called the homecoel. Hemolymph travels through circulatory vessels into cavities or sinuses, which are open spaces surrounding an organ. The reason why this fluid is bathed on organs is so the tissues receive oxygen and nutrients directly.

The circulation of hemolymph is done with muscle contractions because there is no blood pressure, which is the nature of an open system like this.

There are advantages and disadvantages to an open circulatory system. The simplicity of this system makes it suitable for small organisms. An open system requires less energy to distribute nutrients and oxygen throughout organs and tissues. There are also unique features that an open system can give to certain organisms. For an example, spiders use hydrostatic pressure made by their open circulatory system to extend their legs. Some disadvantages to this system is the fact that the animals cannot control their velocity of their blood flow, and oxygen cannot be filtered in and out as easily.

Closed Circulatory System

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In a closed ciruclatory system, the circulatory fluid, blood, never leaves a network of blood vessels. The blood is kept under pressure so it can move to all corners of the body. There is a heart (or heart-like organ) that circulates the blood by pumping it through a network of arteries and veins.

The three main types of vessels that blood passes through are: veins, arteries, and capillaries.

Arteries carry blood from the heart to organs in the body. There are two different types of Arteries; the pulmonary arteries and the systemic arteries. Pulmonary arteries extend out from the right ventricle of the heart and splits into left and right pulmonary arteries that transport de-oxygenated blood from the heart to the lungs (one artery to each lung). The other type of artery is the systemic arterties, which carry oxygenated blood to othfer parts of the body. Arterioles branch out form an artery, converying blood into an organ and into the capillaries.

external image fig18_2.gifCapillaries are extremely thin microscopic vessels that run through all of the body's tissues. Their walls are one cell thick and porous to allow oxygen and nutrients to be passed exchanged with tissues, and to be able to carry waste away. A group of capillaries is called a capillary bed. The dropping off of water and nutrients for the organ happens at capillary beds, while also taking away waste like carbon dioxide to be excreted from the body. It is important to note that from an artery to a capiilary there is a big drop in the velocity that the blood is traveling at.

Vessels that carry blood towards the heart are called veins. As the capillaries are start to exit, they merge into venules, and venules converge into veins. Veins carry deoxygenetated blood from tissues to the heart.

Cardiovascular systems of fish, amphibians, reptiles, and mammals/birds


Fish, rays, and sharks have a cardiovascular system in a singular circulation arrangment. The blood from the body is brought by the veins into the heart; then the heart pumps this deoxygenated blood toward the gills. In the gills the blood becomes loaded with oxygen and releases carbon dioxide. The oxygenated blood from the gills is then distributed by arteries to the body. The oxygen is delivered to the tissues through capillaries, these in turn unite to form veins, and the veins go back to the heart. Thus, blood only goes through one circuit.


Amphibians have a double ciruclation method of blood flow. The heart has two pumps, each pump going to a differnt circuit of blood travel. The right pump delivers deoxygenated blood to the pulmonary circuit, where the blood flows into capillary beds. At the capillary beds, gas exchange is occurring; there is a net movement of oxygen gas into the blood and carbon dioxide out of the blood. The left pump of the heart pushes the oxygenated blood to organs and tissues around the body of the amphibian, starting the systemic circuit of blood flow. The now oxygen poor blood returns back to the heart, completing the circuit.


Most repiles contain a three-chambered heart, which contians two aortas, and one partially divided ventricle. Blood leaving the ventricle passes into one of two vessels. It either travels through the pulmonary arteries leading to the lungs or through a forked aorta leading to the rest of the body. Oxygenated blood returning to the heart from the lungs through the pulmonary vein passes into the left atrium, while deoxygenated blood returning from the body through the sinus venosus passes into the right atrium. Both atria empty into the single ventricle, mixing the oxygen-rich blood returning from the lungs with the oxygen-depleted blood from the body tissues.

Mammals and Birds

Four chambered hearts are found in mammals and birds. They consist of two atria and two ventricles to seperate the flow of oxygenated blood and deoxygentated blood. The right atrium receives oxygen-poor blood and pumps it into the right ventricle. The right ventricle takes this oxygen-depleted blood and pumps it into the lungs via the the pulmonary artery. The left side of the heart is where all the oxygen-rich blood is. The left atrium receives oxygenated blood and pumps the blood to the left ventricle via the mitral valve. The left ventricle reiceves the oxygen-rich blood and pumps it to the rest of the body through the aorta. The left ventricle is thicker because it has to pump blood to all of the body organs through the systemic circuit,

Closer look at mammalian circulation

The human heart is about the size of a clenched fist. The heart pumps at a rhythmic pace. When the heart contracts, it pumps blood, and when it relaxes, the chambers fill up with blood. One cycle of a contraction and relaxing is called the cardiac cycle. The phase where the heart contracts is called the systole, and the relaxation phase is called the diastole.Theexternal image human-blood-circulatory-system.jpeg cardiac output is defined as the volume of blood the right and left ventricles push every minute.The heart rate multiplied by the stroke volume determine what the cardiac output is. The heart rate of a person is the number of beats the heart makes a minute. A stroke volume is the volume of blood a ventricle pumps with each beat of the heart. An average stroke volume for a human is 70ml of blood, and multiplying that number by an average resting heart rate of 72 beats per minute equals 5 L/min of cardiac output. This is a big volume of blood being pumped by a ventricle a minute, so one has to ask, is it possible that some blood could travel backwards at any point during the circulation?

Humans have a method of preventing backflow of blood to keep it moving in the correct direction. This method consists of four valves in the heart that are made of connective tissue. When blood is pushing from a certain side, the valves open, and when blood is pushing from the wrong side, the valves are locked. Separating the atriums and ventricles are atrioventricular valves (AV), which are just valves held by strong fibers. When a ventricle contracts, it generates enough pressure to close and seal an AV so blood does not flow back. Before blood goes out of the ventricles, they are met with Semilunar valves, which are valves that prevent backflow of blood when it gets to the aorta. A contraction by a ventricle opens up the semilunar valve, and the pressure build up in the ventricle when it is relaxed closes the semilunar valve so stop blood flowing backwards.

Regulation of the Heartbeat

The contractions and relaxations in the heart are autorhythmic thanks to a group of cells located in the wall of the right atrium. These cells make up a tissue called the sinoatrial node (SA), which is the primary pacemaker and is responsible for the electrical activity of the heart. The SA initiates electrical impulses and sets the time and rate that cardiac muscle tissue will contract.SA impulses are very fast and travel at great speeds through heart tissue. This is how the impulses from the SA work: the node initiates an impulse that travels through the walls of the atria, causing both atria to contract simultaneously. While this contraction happens, other impulses reach different tissues located in the walls of the atria, and they reach the atrioventricular node where a delay point is created. The delay is 0.1 seconds so the contraction in the heart apex does not happen at the same time as the contractions in the atria.

There are two division of the nervous system that change the tempo of the heart: the sympathetic division increases the speed of a pacemaker, and the parasympathetic division slows it down. When a person is walking, the sympathetic division speeds up the pacemaker to allow more oxygen to get to the muscles, and when the person i relaxing, the parasympathetic division slows down the tempo of the SA node to decrease the amount of oxygen flow to muscles.

Structure and function of blood vessels

There needs to be as little resistance to blood flow as possible to maximize efficiency of the ciruclatory system. The walls of blood vessels are lined with endothelium, which minimizes resistance to blood flow because of its smooth surface.Surrounding the endoththeilium are tissues that are different for veins, arteries, and capillaries.
Its no surprise that the walls of a capillary are thin because nutrients and oxygen diffuse through the walls constantly. Arteries are handling high volumes of blood being pumped through them by the heart, so naturally they have thick and strong walls to accommodate for this.Veins are not dealing with blood being pumped at high pressure, they just carry the blood back to the heart at a lower pressure so they have thin walls, just able to hold the blood in. The walls of a vein are about as third as thick as that of an artery. Both veins and arteries have two layers of tissue surrounding the endothelium: a layer of smooth muscle and elastic fibers, and a layer of connective tissue with elastic fibers on top of that. The elastic fibers are for elasticity of the vessel, so the vessel and stretch and coil at will.

Regulation of blood pressure

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Blood pressure is the force of blood against the walls of arteries. Blood pressure is recorded as two numbers during a heartbeat: a maximum pressure (systolic) and a minimum pressure (diastolic). Blood pressure for a healthy 20 year old at rest is 120 mm Hg at systole and 70 mm Hg at diastole.
A pulse is the rhythmic bulging of an artery wall against a bone, in tune with the heartbeat.The reason why an artery will bulge is because the high velocity that blood is being pumped at into an artery is too fast for the small opening of the arterioles to carry blood, so bulging will occur.

Vasoconstriciton is the contraction of the muscle walls of a vessel, causing it to narrow. When this happens, blood pressure increases in the arteries. The opposite of vasoconstriction is vasodilation, where the muscle walls of a vessel relax, causing the diameter of vessel to increase, leading to a decrease blood pressure. These are two methods of regulating blood pressure.

Fluid exchange: capillaries, interstitial fluid, and the lymphatic system


There blood flowing to all the parts of the body at all the times because there are a number of capillaries in each body tissue. Blood flow in the capillaries can actually be altered in the arterioles, where there is smooth muscle. The smooth muscle can contract, narrowing the diameter of the vessel, which in turn decreases blood flow to capillary beds. When the smooth muscle relaxes, the arterioles open up and blood can enter the capillaries.The thin endotheial walls of capillaries along with slow blood flow allow critical exchange of oxygen and nutrients between the blood and organs.Blood proteins that make their way to a capillary cannot defuse through the walls becuase they are too big, so they just have to stay there and eventually dissolve. The dissolving effects the osmotic pressure because fluid will want to flow into the capillary (high to low pressure). An increase in blood pressure can drive blood out of the capillaries. Generally, blood pressure is greater than other forces, so fluid is constantly being forced out of the capillaries.

Lymphatic System

The lymphatic system is an important drainage system that returns water and proteins back to the bloodstream. This system uses a network of lymph vessels, which carry lymph (clear fluid). Without this system, the bodies tissues would be swollen. This system is crucial because of the aforementioned fact that there is fluid being constantly leaked out of the capillaries so this system takes the leaked fluid and returns it back to the bloodstream. The lymphatic system also use valves to prevent backflow of lymph. Lymph nodes are important for the body's defenses because they house important cells that attack viruses. The nodes contain white blood cells that duplicate rapidly in case of infection so it could fight it off.

Blood components: structure and function

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Plasma is the liquid matrix in blood that holds important ions and proteins that help regulate osmosis in vessels, stabilize pH, and are part of the humans defense system. The ions act as a buffer in the blood, keeping the pH at 7.4. Antibodies and immunoglobulins are located in the plasma and fight off viruses and infections that invade the body. Some of the proteins travel with lipids because lipids can only travel in blood when they are attached to a protein.

Erythrocytes are the most numerous red blood cells in human body. Their function is to facilitate oxygen transport. Erythrocytes generate their ATP by anerobic metabolism, because they would be inefficient if they used oxygen that they carry to generate energy.One enthrocyte contains about 20 million molecules of hemoglobin, which is a protein that transports the oxygen. Each molecule of hemoglobin binds with four molecules oxygen, making each ethrocyte able to transport a billion oxygen molecules.

Leukocytes are white blood cells. Their main function is to fight viruses and infections. Leukocytes are not only found in the circulatory system, but also protecting the lymphatic system and interstitial fluid.

Stem cells replenish the body's blood cell populations.Stem cells replenish Eryhtocytes, leukocytes, and platalets. Stem cells are biological cells that can split into diverse specialized cell types. Stem cells are located in three spots in the human body: the bone marrow, adipose tissue, and blood. Stem cells in the bone marrow are extracted by drilling into the bone, also known as harvesting. Lipid cells also contain stem cells, which can be extracted by liposuction. Pheresis is used to extract stem cells form blood. Phereses is when there is blood drawn from a donor, and the blood is passed through a machine which separates stem cells from the blood.

Cardiovascular disease

Cardiovascular disease is the biggest cause of deaths worldwide. Any disease that infects the cardiovascular system (including heart and blood vessels) is referred to as a cardiovascular disease. Low-density lipoprotein (LDL) transports cholesterol to cells within the bloodstream. High density lipoprotein (HDL) is the largest of the lipoproteins, and they carry cholesterol and triglycerides from the body's tissues to the liver.A high ratio LDL to HDL results in an increased right at heart disease. An injury to the body can also result in cardiovascular disease. Macrophages and leukocytes go to the site of injury to aid in repair of the tissues. This results in inflammation of the tissue which disrupts circulatory function.

Astherosclerosis is the inflammation of artery walls because of the congestion fatty materials like cholesterol. The smooth lining (endothelium) of arteries keep blood flowing smoothly through the vessel, but an infection to the artery could result in a rough lining of the wall, disturbing blood flow. Inflammation comes after the infection; white blood cells are attracted the wall, and they begin to take up cholesterol. A build up of plaque, which is a fatty deposit, starts happening. The plaque increases obstruction of flow in the artery by taking up additional
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cholesterol, resulting in very thick and hard artery walls.

A heart attack is the result of astheroscelerosis. A heart attack happens when there is interrupted flow of blood flow to the heart (usually a blockage of a coronary artery), causing heart cells to die. A stroke is the lack of oxygen to the brain.Nervous tissue starts to breakdown and die in the brain because there is no oxygen. When blood pressure increases in the arteries, the heart has to work harder to pump blood, which is called hypertension.

Gas exchange in aquatic vs. terrestrial organisms

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Gas exchange is the process where gasses in organism are exchanged with the atmosphere. In a mixture of gasses, the partial pressureis the pressure that each gas has. The total pressure of the mixture is just the sum of all the partial pressure of the gasses in the mixture.Gas exchange efficiency depends on the medium that respiratory medium, oxygen or water. respiration through oxygen it is easier because it is less dense than water and a lot more plentiful making it easy to move in small passageways. Oxygen is not as abundant in air than water, so aquatic animals need to use more energy to carry out gas exchange. Salt and other particles in the water will lower oxygen content in water. Aquatic animals have adapted to this increase in energy, making the more efficient at gas exchange. The respirory surafces for aquatic animals are called Glls, which are foldings of the body that create a large surface area for gas exchange.

The respiratory surface is the surface where gasses are being exchanged. oxygen and carbon dioxide transfer across a respiratory surface is almost entirely don by diffusion. Simple animals have small enough bodies that their cells can exchange oxygen directly with the environment.

Closer look at mammalian respiration vs. bird respiration

Mammalian respirtation

Through a system of ducts and passageways air is transported to lungs. Air enter the nasal cavities and goes through a series of humidification and sampling before it goes to the pharynx. The pharynx is the intersection where the paths of food and water cross. The larynx deals with food, it moves upward towards the epiglottis where the food goes through the opening of the trachea and down the esophagus. While this is happening, the glottis is open to allow breathing.

The larynx passes air to the trachea. The walls of the trachea are hardened with cartilage to keep the airway open. Two bronchi branch out form the trachea, one bronchus for each lung. When the bronchi go into the lungs, they branch out into bronchioles, which are very thin tubes. Mucus lines the epithelium walls of the bronchi, which catch dust and little particles. Alveoli are tiny air sacs where gas exchange occurs in the lungs. The alveoli exchange oxygen with red blood cells when an electical impulse is sent to the heart by the SA and AV nodes. Carbon dioxide and oxygen exchange initites between the alveoli and red blood cells, which then return to the hearts left atrium.

Mammals get oxygen into their lungs by negative pressure breathing, which pulls air into the lungs. Lung volume increases as the rib muscles and diaphragm contract.

Bird respiration

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Birds have small lungs along with nine air sacs that play an important role in respiration. Fresh air is not mixed up with old air that was already in the system, unlike mammalian respiration. Every exhalation completely renews the air int he lungs.
To keep air flowing through the lungs, there needs to be nine air sacs lined on the side of the lungs. Unlike mammals where aveoli are the site of gas exchange, passageways called parabronchi in the lungs is where the gas exchange occurs. The respitory system of a bird works like this:
  1. When the bird first inhales, air fills posterior air sacs
  2. When the bird exhales, the posterior sacs push the air into the lungs by a contraction.
  3. A second inhale cause air to pass through the lungs and fill up anterior air sacs.
  4. The second exhale contracts the anterior air sacs and pushes air out of the body of the bird.

Coordination of circulation and gas exchange

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Blood arriving in the lungs has a low partial pressure of O2 and a high partial pressure of carbon dioxide relative to air in the alveoli. In the alveoli, O2 diffuses into the blood and carbon dioxide diffuses into the air. In tissue capillaries, partial pressure gradients favor diffusion of O2 into the interstitial fluids and carbon dioxide into the blood. Increased amounts of oxygen are carried in blood with the aid of respiratory pigments. Respiratory pigments are proteins that transport oxygen.

Hemoglobin is an oxygen-carrying protein that is found in all the red blood cells of vertebrates. One hemoglobin carries four oxygen molecules from the lungs to the rest of the body.The hemoglobin dissociation curve shows that a small change in the partial pressure of oxygen can result in a large change in delivery of O2. Carbon dioxide produced during cellular respiration lowers blood pH and decreases the affinity of hemoglobin for O2; this is called the Bohr shift.

Hemoglobin also helps in carbon dioxide transport.while helping buffer the blood. Carbon dioxide from respiring cells diffuses into the blood and is transported either attatched to hemoglobin, in blood plasma, or as bicarbonate ions (HCO3–).

Respiratory adaptions of diving mammals

Diving mammals have evolved to have very extraordinary respiration systems. A certain seal called the Weddel seal can hold up to twice much oxygen per kilogram of body mass. Because of the small lungs of the seal, the mammal can only hold 5% of the oxygen in the lungs, and 70% of it in the blood! The rest is stored in the muscles, specifically proteins called myoglobin, which holds oxygen This helps prolong the underwater journeys that the seal has. The elephant seal can reach depths of 1,500 meters underwater and stay there for two hours. Humans cannot hold their breath underwater for not even more than a couple minutes! Basically the deep diving mammals just stock up on oxygen while they are exposed to the air, and when they go underwater, they deplete their oxygen supply very slowly. This is what the evolution of great respiratory systems can do for diving mammals.

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