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Transport in Animals

Closed Circulatory System

The closed circulatory system is found in all vertebrates such as fish, reptiles, amphibians, birds and mammals. It is most developed in mammals. The transport fluid known as blood is pumped by a muscular heart through blood vessels. There are three types of blood vessels namely arteries, veins and capillaries. Arteries carry blood away from the heart while veins carry blood to the heart. Capillaries link arteries to veins and provide site for exchange of materials.

 

Closed Circulatory System

The closed circulatory system is found in all vertebrates such as fish, reptiles, amphibians, birds and mammals. It is most developed in mammals. The transport fluid known as blood is pumped by a muscular heart through blood vessels. There are three types of blood vessels namely arteries, veins and capillaries. Arteries carry blood away from the heart while veins carry blood to the heart. Capillaries link arteries to veins and provide site for exchange of materials.


There are two type of closed circulatory systems:Sngle and double

Single circulation

In a single circulatory system blood flows from the heart to the gills then to the body tissues before flowing back to the heart. It therefore passes through the heart only once in a complete circulation e.g. Fish.

Double circulation

In double circulatory system blood flows from the heart to the lungs then back to the heart before proceeding to the body tissues. Blood therefore passes through the heart twice before proceeding to body tissues in a complete circulation e.g.man. This animation shows blood flowing from the left side of the heart to the body and then returns to right side of the heart. Blood then flows from right side of the heart to lungs before returning to the heart.


In closed circulation there is no direct contact between transporting fluid and body tissues. Transporting fluid flows under a high pressure due to flowing through the blood vessels. Blood flows over a long distance at a fast rate. It is more efficient at supplying nutrients to and removing waste products from the tissues. Transport fluid is red due to hemoglobin pigment. The closed circulatory system is found in all vertebrates such as reptiles, amphibians, birds and mammals. It is most developed in mammals. The transport fluid, known as blood is pumped by a muscular heart and is transported to and from all parts of the body through closed vessels known as blood vessels. Veins carry blood to the heart while arteries carry blood away from the heart.

 

TRANSPORT IN ANIMALS

Objectives

By the end of this lesson you should be able to

:

  • Explain the importance of transport in animals.
  • Distinguish between open and closed circulatory systems.
  • Relate the structure of the heart and the blood vessels to their functions.
  • Trace the path taken by blood from the heart to all parts of the body and back to the heart.
  • Name the common diseases of the circulatory system in humans and suggest methods of control/prevention.
  • Relate the structure of the components of blood to their functions.
  • Explain how oxygen and carbon (iv) oxide are transported in blood.
  • Describe the mechanism of blood clotting and its importance
  • Describe the human blood groups and their importance in blood transfusion.
  • Explain immunity and describe immune responses.

Background Information

We have learnt the meaning of transport and its importance. In the previous sub-topic, we emphasized on transport of water, mineral salts and organic substances in plants. Animals, just like plants, need to transport substances. This takes place through an elaborate transport system known as the circulatory system. Circulatory systems are of two types: open and closed. Open circulatory systems are found in invertebrates such as insects. Closed circulatory systems are found in vertebrates.

Blood from the body tissues enters the right side of the heart via the vena cava. The heart then pumps the blood to the lungs through the pulmonary artery for oxygenation. Blood flows back to the left side of heart through the pulmonary vein. From the heart the blood is pumped to the rest of the body via the aorta.

The aorta branches into arteries that serve the various body organs. The blood then returns back to the right side of the heart via the vena cava.

The carotid artery supplies the head with blood while the carotid vein drains blood out of the head.

Hepatic artery supplies blood to the liver while hepatic vein drains blood out from the liver.

Mesenteric artery supplies blood to the gut while hepatic portal vein drains blood from the gut to the liver

The renal artery supplies the kidneys with blood while renal vein drains blood from the kidney

The iliac artery supplies the trunk and limbs with blood while iliac vein drains blood away.

The subclavian artery supplies the arms with blood while subclavian vein drains blood away from the arms.

The coronary artery supplies the heart muscles with blood while the coronary vein drains blood away from the heart muscles.

External structure of the heart
This illustration shows the key external features of a human heart seen from the ventral side. Note the vessels spreading over the heart tissue. These are known as the coronary vessels. They supply blood rich in nutrients and oxygen to the heart muscles through the coronary artery and take away blood rich in waste products and carbon (IV)oxide through the coronary vein.

The heart is enclosed in a pericardial membrane. This secretes a fluid that acts as a lubricant to lubricate the heart during its contractions. It also keeps the heart in position and prevents overdilation of the heart

The outer part of the heart is covered by a fatty tissue that acts as a shock absorber. The heart is made up specialised muscles known as cardiac muscles which are myogenic, that is, they initiate and sustain the heartbeat.

Internal Structure

Red Blood Cells

The red blood cells are also called erythrocytes. There are about 70000 RBC per mm3 of blood. They are formed in the bone marrow of the short bones e.g ribs, sternum and bones of the backbone (vertebrae). In the embryo red blood cells are formed in the liver.

These cells have the following characteristics that make them suitable for the transportation of respiratory gases:

  • Are small in size 8-10mm ( about 0.008 mm) and 3-4 mm thick(about 0.0003 mm) to increase eth surface area for gaseous exchange.
  • Are biconcave in shape (disc shaped to increase tha surface area for gaseous exchange
  • Are flexible to enable them squeeze through narrow capillaries
  • Lack nucleus to increase the space for haemoglobin
  • Contain haemoglobin pigment which has high affinity for oxygen.
  • Are numerous per unit volume to increase the surface area for gaseous exchange.
  • Lined with a thin plasma membrane to reduce the distance which diffusion of gases occur.
  • Have an enzyme carbonic anhydrase which enhances the carbon IV oxide transport.

Circulation in the Heart
This illustration shows movement of blood through the heart.

Circulation in the Heart

Deoxygenated blood from all parts of the body enters the right auricle through the vena cava. It then flows via the tricuspid valve into the right ventricle. The right ventricle contracts forcing the semi-lunar valves to open. This allows the blood to flow to the lungs through the pulmonary artery.

Contraction of the left ventricle forces the blood to flow from the ventricle via the semi-lunar valves through the aorta to the tissues. Oxygenated blood flows from the lungs into the left auricle and then to the left ventricle via the bicuspid valve.

Note that this kind of circulation where the blood passes through the heart twice for each circulation is referred to as double circulation.

White Blood Cells
White blood cells are also called leucocytes. They are formed in the bone marrow of long bones like the femur (thigh bone).

They have a nucleus. Unlike the red blood cells, they lack haemoglobin and hence are colourless. They are fewer in number as compared to red blood cells. In a healthy human body, the ratio of white blood cells to red blood cells is 1: 600. However their number increases during infection. They are grouped into two types namely granulocytes and agronulocytes.

Granulocytes have lobbed nucleus with granules in the cytoplasm while agranulocytes have large rounded nucleus and lack granules.

Granulocyte and agranulocyte.

Functions of White Blood Cells

Granulocytes

Granulocytes are also called phagocytes. They fight infections by engulfing and digesting the pathogens as shown in the animation.

This animation illustrates a granulocyte cell carrying out phagocytosis. Foreign particles in blood are taken in and digested.

 

Agranulocytes

Agranulocytes consist of lymphocytes and monocytes.Lymphocytes fight infection by producing antibodies which destroy the pathogens.The monocytes fight infection by phagocytosis.

Comparison between Closed and Open Circulatory Systems

Open Circulation

  • In Open circulation transporting fluid is in contact with the body tissues.
  • Transporting fluid flows at a low pressure due to presence of sinuses.
  • Blood is circulated over a short distance at a slow rate.
  • It is less efficient at supplying nutrients to and removing waste products from the tissues.
  • Transport fluid is colorless due to lack of hemoglobin.

Closed Circulation

  • In Closed circulation there is no direct contact between transporting fluid and body tissues.
  • Transporting fluid flows under a high pressure due to flowing through the blood vessels.
  • Blood flows over a long distance at a fast rate.
  • It is more efficient at supplying nutrients to and removing waste products from the tissues.
  • Transport fluid is red due to hemoglobin pigment.

Pumping Mechanism of the Heart

The heart undergoes contraction and relaxation movements to pump blood to all parts of the body. The contractions are known as systole movements while the relaxations are known as diastole movements.

Systolic and diastolic movements.

Heart beat

 

Diastolic movements

Ventricle muscles relax while auricle muscles contract. This causes ventricle volume to increase while auricle volume decreases.Ventricle pressure decreases while auricle pressure increases. The atrio-ventricular valves open allowing deoxygenated blood from the body tissues to flow from the right auricle into the right ventricle while oxygenated blood from the lungs flows from the left auricle into the left ventricle. At the same time. Semi-lunar valves close preventing backflow of blood into ventricles.

 

Systolic movements

Ventricle muscles contract while auricle muscles relax. This causes ventricle volume to decrease while auricle volume increases. Ventricle pressure increases while auricle pressure decreases. The atrio-ventricular valves close while the semi-lunar valves open. Deoxygenated blood from the right ventricle flows through the pulmonary artery to the lungs via the semi-lunar valves while oxygenated blood from the left ventricle flows through the aorta via the semi-lunar valves.

Platelets

Platelets are also called thrombocytes. They are formed from disintegrated large cells of the bone marrow. They are relatively smaller than the red and white blood cells and also irregular in shape.Platelets are mainly involved in blood clotting process. They initiate blood clotting process by producing an enzyme called thrombokinase, also called thromboplastin, when exposed to air or tissue is damaged.

 

Heart Beat

This is the ryhthmic contraction and relaxation of the heart. The human heart beats at an average rate of 70-75 times per a minute at rest. The heart beat is initiated by a special tissue called Sino atrial Node (SAN) located in the wall of the right auricle. The heart beat can be felt as a pulse where the artery is close to the skin surface e.g. wrist

Contraction of the heart originates from the myogenic cardiac muscles. In the wall of the right atrium, close to where the vena cava enters the heart, there is a specialised muscle node made of fine fibres. This is the Sino-Atrial Node (SAN). The SAN sets the pace at which the heart muscles contract and relax and hence sets the pace of the heart beat. It is therefore called the pacemaker.

During heartbeat, the SAN initiates a few electrical impulses which flow thorough the muscles to the atria, and spreads to the junction between atria and ventricles. At this junction it excites another node of fibres called atrio-ventricular node (AVN). A specially modified cardiac muscle originates from the AVN and continues to the base of the heart. This muscle bundle is called Purkinje tissue. It has fibres radiating into the walls of both ventricles. When the AVN receives excitation from the atria, it sends impulses down the Purkinje tissue. The impulse excitation spreads through the walls of the ventricle and initiates muscular contraction and hence heartbeat.

Transport of Respiratory Gases

The respiratory gases are oxygen and carbon(IV) oxide. Oxygen is used during respiration while carbon(IV) oxide is produced during the process.

Oxygen transport

Oxygen diffuses into the red blood cell through its thin membrane to form an unstable compound with haemoglobin called oxyhaemoglobin. This occurs in the lungs where oxygen concentration is high. The red blood cells transport the oxygen to other body parts where oxyhaemoglobin breaks down to release oxygen due to low oxygen concentastion i these areas. The oxygen diffuses out of the RBC through the capillary walls into the tissue cells.

 

 

Carbon IV Oxide transport

Carbon IV Oxide is transported by the Red Blood Cells in two ways:

  • As carbaminohaemoglobin in red blood cells. This occurs when carbon(IV) oxide diffuses into the red blood cells from the respiring cells due to diffusion gradient to form carbaminohaemoglobin compound with haemoglobin.
  • As bicarbonate ions in red blood cells. This occurs when carbon(IV) oxide diffuses from the respiring cells into the red blood cells due to diffusion gradient. It then dissolves in water to form weak carbonic acid . This reaction is catalysed by the enzyme carbonic anhydrase. Carbonic acid is then converted to bicarbonate ions by the same enzyme.

A little carbon(IV) oxide is also transported as carbonic acid by the plasma.

Exchange of Materials between the Blood and Body Tissues

Exchange of materials between the blood and the body tissues occur across the walls of the capillaries by diffusion. This is made possible by the fact that capillary walls are one cell thick. This allows easy diffusion of nutrients from the blood into the tissues and metabolic waste from the tissues to the blood. Exchange of these substances occurs by ultra filtration as a result of high pressure of blood in the capillaries. Small molecules filter through the capillary walls into the intracellular spaces forming tissue fluid, through a process known as ultra filtration.

 

 

Tissue fluid is similar to plasma in composition. The only difference is that tissue fluid lacks red blood cells, proteins, and some white blood cells. The cells obtain nutrients from the tissue fluid and give out their waste products to the tissue fluid. This takes place through simple diffusion. Substances from the tissue fluid diffuse back into the blood capillaries. Water molecules move from the tissue fluid into blood through osmosis.

This animation illustrates exchange of materials between blood and body tissues across cappilary walls.

 




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