Transport in Animals

TRANSPORT IN ANIMALS

As organisms increase in size and complexity so the quantity of materials moving in and out of the body increases. The distance that materials have to travel within the body also increases, so that diffusion becomes inadequate as a means for their distribution. There are two circulatory systems which rely on mass flow in animals, names:-

• Vascular system and
• Lymphatic system.

In animals, materials are transported through blood vascular system which is characterized by the following features;

1. Presence of the circulatory fluid eg: blood.
2. Presence of the tubes in which the blood flows eg: blood vessels.
3. Presence of a pumping device such as heart or modified blood vessels.

Composition of blood

1. Plasma: It consists of 90% water and 10% of a variety of substances in solution and suspension. ie: proteins, hormones, mineral salts, gases, wastes, water soluble vitamins B and C.
2. Blood cells:- They include;
   • Red blood cells (RBC) Which are packed with haemoglobin, the oxygen carrying compound which gives blood its red colour. They lack nucleus therefore makes more room for hemoglobin. RBC (erythrocytes) also lacks mitochondria which means they have to respire anaerobically. Therefore they do not use up any of the oxygen they carry. RBCs also contain the enzyme carbonic anhydrase which plays an important role in carbon dioxide transport.
   • White blood cells (leucocytes)- They play an important role in the body’s defence mechanisms against diseases.
   • Platelets (Thrombocytes) – They are responsible for starting the process of blood clotting.

Mechanism of blood clottingThe mechanism of blood clotting is a vital haemostatic process that prevents excessive loss of blood following an injury. The process occurs in three main stages: vascular spasm, platelet plug formation, and blood clotting (coagulation). Here's an explanation of the mechanism:

Vascular Spasm

The immediate constriction of blood vessels in response to injury. Mechanism:

• Triggered by chemicals released from damaged endothelial cells, platelets, and pain receptor reflexes.
• Reduces blood flow to the injured area to minimize blood loss.
• Typically lasts 20–30 minutes, providing time for other clotting mechanisms to activate.

Platelet Plug Formation

Formation of a temporary plug by platelets to seal the break in the blood vessel wall. Mechanism:

• Platelets adhere to exposed collagen fibers in the damaged vessel.
• Aggregated platelets release substances to recruit more platelets, forming a plug (also called the haemostatic plug).
• Endothelial cells release nitric oxide and prostacyclin, which restrict platelet aggregation to the injured site.

Blood Clotting (Coagulation)

Conversion of soluble fibrinogen into insoluble fibrin to stabilize the clot. Stages:

1. Formation of Prothrombin Activator: Triggered by complex chemical reactions in response to vessel damage.
2. Conversion of Prothrombin to Thrombin: Prothrombin activator converts the plasma protein prothrombin into the enzyme thrombin.
3. Formation of Fibrin Mesh:
   • Thrombin hydrolyzes fibrinogen into fibrin, which forms insoluble needle-like fibers.
   • Fibrin fibers create a mesh that traps blood cells and platelets, sealing the damaged site.

This entire mechanism ensures rapid and localized clotting at the injury site while preventing clots from forming elsewhere in the body. The clot remains until the vessel is permanently repaired.

The Heart

The heart is a muscular organ located between the lungs, behind the sternum, functioning as the body’s central circulatory pump. Its role is crucial for distributing oxygen and nutrients via the blood, making it one of the most vital organs.

Structure of the Heart

1. Location and Surrounding Structures:
   • Encased in a double-walled sac called the pericardium, which produces serous fluid to prevent friction and maintain the heart's position.
   • Positioned in the pericardial cavity, with the superior base connected to major blood vessels (aorta, pulmonary veins/arteries, and vena cava) and the inferior apex resting above the diaphragm.
2. Heart Walls:
   • Epicardium: Outer layer, thin and protective.
   • Myocardium: Middle muscular layer, responsible for pumping. Thickness varies; the left side is thicker than the right due to systemic circulation demands.
   • Endocardium: Inner layer, a thin endothelial lining inside the heart chambers.
3. Heart Chambers:
   1. Atria (right and left): Upper chambers receiving blood.
      • Right atrium receives deoxygenated blood from the body.
      • Left atrium receives oxygenated blood from the lungs.
   2. Ventricles (right and left): Lower chambers pumping blood.
      • Right ventricle pumps deoxygenated blood to the lungs.
      • Left ventricle pumps oxygenated blood to the entire body.
4. Heart Valves:
   1. Atrioventricular valves: Separate atria from ventricles.
      • Tricuspid valve (right side).
      • Bicuspid/mitral valve (left side).
   2. Semilunar valves: Separate ventricles from arteries.
      • Pulmonary valve: Right ventricle to pulmonary artery.
      • Aortic valve: Left ventricle to aorta.
   3. Chordae tendineae prevent valve inversion during blood flow.

Blood Flow Through the Heart

1. Deoxygenated blood enters:
   • From the vena cava into the right atrium.
   • Through the tricuspid valve to the right ventricle.
   • Ejected via the pulmonary valve into the pulmonary artery to the lungs.
2. Oxygenated blood returns:
   • From the lungs via pulmonary veins into the left atrium.
   • Through the bicuspid valve to the left ventricle.
   • Pumped via the aortic valve into the aorta for systemic distribution.

Heart Sounds

• "Lub": Closing of atrioventricular valves during ventricular contraction (longer sound).
• "Dup": Closing of semilunar valves after ventricular contraction (shorter sound).

Volume of blood pumped by the heart in one minute.

• Formula: Cardiac Output (CO)=Stroke Volume (SV)×Heart Rate (HR)\text{Cardiac Output (CO)} = \text{Stroke Volume (SV)} \times \text{Heart Rate (HR)}  Cardiac Output (CO)=Stroke Volume (SV)×Heart Rate (HR) Example: For a heart rate of 70 beats/min and stroke volume of 70 mL, CO = 5 L/min.

Function of the Heart

1. Pumps deoxygenated blood to the lungs for oxygenation and oxygenated blood to the body tissues.
2. Coordinates its chambers and valves to ensure unidirectional blood flow.
3. Supports the body's metabolic needs by maintaining efficient circulation

Functions of mammalian Blood

1. Transport of digested food from the small intestine to various parts of the body where they are stored or assimilated and transport from storage areas to places where they are used.

2. Transport of soluble excretory materials to organs of excretion.

3. Transport of hormones from the glands where they are produced to target organs. This allows communication within the body.

4. Distribution of excess heat from the deeply seated organs. This helps to maintain a constant body temperature.

5. Transport of respiratory gases (ie: oxygen and CO2).

6. Defense against diseases. This is achieved in three ways:-.

   • Clotting of blood which prevents excessive blood loss and entry of pathogens
   • Phagocytosis which engulf and digest bacteria.
   • Immunity achieved by antibodies and lymphocytes

7. Maintenance of constant blood solute potential and pH as a result of plasma protein Activity Transplantation

   • Refers to the replacement of diseased tissue or organs by healthy ones.
   • A technique is used increasingly in surgery today.
   • However, when foreign tissue is inserted into another individual it is usually rejected by the recipient because it acts as an antigen, stimulating the immune response in the recipient.

Types of transplant:-

1. Isograft Grafting within the same individual.
2. Autograft Grafting between two individuals who are genetically identical.
3. Allograft Two individuals of the same species.
4. Xenographt – Two individuals of different species.

The details of these are beyond the scope of your level Comparison of the structure and function of:-

Artery	Vein	Capillary
– Transports blood away from the heart.	– Transport blood towards the heart.	– Link arteries to vein. Site of exchange of materials.
– Tunica media thick and composed of elastic & smooth tissues.	– Tunica media relatively thin and only slightly muscular. Few elastic fibres.	– No tunica media. – No elastic fibres.
– No semilunar valves (except where leaves heart).	– Semilunar valves present so as to prevent back flow of blood.	– No semilunar valves.
– Blood plow rapid.	– Blood flow slow.	– Blood flow slowing.
– Low blood volume.	– Much higher blood volume than capillaries or arteries.	– High blood volume.
– Blood oxygenated except in pulmonary artery.	– Blood deoxygenated except in pulmonary vein.	– Mixed oxygenated and deoxygenated blood.

The cardiac cycleRefers to the sequence of events which takes place during the completion of one heart beat. It involves repeated contraction and relaxation of the heart muscle. Contraction is called Systole and relaxation is called Diastole****.**** It occurs is follows;

1. Atrial diastole During the time when the atria and the ventricles are both relaxed, blood returning to the heart enters the two atria. Oxygenated blood enters the left atrium and deoxygenated blood enters the right atrium. At first the bicuspid and tricuspid valve are closed but as the atria are filled with blood the valve are pushed open.
2. Atrial systole When the atriole distole ends, the two atria contract simultaneously. This is termed as atrial systole and results in blood being pumped into the ventricles.
3. Ventricular systole. The ventricles contract and pressure rises in them and forces open the semi-lunar valve of the aorta and pulmonary artery and blood enters these vessels. During ventricular systole the first heart sound described as “Lub” is produced.
4. Ventricular diastole Ventricular systole end and is followed by ventricular diastole. The higher pressure developed in the aorta and pulmonary artery tends to force some blood back towards the ventricles and this closes the semi-lunar valves of the aorta and pulmonary artery. Hence back flow into the heart is prevented. The closing of the valves causes the second heart sound “dub.” The two heart sounds are therefore:

Myogenic contraction of heart rateWhen a heart is removed from a mammal and placed in a well oxygenated salts solution at 37^o^Cit will continue to beat rhythmically for a considerable time without stimuli from the nervous system or hormones. This demonstrates the myogenic nature of the stimulation of the heart, i.e. heart muscle has its own ‘build in’ mechanism for bringing about its contraction. The stimulus for contraction of the heart originates in a specific region of the right atrium called the SINO-atrial node (SAN). This is located near the opening of the vena cavae. It consists of a small number of cardiac muscle fibers and a nerve ending from the automatic nervous system. The SAN can stimulate the heart beat on its own but the rate at which it beats can be varied by stimulation from the automatic nervous system. The cells of the SAN slowly become depolarized during atrial diastole. This means that the charge across the membrane is gradually reduced. At a certain point an action potential is set up in the cell. A wave of excitation similar to a nerve impulse passes across muscle fibers of the heart as the action potential spreads from the SAN. It causes the muscle fibres to contract. The SAN is the PACEMAKER because each wave of excitation begins here and acts as the stimulus for the next wave of excitation. Once contraction has begun, it spreads through the walls of atria through the network of cardiac muscle fibres and both atria contract more or less simultaneously. The atrial musle fibres are completely separated from those of the ventricles except for a region in the right atrium called the atrio-ventricular node (AVN) The structure of the AVN is similar to that of the SAN and is connected to a bundle of specialized muscle fibres, the AV bundle which provides the only route for the transmission of the wave of excitation from the atria to the ventricles. There is a delay of approximately 0.15s in conduction from the SAN to AVN, which means that atrial systol is completed before ventricular systole begins. The AV bundle is connected to the bundle of His (strand of modified cardiac fibres) which gives rise to finer branches known as Purkyne. Impulses are conducted rapidly along the bundle and spread out from there to all parts of the ventricles. Both ventricles are stimulated to contract simultaneously. The wave of ventricular contraction begins at the bottom of the heart and spread upwards squeezing blood out of the ventricles towards the arteries which pass vertically upwards out of the heart. NOTE: The period during which cardiac fibres do not respond is called Absolute refractory period. This period is longer in cardiac muscle than in other types of muscles and enables it to recover fully with becoming fatigued, even when contracting vigorously and rapidly. As muscle recover it passes through a relative refractory period when it will respond only to a strong stimulus. The cardiac muscleThe cardiac muscle is that muscle that forms the walls of the heart. Structure of the cardiac muscle:

• Structurally, the cardiac muscle has fibers, each of which consists of cylindrical short cells arranged in columns. Each cell has a central nucleus, myofibrils and faint transverse striations.
• Adjacent columns are joined by oblique cross connections.
• The cells have abundant sarcoplasm and they are well supplied with blood and mitochondria.

Diagram: Adaptations of the cardiac muscle

1. It is striated to confer strength so that it withstands the pumping pressure of the blood.
2. It is highly vascularized to ensure adequate supply of food and oxygen.
3. The numerous mitochondria supply the necessary energy required to pump the blood.
4. It is myogenic ie: Contraction and relaxation are initiated within itself.
5. It is capable of contracting and relaxing throughout its life without any fatigue.
6. Cells can tolerate high levels of lactate (a product of anaerobic respiration).

The circulatory systems in animals The circulatory system of animals is a combination of circulating fluid such as blood, tubes through which the fluid flows, for example, blood vessels and a pumping organ such as the heart or modified blood vessel. This system is used to transport blood and other materials throughout the body of an animal. Animals have different shapes, sizes, and types of circulatory systems. The animal can have either an open or a closed circulatory blood system. Types of circulatory systems in animals Two distinct types of blood systems are found in animals;

1. Open blood system and
2. Closed blood system.

The open blood system

1. This is a type of blood vascular system in which blood mixes with the body tissues.
2. In this case, blood is pumped by the heart into an aorta which branches into a number of arteries.
3. These open into a series of blood species collectively called Haemocoel.
4. The blood then goes back through the open ended vein. In this system, the flow of blood cannot be adjusted and, the blood flows at a low pressure.
5. This system is found in arthropods and molluscs.

Closed circulatory system

1. This is a type of circulatory system in which the blood is confined to vessels.
2. Blood never mixes up with the body tissues nor does it bath the organs directly.
3. In this case, the speed of the blood can be adjusted, the blood is at high pressure and therefore goes around the body very quickly.
4. This system is found in Annelids and vertebrates.

Question: Summarize the differences between open and closed circulatory systems.

S/N	Open Circulatory System	Closed Circulatory System
1	The blood is not confined to vessels.	The blood is confined to vessels.
2	The organs are in contact with the blood, i.e., they are immersed in blood.	The organs are not immersed in blood, i.e., they are not in contact with the blood.
3	The blood circulates slowly around the body due to low pressure.	The blood circulates rapidly around the body due to high pressure.
4	The direction of blood flow can slightly be controlled.	The direction of blood flow has defined control.
5	The rate of flow cannot be controlled.	The rate of flow can be controlled.
6	The supply and elimination of materials take place slowly.	The supply and elimination of materials take place rapidly.
7	The exchange of materials takes place between blood and sinuses.	The exchange of materials between blood and tissues takes place through capillaries.

Subdivisions of closed circulatory systemThe closed circulatory system is subdivided into: Single circulatory systemThis is a type of circulatory system in which blood passes the heart only once in a single complete circulatory turn. In fish for example blood from the heart first goes to the gills to collect oxygen, but then continues round the whole body before returning to the heart. The deoxygenated blood from various parts of the body passes direct to the heart which pumps it to the gills for being oxygenated for the circulation to begin once more. Diagram: Single circulation of blood in fish General plan of the mammalian circulatory system (Double circulatory system) Double circulatory systemIn this type of circulation, blood passed the heart twice in a single complete circulatory turn.

• Only birds and mammals have true double circulations. It is probably no coincidence that only birds and mammals are warm blooded.
• Warm-bloodedness requires a high metabolic rate and this is only possible if a good supply of oxygen is available for high levels of aerobic respiration. Animals with a high metabolic rate can maintain higher levels of activity than other animals.

Advantage of double circulation systemBlood can be sent to the lungs to pick up oxygen and then be returned to the heart to be pumped again before travelling around the body. Double circulatory system has;

1. Pulmonary circulation and
2. Systemic circulation.
3. Coronary blood circulation.

Pulmonary circulationThis is a circulation between the heart and the lungs. Deoxygenated blood from the heart is carried by the pulmonary artery to the lungs where as oxygenated blood from the lungs to the heart is carried by pulmonary vein. Systematic circulationThe circulation between the heart and all other body parts except the lungs. Deoxygenated blood from various parts of the body is brought to the heart by the vena cava where as oxygenated blood from the heart is pumped to various body parts through the aorta. Coronary circulation:- This is the circulation within the walls of the heart. Features of a human circulation

1. It is a double circulation.
2. The organs are arranged in parallel rather than in series. If they were arranged in series, blood would pass from organ. A to B to C and so on, losing pressure, oxygen and nutrients in each stage. This would be extremely inefficient. Also, any damage done to a blood vessel linking two organs would interrupt the whole circulation.
3. A portal vessel (vessel linking two organs neither of which is the heart) links the gut to the liver i.e.: Gut and liver are linked in series not in parallel.

Advantage of this series linkage is that blood from the gut is variable in composition and it contains other substances such as alcohol. Liver monitors blood passing through it and maintains a constant composition. Eg: Liver removes excess glucose from the blood and stores it as glycogen. **NOTE:**Vessels conveying blood away from the heart are called Arteries. These divide into smaller arteries called arterioles. The arterioles divide many times into capillaries where exchange of materials between blood and tissue takes place. Within the organ or tissue the capillaries reunite to form Venules which begin the process of returning blood to the heart. The venules join to from Veins. Veins carry blood back to the heart. Types of transplant:-

1. Isograft Grafting within the same individual.
2. Autograft Grafting between two individuals who are genetically identical.
3. Allograft Two individuals of the same species.
4. Xenographt Two individuals of different species.

The details of these are beyond the scope of your level Comparison of the structure and function of:- Changes in foetal circulation at birth (The foetal circulation)

• Throughout the development in the uterus the fetal lungs do not function since gaseous exchange and nutrition are provided by the mother via the placenta.
• Most of the oxygenated blood returning to the fetus via the umbilical vein by-passes its liver in a vessel, the ductus venosus which shunts blood into the inferior vena cava and passes it to the right atrium.
• Some blood from the umbilical vein flows directly to the liver, blood entering the right atrium therefore contains a mixture of oxygenated and deoxygenated blood. From here most of the blood passes through an opening. Some blood passes from the right atrium into the right ventrical and into the pulmonary artery but does not pass to the lungs. Instead it pass through the ductus arteriosus directly to the aorta, so by-passing the lungs, pulmonary vein and the atrium and ventricle of the left side of the heart. Blood from the left atrium passes into the left ventricle and into the aorta which supplies blood to the body and the umbilical artery.
• Pressure into the fetal circulatory system is greatest in the pulmonary artery and this determines the fetus and placenta.

• The baby finally acquires an adult’s circulatory system and independent physiology

Changes at birth At birth the sudden inflation of the lungs reduce the resistance to blood flow through the pulmonary capillaries and blood flows through them in preference to the ductus arteriosus; this reduces the pressure in the pulmonary artery. At the same time the typing of the umbilical cord prevents blood from flowing through the placenta, and this increases the volume of blood flowing through the body of the baby and leads to a sudden increase in blood pressure in the aorta, left ventricle and left atrium. This pressure change causes the small valves guarding the foramen ovale which open to the left atrium to close, preventing the short circuiting of blood from right to left atrium. Within a few months these valves fuse to the wall between the atria and close the foramen ovale completely. If this does not occur, the baby is left with a “hole in the heart” and will require surgery to correct the defect. The increased pressure in the aorta and decreased pressure in the pulmonary artery force blood backwards along the ductus arteriosus into the pulmonary artery an d hence to the lungs, thereby boosting its supply. After a few hours, muscles in the walls of the ductus arteriosus constrincts under the influence of the rising of oxygen in the blood and close off this blood vessel. A similar mechanism of muscular contraction closes of the ductus venosus and increasing blood flow through the liver. The mechanism of closing down the ductus venosus is not known but is essential in transporting the ante-natal (before birth) circulation into the post-natal (after birth) condition. Note: Failure of the foramen ovule to close up, results into continued mixing up of the oxygenated and deoxygenated blood. Due to high concentration of CO2in the blood, the baby develops the blue tanks on its skin and it is described as a “Blues baby”. Differences between the foetal and adult blood circulation There are both similarities and differences between the foetus and adult blood circulation systems. The cause of these differences is that most functions that are performed by the gut, kidney, liver and lungs in the adult are all performed by placenta in the foetus. For example, the placenta supplies the nutrients and oxygen in the foetus. In addition, the waste materials, which are returned to the maternal circulation, pass through the placenta. Generally, the differences between foetal and adult circulation systems. between foetal and adult circulatory systems.

S/N	Fetal Circulation	Adult Circulation
1	There are umbilical vessels.	The umbilical vessels are missing.
2	Blood bypasses the liver due to the presence of ductus venosus.	The blood goes to the liver as ductus venosus is sealed.
3	There is no pulmonary circulation due to the presence of ductus arteriosus.	There is pulmonary circulation since the ductus arteriosus is sealed.
4	There is mixing of oxygenated and deoxygenated blood due to the presence of foramen ovale.	The oxygenated and deoxygenated blood do not mix as the foramen ovale is sealed.
5	Lungs are not functional; gaseous exchange takes place at the placenta.	Lungs are functional, and therefore, are sites for gaseous exchange.
6	There is no functional hepatic portal vein.	There is a functional hepatic portal vein.
7	Hemoglobin has a high affinity for oxygen.	Hemoglobin has a low affinity for oxygen.
8	The pressure is higher in the pulmonary artery than in the aorta.	The pressure is higher in the aorta than in the pulmonary artery.