9. Transport in Animals

• All living animal cells need a supply of oxygen and nutrients
• They also need to remove waste products such as carbon dioxide and urea

Size, SA:V and Metabolic Rate:

• These are the three main factors that affect the need for a transport system
• Active organisms usually have high metabolic rates, this requires more oxygen to allow more aerobic respiration to take place
• This is essential so that more ATP can be released to provide energy for the higher level of activity
• Size and SA:V affects the need fora transport system as the larger the organism, the smaller the SA:V which means it is harder for substances to simply diffuse

Single Circulatory System:

• In this system, blood flows through the heart once every time it goes around the body 
• Fish have a single circulatory system
• The blood flows from the heart to the gills and then on to the body before returning to the heart

Heart --> Gills --> Body --> Heart

Double Circulatory System:

• In this system, blood flows through the heart twice for every circuit around the body
• Mammals have developed a circulation that involves two separate circuits
• One circuit carries blood to the lungs to take up oxygen (pulmonary circulation)
• The other circuit carries the oxygen and nutrients around the body to the tissues (systemic circulation)
• The heart is adapted to form two pumps, one for each circulation 

Body --> Heart --> Lungs --> Heart --> Body

Open Circulatory System: Insects have this type of system

In an open system:

• there is no separate tissue fluid
• blood circulates around the organs and cells
• pressure cannot be raised to help circulation
• circulation is affected by body movement 
• oxygenated and deoxygenated blood mixes freely

Closed Circulatory System: Fish and Mammals have this type of system

In a closed system:

• blood is kept in vessels
• pressure can be maintained
• pressure can be higher
• flow can be faster
• flow can be directed to certain tissues or organs 

• Blood flows through a series of vessels

• Each is adapted to its particular role in relation to its distance from the heart

• All types of blood vessel has an inner layer made of cells called endothelium

• This is a thin layer that is particularly smooth to reduce friction with the flowing blood

• Arterioles are small arteries with a spiral layer of smooth muscle

• They distribute blood from the arteries to the capillaries and can constrict to reduce blood flow

• Venules are small veins that collect the blood from the capillaries and lead into the veins

Feature                              Arteries                              Veins                              Capillaries

Function                            Transports blood away       Transports blood            Enables exchange of 

                                           from the heart                     back to the heart            materials in blood

Thickness of Wall             Thick                                  Thin                               One cell thin          

Components of Wall         All comps thick                  All comps thin              One layer of cells

Blood Pressure                  High                                    Low                              Low

Presence of Valves            No                                       Yes                                No

Cause of Flow                   Pressure created by heart    Squeezing of valves     Pressure from action of heart

Blood is the fluid found inside the blood vessels. It consists of:

Tissue fluid surrounds the body cells. It is plasma that has been filtered out of the blood, so it contains all the dissolved elements the blood (except the cells, platelets and plasma proteins as they are too large to pass out of the blood vessels). There may be some phagocytic neutrophils in tissue fluid as these can change shape to squeeze out of the blood vessels. 

The Formation of Tissue Fluid:

• Fluid and dissolved substances can squeeze between the endothelium cells of the capillaries
• The fluid is acted upon by two forces:At the arterial end of the capillary, the hydrostatic pressure created by the heart is still quite high.
  • hydrostatic pressure- gradient between the blood and the tissue fluid, which tends to push fluid out of the capillary
  • oncotic pressure- gradient between the blood and tissue fluid, which tends to move the fluid into the blood because the water potential of the blood is lower than the water potential of the tissue fluid
    
• Therefore there is a steep pressure gradient, which overcomes the oncotic pressure, pushing fluid out of the capillary.
• At the venous end of the capillary, the hydrostatic pressure is lower
• The hydrostatic pressure is less steep than the oncotic pressure and the fluid returns to the capillary
• This is how oxygen and nutrients enter the cells and carbon dioxide and waste is removed from cells
  

• Lymph is excess tissue fluid that is not returned to the blood vessel

• Instead, it is drained into the lymph vessels

• These carry the fluid back to the circulatory system by a different route

• Lymph contains the same substances as tissue fluid but it has less oxygen and glucose as these have been used by the cells

• Lymphocytes produced in the lymph nodes may also be present

• The mammalian heart is a muscular pump that is divided into two sides

• The right side pumps deoxygenated blood to the lungs to be oxygenated

• The left side pumps oxygenated blood to the rest of the body

• On both sides the action of the heart is to squeeze the blood, putting it under and forcing it along the arteries

• The muscle surrounding the two main pumping chambers (the ventricles) is dark red

• Above the ventricles are two thin-walled chambers known as the atrium

• These are much smaller than the ventricles

• On the surface of the heart are coronary arteries 

• These carry oxygenated blood to the heart muscle itself 

• These arteries are important as the heart continually works hard

• If they become constricted, blood flow to the heart muscle is restricted and this can reduce the delivery of oxygen and nutrients

• At the top of the heart are the veins that carry blood into the heart and the arteries that carry the blood out the heart 

The heart is divided into four chambers: two atria and two ventricles

The Atria:

The two upper chambers are atria. These receive blood from the major veins. Deoxygenated blood flows from the vena cava into the right atrium. Oxygenated blood flows from the pulmonary vein into the left atrium. The atria have very thin walls as they do not need to create much pressure. Blood simply flows through the atria into the ventricles. When the ventricles are nearly full, the atrial walls contract just to completely fill the ventricles. 

Deoxygenated Blood: Vena Cava --> Right Atrium

Oxygenated Blood: Pulmonary Vein --> Left Atrium

Both Blood: Atria --> Ventricles

The Ventricles:

The two lower chambers of the heart are the ventricles. Each has a thick muscular wall. The wall contracts to create pressure which pushes the blood into the arteries.

The right ventricle has walls that are thicker than the atrial walls. This enables it to pump blood out of the heart. The right ventricle pumps deoxygenated blood to the lungs. The pressure of the blood must not be too high to prevent the capillaries in the lungs bursting.

The walls of the left ventricle are often two or three times thicker than the right ventricle. The blood from the left ventricle is pumped out through the aorta and needs sufficient pressure to propel it all the way around the body.

The ventricles are separated from each other by a wall of muscle called the septum. This ensures that the oxygenated blood on the left side is kept separate from the deoxygenated blood of the right side.

Deoxygenated Blood: Right Ventricle --> Lungs

Oxygenated Blood: Left Ventricle --> Aorta --> Whole Body

Deoxygenated Blood: Vena Cava --> Right Atrium --> Righ Ventricle --> Lungs

Oxygenated Blood: Pulmonary Vein --> Left Atrium --> Left Ventricle --> Aorta --> Whole Body

1. Blood returns to the heart from the body (via the vena cava) and lungs (via the pulmonary vein). Both the atria fill at the same time. The valves between the atria and the ventricles (the atrioventricular valve) are open to allow blood to flow straight through into the ventricles.

2. Once the ventricles are nearly full, the sinoatrial node (SAN) initiates a new heartbeat. It creates a wave of excitation which spreads over the walls of the atria. The walls contract, pushing a little extra blood from the atria into the ventricles. The wave of excitation is stopped by a layer of non-conducting fibres between the atria and the ventricles. The wave of excitation can only pass through the atrioventricular node (AVN), where it is delayed a little. This allows time for the ventricles to fill.

3. After the delay, the wave of excitation passes down the bundle of His in the septum between the ventricles. At the base of the septum, the bundle splits into separate fibres call Purkyne tissue that carries the excitation up the walls of the ventricles, causing a contraction from the base upwards. The walls of the two ventricles contract together. As the pressure rises, the AV valves are pushed shut which prevents them from inverting.

4. The blood pressure in the ventricles rises quickly until it rises above the pressure of the aorta and pulmonary artery. This pushes the semilunar valves open and blood is pushed into the main arteries.

5. Once the contraction is complete, the muscles relax and the heart goes back to the original size and shape

The changes in pressure in the heart can be represented by a graph.

The important points where one line crosses another as this is where the pressure in one chamber rises above that in another chamber, causing a valve to open or close. 

An electrocardiogram (EEG) records the electrical activity of the heart

• Wave P is the excitation of the atria
• Wave QRS is the excitation of the ventricles
• Wave T is associated with ensuring the muscles have time to rest

Abnormal heart activity can often be identified by an abnormal EEG trace

The waves may be smaller, inverted or further apart

Haemoglobin- the red pigment that transports oxygen

Oxyhaemoglobin- The product that is formed when oxygen combines with the haemoglobin in the blood

Transport of Oxygen

Oxygen enters the blood in the lungs. Oxygen diffuses into the blood plasma and red blood cells. The oxygen associates with the haemoglobin (Hb) to form oxyhaemoglobin. 

Haemoglobin is a complex protein with four subunits. Each subunit contains a haem group that contains a single iron ion. This attracts and holds one oxygen molecule. The haem group has an affinity (attraction) to oxygen. Each haemoglobin molecule can carry four oxygen molecules. 

Transport of Carbon Dioxide

Carbon dioxide released from respiring tissues must be removed from the tissues and transported to the lungs. Carbon dioxide in the blood is transported in three ways:

• as hydrogencarbonate ions (HCO3-) in the plasma (85%)
• combinded directly with haemoglobin to form carbaminohaemoglobin (10%)
• dissolved directly in the plasma (5%)

Oxygen Dissociation Curve

The oxyhemoglobin dissociation curve relates oxygen saturation (SO₂) and partial pressure of oxygen in the blood (PO₂), and is determined by what is called "haemoglobin's affinity for oxygen," that is, how readily haemoglobin acquires and releases oxygen molecules from its surrounding tissue

Fetal Haemoglobin

Fetal haemoglobin is a modified form of haemoglobin found in the mammalian fetus. This haemoglobin has a higher affinity for oxygen compared to adult haemoglobin. This is because the fetus haemoglobin has to take up oxygen from the mother's haemoglobin. 

The oxyhaemoglobin dissociation curve for fetal haemoglobin is to the left of the curve for adult haemoglobin. 

• As carbon dioxide diffuses into the blood, some of it diffuses into the red blood cells

• It combines with water to form a weak acid (carbonic acid)

• This reaction is catalysed by the enzyme carbonic anhydrase

• CO2 + H20 --> H2CO3

• The carbonic acid dissociates to release hydrogen ions (H+) and hydrogencarbonate ions (HCO3-)

• H2CO3 --> H+ + HCO3-

• The hydrogencarbonate ions diffuse out of the red blood cells into the plasma

• The charge inside the red blood cells is maintained by the movement of chloride ions (Cl-) from the plasma into the red blood cells. This is called chloride shift

• The hydrogen ions could cause the content of the red blood cells to become acidic. To prevent this, the hydrogen ions are taken up by haemoglobin to produce haemoglobinic acid. The haemoglobin is acting as a buffer (a compound that maintains the pH) 

• The hydrogen ions produced are absorbed by the haemoglobin, however, in very active tissues a lot of carbon dioxide is released and therefore a lot of hydrogen ions are created

• As the concentration of hydrogen ions increase, this decreases the pH of the cytoplasm

• The change in pH alters the structure of the haemoglobin, reducing the affinity for oxygen

• Therefore more oxygen is released

• The haemoglobin dissociation curve shifts to the right (the Bohr effect)

• This also frees up more haemoglobin molecules to absorb the additional hydrogen ions