Transport in Mammals Notes

Transport in Mammals

B7.01 The Circulatory System

The circulatory system, also known as the blood system, is the main transport system in mammals, including humans.
It consists of a network of tubes called blood vessels and a pump, the heart, that keeps the blood flowing.
Valves in the heart and blood vessels ensure the blood flows in the right direction.
Blood flows into the left-hand side of the heart from the lungs, goes out to the rest of the body, returns to the right-hand side of the heart, and then goes back to the lungs.

Oxygenating the Blood

Blood in the left-hand side of the heart comes from the lungs and contains oxygen picked up by capillaries surrounding the alveoli; this is called oxygenated blood.
Oxygenated blood is sent around the body, where body cells take up some of the oxygen for respiration. The blood then becomes deoxygenated.
Deoxygenated blood returns to the right-hand side of the heart and goes to the lungs to become oxygenated again.

The Double Circulatory System

The circulatory system is a double circulatory system, meaning blood passes through the heart twice in one complete circuit of the body.
The two parts of this system are:
- The pulmonary system: blood vessels that take blood to the lungs and back.
- The systemic system: blood vessels that take blood to the rest of the body and back.
Double circulatory systems are found in mammals, birds, and reptiles, while fish have a single circulatory system where blood passes through the heart only once in a complete circuit.

Advantages of a Double Circulatory System

Blood loses pressure when flowing through tiny blood vessels in the lungs or gills.
In mammals, low-pressure blood returns to the heart to have its pressure raised before being sent to the rest of the body.
In fish, low-pressure blood continues around the body, meaning blood travels more slowly to the fish's body organs compared to mammals.
This is important for the delivery of oxygen for respiration, which is more effective in mammals due to the quicker delivery to metabolically active tissues.

B7.02 The Heart

The function of the heart is to pump blood around the body and is made of cardiac muscle that contracts and relaxes regularly throughout life.
The heart is divided into four chambers: two upper chambers called atria and two lower chambers called ventricles.
The chambers on the left-hand side are separated from those on the right-hand side by a septum.

Blood Flow Through the Heart

Blood flows into the heart at the top, into the atria.
The left atrium receives blood from the pulmonary veins, which come from the lungs.
The right atrium receives blood from the rest of the body through the venae cavae.
From the atria, blood flows into the ventricles, which then pump it out of the heart.
The strong cardiac muscle contracts with considerable force, squeezing inwards on the blood inside the heart and pushing it out.
The blood in the left ventricle is pumped into the aorta, which takes the blood around the body.
The right ventricle pumps blood into the pulmonary artery, which takes it to the lungs.
The left ventricle has an especially thick wall of muscle to enable it to pump blood all around the body; blood flowing to the lungs in the pulmonary artery has a much lower pressure than the blood in the aorta.

Functions of Atria and Ventricles

The atria receive blood from the lungs or the body and supply it to the ventricles.
The ventricles pump blood out of the heart to other parts of the body and have much thicker, more muscular walls than the atria.
The right ventricle pumps blood to the lungs, which are very close to the heart, while the left ventricle pumps blood all around the body.

Coronary Heart Disease

Coronary arteries are blood vessels on the outside of the heart that supply blood to the heart muscles. These muscles are so thick that nutrients and oxygen in the blood inside the heart would not be able to diffuse to all the muscles quickly enough.
If a coronary artery gets blocked, for example, by a blood clot, the cardiac muscles run short of oxygen.
They cannot respire, so they cannot obtain energy to allow them to contract, and the heart therefore stops beating. This is called a heart attack or cardiac arrest.
Blockage of the coronary arteries is called coronary heart disease (CHD), a common cause of illness and death, especially in developed countries.

Risk Factors for Coronary Heart Disease

Smoking cigarettes: Several components of cigarette smoke, including nicotine, cause damage to the circulatory system. Stopping smoking is the single most important thing a smoker can do to reduce their chances of getting coronary heart disease.
Diet: A diet high in salt, saturated fats (fats from animals), or cholesterol increases the chances of getting coronary heart disease. Eating a diet containing a wide variety of foods, with not too many fats, is advisable. Oils from plants and fish can help to prevent heart disease.
Obesity: Being very overweight increases the risk of coronary heart disease. Keeping your body weight at a suitable level and taking plenty of exercise helps to maintain the coronary arteries in a healthy condition.
Stress: Unmanageable or long-term stress appears to increase the risk of developing heart disease. Avoiding severe or long-term stress is a good idea; otherwise, finding ways to manage stress is important.
Genes: Some people have genes that make it more likely they will get coronary heart disease, called a genetic predisposition. In that case, it is important to try hard to reduce the other risk factors by having a healthy life-style.

Heart Beat

Most people's hearts beat about 60 to 75 times a minute when they are resting.
Each complete 'lub-dup' sound represents one heart beat and the sounds of the valves closing with each heart beat.
A good way to measure the rate of your heart beat is to take your pulse rate, caused by the expansion and relaxation of an artery.
When a person exercises, their heart beats faster because their muscles are using up oxygen more quickly in respiration, to supply the energy needed for movement.

Valves in the Heart

There are one-way valves between the left atrium and ventricle and between the right atrium and ventricle, called atrioventricular valves.
These valves stop blood flowing from the ventricles back to the atria, ensuring blood is pushed up into the arteries when the ventricles contract.
The rate at which the heart beats is controlled by a patch of muscle in the right atrium called the pacemaker, sending electrical signals through the walls of the heart at regular intervals.
The pacemaker's rate changes according to the needs of the body; for example, during exercise, when extra oxygen is needed by the muscles, the brain sends impulses along nerves to the pacemaker to make the heart beat faster.

Factors Affecting Heart Rate During Exercise

During exercise, muscles respire more quickly than usual, releasing energy needed for movement. This increases the production of carbon dioxide, which dissolves in the blood, forming a weak acid and lowering the pH of the blood.
Receptor cells in the brain sense this drop in pH, triggering an increase in the frequency of nerve impulses sent to the pacemaker.

Heart Pumping Mechanism

The heart beats as the cardiac muscles in its walls contract and relax. When they contract, the heart becomes smaller, squeezing blood out. When they relax, the heart becomes larger, allowing blood to flow into the atria and ventricles.

B7.03 Blood Vessels

Types of Blood Vessels

There are three main kinds of blood vessels: arteries, capillaries, and veins.
Arteries carry blood away from the heart and divide again and again, eventually forming very tiny vessels called capillaries.
The capillaries gradually join up with one another to form large vessels called veins.
Veins carry blood towards the heart.

Arteries

When blood flows out of the heart, it enters the arteries, then at very high pressure because it has been forced out of the heart by the contraction of the muscular ventricles.
Arteries therefore need very strong walls to withstand the high pressure of the blood flowing through them.
The blood does not flow smoothly through the arteries but pulses through as the ventricles contract and relax.
The arteries have elastic tissue in their walls, which can stretch and recoil with the force of the blood, helping to make the flow of blood smoother.

Capillaries

The arteries gradually divide to form smaller and smaller vessels. No cell is very far away from a capillary.
The function of the capillaries is to take nutrients, oxygen, and other materials to all the cells in the body and to take away waste materials.
To do this, their walls must be very thin so that substances can get in and out easily, where the walls of the smallest capillaries are only one cell thick.

Veins

The capillaries gradually join up again to form veins. By the time the blood gets to the veins, it is at a much lower pressure than it was in the arteries.
The blood flows more slowly and smoothly now, and there is no need for veins to have such thick, strong, elastic walls.
The space inside the veins, called the lumen, is much wider than the lumen of the arteries to help keep the blood moving easily through them.
Veins have valves in them to stop the blood flowing backwards, unlike arteries, because the force of the heart beat keeps blood moving forwards through them.
Contraction of skeletal muscles around the veins also keeps blood moving in the veins, as the large veins in your legs are squeezed by your leg muscles when you walk, this helps push the blood back up to your heart.

Naming Blood Vessels

Figures B7.09 and B7.10 illustrate the positions of the main arteries and veins in the body.

B7.04 Blood

Components of Blood

The liquid part of blood is called plasma, containing cells, mostly red blood cells, in it. A much smaller number are white blood cells. There are also small fragments formed from special cells in the bone marrow called platelets.
Plasma is mostly water, containing many dissolved substances, soluble nutrients such as glucose, amino acids, and mineral ions, hormones, and carbon dioxide.
Red blood cells are made in the bone marrow of some bones, including the ribs, vertebrae, and some limb bones. They are produced at a very fast rate - about 9000 million per hour.
Red cells have to be made so quickly because they do not live for very long, about 4 months, as they don't have a nucleus.
Red cells are red because they contain the pigment haemoglobin, which carries oxygen and contains iron, which readily combines with oxygen where the gas is in good supply and just as readily gives it up where the oxygen supply is low, as in active tissues.
The lack of a nucleus in a red blood cell means more space for packing millions of molecules of haemoglobin.
Another unusual feature of red blood cells is their shape; they are biconcave discs, like a flat disc that has been pinched in on both sides. This, together with their small size, gives them a relatively large surface area compared with their volume, speeding up the rate at which oxygen can diffuse in and out of the red blood cell.
The small size of the red blood cell is also useful in enabling it to squeeze through even the tiniest capillaries, meaning oxygen can be taken very close to every cell in the body.
White cells are easily recognized because, unlike red blood cells, they do have a nucleus, which is often quite large and lobed. They can move around and squeeze out through the walls of blood capillaries into all parts of the body, fighting pathogens (disease-causing bacteria and viruses) and clearing up any dead body cells, taking in and digesting bacteria in phagocytosis, other produce chemicals called antibodies.
Platelets are small fragments of cells, with no nucleus.
They are made in the red bone marrow and are involved in blood clotting, stopping pathogens getting into the body through breaks in the skin and preventing too much blood loss.