Transport in humans and Excretion

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Blood and its components

There are four main components in blood: Plasma, platelets, red blood cells, and white blood cells.

Plasma is the liquid part of the blood, it is pale yellow and carries all the other components of the blood. It also carries hormones, urea, digested food molecules (such as glucose,) carbon dioxide from respiration, and finally heat energy.

Red blood cells carry oxygen, and transport it around the body, which is vital for respiration. RED BLOOD CELLS CONTAIN HAEMOGLOBIN, which gives blood a red colour. In the lungs, the haemoglobin reacts with oxygen to form oxyhaemoglobin, and when the red blood cells reach cells in the body, the reverse reaction happens which releases oxygen into the cells. Red blood cells also don't have a nucleus, and this means the cell can carry more haemoglobin, and thus more oxygen. Also, their shape helps them to optimise oxygen levels, as a bioconcave shape gives a larger surface area for absorbing and releasing oxygen

PAPER TWO- when a blood vessel is damaged, the platelets clump together around the area. In the clot, a protein called fibrin holds the platelets together. This is known as blood clotting and prevents further blood loss. This process is also useful, since it prevents microorganisms entering and possibly infecting the wound. 

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Pathogens reproduce rapidly in the body, unless destroyed (the immune system's job.) 

There are two types of white blood cell: Lymphocytes and Phagocytes

Phagocytes work by detecting the pathogen and then engulfing and digesting it. They can also send chemical messages to call in more phagocytes. 

Lymphocytes work by producing antibodies. On every pathogen's surface, there are unique molecules called antigens. When the lymphocyte encounters the foreign body, it will produce antibodies to match the antigens. These bind to the antigens/pathogen to destroy them, and only specific antibodies fit the antigens. Antibodies are then produced rapidly to mark all similar pathogens, and lymphocytes reproduce rapidly. Antibodies can also coat the pathogens so that they clump and can easily be digested by a phagocyte. 

After the pathogens have been destroyed, some lymphocytes remain in the blood as memory cells. This means that if the same antigen enters the body again, the memory cells can quickly reproduce, since they carry a memory of the antigen. Therefore, you are immune to most diseases if you've already had them. 

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PAPER TWO- Vaccination protects from further infections.

When you are infected, lymphocytes take time to produce the suitable antibodies for the antigens, and during this time you can become seriously ill or die. Vaccination involves injecting inactive or dead pathogens into the body. They are harmless but still carry antigens, so they trigger an immune response. Therefore, your lymphocytes produce antibodies to attack them. 

Like a normal pathogen invasion, once they have been destroyed, memory cells will remain in the blood so that there is a quicker response in the future. This means that antibodies can be produced in a shorter time, sometimes preventing death if the real disease occurs. 

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Blood Vessels

Arteries- carry blood AWAY FROM THE HEART.

They carry blood at a high pressure, since they carry the blood pumped from the heart and the blood is always oxygenated. Therefore, the walls of the artery are thick, strong and elastic. The walls are very thick, compared to the smaller lumen, and they contain thick layers of muscle.

Veins- carry blood TO THE HEART.

The blood is under lower pressure in the veins, so the lumen is bigger to help the blood flow, and the walls are not as thick as the arteries. Veins also have valves in them to help the blood flow in the correct direction, and prevent backflow.

Capillaries- exchange materials at the tissues.

Arteries branch into capillaries, which then branch into veins. They are tiny, and too small to see. Capillaries have permeable walls, so substances can diffuse in and out.They carry blood close to every cell, so that they can supply oxygen and nutrients, but take away carbon dioxide. Their walls are usually one cell thick, which reduces diffusion distance/time. 

The pulmonary artery and vein are different to normal veins and arteries

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Heart Rate

Exercise increases heart rate, because when you are exercising, you are respiring more to provide the energy. Therefore, more oxygen is needed around your body, so your heart pumps more frequently to carry the oxygen to your cells. Also, you must remove carbon dioxide just as quickly. 


Exercise increases the concentration of carbon dioxide in the blood, and this is detected by receptors in the aorta and cartoid artery (neck.) These receptors send signals to the brain, which instructs the heart to beat more frequently and with greater force.

Heart rate also increases when the body feels threatened. This is because in this situation, the adrenal glands release the hormone adrenaline. This hormone binds to specific receptors in the heart. Therefore the cardiac muscle contracts more frequently to pump more blood. This supplies more oxygen to the tissues, getting the body ready for action. 

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The Heart


The main artery in the body is the aorta, and the main vein is the vena cavaThe right atrium recieves deoxygenated blood from the vena cava and the body. This blood travels through the right ventricle to the pulmonary artery, and into the lungs. The blood is then oxygenated in the lungs, and travels through the pulmonary vein to the left atrium. The blood then goes through the left ventricle, to the aorta, and to the body cells. The valves in the heart prevent blackflow, and the left ventricle wall is much thicker, as it has to pump the blood to the whole body, compared to the right ventricle (just the lungs.)

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The Circulation System

Hepatic- Liver related

Pulmonary- Lung related

Renal- Kidney related

For example, the renal artery takes blood away from the heart and to the kidneys. The renal vein therefore takes blood from the kidneys and to the heart.

This works similarly for the other organs.


It takes the blood away from the gut, to the liver. This is because if there are any toxins or poisons absorbed from the gut, they can be broken down by the liver before entering the main blood stream and going round the whole body.

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The removal of waste products is called excretion and is carried out by the skin (sweat or water,) the lungs (water and carbon dioxide,) and also the KIDNEYS:

  • They remove urea which is formed in the liver from an excess of amino acids. 
  • They adjust salt and water levels in the blood.
  • They excrete excess minerals.
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The Kidney


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Each kidney contains thousands of nephrons which filrate the blood.

1) Ultrafiltration: Blood from the renal artery flows through the glomerulus in the cortex (a knot of capillaries at the start of the nephron.) The high pressure means that the capillaries squeeze water, urea, salts and glucose from the blood into the Bowman's capsule. The membranes between the blood vessels in the glomerulus and the Bowman's capsule are semi-permeable and do not allow larger molecules like protein and blood cells to be squeezed out. Therefore, these molecules stay in the blood. The filtered liquid in the Bowman's capsule is known as the glomerular filtrate. 

2) Reabsorption: The glomerular filrate moves along the nephron, and useful substances such as all glucose is re absorbed by the proximal convoluted tubule in the cortex (this requires active transport and energy since it is against the concentration gradient,) sufficient salt, and sufficient water is reabsorbed( from the collecting duct from other nephrons into the blood.)

3) Release of wastes: The excess or unrequired substances such as urea, salts and water, form urine which continues down the nephron, ureter, and through the bladder. Finally it is released by the urethra.

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A Nephron


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Osmoregulation and ADH

Kidneys are vital for adjusting the body's water content. Water enters the body through food and drink, but exits the body in three main ways: sweating, breathing and urinating. The body has to balance the water in, with the water out. This is called OSMOREGULATION. A way to do this is to alter the volume of water excreted by the kidneys.

ADH: The Anti-Diuretic Hormone is found in the pituitary gland in the region of the brain called the Hypothalamus. It controls the amount of water reabsorbed back into the blood by targeting the collecting duct. The nephrons will become more permeable if ADH is released to absorb more water. The brain monitors the level of water in the blood, and therefore instructs how much ADH should be released. 


Brain detects water LOSS, and there is low water potential ---> the Pituitary gland releases more ADH ---> ADH makes the nephrons more permeable to water, and so more water is reabsorbed. 

Brain detects water GAIN, and there is high water potential ---> Pituitary gland releases less ADH ---> the nephrons reabsorb less water, and the urine is more concentrated with urea and minerals. 

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