blood. This is partly because the blood has less far to go (the lungs are right next to the heart), but also because a lower pressure in the pulmonary circulation means that less fluid passes from the capillaries to the alveoli. The heart is made of cardiac muscle, composed of cells called myocytes. When myocytes receive an electrical impulse they contract together, causing a heartbeat. Since myocytes are constantly
active, they have a great requirement for oxygen, so are fed by numerous capillaries from two coronary arteries. These arise from the aorta as it leaves the heart. Blood returns via the coronary sinus, which drains directly into the right atrium.
Exchange surfaces usually have:
- Large surface area to volume ratio to enable more efficient exchange by increasing the surface area and reducing the diffusion distance. This may be achieved by folding of tissues (eg: villi) or by presence of microvilli / brush borders on cell surfaces (eg: epithelial cells in proximal convoluted tubule); outgrowths – root hair cells.
- Thin wall– to decrease diffusion distance(eg: alveoli).Single layer of epithelium.
- Large number of mitochondria – to provide ATP (energy) for active uptake. Eg: intestinal villi, proximal convoluted tubule, root hair cells.
- understand the special features of gas exchange surfaces;
All gas exchange surfaces have some common features which enable more efficient / rapid exchange of gases by diffusion:
- Large SA : volume ratio – more surface for diffusion.
- Thin walled: decreases diffusion distances.
- Moist and permeable - so that gases first dissolve in moisture and then diffuse, the moisture acts as a diffusion medium.
- They are able to maintain a concentration gradient – to increase rate of diffusion.
Eg: mesophyll cells of leaves, alveoli in lungs, gills in fishes.
Thorax/thoracic cavity is the region of the body cavity (coelom) lying above the diaphragm. It is surrounded by ribs, sternum and vertebral column (rib cage). The lungs and heart are found in the thoracic cavity. Ventilation (breathing movements) is achieved by the combined action of ribs, intercostal muscles and diaphragm.
Pleural membranes surround the lungs. There are 2 pleural membranes – the outer parietal pleural membrane and the inner visceral pleural membrane. The membranes secrete a fluid, which remains in the pleural cavity, between both membranes. This lubricates the pleura and reduces friction as the membranes rub against each other during breathing movements.
The main role of alveoli is gas exchange between air and blood. Each alveolus is surrounded by a network of capillaries. The alveolus and capillaries are made up of a single layer of epithelial cells. This decreases the diffusion distance. The flow of blood in the capillaries and ventilation helps to maintain a concentration gradient between air in alveolus and blood in the capillaries. This ensures that Oxygen diffuses into the blood and CO2 diffuses out of the blood.
The surfactant is a mixture of phospholipid molecules present on the inner surface of the alveoli. The lung surfactant prevents the alveoli from collapsing (due to cohesive forces of water molecules) during exhalation. This makes inflation of the alveoli easy. Some babies born without surfactant in the lungs die from exhaustion. This is because they need to spend too much energy to inflate the lungs. This is called the respiratory distress syndrome.
describe the structure of erythrocytes and understand their role in transport
Red blood cells / erythrocytes are disc shaped biconcave cells, without a nucleus. The cells are filled with a red pigment called haemoglobin. The main function of haemoglobin is to carry oxygen. It also carries some amount of CO2 , as carbamino-haemoglobin. Haemoglobin consists of 4 polypeptide chains: 2 alpha chains and 2 beta chains. Each chain is attached to a haem (iron containing) group, which can combine with one O2 molecules.
- understand the transport of oxygen and carbon dioxide;
The shape of RBCs increases the surface area to volume ratio for exchange of respiratory gases. The lack of nucleus also enables the cells to pass through the narrow lumen of the capillaries.
Transport of oxygen: oxygen is carried from lungs to the body tissues in the form of oxyhaemoglobin. Hamemoglobin combines with O2 in the lungs, where the O2 partial pressure is high, to form oxyhaemoglobin. This is carried to the tissues, where the O2 partial pressure is low p(O2) or high p(CO2), haemoglobin has a lower affinity for O2. So oxyhaemoglobin dissociate to form oxygen and haemoglobin. The oxygen then diffuses into the tissues and is used for respiration.
(Loading of oxygen)
High p(oxygen)-in lungs
Haemoglobin + Oxygen Oxyhaemoglobin
Low p(oxygen)-in tissue
(Unloading of oxygen)
Transport of carbon dioxide:
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About 5% of CO2 dissolves in plasma and is transported from tissues to lungs as molecular CO2.
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About 10% of CO2 combines with the amino group of haemoglobin to form carbamino-haemoglobin, which dissociates in lungs to release CO2.
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About 85% of CO2 is carried in the form of hydrogen carbonates, dissolved in the plasma.
Digestive system
The alimentary canal is a tube running from the mouth to the anus. It is specially adapted for digestion and absorption of food. Digestion begins in the mouth and is completed in the ileum. Digestion may be physical (without changing the chemical composition eg: chewing / mastication and emulsification of fats by bile) or chemical (using hydrolytic enzymes to convert large complex organic molecules into simpler organic molecules). Enzymes are secreted by the walls of the alimentary canal (as in stomach and ileum) or by exocrine gland (like salivary glands and pancreas). The liver secretes bile for emulsification of fats. The main parts of the alimentary canal are mouth, pharynx, oesophagus, stomach, small intestine, (duodenum and ileum), large intestine, rectum and anus. The wall of the alimentary canal has the same basic structure throughout its length. Epithelial cells line the lumen of the gut. It is glandular and secretes mucus and specific enzymes for digestion. In many regions the gut lining is highly folded into villi to increase the surface area for digestion and absorption.
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Describe mastication and movement of food along the gut.
Mastication is the chewing of food. This increases the surface area for enzyme action and also makes it easy to swallow. The teeth play an important role in grinding of food. The tongue helps to manipulate the food during mastication. The food is made into a bolus and swallowed. It is then squeezed down the oesophagus by rhythmic contraction of circular and longitudinal muscles of the oesophagus, called Peristalsis.
- describe the histology of the ileum wall; understand the sources and effects of secretions concerned with the digestion of carbohydrates;
