physiology Flashcards
The human digestive system
- Esophagus: food is swallowed and moves down to the stomach
- Stomach: Digestive enzymes are secreted from its walls which initiate protein digestion under low (acidic) pH.
- Small intestine: digestion is completed in the first part of the small intestine and absorption of digested molecules takes place in the final part.
- Large intestine: Undigested material proceeds, faeces are formed and water is absorbed. Solid faeces are egested out through the anus.
- Liver: Among many other functions, it synthesizes and releases bile into the gall bladder.
- Gall bladder: it secretes bile into the small intestine to help lipid digestion
- Pancreas: the exocrine part of the pancreas secretes digestive enzymes into the small intestine which break down various macromolecules.
Peristalsis
It is the movement of mixing and churning the food in order to move along the gut. This takes place through contractions of smooth muscle layers which line the alimentary canal. Smooth muscle is responsible for doing involuntary movements while striated muscle attached to bones is doing voluntary movements. There are two types of smooth muscle in the gut, circular (inside) and longitudinal (outside). Peristalsis is the muscle contractions in waves which pass along the intestine.
Contraction of circular muscle behind the food constricts the gut to prevent food from being pushed backwards. Contraction of longitudinal muscle where food is located moves it along the gut. Contraction of both layers of muscle mixes food with enzymes in the small intestine.
Digestion in the small intestine
The main types of food molecules that need to be digested are carbohydrates (mainly starch and glycogen since cellulose remains undigested), proteins, triglycerides (fats and oils) and nucleic acids.
Digestion of these large molecules happens naturally at body temperature, but only at a very slow rate. Enzymes are essential to speed up the process. The pancreas secretes enzymes (pancreatic juice) into the lumen of the initial part of the small intestine called the duodenum:
Details of starch digestion
Recall that starch contains two types of molecules, amylose and amylopectin differing in the types of bonds created between the glucose monomers (see notes in topic 2, p.8). Amylase is an enzyme found in saliva and in the pancreatic juice (salivary and pancreatic amylase) which has the ability to break amylose molecules into maltose. Amylase cannot recognize and thus break amylopectin. Fragments of amylopectin are called dextrins. Dextrinase breaks amylopectin into glucose and maltase break maltose into glucose.
Glucose is then carried into the blood stream through the villi of the small intestine and then through the hepatic portal vein into the liver. In the liver excess glucose is absorbed by liver cells (hepatocytes) converted into glycogen and stored.
1.4. Absorption in the small intestine-the role of the villi
Absorption is the process where food monomers exit the digestive system and enter the bloodstream. Small finger-like projections from the wall of the small intestine called villi (see figure below) are specially adapted to absorb food molecules. Molecules are absorbed by the epithelial cells which form the inner lining of the mucosa. These molecules may be food monomers from digestion, minerals, ions and vitamins. After food has been absorbed it is assimilated – it becomes part of the tissues of the body.
▪ Villi increase the surface area over which food is absorbed.
▪ An epithelium, consisting of only one thin layer of cells, is all that foods have
to pass through to be absorbed.
▪ Protrusions of the exposed part of the plasma membranes of the epithelium
cells increase the surface area for absorption. These projections are called
microvilli.
▪ Protein channels in the microvilli membranes allow rapid absorption of foods
by facilitated diffusion and pumps allow rapid absorption by active transport.
▪ Mitochondria in the epithelium cells provide the ATP needed of active
transport.
▪ Blood capillaries inside the villus are very close to the epithelium so the
distance for diffusion of foods is very small.
A lacteal (a branch of the lymphatic system) in the centre of the villus carries away fats after absorptio
Mechanisms of absorption
Recall that absorption takes place through the membranes of the epithelial cells of the villi which have been extended for this purpose by microvilli. There are various methods for absorption:
1. Simple diffusion
Example: hydrophobic nutrients such as fatty acids and monoglycerides. 2. Facilitated diffusion
Example: hydrophilic molecules such as fructose
3. Active transport with the use of pumps
Example: mineral ions such as sodium, calcium and iron
4. Exocytosis
Example: droplets of fat substances reformed into the epithelial cells called chylomicrons exit the cells and move into the lacteal (lymphatic vessel).
5. Co-transport of glucose together with sodium ions.
Example: glucose moves against the concentration gradient together with a co- transporter protein which transports sodium into the cells through (facilitated) diffusion.
- THE CARDIOVASCULAR SYSTEM
In complex animal bodies diffusion cannot supply enough raw materials to all of the cells, therefore specialised circulatory structures are required to transport materials. These structures make up the circulatory system. A circulatory system typically consists of the following:
• Blood- a fluid which contains a variety of cells
• A system of blood vessels in which the blood circulates
• A pumping organ-generally a heart
2.1. Blood
Blood is composed of the plasma, the red blood cells (erythrocytes), the white blood cells (leucocytes) and the platelets.
❑ The plasma is a watery fluid which contains a variety of substances such as proteins (antibodies for defense is an example of such proteins), salts, and materials for transport like nutrients, hormones, waste products of metabolism such as urea and gases (carbon dioxide). Heat is also transferred through plasma.
❑ The erythrocytes (red blood cells) are mainly responsible for the transport of oxygen to tissues through the major transport protein they include, haemoglobin. They also transport carbon dioxide.
❑ The leucocytes (white blood cells) are of several different types, phagocytes (causing phagocytosis of microorganisms), monocytes, and lymphocytes (responsible for the immune response of the body). Their function is mainly in the different kinds of defense of a body against disease, infection and inflammation.
❑ The platelets are fragments of cells enclosed in a membrane. Their function is to initiate the process of blood clotting after a wound has occurred. This has as an effect the avoidance of excessive bleeding.
Blood clotting mechanism
Platelets, protein factors, vitamin K and Ca ions are needed for the blood to clot after a serious or superficial wound. The stages are as following:
1. When the epithelium is damaged, platelets become activated
2. Clotting factors are released from damaged tissues (such as factor VI and others)
3. These factors help activating the enzyme prothrombin to become thrombin
4. Active thrombin catalyses the conversion of soluble plasma fibrinogen into insoluble fibrin
Fibrin is an insoluble thread-like structure which covers the wound and traps any released red blood cells and white blood cells. The clot quickly dries upon exposure to air and produces a protective scab, which remains until the wound is healed.
2.2. Blood vessels
a. ARTERIES Carry oxygenated blood away from the heart thicker than veins three layers: inner endothelium middle smooth muscle outer connective tissue (arteriole= small artery)
b. Veins
Carry deoxygenated blood toward the heart
same three layers
thinner and more expansive than arteries
contain valves to flow blood back to the heart
small vein= venule
c. Capillaries
site of gas exchange with tissues
connect arterioles and venules
site of exchange: gases, nutrients, wastes
can be closed off when not needed
the double circulation
William Harvey predicted that in humans there is a double circulation. One circulation is from the heart to lungs-the pulmonary circulation. The other is the circulation of blood moving from the heart to all of the other organs of the body- systemic circulation. The heart is separated into two parts- the right side pumps deoxygenated blood to the lungs via the pulmonary artery. Oxygenated blood returns to the heart via the pulmonary vein. The left side of the heart is carrying oxygenated blood to the organs via the aorta.
2.4. Structure of the heart
The mammalian heart consists largely of cardiac muscle (the myocardium), a specialized tissue which is capable of rhythmical contraction and relaxation over a long period without fatigue. The muscle is supplied with blood vessels and also contains connective tissue which gives strength and helps to prevent it from tearing. The heart is made up of two thin-walled atria which are elastic and distend as blood enters them. The left atrium receives oxygenated blood from the pulmonary veins (coming from the lungs) while the right atrium receives deoxygenated blood from the superior and inferior vena cava (coming from the body circulation). Blood enters the ventricles through the atria. The right ventricle then pumps blood to the lungs through the pulmonary arteries, and the left ventricle pumps blood all around the body through the aorta. The left ventricle has a much thicker wall than the right ventricle, since it has to pump blood with high pressure to the extremities of the body. Atrioventricular valves, found between the atria and ventricles, preventing backflow of blood into the atria when the ventricles contract. Valves are also found between the ventricles and the large blood vessels leaving the heart, called the semi- lunar valves.
The cardiac cycle and heart beat
- ATRIA DIASTOLE: Atria relaxed, filling with blood. Atrioventricular valves
CLOSED - ATRIA SYSTOLE: Atria contract – blood pushed through open atrioventricular valves into ventricles. Atrioventricular valves OPEN
- VENTRICULAR SYSTOLE: Ventricles contract, pressure inside them rises, semilunar valves OPEN, blood spurts into the aorta and pulmonary artery. Atrioventricular valves CLOSED.
- VENTRICULAR DIASTOLE: Ventricles relax, semilunar valves are CLOSED
Control of heart rate
Heart muscle tissue has a special property-it can contract on its own without being stimulated by a nerve. One region is responsible for initiating each contraction. This region is called the Sino-Atrial node (SA node, known also as the pacemaker of the heart) and is located in the wall of the right atrium. Each time the pacemaker sends out a signal the heart carries out a contraction or beat. Nerves and hormones can transmit messages to the pacemaker.
➢ One nerve carries messages from the brain stem (medulla) to the pacemaker that tell the pacemaker to speed up the beating of the heart.
➢ Another nerve carries messages from the brain to the pacemaker that tell the pacemaker to slow down the beating.
➢ The hormone epinephrine (adrenalin), carried to the pacemaker by the bloodstream tells the pacemaker to speed up the beating of the heart and help prepare the body for physical activity.
THE DEFENSE SYSTEM 3.1. Barriers to infection
Pathogens are various types of organisms (viruses, bacteria, protozoa, fungi and macroorganisms –worms) that can cause disease. The skin and mucous membranes act as barriers against microbes.
