Homeostasis and Immunology Flashcards
What is homeostasis?
Homeostasis is the tendency for an organism or cell to maintain a constant internal environment within tolerance limits.
what is negative feedback?
Negative feedback decreases the gap between an original level and a new level, so the original level is restored. A set-point can be chosen and any deviations from it can be reversed, keeping the variable close to the set-point. Negative feedback mechanisms therefore promote balance and they form the basis of homeostasis in the body. Large amounts of energy have to be used, but for many multicellular organisms this is worthwhile so body cells are kept in ideal and stable conditions, despite fluctuations in the external environment. This allows extreme and hostile environments to be inhabited so there are very few parts of Earth where life is totally absent.
what is positive feedback?
Positive feedback increases the gap between an original level and a new level. Positive feedback promotes change rather than stability so it is unsuitable for homeostasis.
Responses to high blood glucose concentrations
Insulin is secreted by beta cells in the islets of Langerhans in the pancreas. Insulin stimulates the liver and muscle cells to absorb glucose and convert it to glycogen. Granules of glycogen are stored in these cells. Other body cells are stimulated to absorb glucose and use it in cell respiration instead of fat. These processes lower the blood glucose. This is glycogenesis
Responses to low blood glucose concentrations
Glucagon is secreted by alpha cells in the pancreas, and stimulate liver cells to break glycogen down into glucose and release the glucose. This raises the blood glucose levels. Glycogenolysis
Causes/ risk factors and prevention of type 1 diabetes
The bodys own immune system destroys beta cells in the pancreas, so insulin secretion becomes insufficient. This can happen over a short period with severe and obvious symptoms of the disease starting rather suddenly. The causes are still being researched.
Youth- type 1 diabetes usually develops in children, teenagers or young adults
Family history of type 1 diabetes or another autoimmune disease
Blood glucose concentration is tested regularly, and insulin is injected, often before a meal, to prevent peaks of blood glucose as food is digested and absorbed. Implanted devices can release insulin into the blood when necessary. In the future, stem cells may be used to replace lost beta cells
Causes/ risk factors and prevention for type 2 diabetes?
Target cells become insensitive to insulin because of a deficiency of insulin receptors or glucose transporters. Onset is gradual and may go unnoticed for many years. It usually happens in older adults but earlier onset now sometimes occurs
- diets rich in fat and low in fibre
- obesity due to over-eating and lack of exercise
- genetic factors that affect fat metabolism
Diet can reduce the peaks and troughs of blood glucose. Sugar should be avoided and also starchy foods unless they have a low glycemic index indicating slow digestion. Strenuous exercise and weight loss are beneficial because they improve insulin uptake and action.
What is thermoregulation, and how is it controlled?
Thermoregulation is control of core body temperature to keep it close to a set point despite fluctuations in external temperature. Humans have a set-point close to 37 degrees celsius. Thermoregulation is achieved through negative feedback.
Body temperature is monitored by nerve endings of sensory neurons in the skin that act as peripheral thermoreceptors. They anticipate rates of heat loss from the body and therefore changes in core temperature. Central thermoreceptors are located in the core of the body including the hypothalamus.
The hypothalamus is the integrating centre for thermoregulation.
Heat is generated by metabolism in cells. The metabolic rate can be increased or decreased to raise or lower the amount of heat generated. Thyroxin increases the metabolic rate of cells. All cells respond, but the most metabolically active such as liver, muscle and brain are the main targets.
Responses to the cold in humans
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Vasoconstriction
Arterioles are branches of arteries. Contraction of circular muscle in an arteriole wall lumen narrows the lumen (vasoconstriction), so less blood flows to the tissues served by the arteriole. Vasoconstriction of arterioles supplying the skin reduces blood flow so the skin cools below core body temperature and less heat is lost to the environment. -
Shivering
Muscle contraction generates heat. Sometimes many small, involuntary muscle contractions and relaxations are carried out at a rapid rate solely to generate heat.
This is shivering. -
Uncoupled respiration
Brown adipose tissue is a modified version of the white adipose tissue that is used for fat storage. The brown colour is due to the cells containing less fat and more mitochondria. These mitochondria oxidize fat by norma metabolic pathways but whereas the oxidation reactions are normally coupled to ATP production, in brown adipose tissue all the energy released by the oxidation is transformed into heat and no ATP is produced. This is known as uncoupled respiration. During childhood, the amount of brown adipose tissue decreases, but even in adulthood some is retained to generate heat and help prevent hypothermia. -
Hair erection
In mammals with a thick coat, the air between the hairs acts as a thermal insulator. Erector muscles can move the hairs to make the coat thicker and the insulating effect greater. During human evolution, the amount of hair over most of the body has been reduced to a few short hairs. The erector muscles can still make the hairs stand up, but they do not trap air well enough to insulate the body. This ineffectual response to cold is also known as goosebumps.
Responses to the heat in humans
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Vasodilation
Relaxation of circular muscle cells in the walls of arterioles supplying the skin causes widening (vasodilation), increasing blood flow. This warms the skin to core temperature, so more heat is lost to the environment. Important: Capillaries do not move closer to the skin surface it is the rate of blood flow through them that changes. -
Sweating
Sweat is secreted by glands in the skin and then passes through narrow ducts to the skin surface, where water in the sweat evaporates. Solutes in the sweat, especially ions such as sodium, are left on the skin surface and can sometimes be detected by their salty taste. Water has a high latent heat of vaporization as hydrogen bonds have to be broken for water molecules to separate. Evaporation of water from sweat therefore causes significant cooling of the skin. Blood flowing through the skin loses heat and can then cool other parts of the body.
Sweat secretion is controlled by the hypothalamus. If the body is overheated, the hypothalamus stimulates the sweat glands to secrete up to two litres of sweat per hour. Usually, no sweat is secreted if body temperature is below the set-point, but epinephrine can cause sweat secretion in anticipation of a period of intense activity that will cause overheating. - Behavioural responses:
Humans respond to overheating by a variety of behavioural responses:
- removing layers of clothing
- moving from an area of sunshine to the shade
- reducing physical activity so less heat is generated by muscle contraction.
The converse of these actions are responses to feeling cold
How is blood supply to organs changed during:
intense physical activity, wakeful rest and sleep
Intense physical activity:
- Greatly increased supply to skeletal muscles and to the brain to supply more O, and glucose for muscle contraction. Increased supply to the brain as mental activity is heightened.
- Reduced supply to the gut and kidneys as digestion and excretion can be temporarily suspended.
During wakeful rest:
- Maximal supply to the kidney to remove waste products rapidly.
- Moderate supplies to the brain for mental activity and to skeletal muscles for maintaining posture while standing or sitting.
Variable supplies to the digestive system-less after fasting and more with food in the gut after feeding.
During sleep:
- Increased supply to the brain to remove toxins. Reduced supply to skeletal muscles as muscle contractions are limited while lying prone (lying flat on stomach).
- Reduced supply to kidneys so avoiding waking to urinate. Variable supply to the digestive system depending on whether food was recently eaten.
Osmoregulation and excretion
The kidney has the twin roles of osmoregulation and excretion. It achieves these roles by filtering about 20% of the water and solutes from blood plasma and then selectively reabsorbing the substances in the filtrate that the body requires.
Osmoregulation is keeping the osmotic concentrations of body fluids within narrow limits as a part of homeostasis. Osmotic concentration is the overall concentration of the solutes that can affect movement of water by osmosis. The kidney carries out osmoregulation by varying the relative amounts of water and salts that pass out of the body in urine.
Excretion is removal of the toxic waste products of metabolism from the body. An example is removal of nitrogen compounds from the breakdown of excess amino acids such as urea. They would become toxic if they accumulated.
The kidneys also remove substances passively absorbed from food in the gut that are not used by the body-for example, many drugs and pigments from food.
The role of the kidney in excretion
Ultrafiltration is the first stage in the production of urine.
It happens in many small structures in the cortex of the kidney that each consist of a glomerulus surrounded by a Bowman’s capsule. The glomerulus is a ball-shaped network of blood capillaries. Blood flows into it through an afferent arteriole and away in an efferent arteriole to other capillaries in the kidney.
Fluid is filtered out through the walls of all capillaries to produce tissue fluid. A much larger proportion of the blood plasma is filtered out in glomerular capillaries. This is because of high blood pressure and very permeable capillary walls. Two factors cause high blood pressure:
- the efferent arteriole being narrower than the afferent arteriole
- the contorted route that blood must follow to pass through the glomerulus.
High permeability is due to the presence of fenestrations, which are pores between capillary wall cells that are unusually wide (100 mm diameter) and numerous. The fluid forced out of the blood plasma is called glomerular filtrate.
How is ultrafiltration carried out?
Ultrafiltration is carried out by two layers:
- The basement membrane that covers and supports the wall of the glomerular capillaries. It is a non-cellular gel, made of negatively charged glycoproteins that are cross-linked to form a mesh.
Most plasma proteins cannot pass through, due to their size and negative charges. - The inner wall of Bowman’s capsule consists of cells with branching outgrowths that wrap around the glomerular capillaries. The cells are podocytes and the branches are foot processes. Very narrow gaps between adjacent foot processes help prevent proteins from being filtered out of blood in the glomerulus.
What happens after ultrafiltration?
The glomerular filtrate is collected by the cup-shaped Bowman’s capsule and then flows on into the nephron.
This is a tubular structure with associated peritubular blood capillaries. A large total volume of glomerular filtrate containing many useful substances is produced per day (about 180 litres). Most of these useful substances are selectively reabsorbed from the filtrate to the blood as the filtrate flows through the nephrons. Water products such as urea and excess water and salts are not reabsorbed leaving about 1.5 litres of urine to be excreted per day.
Much of the selective reabsorption happens in the proximal convoluted tubule at the start of the nephron.
The convolutions increase the length of this part of the nephron, so the filtrate takes longer to flow through and there can be more reabsorption.
- Sodium ions (Na+) are reabsorbed by active transport.
- Chloride ions (Cl-) follow N+ ions passively due to their negative charge.
- Glucose is reabsorbed by sodium cotransport (see Section B2.1.16).
- Amino acids are also reabsorbed by cotransport.
- Water is reabsorbed by osmosis because reabsorption of solutes lowers the osmotic concentration of the filtrate inside the proximal convoluted tubule.
By the end of the proximal tubule, all glucose and amino acids and 80% of the water, sodium and other mineral ions have been reabsorbed and the filtrate flows into the loop of Henlé.
what occurs in the loop of henle
The kidney has two main regions- an outer cortex and an inner medulla. The cortex contains the glomeruli and Bowman’s capsules and also the proximal and distal convoluted tubules. The medulla contains loops of Henle and collecting ducts. The role of the loop of Henle is to maintain an osmotic concentration gradient from a normal 300mOsm near the cortex to a much higher concentration near the centre of the kidney. Filtrate enters the loop of Henle from the proximal. convoluted tubule and first flows towards the centre of the kidney in the descending limb. It then does a U-turn and flows back to the cortex in the ascending limb.
The ascending and descending limbs of the loop of Henle act as a countercurrent mechanism that works due to differences in permeability. The descending limb is permeable to water but impermeable to Na+ ions and the ascending limb is impermeable to water and permeable to Na+ ions. As filtrate flows down the descending limb, the osmotic concentrations in the interstitial fluid are increasingly high, so water is continually reabsorbed from the filtrate by osmosis (into the vasa recta), making it hypertonic to normal body fluids. As the filtrate then flows up the ascending limb, Na+ ions are transferred out of the filtrate into the interstitial fluid by active transport. The Na+ protein pumps can increase the Na+ concentration of the interstitial fluid by 200 mOsm, but because of the countercurrent mechanism, the loop of Henle can increase the concentration in the deepest parts of the medulla by much more than this. The longer the loops of Henle in the kidneys, the greater the concentration taht can be achieved. In humans the concentration can reach 1200 mOsm (4x hypertonic to normal body fluids).
What happens in the collecting ducts?
After the loop of Henle the filtrate passes through the distal convoluted tubule, where K+ and other ions can be exchanged between the filtrate and the blood to adjust blood concentrations. The filtrate then passes through the collecting duct, with increasing osmotic concentrations of interstitial fluid generated by the loop of Henle. The collecting duct carries out osmoregulatory functions in the kidney. Osmoreceptors in the hypothalamus monitor the osmotic concentration of the blood and adjust the amount of hormone ADH secreted by the pituitary gland. ADH concentrations determine the permeability to water of the cells in the collecting duct. Aquaporins can be added to the plasma membranes of wall cells by fusion of vesicles or they can be removed from the membrane by formation of vesicles from plasma membrane containing aquaporins.
What are pathogens
- pathogens are organisms that cause infectious diseases
- the organism which is infected and develops the disease is the host
- a broad range of pathogens can cause diseases in humans. The main groups are viruses, bacteria, fungi and protists. The bacteria which cause disease in all humans are eubacteria
- Archea are the other domain of bacteria and aren’t known to cause any infectious diseases in humans. (more due to lack of research into this area)
What is the primary defence against pathogens?
The skin and mucous membranes are the primary defence against infection by pathogens. They form physical and chemical barriers that few organisms can penetrate.
- the outer layers of the skin are tough and form a physical barrier. Sebaceous glands in the skin secrete lactic acid and fatty acids, which make the surface acidic. This prevents the growth of most pathogenic bacteria and therefore acts as a chemical barrier.
- Mucous membranes are soft areas of skin that are kept moist with mucus. There are mucous membranes in the nose, trachea, penis and vagina and urethra. Although these don’t form a strong physical barrier, mucus secreted by these membranes contains lysozyme, an enzyme that kills many bacteria, so it acts as a chemical barrier.
Why does blood clot, and how did it happen?
When the skin is cut and blood leaks from blood vessels, the blood is quickly sealed by a blood clot.
At the start of the clotting process, platelets (small cell fragments in blood plasma) are attracted to the wounded tissues, where they release clotting factors. These clotting factors initiate a cascade of reactions in which the product of each reaction is the catalyst of the next reaction. This system helps to ensure that blood only clots when necessary, and also that it is a very rapid process. The penultimate reaction in the cascade produces thrombin. In the last reaction thrombin converts fibrinogen, a soluble plasma protein, into long protein fibres called fibrin.
Fibrin forms a mesh of fibres across the wounds. Blood cells are caught in the mesh and soon form a semi-solid clot to seal the cut and prevent entry of pathogens. If exposed to air, the clot dries to form a protective scab, which remains until the wound has healed.
What are the two main parts of the immune system?
- The innate immune system: non specific, as different pathogens are all responded to in the same way and the responses do not change during an organisms life. Phagocytes are part of the innate immune system, they have a lobed nucleus.
- The adaptive immune system: specific, as each pathogen encountered is dealt with differently, and elicits a different immune response, immune responses develop during an organisms life. Lymphocytes are part of the adaptive immune system.
what is phagocytosis
Phagocytes engulf (ingest) all pathogens that they encounter in the body. This happens by endocytosis. Once the pathogen is inside a vacuole in the cytoplasm of the phagocyte, lysosomes fuse with the vacuole, to add enzymes which digest and kill the pathogen. Phagocytes can ingest pathogens in the blood. They can also squeeze out through the walls of blood capillaries and move through tissues to sites of infection. Their movement is amoeboid (like an amoeba). Pus that forms at sites of infection consists of large numbers of phagocytes.
what is the role of lymphocytes?
- lymphocytes have a rounded nucleus and until activated only have a small amount of cytoplasm
- Lymphocytes circulate in the blood. There are also large numbers of them in lymph nodes of the lymphatic system. This system consists of vessels that drain excess fluid from body tissues. Lymph nodes are small bean-shaped structures that develop at intervals along lymph vessels.
- Lymphocytes of different types cooperate to produce antibodies. The main types are B-lymphocytes and helper T-lymphocytes
- Antibodies are very large Y shaped proteins. The two arms are variable, with hypervariable parts that recognise and bind to a specific molecule on a pathogen. The stem of the Y helps the body to destroy the pathogen (make it more recognisable to the phagocyte to engulf it)
- Humans can be infected by many different pathogens, including new strains which have recently evolved.
- The immune system as a whole can produce a vast array of different antibodies, but each individual B-lymphocyte can produce only one type of antibody.
- Only a small number of B-lymphocytes for producing each type of antibody exist in the body. When a new pathogen infects the body, the cells that can produce the appropriate antibody divide to form a large clone of antibody producing cells.
How do antigens trigger antibody production
Lymphocytes have to distinguish between body cells (self-cells) and non body cells (non self cells) such as invading pathogens. Lymphocytes recognise pathogens by difference between their molecules and those of body cells. The molecules used for recognition are called antigens. They are mostly proteins, glycoproteins or large polysaccharides and are usually located on the surface of the pathogen.
The immune response to an antigen is the production of specific antibodies. Any molecule that causes this response is an antigen. There are antigens on the surface of bacteria, viruses, parasites and cancer cells. They also occur on the surface of cells from another human with a different tissue type. This explains the need to match tissue types for organ transplants. If the wrong blood type is transfused into a patient, molecules on the surface of red blood cells act as antigens and trigger antibody production.
Lymphocytes produce antibodies that bind specifically to the antigen. This is dependent on matching shapes and chemical properties, so it is similar to the binding of a ligand to a receptor, or the binding of a substrate to the active site of an enzyme. However, unlike the binding of ligands to receptors, antibody to antigen is irreversible. Unlike the binding of substrates to enzymes, the antigen isn’t changed chemically.
The part of an antibody that binds to the antigen is the hypervariable region. As the name suggests there is immense variation in the hypervariable regions of antibodies and the immune system can generate new versions. However, one lymphocyte only produces antibodies with one type of hypervariable region.
