Animal Physiology Flashcards

Question

Describe the first line of Defense against disease

Answer 1

Skin Protects external structures when intact (outer body areas) Consists of a dry, thick and tough region composed predominantly of dead surface cells Contains biochemical defence agents (sebaceous glands secrete chemicals and enzymes which inhibit microbial growth on skin) The skin also secretes lactic acid and fatty acids to lower the pH (skin pH is roughly ~ 5.6 – 6.4 depending on body region) Mucous Membranes Protects internal structures (i.e. externally accessible cavities and tubes – such as the trachea, oesophagus and urethra) Consists of a thin region of living surface cells that release fluids to wash away pathogens (mucus, saliva, tears, etc.) Contains biochemical defence agents (secretions contain lysozyme which can destroy cell walls and cause cell lysis) Mucous membranes may be ciliated to aid in the removal of pathogens (along with physical actions such as coughing / sneezing)

Answer 2

Air enters the respiratory system through the nose or mouth and passes through the pharynx to the trachea The air travels down the trachea until it divides into two bronchi (singular: bronchus) which connect to the lungs The right lung is composed of three lobes, while the left lung is only comprised of two (smaller due to position of heart) Inside each lung, the bronchi divide into many smaller airways called bronchioles, greatly increasing surface area Each bronchiole terminates with a cluster of air sacs called alveoli, where gas exchange with the bloodstream occurs

Answer 3

-They have a very thin epithelial layer (one cell thick) to minimise diffusion distances for respiratory gases -They are surrounded by a rich capillary network to increase the capacity for gas exchange with the blood -They are roughly spherical in shape, in order to maximise the available surface area for gas exchange - Their internal surface is covered with a layer of fluid, as dissolved gases are better able to diffuse into the bloodstream

Answer 4

Type I pneumocytes Type I pneumocytes are involved in the process of gas exchange between the alveoli and the capillaries They are squamous (flattened) in shape and extremely thin (~ 0.15µm) – minimising diffusion distance for respiratory gases Type I pneumocytes are connected by occluding junctions, which prevents the leakage of tissue fluid into the alveolar air space Type I pneumocytes are amitotic and unable to replicate, however type II cells can differentiate into type I cells if required Type II pneumocytes Type II pneumocytes are responsible for the secretion of pulmonary surfactant, which reduces surface tension in the alveoli They are cuboidal in shape and possess many granules (for storing surfactant components) Type II pneumocytes only comprise a fraction of the alveolar surface (~5%) but are relatively numerous (~60% of total cells)

Answer 5

Type II pneumocytes secrete a liquid known as pulmonary surfactant which reduces the surface tension in alveoli As an alveoli expands with gas intake, the surfactant becomes more spread out across the moist alveolar lining This increases surface tension and slows the rate of expansion, ensuring all alveoli inflate at roughly the same rate

Answer 6

When the pressure in the chest is less than the atmospheric pressure, air will move into the lungs (inspiration) When the pressure in the chest is greater than the atmospheric pressure, air will move out of the lungs (expiration) Inspiration The muscles responsible for inspiration are the diaphragm and external intercostals (plus some accessory muscles) Diaphragm muscles contract, causing the diaphragm to flatten and increase the volume of the thoracic cavity External intercostals contract, pulling ribs upwards and outwards (expanding chest) Additional muscle groups may help pull the ribs up and out (e.g. sternocleidomastoid and pectoralis minor) Expiration The muscles responsible for expiration are the abdominal muscles and internal intercostals (plus some accessory muscles) Diaphragm muscles relax, causing the diaphragm to curve upwards and reduce the volume of the thoracic cavity Internal intercostal muscles contract, pulling ribs inwards and downwards (reducing breadth of chest) Abdominal muscles contract and push the diaphragm upwards during forced exhalation Additional muscle groups may help pull the ribs downwards (e.g. quadratas lumborum)

Answer 7

Emphysema is a lung condition whereby the walls of the alveoli lose their elasticity due to damage to the alveolar walls The loss of elasticity results in the abnormal enlargement of the alveoli, leading to a lower total surface area for gas exchange The degradation of the alveolar walls can cause holes to develop and alveoli to merge into huge air spaces (pulmonary bullae) The major cause of emphysema is smoking, as the chemical irritants in cigarette smoke damage the alveolar walls The damage to lung tissue leads to the recruitment of phagocytes to the region, which produce an enzyme called elastase This elastase, released as part of an inflammatory response, breaks down the elastic fibres in the alveolar wall Elastase activity can be blocked by an enzyme inhibitor (α-1-antitrypsin), but not when elastase concentrations are increased A small proportion of emphysema cases are due to a hereditary deficiency in this enzyme inhibitor due to a gene mutation

Answer 8

Total lung capacity – Volume of air in the lungs after a maximal inhalation (~ 6 litres in a normal adult male) Vital capacity – Volume of air that can be exchanged by the lungs via a maximal inhalation and exhalation Residual volume – Volume of air that is always present in the lungs (~ 20% of total lung capacity) Tidal volume – Volume of air that is exchanged via normal breathing (~ 500 ml per breath)

Answer 9

Asthma is a common, chronic inflammation of the airways to the lungs (i.e. bronchi and bronchioles) Inflammation leads to swelling and mucus production, resulting in reduced airflow and bronchospasm During an acute asthma attack, constriction of the bronchi smooth muscle may cause significant airflow obstruction Common symptoms of an asthma attack include shortness of breath, chest tightness, wheezing and coughing Severe cases of asthma may be life threatening if left untreated Asthma may be caused by a number of variable and recurring environmental triggers, including allergens, smoke, cold air, certain medications and arthropods (e.g. dust mites)

Answer 10

Via simple observation (counting number of breaths per minute) Chest belt and pressure meter (recording the rise and fall of the chest) Spirometer (recording the volume of gas expelled per breath)

Answer 11

When a specific B cell first encounters the allergen, it differentiates into plasma cells and makes large quantities of antibody (IgE) The IgE antibodies attach to mast cells, effectively ‘priming’ them towards the allergen Upon re-exposure to the allergen, the IgE-primed mast cells release large amounts of histamine which causes inflammation The release of histamine from IgE-primed mast cells causes an inflammatory response that results in allergic symptoms Inflammation improves leukocytes mobility to infected regions by triggering vasodilation and increasing capillary permeability Vasodilation is the widening of blood vessels to improve the circulation of blood to targeted regions Vasodilation causes redness (as vessel expansion moves blood closer to the skin) and heat (which is transported in blood) Increased capillary permeability leads to swelling (more fluid leaks from the blood) and pain (swelling causes compression of nerves) Redness, heat, swelling and localised pain are all typical symptoms of an allergic response

Answer 12

All nucleated cells of the body possess unique and distinctive surface molecules that identify it as self These self markers are called major histocompatibility complex molecules (MHC class I) Any substance that is recognised as foreign and is capable of triggering an immune response is called an antigen (non self)

Answer 13

Direct contact – the transfer of pathogens via physical association or the exchange of body fluids Contamination – ingestion of pathogens growing on, or in, edible food sources Airborne – certain pathogens can be transferred in the air via coughing and sneezing Vectors – intermediary organisms that transfer pathogens without developing disease symptoms themsel

Answer 14

Pathogens are generally species-specific in that their capacity to cause disease (pathogenesis) is limited to a particular species Certain pathogens may cross the species barrier and be able to infect and cause disease in a range of hosts Diseases from animals that can be transmitted to humans are called zoonotic diseases (or zoonoses)

Answer 15

Memory cells are produced to prevent this delay in subsequent exposures and hence prevent disease symptoms developing When a B lymphocyte is activated and divides to form plasma cells, a small proportion will differentiate into memory cells Memory cells are long living and will survive in the body for many years, producing low levels of circulating antibodies If a second infection with the same pathogen occurs, memory cells will react more vigorously to produce antibodies faster As antibodies are produced faster, the pathogen cannot reproduce in sufficient amounts to cause disease symptoms Hence, because pathogen exposure no longer causes the disease to occur, the individual is said to be immune

Answer 16

ccinations induce long-term immunity to specific pathogenic infections by stimulating the production of memory cells A vaccine is a weakened or attenuated form of the pathogen that contains antigens but is incapable of triggering disease The antigenic determinants in a vaccine may be conjugated to an adjuvant, which functions to boost the immune response The body responds to an injected vaccine by initiating a primary immune response, which results in memory cells being made When exposed to the actual pathogen, the memory cells trigger a more potent secondary immune response As a consequence of this more potent immune response, disease symptoms do not develop (individual is immune to pathogen) Herd immunity is when individuals who are not immune to a pathogen are protected from exposure by the large amounts of immune individuals within the community

Answer 17

Examples of Active Immunity Natural – Producing antibodies in response to exposure to a pathogenic infection (i.e. challenge and response) Artificial – Producing antibodies in response to the controlled exposure to an attenuated pathogen (i.e. vaccination) Examples of Passive Immunity Natural – Receiving antibodies from another organism (e.g. to the foetus via the colostrum or a newborn via breast milk) Artificial – Receiving manufactured antibodies via external delivery (e.g blood transfusions of monoclonal antibodi

Answer 18

Skeletons are a rigid framework that function to provide support and protection for body organs. Skeletons provide a surface for muscle attachment and thus facilitate the movement of an organism

Answer 19

Joint capsule – Seals the joint space and provides stability by restricting the range of possible movements Cartilage – Lines the bone surface to facilitate smoother movement, as well as absorbing shock and distributing load Synovial fluid – Provides oxygen and nutrition to the cartilage, as well as lubrication (reduces friction)

Answer 20

There are six main types of synovial joints that allow for different ranges of movement, which are (in order of mobility): Plane joints (least mobility), hinge joints, pivot joints, condyloid joints, saddle joints, ball and socket joints (most mobility)

Answer 21

Skeletal muscles exist in antagonistic pairs (when one contracts, the other relaxes) to enable opposing movements Opposing movements may include: flexion vs extension When the flexor muscle contracts, the extensor muscle relaxes and the tibia and femur are brought closer together This retracts the hind quarters in preparation for pushing off the ground When the extensor muscle contracts, the flexor muscle relaxes and the tibia is pushed away from the femur This extends the hind quarters and causes the insect to jump

Answer 22

Skeletal muscles consist of tightly packaged muscular bundles (fascicles) surrounded by connective tissue (perimysium Each bundle contains multiple muscle fibres, which are formed when individual muscle cells fuse together Muscle fibres contain tubular myofibrils that run the length of the fibre and are responsible for muscular contraction The myofibrils can be divided into repeating sections called sarcomeres, each of which represent a single contractile unit

Answer 23

They are multinucleate (fibres form from the fusion of individual muscle cells and hence have many nuclei) They have a large number of mitochondria (muscle contraction requires ATP hydrolysis) They have a specialised endoplasmic reticulum (it is called the sarcoplasmic reticulum and stores calcium ions) They contain tubular myofibrils made up of two different myofilaments – thin filament (actin) and thick filament (myosin) The continuous membrane surrounding the muscle fibre is called the sarcolemma and contains invaginations called T tubules

Answer 24

1. Depolarisation and Calcium Ion Release An action potential from a motor neuron triggers the release of acetylcholine into the motor end plate Acetylcholine initiates depolarisation within the sarcolemma, which is spread through the muscle fibre via T tubules Depolarisation causes the sarcoplasmic reticulum to release stores of calcium ions (Ca2+) 2. Actin and Myosin Cross-Bridge Formation On actin, the binding sites for the myosin heads are covered by a blocking complex (troponin and tropomyosin) Calcium ions bind to troponin and reconfigure the complex, exposing the binding sites for the myosin heads The myosin heads then form a cross-bridge with the actin filaments 3. Sliding Mechanism of Actin and Myosin ATP binds to the myosin head, breaking the cross-bridge between actin and myosin ATP hydrolysis causes the myosin heads to change position and swivel, moving them towards the next actin binding site The myosin heads bind to the new actin sites and return to their original conformation This reorientation drags the actin along the myosin in a sliding mechanism The myosin heads move the actin filaments in a similar fashion to the way in which an oar propels a row boat 4. Sarcomere Shortening The repeated reorientation of the myosin heads drags the actin filaments along the length of the myosin As actin filaments are anchored to Z lines, the dragging of actin pulls the Z lines closer together, shortening the sarcomere As the individual sarcomeres become shorter in length, the muscle fibres as a whole contracts

Answer 25

The thick filament (myosin) contains small protruding heads which bind to regions of the thin filament (actin) Each individual sarcomere is flanked by dense protein discs called Z lines, which hold the myofilaments in place The actin filaments radiate out from the Z discs and help to anchor the central myosin filaments in place The centre of the sarcomere appears darker due to the overlap of both actin and myosin filaments (A band) The peripheries of the sarcomere appear lighter as only actin is present in this region (I band) The dark A band may also contain a slightly lighter central region where only the myosin is present (H zone)

Answer 26

Water levels within an organism are constantly changing as a result of metabolic activity The concentration of water within cells (osmolarity) will impact tissue viability Osmoconformers maintain internal conditions that are equal to the osmolarity of their environment By matching internal osmotic conditions to the environment, osmoconformers minimise water movement in and out of cells Less energy is used to maintain internal osmotic conditions within an osmoconformer Osmoregulators keep their body’s osmolarity constant, regardless of environmental conditions While osmoregulation is a more energy-intensive process, it ensures internal osmotic conditions are always tightly controlled Osmoregulators can maintain optimal internal conditions whereas osmoconformers are affected by environmental conditions

Answer 27

Excretion is the removal from the body of the waste products of metabolic activity

Answer 28

Nitrogenous wastes are toxic to the organism and hence excess levels must be eliminated from the body The type of nitrogenous waste in animals is correlated with the evolutionary history of the animal and the habitat Most aquatic animals eliminate their nitrogenous wastes as ammonia (NH3) Ammonia is highly toxic but also very water soluble and hence can be effectively flushed by animals in aquatic habitats Terrestrial animals have less access to water and hence must package nitrogenous waste in less toxic forms Mammals eliminate their nitrogenous wastes as urea, which is less toxic and hence can be stored at higher concentrations Reptiles and birds eliminate wastes as uric acid, which requires more energy to make but is relatively non-toxic and requires even less water to flush (it is eliminated as a semi-solid paste)

Answer 29

Insects have a circulating fluid system called hemolymph that is analogous to the blood system in mammals Malpighian tubules branch off from the intestinal tract and actively uptake nitrogenous wastes and water from the hemolymph The tubules pass these materials into the gut to combine with the digested food products Solutes, water and salts are reabsorbed into the hemolymph at the hindgut, whereas nitrogenous wastes (as uric acid) and undigested food materials are excreted via the anus

Answer 30

Blood in the renal vein (i.e. after the kidney) will have: Less urea (large amounts of urea is removed via the nephrons to form urine) Less water and solutes / ions (amount removed will depend on the hydration status of the individual) Less glucose and oxygen (not eliminated, but used by the kidney to generate energy and fuel metabolic reactions) More carbon dioxide (produced by the kidneys as a by-product of metabolic reactions)

Answer 31

Bowman’s capsule – first part of the nephron where blood is initially filtered (to form filtrate) Proximal convoluted tubule – folded structure connected to the Bowman’s capsule where selective reabsorption occurs Loop of Henle – a selectively permeable loop that descends into the medulla and establishes a salt gradient Distal convoluted tubule – a folded structure connected to the loop of Henle where further selective reabsorption occurs

Answer 32

Structure of the Bowman’s Capsule As the blood moves into the kidney via afferent arterioles it enters the glomerulus This glomerulus is encapsulated by the Bowman’s capsule, which is comprised of an inner surface of cells called podocytes Between the podocytes and the glomerulus is a glycoprotein matrix called the basement membrane that filters the blood Basement Membrane Glomerular blood vessels are fenestrated (have pores) which means blood can freely exit the glomerulus The podocytes of the Bowman’s capsule have gaps allowing for fluid to move freely into the nephron The basement membrane is size-selective and restricts the passage of blood cells and large proteins Hydrostatic Pressure Ultrafiltration involves blood being forced at high pressure against the basement membrane, optimising filtration This high hydrostatic pressure is created in the glomerulus by having a wide afferent arteriole and a narrow efferent arteriole The glomerulus forms extensive narrow branches, which increases the surface area available for filtration

Answer 33

It involves the reuptake of useful substances from the filtrate and occurs in the convoluted tubules (proximal and distal) The majority of selective reabsorption occurs in the proximal convoluted tubule, which extends from the Bowman’s capsule The proximal convoluted tubule has a microvilli cell lining to increase the surface area for material absorption from the filtrate The tubule is a single cell thick and connected by tight junctions, which function to create a thin tubular surface with no gaps There are also a large number of mitochondria within these tubule cells, as reabsorption involves active transport The tubules reabsorb all glucose, amino acids, vitamins and hormones, along with most of the mineral ions (~80%) and water Mineral ions and vitamins are actively transported by protein pumps and carrier proteins respectively Glucose and amino acids are co-transported across the apical membrane with sodium (symport) Water follows the movement of the mineral ions passively via osmosis

Answer 34

The function of the loop of Henle is to create a high solute (hypertonic) concentration in the tissue fluid of the medulla The descending limb of the loop of Henle is permeable to water but not salts The ascending limb of the loop of Henle is permeable to salts but not water This means that as the loop descends into the medulla, the interstitial fluid becomes more salty and hypertonic

Answer 35

Water Reabsorption As the collecting duct passes through the medulla, the hypertonic conditions of the medulla will draw water out by osmosis The amount of water released from the collecting ducts to be retained by the body is controlled by anti-diuretic hormone (ADH) ADH is released from the posterior pituitary in response to dehydration (detected by osmoreceptors in the hypothalamus) ADH increases the permeability of the collecting duct to water, by upregulating production of aquaporins (water channels) This means less water remains in the filtrate, urine becomes concentrated and the individual urinates less (i.e. anti-diuresis) When an individual is suitably hydrated, ADH levels decrease and less water is reabsorbed (resulting in more dilute urine) Remember: ADH is produced when you Are DeHydrated

Answer 36

Dehydration Dehydration is a loss of water from the body such that body fluids become hypertonic Individuals will experience thirst and excrete small quantities of heavily concentrated urine (to minimise water loss) Blood pressure will drop (less water in plasma) and the heart rate will increase to compensate for this The individual will become lethargic and experience an inability to lower body temperature (due to lack of sweat) Severe cases of dehydration may cause seizures, brain damage and eventual death Overhydration Overhydration is a less common occurrence that results when an over-consumption of water makes body fluids hypotonic Individuals will produce excessive quantities of clear urine in an effort to remove water from the body The hypotonic body fluids will cause cells to swell (due to osmotic movement), which can lead to cell lysis and tissue damage Overhydration can lead to headaches and disrupted nerve functions in mild cases (due to swelling of cells) In severe cases, overhydration may lead to blurred vision, delirium, seizures, coma and eventual death

Answer 37

With more drought conditions the henle increase in length to preserve more water

Answer 38

Glucose: The presence of glucose in urine is a common indicator of diabetes (high blood glucose = incomplete reabsorption) Proteins: High quantities of protein in urine may indicate disease (e.g. PKU) or hormonal conditions (e.g. hCG = pregnancy) Blood cells: The presence of blood in urine can indicate cancer Drugs / toxins: Many drugs pass through the body into urine and can be detected (e.g. performance enhancing drugs)

Answer 39

Kidney dialysis involves the external filtering of blood in order to remove metabolic wastes in patients with kidney failure Blood is removed and pumped through a dialyzer, which has two key functions that are common to biological membranes: It contains a porous membrane that is semi-permeable (restricts passage of certain materials) It introduces fresh dialysis fluid and removes wastes to maintain an appropriate concentration gradient Kidney dialysis treatments typically last about 4 hours and occur 3 times a week – these treatments can be effective for years The best long-term treatment for kidney failure is a kidney transplant: The transplanted kidney is grafted into the abdomen, with arteries, veins and ureter connected to the recipient’s vessels Donors must typically be a close genetic match in order to minimise the potential for graft rejection Donors can survive with one kidney and so may commonly donate the second to relative suffering kidney failure

Answer 40

Spermatogenesis describes the producton of spermatozoa (sperm) in the seminiferous tubules of the testes The process begins at puberty when the germline epithelium of the seminiferous tubules divides by mitosis These cells (spermatogonia) then undergo a period of cell growth, becoming spermatocytes The spermatocytes undergo two meiotic divisions to form four haploid daughter cells (spermatids) The spermatids then undertake a process of differentiation in order to become functional sperm cells (spermatozoa)

Answer 41

Oogenesis describes the production of female gametes (ova) within the ovaries (and, to a lesser extent, the oviduct) The process begins during foetal development, when a large number of primordial cells are formed by mitosis (~40,000) These cells (oogonia) undergo cell growth until they are large enough to undergo meiosis (becoming primary oocytes) The primary oocytes begin meiosis but are arrested in prophase I when granulosa cells surround them to form follicles The primary oocytes remain arrested in prophase I until puberty, when a girl begins her menstrual cycle Each month, hormones (FSH) will trigger the continued division of some of the primary oocytes These cells will complete the first meiotic division to form two cells of unequal size One cell retains the entirety of the cytoplasm to form a secondary oocyte, while the other cell forms a polar body The polar body remains trapped within the follicle until it eventually degenerates The secondary oocyte begins the second meiotic division but is arrested in metaphase II The secondary oocyte is released from the ovary (ovulation) and enters into the oviduct (or fallopian tube) The follicular cells surrounding the oocyte form a corona radiata and function to nourish the secondary oocyte If the oocyte is fertilised by a sperm, chemical changes will trigger the completion of meiosis II and the formation of another polar body (the first polar body may also undergo a second division to form a third polar body) Once meiosis II is complete the mature egg forms a ovum, before fusing its nucleus with the sperm nucleus to form a zygote

Answer 42

1. Number of cells produced In spermatogenesis, the cells divide equally during meiosis to produce four functional gametes In oogenesis, the cells do not divide equally and as a result only one functional gamete is formed (plus 2 – 3 polar bodies) 2. Size of cells produced In spermatogenesis, the cells that are formed following differentiation are all of equal size with equal amounts of cytoplasm In oogenesis, one daughter cell (the ovum) retains all of the cytoplasm, while the other daughter cells form polar bodies The polar bodies remain trapped within the surrounding layer of follicle cells until they eventually degenerate 3. Timing of the process In spermatogenesis, the production of gametes is a continuous process that begins at puberty and continues until death In oogenesis, the production of gametes is a staggered and finite process: It begins before birth (prenatally) with the formation of a fixed number of primary oocytes (~40,000) It continues with the onset of puberty according to a monthly menstrual cycle It ends when hormonal changes prevent the further continuance of the menstrual cycle (menopause)

Answer 43

1. Capacitation Capacitation occurs after ejaculation, when chemicals released by the uterus dissolve the sperm’s cholesterol coat This improves sperm motility (hyperactivity), meaning sperm is more likely to reach the egg (in the oviduct) It also destabilises the acrosome cap, which is necessary for the acrosome reaction to occur upon egg and sperm contact 2. Acrosome Reaction When the sperm reaches an egg, the acrosome reaction allows the sperm to break through the surrounding jelly coat The sperm pushes through the follicular cells of the corona radiata and binds to the zona pellucida (jelly coat) The acrosome vesicle fuses with the jelly coat and releases digestive enzymes which soften the glycoprotein matrix The sperm then pushes its way through the softened jelly coat and binds to exposed docking proteins on the egg membrane The membrane of the egg and sperm then fuse and the sperm nucleus (and centriole) enters the egg 3. Cortical Reaction The cortical reaction occurs once a sperm has successfully penetrated an egg in order to prevent polyspermy Cortical granules within the egg’s cytoplasm release enzymes (via exocytosis) into the zona pellucida (jelly coat) These enzymes destroy sperm binding sites and also thicken and harden the glycoprotein matrix of the jelly coat This prevents other sperm from being able to penetrate the egg (polyspermy), ensuring the zygote formed is diploid

Answer 44

Blastocyst Formation Following the fusion of an egg and sperm (fertilization), an influx of Ca2+ into the ova prompts the completion of meiosis II The egg and sperm nuclei combine to form a diploid nuclei and the fertilized cell is now called a zygote The zygote will undergo several mitotic divisions to form a solid ball of cells called a morula. As the morula continues to divide, it undergoes differentiation and cavitation (cavity formation) to form a blastocyst Implantation of Blastocyst The final stage of early embryo development is the implantation of the blastocyst into the endometrial lining of the uterus The blastocyst breaches the jelly zona pellucida that was surrounding it and preventing its attachment to the endometrium Digestive enzymes are released which degrade the endometrial lining, while autocrine hormones released from the blastocyst trigger its implantation into the uterine wall from the trophoblastic end Only once the blastocyst is embedded within the uterine wall can the next stage of embryogenesis occur The growing embryo will gain oxygen and nutrients from the endometrial tissue fluid, ensuring its continued development The entire process (from fertilization to implantation) takes roughly 6 – 8 days

Answer 45

hCG promotes the maintenance of the corpus luteum within the ovary and prevents its degeneration As a consequence of this, the corpus luteum survives and continues to produce both oestrogen and progesterone The levels of hCG are maintained for roughly 8 – 10 weeks while the placenta is being developed After this time, the placenta becomes responsible for progesterone secretion and nourishing the embryo At this point the corpus luteum is no longer required and begins to degenerate as hCG levels drop

Answer 46

The chorionic villi extend into the intervillous space (lacuna) and exchange materials between the mother and foetus Chorionic villi are lined by microvilli to increase the available surface area for material exchange Foetal capillaries within the chorionic villi lie close to the surface to minimise diffusion distance from blood in the lacunae Materials such as oxygen, nutrients, vitamins, antibodies and water will diffuse from the lacunae into foetal capillaries Foetal waste (such as carbon dioxide, urea and hormones) will diffuse from the lacunae into the maternal blood vessels

Answer 47

After 9 months, the baby is fully grown and stretches the walls of the uterus – placing a strain on both mother and infant This stress induces the release of chemicals which trigger a rise in the levels of estrogen which prepares the smooth muscle of the uterus for hormonal stimulation by increasing its sensitivity to oxytocin Estrogen also inhibits progesterone, which was preventing uterine contractions from occurring while the foetus developed Now that the uterus is primed for childbirth, the brain triggers the release of oxytocin from the posterior pituitary gland. Oxytocin stimulates the uterine muscles to contract, initiating the birthing process (it also inhibits progesterone secretion) The foetus responds to this uterine contraction by releasing prostaglandins, which triggers further uterine contractions As the uterine contractions trigger the release of chemicals that cause further contractions, a positive feedback loop ensues Contractions will stop when labour is complete and the baby is birthed (no more stretching of the uterine wall)

Answer 48

Animal size / mass – larger animals tend to have longer gestation periods (as they tend to produce larger offspring) The level of development at birth – more developed infants will typically require a longer gestation period

Answer 49

The fluid is largely incompressible and good at absorbing pressure, and so protects the foetus from impacts to the uterus The fluid also creates buoyancy so that the foetus does not have to support its own weight while a skeletal system develops Amniotic fluid prevents the dehydration of foetal tissues

Answer 50

Estrogen and Progesterone