NCUK Weeks 9 & 10 - Biology PDF

NCUK – WEEK 9,10 Part I: Part II: Fertilization, implantation and role of placenta. Execratory system Part I Fertilization, implantation and role of placenta. Fertilisation Fertilization, the union of sperm and egg, produces a diploid zygote. Although most mammals reproduce only during certain seaso...

NCUK – WEEK 9,10 Part I: Part II: Fertilization, implantation and role of placenta. Execratory system Part I Fertilization, implantation and role of placenta. Fertilisation Fertilization, the union of sperm and egg, produces a diploid zygote. Although most mammals reproduce only during certain seasons of the year, men produce sperm more or less continuously, and women ovulate about once a month. During Fertilization, the Sperm and Egg Nuclei unit  During intercourse, the penis releases sperm into the vagina.  The sperm move through the cervix, into the uterus, and finally enter the uterine tubes.  Sperm, under ideal conditions, may live for 2 to (rarely) 4 days inside the female reproductive tract, and an unfertilized egg remains viable for a day or so.  Therefore, if copulation occurs within a day or two of ovulation, the sperm may meet an egg in one of the uterine tubes.  When it leaves the ovary, the egg is surrounded by accessory follicle cells. These cells, now called the corona radiata, form a barrier between the sperm and the egg.  A second barrier, the jelly-like zona pellucida (“clear area”), lies between the corona radiata and the egg. Follow: fertilization  In the uterine tube, hundreds of sperm surround the corona radiata.  Each sperm releases enzymes from its acrosome.  These enzymes weaken both the corona radiata and the zona pellucida, allowing the sperm to wriggle through to the egg.  If there aren’t enough sperm, not enough enzymes are released, and none of the sperm will reach the egg.  Perhaps 1 in 100,000 of the sperm deposited in the vagina reaches the uterine tube, and 1 in 20 of those encounters the egg, so only a few hundred join in attacking the barriers around the egg. Cell Differentiation, gastrulation, and Organogenesis Occur During the First Two Months  Fertilization of a human egg usually takes place in the uterine tube.  The resulting zygote undergoes a few cleavage divisions in the uterine tube, becoming a morula on its way to the uterus.  By about the fifth day after fertilization, the zygote has developed into a hollow ball of cells, known as a blastocyst.  A human blastocyst consists of an outer layer of cells surrounding a cluster of cells called the inner cell mass.  Beginning around the sixth to ninth day after fertilization, the outer cell layer attaches to, and then burrows into, the lining of the uterus (the endometrium), a process called implantation.  The outer cell layer of the blastocyst will become the chorion.  The complex intermingling of the chorion and the endometrium forms the placenta, which will be described shortly.  All the cells of the inner cell mass have the potential to develop into any type of tissue in the human body.  This remarkable flexibility allows the inner cell mass to produce both the entire embryo and the amnion, allantois, and yolk sac.  Although they still retain the potential to develop into any cell type, the cells of the inner cell mass of an early blastocyst start to differentiate through the process of induction.  For example, the cells in contact with the outer cell layer usually produce the embryo proper, while the cells exposed to the blastocyst fluid produce extraembryonic membranes, especially the yolk sac. Gastrulation Occurs After Implantation During the second week of development, the inner cell mass grows and splits, forming two fluid-filled sacs that are separated by a double layer of cells called the embryonic disk. One layer of cells is continuous with the yolk sac. The second layer of cells is continuous with the amnion. Gastrulation begins near the end of the second week. Cells migrate in through a slit in the amnion side of the embryonic disk. This slit is the disk’s equivalent of the amphibian blastopore. Once inside the disk, the migrating cells form mesoderm, endoderm, and the allantois. The cells remaining on the surface become ectoderm Organogenesis Begins During Weeks Three to eight During the third week of development, the embryo begins to form the spinal cord and brain. The heart starts to beat about the beginning of the fourth week. At this time, the embryo, bathed in fluid contained within the amnion, bulges into the uterine cavity. Meanwhile, the umbilical cord forms from the fusion of the yolk stalk and body stalk. The yolk stalk connects the yolk sac to the embryonic digestive tract. Role of placenta: The Placenta exchanges Materials Between Mother and embryo  During the first few days after implantation, the embryo obtains nutrients directly from the endometrium of the uterus.  During the following week or so, the placenta begins to develop from interlocking structures produced by the embryo and the endometrium.  The outer layer of the blastocyst forms the chorion, which grows finger-like chorionic villi that extend into the endometrium.  Blood vessels of the umbilical cord connect the embryo’s circulatory system with a dense network of capillaries in the villi.  Meanwhile, some of the blood vessels of the endometrium erode away, producing pools of maternal blood that bathe the chorionic villi.  The embryo’s blood and the mother’s blood remain separated by the walls of the villi and their capillaries, so the two blood supplies do not actually mix to any great extent.  The walls of the capillaries and chorionic villi restrict the movement of many substances, including most large proteins and cells.  Many small molecules, on the other hand, readily move between the mother’s blood and the embryo’s blood.  Oxygen diffuses from the mother to the embryo.  Nutrients, many aided by active transport, also travel from the mother to the embryo.  Carbon dioxide and other wastes, such as urea, diffuse from the embryo to the mother.  Certain types of antibodies, even though they are quite large, are selectively transported across the placenta from mother to embryo, especially late in pregnancy, and play an important role in defending the newborn infant against disease.  Although the placenta isolates the fetus from many assaults, it does not provide complete protection: Some disease-causing organisms and many harmful chemicals can pass through the placenta.  The placenta allows exchange of wastes and nutrients between fetal capillaries and maternal blood pools, while keeping the fetal and maternal blood supplies separate.  The umbilical arteries carry deoxygenated blood from the fetus to the placenta, and the umbilical vein carries oxygenated blood back to the fetus. Part II Human execratory system The human urinary system and its blood supply  Urinary systems are organ systems that produce and eliminate urine.  Urine is a watery fluid that contains a variety of substances that have been removed from the blood or (in many invertebrates) from the interstitial fluid that bathes all cells.  Urine contains waste products from proteins, various ions and other water-soluble nutrients in excess of the body’s needs, and certain foreign substances (such as drugs or their metabolic by-products).  By producing and eliminating urine, urinary systems play two major roles in most animals: They excrete cellular wastes and they help maintain homeostasis. Urinary systems excrete cellular Wastes * Excretion is a general term that encompasses the elimination of any form of waste from the body. Urinary systems excrete cellular wastes, primarily the nitrogenous wastes ammonia, urea, and uric acid. The mammalian urinary system  consists of the paired kidneys and ureters as well as a single bladder and urethra.  These organs filter small nutrient and waste molecules and ions out of the blood,  And then help maintain homeostasis by returning essential ions and nutrients to the blood, while collecting and excreting excess substances and cellular wastes. The structure and blood supply of the human kidney: Yellow arrows show the path of urine flow. Structures of the human Urinary system produce, store, and excrete Urine. Human kidneys are fist-sized organs located at about waist level on either side of the spinal column. The outermost layer of each kidney is the renal cortex. Beneath the renal cortex lies the renal medulla (“kidney marrow”), which allows the kidney to produce concentrated urine, thus conserving water. The renal medulla surrounds a branched, funnel-like chamber called the renal pelvis (“kidney bucket”), which collects urine and conducts it into the ureter. The ureter is a narrow, muscular tube that contracts rhythmically to propel the urine from the kidney to the bladder, a hollow, muscular chamber that collects and stores urine. The average adult bladder can hold about a pint (500 milliliters) of urine, but the desire to urinate is triggered by smaller amounts. As accumulating urine expands the bladder wall, the pressure eventually activates stretch receptors that trigger reflexive contractions. Urine is retained in the bladder by two circular sphincter muscles. The internal sphincter, located at the junction of the bladder and the urethra, opens automatically during these contractions. The external sphincter, slightly below the internal sphincter, is under voluntary control, allowing the brain to suppress urination unless the bladder becomes overly full. Urine exits the body through the urethra, a single narrow tube about 1.5 inches long in women and about 8 inches long in men (because it extends through the prostate gland and penis). Water balance is essential for homeostasis An important function of the kidney is osmoregulation. Osmoregulation is the process of maintaining blood. osmolarity—the concentration of ions and other solutes in the blood plasma—within very strict limits.  If a person consumes excess water faster than the kidneys can excrete it, the surplus water in the blood will move by osmosis into the interstitial fluid and then into cells, causing them to swell.  Swelling in brain cells causes headaches, nausea and vomiting, seizures, coma, and sometimes death.  In contrast, if a person becomes dehydrated (if water is unavailable or illness causes prolonged diarrhea and vomiting), blood osmolarity increases and blood volume decreases.  If the kidneys cannot conserve enough water, dehydration can cause low blood pressure, dizziness, and confusion.  In extreme cases, loss of water in brain cells can lead to coma and death. An individual nephron and its blood supply Nephrons in the kidneys Filter blood and produce Urine  The entire blood volume passes through the kidneys about 60 times daily, allowing them to fine-tune its composition.  Each kidney contains roughly 1 million microscopic urineforming units called nephrons.  Nephrons are packed together in the renal cortex, with a thin extension of each nephron extending into the renal medulla.  Each nephron has two major parts: the renal corpuscle and the renal tubule.  The role of the renal corpuscle is to pressure-filter the blood and collect the resulting fluid, called filtrate.  The renal corpuscle consists of two parts: the glomerulus and the glomerular capsule.  The glomerulus is a knot of exceptionally porous capillaries that allow water and small molecules dissolved in the blood plasma to ooze out as blood flows through them.  The surrounding cup-shaped glomerular capsule captures this blood filtrate.  The filtrate then enters the renal tubule, which conducts the filtrate as it is converted to urine.  The renal tubule consists of three parts: The first portion is the proximal tubule, which returns water and most essential molecules and ions to the blood.  The filtrate then enters the second portion of the tubule, the nephron loop (also called the loop of Henle).  In most nephrons, the nephron loop extends from the renal cortex into the uppermost portion of the renal medulla, but some extend much more deeply, as described later.  The main function of the nephron loop is to produce and maintain a high concentration of salt ions (Na+ and Cl-) in the interstitial fluid of the renal medulla.  This high interstitial concentration of solutes helps the kidney produce concentrated urine and maintain water in the blood, as described later.  The filtrate is finally converted into urine in the distal tubule, where more substances are removed from and secreted into the blood.  The distal tubule empties urine into a collecting duct, a larger tube adjacent to the nephron.  There are thousands of collecting ducts within the kidney; each receives urine from many nephrons.  Collecting ducts conduct urine from the renal cortex, through the renal medulla, and into the renal pelvis. Blood vessels support the Nephron’s role in Filtering the blood Blood is carried to the kidney by the renal artery, which gives rise to thousands of microscopically narrow arterioles. Each arteriole supplies blood to a nephron. Within the renal corpuscle, the arteriole branches to form the capillaries of the glomerulus. These empty into an outgoing arteriole. The outgoing arteriole gives rise to peritubular capillaries which form a network surrounding the renal tubule. The peritubular capillaries conduct the blood into a venule that joins the renal vein. How is urine formed?  Urine is produced in the nephrons of the kidneys by three processes: Filtration, Reabsorption, and Secretion.  As urine is formed, dissolved substances move between the parts of the nephron and the interstitial fluid that surrounds these structures.  The interstitial fluid in turn exchanges substances with a nearby network of microscopic capillaries. Filtration removes small Molecules and Ions from the blood  Filtration, the first step in urine formation, occurs when fluid is forced by blood pressure through the walls of the nephron’s glomerular capillaries.  Two factors facilitate glomerular filtration: First, the glomerular capillaries are far more porous than most other capillaries, and second, the arterioles that collect blood from the capillaries are narrower than the arterioles that supply them, creating an unusually high pressure within the glomerular capillaries.  As a result, about 20% of the blood’s fluid, along with its small dissolved molecules, is forced out through the glomerular capillary walls.  Blood cells and plasma proteins, which are too large to penetrate the capillary walls, remain in the blood.  The filtrate (essentially plasma minus its large proteins) is collected by the surrounding glomerular capsule, which conducts it into the proximal tubule.  Urea makes up about 40% of the solutes in the glomerular filtrate. Reabsorption returns Important substances to the blood Reabsorption returns to the blood nearly all the water, ions (Na+, Cl-, K+, Ca2+, HCO3-), and organic nutrients such as vitamins, glucose, and amino acids that were previously removed during filtration.  The ions Na+, Cl-, K+, Ca2+ are critical for nerve and muscle function, and Na+ levels in blood exert a major influence on blood volume and pressure.  The bicarbonate ion (HCO3-) is crucial for maintaining the constant pH required for metabolic reactions.  The reabsorbed molecules move by diffusion or active transport through the walls of the renal tubule and into the peritubular capillaries, which return them to the bloodstream.  Most reabsorption takes place in the proximal tubule, but reabsorption also occurs in the nephron loop and the distal tubule.  In the proximal tubule, reabsorption is generally not under hormonal control, but in the distal tubule, it is under the control of hormones that help maintain homeostasis.  Thus, the distal tubule fine-tunes blood composition by regulating the reabsorption of water and ions to maintain homeostasis based on the changing needs of the body.  The fluid that has travelled through the nephron becomes urine as it leaves the distal tubule. Secretion Actively Transports substances into the renal Tubule for excretion Secretion, which occurs mainly through active transport, moves wastes and excess ions from the blood into the renal tubule.  Secreted substances include excess K+ and H+, small quantities of ammonia, some medicinal and recreational drugs or their breakdown products (including penicillin, aspirin, morphine, nicotine, and cocaine), as well as certain food additives and pesticides.  Secretion occurs primarily in the proximal tubule, but some also occurs in the distal tubule during the final stages of urine formation.  As with reabsorption, secretion by the distal tubule is regulated by circulating hormones to maintain homeostasis. Concentration of Urine occurs in the Distal Tubule and collecting Duct  When the filtrate enters the distal tubule, about 80% of its water has already been reabsorbed in the proximal tubule and nephron loop, but the filtrate is still considerably more dilute than the surrounding interstitial fluid in the renal cortex.  From this point on, additional reabsorption of water is precisely regulated to maintain the blood’s osmolarity within narrow limits.  If fluid intake has been high, more water will be left behind in the filtrate, and watery urine will be produced until the normal blood volume is restored.  If fluid intake has been low, concentrated urine will be produced. This occurs because the distal tubule and collecting duct will become more permeable to water, which will leave the urine by osmosis and be returned to the blood.  Returning water to the blood to avoid dehydration is a major function of the collecting duct.  The key to producing concentrated urine lies in the elevated solute concentration of the surrounding fluid.  The interstitial fluid within the renal medulla contains high concentrations of salt and urea (because salt enters the fluid from the nephron loop, and some urea diffuses into the interstitial fluid from the collecting duct).  The collecting duct carries urine through increasingly concentrated interstitial fluid within the renal medulla.  When the collecting duct is water permeable, the difference in osmolarity between the urine and the interstitial fluid causes water to leave the urine and enter the interstitial fluid by osmosis. Details of urine formation in the nephron  The concentration of dissolved substances in the filtrate is indicated by the intensity of yellow color; black arrows indicate the direction of filtrate flow.  Outside the nephron, darker shades of beige represent higher concentrations of salt and urea in the surrounding interstitial fluid.