Homeostasis & Body Systems

Homeostasis

• A state of relative constancy achieved through automatic mechanisms compensating for changes.
• Requires sensors to detect internal and external changes.
• Also needs a control center (brain and spinal cord) and effectors.
• Health depends on proper homeostatic function.
• Illnesses and diseases often result when these mechanisms break down, such as diabetes.
• External factors like stress can impair homeostatic efficiency.

Scientific Language

• Cardiovascular: Relates to the heart and blood vessels.
• Homeostasis: Balance within all body systems.
• Cytoplasm: Liquid inside a cell giving it form.
• Genome: DNA.
• Embryo: Early developmental stage of a multicellular organism.
• Proliferating: Rapidly reproducing (cells, structures, organisms).

Endocrine System

• Hypothalamus acts as the link between nervous and endocrine systems.
• The system releases hormones to maintain homeostasis.
• The nervous system regulates the body via neurotransmitters.
• Pathways include: Fight/flight (nerve stimulus hormone), common response (hormone to gland), and simple response (nutrient levels stimulate hormones).
• Hormones are chemical messengers produced by endocrine glands, traveling via the bloodstream.
• Function to maintain homeostasis, control development/growth, and control reproduction.

Types of Hormones

• Tropic: Influences other glands, either inhibiting or enhancing hormone production (TSH, LH, FSH, ACTH, GH).
• Non-Tropic: Targets cells instead of glands, stimulating cell growth and metabolism.

Feedback Loops

• Negative Feedback Loop: Events cause effects in the opposite direction from stimuli, returning the system to equilibrium.
• Positive Feedback Loop: Events cause effects that bring the body out of equilibrium.
• Positive feedback loops trigger essential changes, needed for reproduction but not for maintaining homeostasis.

Male Reproductive System

• Testes within scrotum produce sperm and testosterone.
• Epididymis stores and matures sperm, expelling it via muscle contractions.
• Vas deferens carries sperm from the epididymis to the ejaculatory duct.
• Seminal vesicles secrete a milky substance that activates sperm.
• Ejaculatory duct connects the vas deferens and seminal vesicles; semen exits via the urethra.
• Prostate gland produces prostate fluid (part of semen) that nourishes, protects sperm, and aids in expulsion.
• Bulbourethral glands secrete pre-ejaculatory fluid to lubricate the urethra and neutralize acidic urine residue.
• Penis functions for urination and ejaculation, with spongy tissue that engorges with blood during an erection.

Female Reproductive System

• Functions to produce eggs, facilitate fertilization and pregnancy, and enable menstruation.
• Vagina extends from the cervix to the exterior body, serving as the birth canal and receiving the penis during intercourse.
• Uterus, located between the urinary bladder and rectum, receives, retains, and nourishes the fertilized egg.
• Endometrium lines the uterus, allowing implantation of a fertilized egg and shedding during menstruation.
• Fallopian tubes connect the uterus to the ovaries, providing the environment for fertilization.
• Fimbriae at the ends of the fallopian tubes help capture eggs from the ovary.
• Ovaries are sac-like structures connecting to the fallopian tubes, producing eggs and hormones.
• Follicles within the ovaries consist of an oocyte (immature egg) and follicular cells.
• The ovaries release eggs during ovulation and secrete progesterone to support pregnancy.
• Females are born with all the eggs they will ever have (around 1-2 million), with no eggs maturing during childhood but activating at puberty.
• Ovulation stops around age 50 due to menopause.
• Vulva refers to the external genitalia.
• Breasts produce milk after childbirth. Regulated by estrogen, they are considered secondary sexual characteristics.
• Immature eggs are in follicles located in the ovaries.

Spermatogenesis

• Production of sperm cells begins at puberty and continues throughout lifetime.
• Occurs in the seminiferous tubules.
• Stem cells undergo rapid mitosis to produce more stem cells (spermatogonia) before puberty.
• FSH modifies spermatogonia division.
• Type A daughter cell is a stem cell.
• Type B daughter cell becomes a primary spermatocyte.
• Primary spermatocytes undergo meiosis.
• One primary spermatocyte produces four haploid spermatids with 23 chromosomes.

Oogenesis

• Oogonia are female stem cells found in a developing fetus.
• Oogonia undergo mitosis to produce primary oocytes.
• Primary oocytes are surrounded by cells that form primary follicles in the ovary.
• Oogonia no longer exist by the time of birth.
• Meiosis starts inside the maturing follicle, producing a secondary oocyte and the first polar body.
• Follicle development to the vesicular follicle stage takes about 14 days.
• Ovulation of a secondary oocyte occurs with the release of luteinizing hormone (LH).
• Secondary oocyte is released and surrounded by a corona radiata.
• Meiosis completes over ovulation only if sperm penetrates, producing an ovum.
• Two additional polar bodies are produced.
• Once the ovum is formed, the 23 chromosomes combine with those of the sperm to form a zygote.
• The secondary oocyte degenerates if sperm doesn't penetrate it, and meiosis doesn't complete to form an ovum.

Reproductive Hormones

• Gonadotropin Releasing Hormone (GnRH): Stimulates the anterior pituitary gland to produce LH and FSH.
• Luteinizing Hormone (LH): Regulates the ovarian cycle and estrogen production, stimulates androgen production.
• Follicle Stimulating Hormone (FSH): Regulates the ovarian cycle and estrogen production and sperm production.
• Androgens/Testosterone: Development of male secondary sex characteristics and sperm production.
• Estrogen: Development of female secondary sex characteristics and regulates the uterine cycle.
• Progesterone: Prepares the uterus for implantation of the fertilized ovum, maintains pregnancy, and promotes mammary gland development.
• Oxytocin: Stimulates contractions on smooth muscle in the uterus during labor and ejection of milk from breasts during nursing.
• Prolactin: Stimulates mammary glands to produce milk.
• Relaxin: Softens the cervix and relaxes pelvic ligaments, causing smooth muscle relaxation during childbirth.
• Inhibin: Secretes FSH, exerts negative feedback control on the hypothalamus and pituitary by increasing spermatogenesis corpus luteum activity increase.
• Human Chorionic Gonadotropin (hCG): Maintains output of estrogen and progesterone by the corpus luteum during pregnancy, maintains the uterine lining.

Embryology (Scientific Language)

• Fuse: Process where structures form a single entity.
• Ovum: Egg cell.
• Differentiation: Unspecialized cells become specialized.
• Zygote: First diploid cell formed by fusion of gametes.
• Morula: Early embryo stage, 8-32 cells.
• Blastula: Hollow sphere of cells, early embryonic stage.
• Gastrula: Blastula reforms into a multidimensional structure.
• Embryo: Early developmental stages of a developing organism.
• Organogenesis: Formation of organs from germ layers.
• Neurulation: Formation of the neural tube.
• Fertilization: Sperm cell fuses with an egg cell, forming a zygote.

Fertilization and Implantation

• The egg travels into the fallopian tube during ovulation while waiting for sperm.
• Sperm breaks enzymes to penetrate the outer layer (zona pellucida) to fertilize it and create a zygote.
• The zygote divides while traveling down the fallopian tube; becomes a blastocyst and attaches to the uterine lining (implantation).
• Mobile sperm chases the egg through complicated structure.
• The egg must be fertilized it within 24 hours.
• Fimbriae guide the egg into the fallopian tube the egg while cilia push it through.
• Large egg carries nutrients and energy for survival after fertilization.
• Sperm has many mitochondria for high mobility.
• Only one sperm can penetrate the egg.
• The hardening of the egg post sperm entry is called a cortisol reaction, it releases enzymes that kill other sperm.
• Cleavage: Cell division; the zygote divides into more and more cells until it is a blastocyst.
• Breakdown of implantation in steps:
• Hatching: Blastocyst gets rid of zona pellucida shell.
• Apposition: Blastocyst picks a place to attach on the uterine lining.
• Adhesion: Blastocyst latches onto the uterine lining.
• Invasion: Trophoblast breaks into endometrium, connects to mothers blood supply.
• Locked in: Blastocyst is fully implanted; trophoblast releases hormones and nutrients.

Ectopic Pregnancy

• Blastocyst implants in the wrong place, not the uterus, such as the fallopian tube (tubal pregnancy), cervix, ovary, or abdominal cavity.
• Blocked or damaged fallopian tubes, hormonal imbalances, smoking, pelvic infections, or previous ectopic pregnancies increase the risk.
• Fallopian tubes could rupture, leading to internal bleeding from not being built for pregnancy.
• Treatment in surgery or medicine stops cell division to prevent continuation.
• Future pregnancies remain possible though riskier.
• Ampulla, the largest section of the fallopian tubes, is where fertilization usually occurs, thus ectopic pregnancies occur when not moving through properly.

In Vitro Fertilization (IVF)

• Helps people conceive, reasons include blocked tubes, low sperm count, egg quality issues, unexplained infertility, same sex couples, single parents

In Vitro Fertilization (IVF) process

• Ovarian Stimulation: Multiple eggs mature at the same time.
• Egg Retrieval: Doctor retrieves eggs via needle.
• Sperm Collection: Partner or donor provides sample.
• Fertilization: Egg and sperm are introduced in lab dish, injecting directly.
• Embryo Development: Fertilized eggs grow in lab for a few days, on day 3-5 the strongest is picked.
• Embryo Transfer: Transferred back into the uterus. Not always successful Female/Male infertility - ovulation disorders, fallopian tube blockage, Uterine issues, low sperm count, hormonal imbalance, physical blockage

Differentiation

• When cells in the embryo transform from unspecialized to specialized cells
• Occurs during gastrulation when embryo forms layers (mesoderm, ectoderm and endoderm)

Germ Layers

• Ectoderm: Makes skin, hair, nails, glands, nervous system (brain, spinal cord, nerves), and sense organs.
• Mesoderm: Makes muscles, bones, cartilage, circulatory, reproductive, and excretory systems.
• Endoderm: Makes digestive and respiratory systems, glands, bladder, and urethra.
• Layers start at 3 weeks, they develop their tissues and organs from here.

Gastrulation and Neurulation

• Gastrulation: When embryo goes from a blastula (hollow ball of cells) to a gastrula (structure with the three layers of differentiation)
• Formation of primitive streak: streak forms along the epiblast, top layer of embryo
• Cell migration: cells migrate through streak before diving inward (invagination) and form the 3 germ layers
• Trilaminar Embryo: now is a trilaminar structure, has three layers
• Neurulation: occurs after gastrulation, when the nervous system starts forming,creation of the neural tube
• Formation of the neural plate: part of ectoderm thickens to form the neural plate
• Shaping of the neural groove: neural plate begins folding inwards creating the neural groove, edges of plate rise up
• Fusion of the neural tube: neural folds come together forming the neural tube, top part becoming the brain and the bottom part becoming the spinal cord
• Neural crest cells: break away cells are called neural crest cells, create the peripheral nerves, melanocytes, facial cartilage and bones

Gastrulation vs Neurulation

• Gastrulation: Happens first (week 3 of development), forms the three germ layers, and key structure is the primitive streak.
• Neurulation: Happens after (week 3-4 of development), forms the neural tube (spinal cord and brain), focuses on the nervous system, and key structure is the neural tube.

Organogenesis

• Process where the three germ layers build the organs
• Week 4 to week 8 of development
• Induction: cells send signals (morphogens) to tell each layer what to create
• Morphogenesis: cells start moving, folding, arranging themselves into 3d structures, practising to form organ shapes
• Cell differentiation: once cells are in correct location, become specialised and start creating
• Nervous system starts forming first, brain and spinal cord form the neural tube, ectoderm
• Heart and blood vessels develop next, heart begins beating at around 4 weeks - mesoderm
• Digestive and respiratory systems next, gut tube forms the stomach, liver, intestines etc, then lungs begin to bud - endoderm
• Skeletal and muscular systems start developing, cartilage and bone forms - mesoderm
• Sense organs develop next, eyes, ears etc - ectoderm
• Organogenesis is what make the three germ layers actually develop

Childbirth (Parturition)

• Labor involves a series of events that expel the infant from the uterus.
• Initiation of labor: Estrogen levels rise, the placenta releases prostaglandins, oxytocin is released, and uterine contractions begin.
• Stages of Labor include: Dilation of the cervix, expulsion, and delivery of the placenta.
• Expulsion: The infant passes through the cervix and vagina, can last as long as 2 hours but usually around 50 minutes, normal delivery is head first, breech presentation is buttocks first.
• Placenta Stage: Delivery of the placenta, usually within 15 minutes of birth of the infant, all placental fragments should be removed to avoid postpartum bleeding.

Stages of Labor - Positive Feedback Loop

• Stretching of the cervix: Baby's head pushes against the cervix, stretches and sends a signal to the brain.
• Release of oxytocin: Brain (hypothalamus) releases oxytocin into the bloodstream via the posterior pituitary (where oxytocin is stored), travels into the uterus and creates stronger contractions.
• Stronger Uterine Contractions: Contractions become harder and more frequent, pushing the baby further into the birth canal, further stretching the cervix.
• The cycle continues until the baby is delivered, then the positive feedback loop ends as without the loop, contraction may not be strong enough to push the baby out.
• Counting Weeks of Pregnancy (Obstetric)
• First day of pregnancy is the first day of the last mensterual cycle.

When calculating the dates, you:

Add 7 days

• Subtract 3 months
• Add 1 year

Breast Feeding - Positive Feedback Loop

• Baby creates suction on nipple and areola of breast: stimulates nerves, signals get sent to the brain
• The brain (hypothalamus) sends signal to posterior pituitary gland to release oxytocin and prolactin
• Oxytocin gets the milk to begin to flow, prolactin makes sure the milk continues to flow
• The more the baby stimulates the breast, the more milk is produced