The human body is composed of trillions of cells. There are over 200 different cell types, each specializes to perform essential functions, bone cells, form and regenerate the bones in the body. Nervous system cells send messages within the brain and throughout the body to produce actions. Cells in the blood carry oxygen and help the immune system. Male and female sex cells unite to produce offspring. The genetic information inside each type of cell acts as an instruction manual telling a cell how to function and replicate a fractured bone. Bleeds bone repair begins as over a period of hours, blood at the fracture site, clots and forms a hematoma. The healing begins as osteoclasts in the area remove dead bone tissue while fibroblasts penetrate the area and build a fibrocartilaginous callus that bridges the broken bone over many weeks, osteoblasts transform the callus into bone, continued remodeling and the mechanical stress put on the bone, creates and shapes New bone over a period of months or years, the bone regains its original strength, shape and internal structure, soft tissue organs in the body repair injury through a multi step process that begins as platelets from torn vessels, work to form a mesh like clot that prevents blood Loss inflammation in the area occurs as tissue repair begins in the damaged area. Mast cells, release, histamine, that dilates blood vessels and increases blood flow to the repair site. White blood cells called neutrophils and macrophages work to consume bacteria and remove damaged tissue and debris.
As bacteria and dead cells are removed, the proliferative phase of wound healing begins: fibroblasts build new tissue by secreting collagen that takes the shape of the original tissue during remodeling. The final phase of wound healing the tissue created by the fibroblasts matures and regains its normal function. The bloodstream delivers oxygen to cells and removes carbon dioxide waste through a process called internal respiration in the lungs oxygen is absorbed into the erythrocytes in the bloodstream. These cells commonly called red blood cells, contain gas transporting molecules called hemoglobin oxygen. Binds to these molecules, the oxygenated blood then moves through the vasculature at the narrow capillaries within body tissues, red blood cells release oxygen, which then diffuses through the capillary wall into tissues. Meanwhile, the waste product, carbon dioxide diffuses into the bloodstream, where it is carried inside red blood cells and in plasma, the deoxygenated blood travels through the vasculature and back to the lungs where carbon dioxide is expelled from the body when bacteria or other pathogens are present. In the body, certain white blood cells categorized as phagocytes consume the microorganisms to protect the body from infection. The process of consuming the pathogen begins when the phagocyte is attracted to chemicals from the pathogen and through receptors binds to it the factor site ingests, the microbe encasing it in a compartment called a vacuole lysosomes in the phagocyte merge with the vacuole. Its enzymes then kill and digest the pathogen and release any indigestible remains from the cell.
This cell process called phagocytosis protects the body by removing pathogens cell fragments, called platelets or thrombocytes, make up about two percent of blood platelets stop blood loss from damaged vessels when a blood vessel, tears platelets at the site adhere to the wall of the vessel. To close the tear the shape of the platelets changes as they liberate the contents of their vesicles, this enables them to connect to one another. Platelets also release chemicals that activate the coagulation system to promote blood clotting blood proteins, known as clotting factors form fibrin threads. Millions of platelets, together with the fibrin threads, form a platelet plug. If the tear is small enough, the plug can stop blood loss, completely structures of the nervous system transfer messages throughout the body specialized nervous system cells called neurons. Make this messaging possible at one end of a neuron are dendrites that receive a messaging signal in the neurons axon. A chain reaction moves the message to the synapse, where the signal is transferred before this process begins. The neuron is polarized, with a net positive charge outside the neuron and a negative charge inside, as the signal passes along the axon, the membrane, depolarizes and repolarizes again passing positive charges in and then back out. The depolarizing signal creates a chain reaction that passes in a wave along the axon when the signal reaches the accent terminal. The change in polarity triggers the release of neurotransmitters from the presynaptic cell. These chemicals travel across a small space and activate another neuron, or they can transfer the signal to tissue like muscle and create an action.
The human body develops replaces old cells with new cells and repairs itself, with the help of cells that constantly multiply by dividing the process begins in the cell nucleus, identical copies of dna molecules organize into chromatid pairs within the chromosome structure. These pairs are connected to each other at the chromosomes centromere in the next steps, spindle fibers attach to the centromeres and the chromatids line up along the cells midline. The chromatids are then pulled apart, creating two identical sets of chromosomes. A nuclear membrane forms around each of the two identical sets of chromosomes and eventually the cell cleaves in two, with each daughter cell, containing one of the two nuclei. The process through which a cell divides the genetic information within its nucleus into two identical copies within independent nuclei is called mitosis. Red blood cells also called erythrocytes make up forty to forty five percent of blood volume and function to transport oxygen from the lungs to the cells of the body. Red blood cells are produced inside the bones of the skeletal system, particularly in the vertebrae, sternum ribs and pectoral, and pelvic girdles inside the red bone marrow of spongy bone stem cells, known as hemocytoblasts, give rise to the different types of blood cells, including red blood cells. During the development process, the hemocytoblast ejects its nucleus, allowing the cell to carry more oxygen to the tissues mature red blood cells, enter the bloodstream via enlarged capillaries, known as sinusoids. Substances such as nutrients gases and waste are constantly moving into and out of cells.
When the concentration of a substance is higher on one side of the cells selectively, permeable membrane, certain molecules may move by osmosis or diffusion through the membrane without the cell using any energy. This process is called passive transport when the cell needs to move molecules from an area of low concentration to one of high concentration, it uses specialized channels or carriers in the cell membrane. This process expends energy and is called active transport. The ovaries produce secondary oocytes the female sex cells during fetal development, stem cells called oogonia go through mitosis. Some of these cells develop into 46 chromosome primary oocytes at the onset of female puberty. Tens of thousands of primary oocytes each form part of a primordial follicle in the ovaries between menarche and menopause. Each primordial follicle develops into a mature follicle and its primary oocyte completes meiosis one. Some primary oocytes become secondary, oocytes 23 chromosome cells. That begin, meiosis ii then pause each month. One secondary oocyte is released into the uterine tube meiosis ii is completed when the oocyte is fertilized. The resulting ovum can develop into a zygote the testes constantly produce sperm. The male sex cells production begins within the seminiferous tubules, where stem cells called spermatogonia develop into immature sperm. Each 46 chromosomes per metagonium divides through mitosis to produce primary spermatocytes. These cells divide by meiosis to become 23 chromosome cells, called secondary spermatocytes that develop into spermatids. This multi step process called spermatogenesis, produces the 23 chromosome immature sperm cells that mature in the epididymis after fertilization, the zygote divides through cleavage as the cells multiply, they produce a sphere called a morula which moves towards the uterine cavity about six days after fertilization.
The morula has developed into a blastocyst that can implant on the wall of the uterus and begin the embryonic period the developing cells differentiate into layers that give rise to structures by the fourth week of development. The embryo has developed into an oblong body that further develops limbs, vertebrae and organs by week 10. The embryo is a fetus. Growth is supplied by nutrient rich blood, passing from the placenta through the umbilical cord over the course of about nine months or 36 weeks. Bones muscles skin and connective tissues, form body systems develop and limbs, and facial features take shape. The development of the skeletons flat bones begins as certain mesenchymal cells at specific points within embryonic fibrous, connective tissue, develop into osteoblasts the osteoblasts cluster together secrete bone matrix and develop into osteocytes that deposit, calcium and other mineral salts that harden the forming bone matrix. The bony regions spread out into thin columns called trabeculae and form spongy bone tissue. A fibrous covering called the periosteum forms on the surface of the bone and a layer of compact bone tissue replaces the upper layers of spongy bone. This process, in which flat bones develop from connective tissue, is called intramembranous ossification. The largest component of blood is plasma, a yellowish liquid, that is, 90 water, plasma, carries suspended blood cells and other substances nutrients that pass from the digestive system into the body are transported in the blood plasma. These nutrients include glucose amino acids, vitamins, minerals and fatty acids.
In the plasma of the blood that enters the kidneys are waste products from body tissue, the kidneys process, blood by filtering waste products such as urea, uric, acid and creatinine out of the blood plasma and into the urinary system. Plasma also transports electrolytes components of the immune system, enzymes and hormones, and it maintains homeostasis by releasing heat, cardiac muscle, which is found only in the heart wall contracts constantly to pump blood throughout the body. The heart wall is composed of three layers: a thin layer called the epicardium or visceral pericardium forms the outermost part of the heart wall. This layer adheres the heart to the pericardium, a sack of tissue that protects the heart from friction as it beats beneath the epicardium lies the thick cardiac muscle of the myocardium. The myocardium is responsible for the hearts pumping action, making powerful, continuous contractions possible this striated tissue contracts involuntarily in response to signals from the hearts own conduction system beneath the myocardium lies the endocardium, which forms the innermost layer of the heart wall, the endocardium lines, the hearts Internal structures and is continuous, with the lining of blood vessels that attach to the heart together these three layers of the heart wall, aid contraction and relaxation as the heart beats during embryonic development cells called chondroblasts begin secreting cartilage. That develops in the shape of the skeletons long bones, the bluish, transparent, avascular tissue formed, is called hyaline cartilage as the cartilage grows, the chondroblasts in the interior calcify and die blood vessels penetrate the structure and deliver osteoblasts.
These cells lay down bone material as the cartilage ossifies in spongy bone forms. A medullary cavity develops at the ends of a long bone. Ossification from cartilage also occurs. This process in which hyaline cartilage becomes bone and grows is called endochondral. Ossification bone growth continues through childhood. After puberty bone tissue replaces the remaining hyaline cartilage and the shaft reaches its adult length. Two is in the walls of hollow organs throughout the body. The tissue contracts and relaxes to contain substances and moves substances through the body. Smooth muscle, contractions are involuntary actions managed by impulses that travel through the autonomic nervous system to the smooth muscle tissue. The arrangement of cells in smooth muscle tissue provides for contraction and relaxation with great elasticity the smooth muscle in the digestive tract contracts and relaxes in peristaltic waves that move swallowed, food and nutrients from the mouth through the stomach and the intestines artery walls include smooth muscle That relaxes and contracts as pressure changes, move changing volumes of blood through the vessel.