Capillaries act as a semipermeable membrane allowing the diffusion of gases and transfer of nutrients and waste products. The single layer of flattened endothelial cells of the capillaries facilitate the exchange of substances between capillaries and tissues. Gases, such as O2 and CO2, metabolic waste products, lactate, glucose and other nutrients are transferred across the walls of the capillaries through small slits in the endothelial cells known as pores or fenestrations.
To prevent capillaries from losing vital substances such as plasma proteins, the slits in the endothelial cells are smaller than these proteins. How do gas exchange and nutrient transfer happen between capillaries and tissues?
According to the Starling principle named after physiologist Ernest Starling who described it in , fluid movement through the capillary walls is governed by hydrostatic pressure and oncotic pressure. Like any fluid pushed through a confined space, blood in a capillary exerts pressure on the wall of the vessel because of the pressure exerted upstream by the blood coming from the arteriole.
The blood pressure BP generates hydrostatic pressure, which expels fluid from the pores of the capillary into the interstitial compartment. The size of the pores in the capillary dictates whether particular nutrients are delivered to particular tissues. Hydrostatic pressure is highest at the arterial end, and lowest at the venous end, of the capillary.
The other influencing force is oncotic pressure, which is underpinned by the principle of osmosis; this is the passive movement of water through a semipermeable membrane from a region of low solute concentration to one of high solute concentration, with the aim of achieving equilibrium. In blood, plasma proteins — which cannot easily pass through the capillary walls — exert an osmotic pressure that tends to pull fluid from the surrounding tissue which has a higher water concentration into the capillary which has a lower water concentration.
This is referred to as oncotic pressure.
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Fig 2 illustrates the interplay between hydrostatic and oncotic pressure. At the arterial end of the capillary, hydrostatic pressure exceeds oncotic pressure, so fluid moves out of the capillary into the interstitial compartment. At the venous end of the capillary, the two forces are reversed, so fluid moves back from the tissue into the capillary. In recent years, the Starling principle has been contested. More work is required to fully understand the complex processes taking place in the capillaries Levick and Michel, Another important factor is the architecture of capillaries, which varies according to their site in the body and affects their permeability Table 3.
There are site-specific variations in fluid transfers between capillaries; for example, in the glomerulus where capillaries supply and drain the individual kidney units , the capillaries are porous and therefore highly permeable. Conversely, at the blood-brain barrier in the brain, the very tight architecture of capillaries reduces their permeability. BP can be affected by a change in cardiac output or PVR. An important measure is the mean arterial pressure MAP , which is the pressure that propels blood towards tissues with each cardiac cycle and generates perfusion pressure to the organs.
There are various short- and long-term physiological mechanisms that regulate BP, summarised in Fig 3 and outlined below. The vasomotor centre in the medulla oblongata of the brain, which hosts most of the sympathetic neurons of the nervous system, has a key role in regulating vascular tone.
It transmits signals along sympathetic nerve fibres to vascular smooth muscle, mostly at the level of the arterioles. This results in vasoconstriction or vasodilation, with corresponding effects on BP and blood flow to the tissues. Alterations in BP are detected by mechanical pressure sensors baroreceptors found in the arterial wall of the carotid sinus the site between the internal and the external carotid arteries and the aortic arch.
If BP suddenly rises, the walls of these vessels expand, which increases the frequency of nerve impulses sent to the vasomotor centre. The vasomotor centre is inhibited, causing reflex vasodilation reduced vascular tone due to less sympathetic nerve activity and a decrease in BP. Conversely, if BP falls, the decreased stretch on the arterial walls causes a decrease in baroreceptor firing and culminates in reflex vasoconstriction and an increase in BP.
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This is the short-term baroreceptor response regulating BP. A similar phenomenon occurs through a chemically induced reflex via chemoreceptors, which are found in specialised cells in the arteries of the neck common carotid arteries and in the aortic arch. These peripheral chemoreceptors predominantly detect changes in oxygen, carbon dioxide levels and pH only carotid bodies. Along with the central chemoreceptors found in the brain they act to control respiration and maintain oxygen and acid-base status. However, they can also affect the cardiovascular function either directly by controlling the vasomotor centre in the brain or indirectly via the pulmonary stretch receptors Klabunde, The kidneys and adrenal glands play a crucial role in the long-term regulation of BP, in which a hormonal system known as the renin-angiotensin-aldosterone system RAAS is involved.
The RAAS activates the sympathetic nervous system and regulates plasma sodium and BP, and is targeted by many drugs designed to control BP and treat cardiac conditions, including the angiotensin-converting enzyme ACE inhibitor ramipril or the angiotensin II receptor blocker irbesartan.
The RAAS begins with the breakdown of angiotensinogen a plasma protein produced by the liver by renin an enzyme produced by the kidneys. Specialised cells that make up the juxtaglomerular apparatus of the kidney can sense changes in BP. When it is low, renin is released, triggering a cascade of enzymatic reactions: angiotensinogen produces an inactive peptide called angiotensin I; ACE an enzyme produced by the lungs converts angiotensin I into angiotensin II, a potent vasoconstrictor, which triggers an increase in BP.
Angiotensin II can also trigger the adrenal gland to produce aldosterone, a mineralocorticoid hormone that sends signals via its receptor in the kidneys. They are clothed in hair and their skin contains glands which secrete sweat. Some of these glands are specialized as mammary glands , producing milk to feed the young.
Mammals breathe with lungs and have a muscular diaphragm separating the thorax from the abdomen which helps them draw air into the lungs. The mammalian heart has four chambers and oxygenated and deoxygenated blood are kept entirely separate. Nitrogenous waste is excreted primarily as urea.
Mammals are amniotes , and most are viviparous , giving birth to live young. The exception to this are the egg-laying monotremes , the platypus and the echidnas of Australia. Most other mammals have a placenta through which the developing foetus obtains nourishment, but in marsupials , the foetal stage is very short and the immature young is born and finds its way to its mother's pouch where it latches on to a nipple and completes its development.
Anatomy And Physiology I
Humans have the overall body plan of a mammal. Humans have a head , neck , trunk which includes the thorax and abdomen , two arms and hands , and two legs and feet. Generally, students of certain biological sciences , paramedics , prosthetists and orthotists, physiotherapists , occupational therapists , nurses , podiatrists , and medical students learn gross anatomy and microscopic anatomy from anatomical models, skeletons, textbooks, diagrams, photographs, lectures and tutorials, and in addition, medical students generally also learn gross anatomy through practical experience of dissection and inspection of cadavers.
The study of microscopic anatomy or histology can be aided by practical experience examining histological preparations or slides under a microscope. Human anatomy, physiology and biochemistry are complementary basic medical sciences, which are generally taught to medical students in their first year at medical school. Human anatomy can be taught regionally or systemically; that is, respectively, studying anatomy by bodily regions such as the head and chest, or studying by specific systems, such as the nervous or respiratory systems.
Academic anatomists are usually employed by universities, medical schools or teaching hospitals. They are often involved in teaching anatomy, and research into certain systems, organs, tissues or cells. Invertebrates constitute a vast array of living organisms ranging from the simplest unicellular eukaryotes such as Paramecium to such complex multicellular animals as the octopus , lobster and dragonfly.
By definition, none of these creatures has a backbone. The cells of single-cell protozoans have the same basic structure as those of multicellular animals but some parts are specialized into the equivalent of tissues and organs. Locomotion is often provided by cilia or flagella or may proceed via the advance of pseudopodia , food may be gathered by phagocytosis , energy needs may be supplied by photosynthesis and the cell may be supported by an endoskeleton or an exoskeleton.
Some protozoans can form multicellular colonies.
Metazoans are multicellular organism, different groups of cells of which have separate functions. The most basic types of metazoan tissues are epithelium and connective tissue, both of which are present in nearly all invertebrates. The outer surface of the epidermis is normally formed of epithelial cells and secretes an extracellular matrix which provides support to the organism. An endoskeleton derived from the mesoderm is present in echinoderms , sponges and some cephalopods. Exoskeletons are derived from the epidermis and is composed of chitin in arthropods insects, spiders, ticks, shrimps, crabs, lobsters.
Calcium carbonate constitutes the shells of molluscs , brachiopods and some tube-building polychaete worms and silica forms the exoskeleton of the microscopic diatoms and radiolaria. The outer epithelial layer may include cells of several types including sensory cells, gland cells and stinging cells. There may also be protrusions such as microvilli , cilia, bristles, spines and tubercles.
Marcello Malpighi , the father of microscopical anatomy, discovered that plants had tubules similar to those he saw in insects like the silk worm. He observed that when a ring-like portion of bark was removed on a trunk a swelling occurred in the tissues above the ring, and he unmistakably interpreted this as growth stimulated by food coming down from the leaves, and being captured above the ring.
Arthropods comprise the largest phylum in the animal kingdom with over a million known invertebrate species. Insects possess segmented bodies supported by a hard-jointed outer covering, the exoskeleton , made mostly of chitin. The segments of the body are organized into three distinct parts, a head, a thorax and an abdomen.
The thorax has three pairs of segmented legs , one pair each for the three segments that compose the thorax and one or two pairs of wings. The abdomen is composed of eleven segments, some of which may be fused and houses the digestive , respiratory , excretory and reproductive systems. Spiders a class of arachnids have four pairs of legs; a body of two segments—a cephalothorax and an abdomen. Spiders have no wings and no antennae.
They have mouthparts called chelicerae which are often connected to venom glands as most spiders are venomous. They have a second pair of appendages called pedipalps attached to the cephalothorax. These have similar segmentation to the legs and function as taste and smell organs. At the end of each male pedipalp is a spoon-shaped cymbium that acts to support the copulatory organ. In BCE, the Edwin Smith Papyrus , an Ancient Egyptian medical text , described the heart , its vessels, liver , spleen , kidneys , hypothalamus , uterus and bladder , and showed the blood vessels diverging from the heart.
The Ebers Papyrus c. Ancient Greek anatomy and physiology underwent great changes and advances throughout the early medieval world. Over time, this medical practice expanded by a continually developing understanding of the functions of organs and structures in the body.
Phenomenal anatomical observations of the human body were made, which have contributed towards the understanding of the brain, eye, liver, reproductive organs and the nervous system. The Hellenistic Egyptian city of Alexandria was the stepping-stone for Greek anatomy and physiology.
Alexandria not only housed the biggest library for medical records and books of the liberal arts in the world during the time of the Greeks, but was also home to many medical practitioners and philosophers. Great patronage of the arts and sciences from the Ptolemy rulers helped raise Alexandria up, further rivalling the cultural and scientific achievements of other Greek states.
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Some of the most striking advances in early anatomy and physiology took place in Hellenistic Alexandria. These two physicians helped pioneer human dissection for medical research. They also conducted vivisections on the cadavers of condemned criminals, which was considered taboo until the Renaissance—Herophilus was recognized as the first person to perform systematic dissections. Herophilus's knowledge of the human body has provided vital input towards understanding the brain, eye, liver, reproductive organs and nervous system, and characterizing the course of disease. He was able to distinguish the sensory and the motor nerves in the human body and believed that air entered the lungs and heart, which was then carried throughout the body.
His distinction between the arteries and veins—the arteries carrying the air through the body, while the veins carried the blood from the heart was a great anatomical discovery.