In addition to the epithelial, connective and nervous tissues already mentioned, the skin also contains muscle tissue. Tiny muscled called arrector pili attached to the hair follicles can cause the hairs of the skin to ‘stand on end’ in response to signals from motor neurons. Although this reaction is useless for heat retention in humans, most mammals are able to increase the thickness of their insulating fur in cold weather by erecting the individual hairs.
Beneath the dermis there is also a layer of subcutaneous tissue containing large numbers of adipose cells. These cells store fat and form an important insulating layer, preventing heat loss in humans, as well as a protective cushion against knocks.
The kidneys are the fist-sized organs found at each side of the lower back, they are part of the urinary system, which also includes the ureters, bladder & urethra. The kidney has two main functions, it removes metabolic waste from the body through the process of excretion and it regulates the water and ion content in the blood.
Each of the two kidneys are supplied with oxygenated blood via the renal artery which branches from the aorta. Deoxygenated blood returns from the kidneys via the renal veins. Urine produced in the kidneys flows down the ureters to the bladder where it can be stored. The exit from the bladder is self controlled through the sphincter muscle, which when relaxed allows urine to flow from the bladder along the urethra to the outside.
Each kidney contains about 1.2 million tubules called nephrons. Each nephron consists of a long tubule and a associated small blood vessels. First, blood is carried off by an afferent arteriole to a tuft of capillaries in the renal cortex, the glomerulus. Here the blood is filtered as the blood pressure forces fluid through the porous capillary walls.
Blood cells and plasma proteins are too large to enter this glomerular filtrate but large amounts of water and dissolved molecules leave the vascular system at this step. The filtrate immediately enters the first region of the nephron tubules. After the filtrate enters the bowman’s capsule it goes into a portion of the nephron called the proximal convoluted tubule, located in the cortex. Approximately two thirds of the NaCl and water filtered into the bowman’s capsule are immediately reabsorbed across the walls here. Although one third of the initial volume of filtrate remains in the nephron tubule after the reabsorption of NaCl and water . The majority of this water is also reabsorbed.
The reabsorption is driven by the active transport of the Na+ out of the filtrate and into surrounding petibular capillaries. Cl- follows Na+ passively because of the electrical attraction, and water follows via osmosis. Active extrusion of the ascending limb of the loop of Henle creates the osmotic gradient required for the absorption of water from the collecting duct. The osmotic gradient is usually constant, but the permeability of the of the collecting duct to water is adjusted by a hormone, anti-diuretic hormone (ADH). When water needs to be conserved, the posterior pituitary gland secretes more ADH, and this hormone increases the number of water channels in the plasma membranes of the collecting ducts to water so that more water is reabsorbed and less is excreted in the urine. As well as regulating water balance, the kidneys also regulate the balance of electrolytes in the blood, also through reabsorption and secretion. This regulation is called homeostasis.
Homeostasis provides cells within the body with a relatively constant environment, and this helps them to work efficiently, no matter what is going on outside the body. Whatever the air temperature around you, the temperature around a cell in your liver is always just below 38oC. However much or little carbohydrate you have eaten, the concentration of glucose in your body fluids does not normally fluctuate very far from 800mg per dm3.
Processes which aim to keep a potentially fluctuating feature within narrow limits, use negative feedback mechanisms. These maintain the organism's internal environment within tolerance limits - the narrow range of conditions where cellular processes are able to function at a level consistent with the continuation of life.
For example, the human body uses a number of processes to control its temperature, keeping it close to an average value or norm of 98.6 degrees Fahrenheit. One of the most obvious physical responses to overheating is sweating, which cools the body by making more moisture on the skin available for evaporation. On the other hand, the body reduces heat-loss in cold surroundings by sweating less and reducing blood circulation to the skin. Thus, any change that either raises or lowers the normal temperature automatically triggers a counteracting, opposite or negative feedback .
In a negative feedback system there needs to be a detector, which measures the value of the feature to be controlled ie the temp of your blood. If the detector finds that the temp is higher than in should be, it sends a message to an effector, which would then do something to lower the temp back to the correct level. It would keep doing this until the detector, which is constantly measuring the value, finds that the level is now too low, and then sends a message to the effector to stop what it’s doing, and now increase the level.
Negative feedback mechanisms in living organisms do not usually succeed in keeping a particular feature absolutely constant. There is usually some fluctuation between set parameters. This happens because it takes time for information to be passed from the detector to the effector, and for the actions of the effectors to have their desired effect.
In humans the detectors are specialised cells. Some of these cells are in the brain, but cells in other organs such as the pancreas also act as detectors for particular substances. Many different organs act as effectors. The skin is an effector on temp regulation, while the kidneys are effectors in the regulation of water and ion content. The information passes from detectors to effectors via nerves or via the blood as chemicals called hormones.
These two systems, the nervous & endocrine (hormone), are devoted exclusively to the regulation of the body organs. Both release regulatory molecules that control the body organs by first binding to the receptor proteins in the walls of those organs.
Most living organisms use a network of nerve cells to gather information about the body’s condition and the external environment, to process and integrate that information, and to issue commands to the body’s muscles and glands. Bundles of nerves called neurones connect every part of the body via the peripheral nervous system (PNS) to its command and control centre, the central nervous system (CNS) – the brain and spinal cord. They control complex reflexes and higher associative functions including learning and memory. All data is analysed here and commands are issued to the muscles and glands.
Fibres carrying impulses away from the cell body are called axons; those carrying towards the cell body are called dendrons. The axons of neurones secrete chemical messengers called neurotransmitters into the synaptic cleft. These chemicals diffuse only a short distance to the post-synaptic membrane, where they bind to receptor proteins and stimulate the post-synaptic cell (another neurone or muscle gland cell). Synaptic transmission generally effects only the one post-synaptic cell that receives the neurotransmitter.
Nerve impulses occur when the resting potential across a membrane of a neurone has a sufficiently high stimulus.
A hormone is a regulatory chemical that is secreted into the blood by an endocrine gland or an organ of the body exhibiting an endocrine function. The blood carries the hormone to every cell in the body, but only the target cells for a given hormone can respond to it. Thus, the difference between a neurotransmitter & a hormone is not in the chemical nature of the regulatory molecule, but rather in the way it is transported to it’s target cells, and it’s distance from these cells.
A chemical regulator called norepinephrine, for example, is released as a neurotransmitter by sympathetic nerve endings and is also secreted by the adrenal gland as a hormone. Some specialised neurones secrete chemical messengers into the blood rather than into a narrow synaptic cleft. In these cases, the chemical that the neurones secrete is sometimes called a neurohormone. The distinction between the nervous system and the endocrine system blurs when it comes to such molecules.
The endocrine system includes all of the organs that function exclusively as endocrine glands – such as the thyroid gland, the pituitary gland, adrenal gland and so on, as well as organs that secrete hormones in addition to other functions. Endocrine glands lack ducts and thus must secrete into surrounding blood capillaries. Hormones secreted by endocrine glands belong to four different chemical categories; Polypeptides (ADH), Glycoproteins (FSH), Amines, derived from amino acids tyrosine and tryptophan, and steroids such as testosterone and progesterone.
The endocrine system is an extremely important regulatory system in it’s own right, but it also interacts and co-operates with the nervous system to regulate the activities of other organ systems in the body. The secretory activity of many endocrine glands is controlled by the nervous system. Among such glands are the adrenal medulla, posterior pituitary, and pineal gland. These three glands are derived from the neural ectoderm, the same embryonic tissue layer that forms the nervous system.
The major site for neural regulation of the endocrine system, however, is the brain’s regulation of the anterior pituitary gland. The hypothalamus controls the hormonal secretions of the anterior pituitary, which in turn regulates other endocrine glands. On the other hand, the secretion of a number of hormones is largely independent of neural control. The release of insulin by the pancreas and aldosterone by the adrenal cortex, for example, are stimulated primarily by increases in the blood concentration of glucose and potassium respectively.
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