Homeostasis, Nervous System, And Excretory System: Answers And Explanations

What is homeostasis?

Homeostasis refers to the ability of the organism to maintain the internal environment in a steady state while considering the external variations. Endocrine system along with the autonomic nervous system plays vital roles in maintaining homeostasis through coordinated action of the hypothalamus as the chief regulator of the feedback mechanisms. Temperature, blood glucose and water are regulated through homeostatic procedures to maintain the stable body conditions in order to adjust to environments that are congenial for survival. Attainment of homeostasis occurs through dynamic equilibrium where the mediators appear to be the nervous and hormonal signals that account for the integrated and coordinated actions through feedback control (Wynn, Chawla and Pollard 2013).

Part 1

Regulation and maintenance of body temperature is necessary to keep the internal environment of the body in steady state condition while the external temperature gets altered. Efficient functioning of the enzymes in addition to the smoother operation of the endocrine and metabolic systems is carried out through homeostasis in living organisms. Maintenance of body temperature in case of humans occurs by means of control rendered by the thermoregulatory centre of hypothalamus that receives impulses from two sets of thermo receptors, one located in the hypothalamus and other at the skin. The hypothalamic receptor is responsible for controlling the temperature of the blood during its course through the brain thereby setting the core temperature. Contrarily the skin receptor is responsible for monitoring the ambient temperature. Adjustment in body temperature is done through the dual working of these thermo receptors in case of temperature alteration (Liedtke 2017).

The normal body temperature in humans is recorded to be 37^?C. Temperature adjustments take place in situations above or below this temperature through hypothalamic regulatory mechanism carried out through neural feedback. At skin temperature above 37^?C, sweating occurs when the heat gets dissipated and rises concomitantly as the skin temperature rises further. The amount of heat generated under such circumstances remains constant although the temperature of the skin undergoes elevation. Conversely when the skin temperature falls below 37^?C, numerous responses pertinent to the conservation of heat with simultaneous increase in the production of heat occurs that include stoppage of bodily perspiration, vasoconstriction to reduce the flow of heat to the skin along with shivering to support thermo genesis in the muscles apart from the secretion of catecholamines, adrenaline and nor-adrenaline in conjunction with thyroxine to account for the production of heat. For lower animals, insulation is facilitated through the erection of body hairs and fur (Romanovsky 2014).

Regulation of body temperature

Part 2

Water is vital to the survival of organisms and is considered crucial to the regulation of homeostasis. Within the body water level is controlled via the hypothalamus. Maintenance of water potential is essential to prevent the loss or gain of water from the cells. Homeostatic function of water includes maintaining the excitability of cells, cellular and extracellular fluid volume, lubrication and moistening along with thermoregulation. Moreover, water being the chief solvent of the body fluid; it serves as medium of carriage of materials in addition to being the reactant as well as medium of certain metabolic reactions like that of hydrolysis and hydration. Therefore it is extremely desirable that the water levels are maintained within the body in order to carry out the vital functioning of the important organs through performance of metabolic reactions.

Osmoreceptors located in the hypothalamus are capable of detecting the changes in water potential of the blood while travelling through the brain. Hypothalamus is responsible for regulating the sensation of thirst along with secretion of the anti diuretic hormone (ADH), a posterior pituitary hormone that in turn performs the vital function of maintaining the water balance in the body through regulation of the excretion of water via urine output. The target cells of the ADH are the endothelial cells located in the collecting ducts of the nephrons of the kidneys and possess the unique property by virtue of which the water molecules can pass through their membranes only by means of water channels called aquaporins instead of passing through the lipid bilayer. ADH thereby acts on these water channels to open up when needed depending upon the situation. ADH stimulates reabsorption of water from the distal and collecting tubules of the nephrons and thereby helps to conserve water in the body by reducing its excretion through urine. When the osmotic concentration of the blood rises due to shortage of water inside the body, the release of ADH from posterior pituitary is stimulated; this in turn reduces the excretion of water through urine so as to conserve the body water. Conversely, when the blood becomes dilute due to presence of excess water in it, the release of ADH is suppressed to excrete the excess water through urine. Therefore, the normal water potential is maintained inside the body following the interplay of ADH and aquaporin water channels through detection made by the osmoreceptors in the hypothalamus (Harrison 1971). 

Homeostasis and water balance

Part 3

The regulation of the blood glucose level is stringently controlled by the endocrine system involving the antagonistic actions of the two pancreatic hormones, insulin and glucagon. Glucose is considered as the chief carbohydrate in the body and its concentration in the blood influences the bodily cells. The blood glucose concentration is maintained within the range of 0.8-1 g/dl of blood while both high glucose concentration (hyperglycaemia) as well as low glucose concentration (hypoglycaemia) are considered detrimental to health and may be fatal at times. The primary control of glucose regulation is rendered by the pancreas whereby the glucose receptor cells are held responsible for maintaining the concentration of glucose in the blood along with the endocrine cells of the Islets of Langerhans that secrete hormones. The α cells of the Islets of Langerhans tissue secrete glucagon while the ? cells secrete insulin hormones respectively that exhibits opposite effects on glucose regulation because of antagonistic functions. Insulin being an anabolic hormone helps to increase the storage of food in the body through stimulating the uptake of glucose by means of the cells for respiration. In the liver, insulin acts to stimulate the conversion of glucose to glycogen by means of glycogenesis thereby lowering the blood glucose level. The hypoglycemic effect of insulin is achieved by virtue of increasing the permeability of different tisssues to glucose so as to facilitate the transport of glucose from blood to tissues. Insulin aids in utilization of glucose in the cells by stimulating the glycogeneis pathway, through lipogeneis allowing the conversion of glucose to fat in addition to oxidation of glucose to yield energy. Inhibition of glucose formation occurs through glycogenolysis and neoglucogenesis in liver and its entry into the blood.  Secretion of insulin from pancreas gets dtimulated when theblood sugar level is raised such as during absorption of food after meals. Thus the absorbed glucose is well utilized and blood glucose level is brought back to normal, otherwise glucose would escape through urine (Nadal et al. 2009).  Contrarily, glucagon being a catabolic hormone performs the vital function of increasing the blood glucose level by stimulating gluycogenolysis which is the breakdown of glycogen to glucose and neuglucogeneiss that account for the formation of glucose from non-carbohydrates like protein or fat in liver. In case of lowering of blood sugar, the secretion of glucagon is stimulated thereby helping the blood sugar to attain a normal level. In extreme cases, synthesis of glucose from pyruvate may also take place under the influence of glucagon. Thus the blood glucose homeostasis is maintained by the actions of insulin and glucagon secretion in response to high and low concentration of blood glucose respectively as detected by the pancreas (Karsli-Uzunbas et al. 2014).

Regulation of blood glucose level

Part 1

(Source: Created by Author)

Part 2

Functions of different parts of the Nervous system:

1. Brain- It is protected by cranium or skull and remains surrounded by meninges- dura mater, arachnoid and pia mater to protect against any external injury. It is the upper expanded part of CNS and divisible into three primary regions like forebrain, midbrain and hind brain comprising of structures like olfactory lobe, cerebrum, diencephalon, optic lobe crura cerebri, cerebellum, pons and medulla oblongata. It acts as the centre for both reflex and voluntary avtions.
2. Spinal Cord- It is located in the neural canal of vertebral column having the white matter outside while the grey matter remains inside. Spinal cord acts as centre for various reflexes. The organs supplied by spinal motor neurons are reflexly controlled by spinal cord. The white matter of spinal cord contains ascending and descending nerve tracts which connects the brain with the peripheral organs innervated by the spinal nerves.
3. Fore Brain- It consists ofolfactory lobe, diencephalon and cerebrum. Diencephalon in turn is divisible into two parts consisting of thalamus and hypothalamus. Cerebrum or cerebral cortex consists of five lobes namely frontal, parietal, temporal, occipital and limbic lobes that in turn act as the seats of co0nscious sensations. Higher associative cognitive functions including learning, memory, emotion, intelligence are attributed to cerebrum apart from controlling skilled voluntary movement as well as speech. Olfactory lobe is responsible for relaying the sense of smell to temporal lobe of cerebrum. Thalamus acts as the relay centre for all sensory inputs and is centre for crude sensations. Hypothalamus is considered the higher centre of autonomic nervous system, centre for thermoregulation and emotional reactions apart from acting as the master gland of the endocrine system.
4. Mid Brain- It consists ofoptic lobe and crura cerebri that are responsible for controlling the reflex movement of head and neck in response to auditory and visual information. It helps in maintenance of posture and equilibrium of the body in addition to regulating the visual reflexes.
5. Hind Brain-It controls the reflex regulation of breathing, salivary secretion, lacrimal secretion, heart rate, cardiac output, blood pressure, secretion of digestive juices and movement of the alimentary canal. Posture and equilibrium are also maintained apart rrom regulating the skilled voluntary movements (Nave and Werner 2014).
6. Sensory cranial nerves-Peripheral nerves containing afferent nerve fibres only. e.g. I, II & VIII cranial nerves.
7. Motor cranial nerves-Peripheral nerves containing the efferent nerve fibres only. e.g. II, IV, VI, XI & XII cranial nerves.
8. Afferent root spinal nerves-Continuation of dorsal horn and are made up of gray matter apart from being sensory in nature. It transmits impulse from sense organ to spinal cord.
9. Efferent root spinal nerves-Continuation of ventral horn and is made up of grey matter apart from being motor in nature. It transmits impulse from spinal cord to effector organ (Llinás 1988).

Removal of nitrogenous wastes present in the body is carried out by virtue of the primary excretory organ, kidney in humans. Accessory excretory organs like lung, skin, liver and intestine apart from performing their specific functions also carry out the removal of wastes from the body (Bradley 2013).

The process of elimination of metabolic waste products from the body is called excretion. The kidneys are the chief excretory organs of man which excrete the water soluble waste products through urine. The kidneys are bean-seed shaped paired organs located on either side of the vertebral column near the posterior wall of lower abdomen. Each kidney is made up of about 1 miilion nephrons which are the structural and functional units of the kidney. Each nephron is divisible into two main parts-the malpighian corpuscle and the renal tubule. The malpighian corpuscle consists of a double layered cup shaped structure, the Bowman’s capsule which encloses a tuft of capillaries, the glomerulus. The renal tubule is further divisible into four parts-the proximal convoluted tubule, the Henle’s lop, the distal convoluted tubule and the collecting duct. The main function of the kidney is to form urine. For this, the Bowman’s capsule forms a protein-free filtrate of plasma from the blood flowing through the glomerular capillaries. As the filtrate passes through the tubule, water and certain other materials are returned to the blood according to their requirement in the body by a process called reabsorption. The tubule also adds certain minerals to the filtrate by a process called secretion. As a result of reabsorption and secretion, the filtrate is converted into urine before it is passed out of the kidney through the ureter. Normal urine is made up of water and materials dissolved in it e.g., organic constituients like urea, uric acid and creatinine, amino acids, hippuric acid etc., and inorganic constituents like chlorides, bicarbonates, phosphates and sulphates of sodium, potassium, calcium, magnesium etc. Under pathological conditions the urine may contain sugar, protein, ketone bodies, blood etc (Dalvie et al. 2009). Hence, the excretory function of kidney through urine formation is executed in this manner.

Fig: Structure of nephron of the chief excretory organ kidney in human

(Source: Jha and Kher 2016)

References

Bradley, T.J., 2013. 10 The Excretory System. Structure and Physiology. Regulation: digestion, nutrition, excretion, 4, p.421.

Dalvie, D., Obach, R.S., Kang, P., Prakash, C., Loi, C.M., Hurst, S., Nedderman, A., Goulet, L., Smith, E., Bu, H.Z. and Smith, D.A., 2009. Assessment of three human in vitro systems in the generation of major human excretory and circulating metabolites. Chemical research in toxicology, 22(2), pp.357-368.

Harrison, D., 1971. Excretion, osmoregulation and homeostasis. In Advanced Biology Notes (pp. 115-122). Macmillan Education UK.

Jha, P.K. and Kher, V., 2016. 1 Physiology: A Clinical Perspective. Manual of Nephrology, p.1.

Karsli-Uzunbas, G., Guo, J.Y., Price, S., Teng, X., Laddha, S.V., Khor, S., Kalaany, N.Y., Jacks, T., Chan, C.S., Rabinowitz, J.D. and White, E., 2014. Autophagy is required for glucose homeostasis and lung tumor maintenance. Cancer discovery, 4(8), pp.914-927.

Levenson, R.W., 2014. The autonomic nervous system and emotion. Emotion Review, 6(2), pp.100-112.

Liedtke, W.B., 2017. Deconstructing mammalian thermoregulation. Proceedings of the National Academy of Sciences, p.201620579.

Llinás, R.R., 1988. Central Nervous System Function.

Melmed, S., Polonsky, K.S., Larsen, P.R. and Kronenberg, H.M., 2015. Williams textbook of endocrinology. Elsevier Health Sciences.

Nadal, A., Alonso-Magdalena, P., Soriano, S., Quesada, I. and Ropero, A.B., 2009. The pancreatic β-cell as a target of estrogens and xenoestrogens: Implications for blood glucose homeostasis and diabetes. Molecular and cellular endocrinology, 304(1), pp.63-68.

Nave, K.A. and Werner, H.B., 2014. Myelination of the nervous system: mechanisms and functions. Annual review of cell and developmental biology, 30, pp.503-533.

Romanovsky, A.A., 2014. Skin temperature: its role in thermoregulation. Acta Physiologica, 210(3), pp.498-507.

Wynn, T.A., Chawla, A. and Pollard, J.W., 2013. Macrophage biology in development, homeostasis and disease. Nature, 496(7446), pp.445-455.