Q.21. Explain the mechanism of respiration in humans. (2013)
Ans. Mechanism of respiration: -
During respiration there is an alternating increase and decrease in thoracic volume. Two components, the ribs and diaphragmatic respiration, work together to produce an ingenious “pump” mechanism. The diaphragm may be regarded as a “piston” in “pump cylinder” of the thoracic cavity. It is pushed in an pulled out, thus pushing air out of the trachea (expiration) or pulling it in (inspiration). Throughout this process the diameter of the thorax continuously alters. In expiration B1, 2, 3 it becomes smaller, and during inspiration it enlarges A1, 2, 3.
Mechanism of rib respiration: -
When none of the respiratory muscles are acting, the elastic thoracic cage lies in the median position of equilibrium, the resting respiratory position. This is influenced by the pull of the lungs and the abdominal muscles. The position of respiratory rest is the position from which inspiration starts A. during inspiration the thoracic cage is actively lifted – as the ribs are positioned obliquely – and enlarged. The sternum, which is attached to the ribs by cartilage, produces a “parallel displacement” of the ends of the ribs. In quiet respiration, after inspiration the elastic thoracic cage passively returns to its resting position. It is only during forced expiration B that the thoracic cage in addition is actively lowered against the elastic force. During artificial respiration compression of the thorax may produce a forced expiration, and the elastic recoil of the thorax to the resting position then produces an inspiration. During quiet rib respiration, the external intercostals and the posterior serratus muscles produce inspiration, and the internal intercostals muscles in the region of the bony ribs are the muscles of expiration. The intercostals muscles hold the intercostals spaces tense against the external air pressure and the negative intrathoracic pressure. In forced rib respiration, the muscles of the shoulder girdle (“accessory muscles of respiration”) act as inspiratory muscles and the abdominal wall muscles and the latissimus dorsi muscle produce expiration.
Mechanism of diaphragmatic respiration: -
The abdominal viscera and abdominal wall muscles are of importance in diaphragmatic respiration. The liver, the “core” of the “pistor”, moves up and down. During inspiration A, the muscles fibres of the diaphragm shorten: the longer fibres, which originate from the back of the diaphragm A4, shorten more than the interior fibres. The central tendon is lowered and the distance between the parts of the diaphragm which ascend to the dome and the chest wall becomes larger, the costodiaphragmatic recess A5, more marked dorsally than ventrally. The lungs expand to fill the extra volume of the virtual space thus opened, the basal parts more than the apices. A left and right anterior diaphragmatic muscles band which lie beneath the pericardium attached to the central tendon, pull down the diaphragm like a pocket under the plane of the xiphoid process A6. The heart AB7 is displaced during diaphragmatic respiration.
In expiration B the diaphragm and the liver move into the thoracic cavity due to contraction of the muscles of the abdominal wall and the pull of thelungs. The excursion of the dome of the diaphragm AB8 varies from 1.5 – 7 cm according to the depth of respiration. In quite respiration about 75% of the change in intrathoracic volume is produced by diaphragmatic movement.
Costodiaphragmatic mechanism – In the adult both mechanism of respiration work together. The essential condition for effective rib respiration is that the diaphragm contracts and is not pulled up into the thoracic cavity by lung suction. Effective diaphragmatic respiration requires stable, tense intercostals spaces. Respiratory position of the thoracic cage and the diaphragm on the left anteriorly and on the right laterally.
Respiratory Dynamics: -
Respiration: - Air pressure exerted through the respiratory pathways A2 forces the lung Al against the chest wall. There is subtamospheric pressure (Donder’s pressure) in the enclosed pleural space A3 which prevents the lung from falling away from the chest wall, and forces it to follow the respiratory movements of the chest wall and diaphragm. The subatmospheric pressure is produced by the pull (which is still present in expiration) that results from stretching the elastic fiber networks of the lung, and it is increased by inspiration.
Costodiaphragmatic recess A4. In expiration the relaxed muscle fibers of the diaphragm rise steeply upward in the thorax into the dome of the diaphragm. Contraction of these fibers displaces the diaphragm from the chest wall and forces the central tendon of the diaphragm into the abdominal cavity. This frees a complementary space between the diaphragm and the chest wall, the costodiaphragmatic recess A4: inspiration. Inspiratory enlargement in all the thoracic diameters contributes to opening the recess. The increase in lung volume causes air to be drawn in through the respiratory pathways — respiratory (tidal) volume, forced inspiration = inspiratory reserve volume. In expiration the tidal volume is breathed out and forced expiration further displaces the expiratory reserve volume. This only leaves the residual volume of air in the lung.
The costomediastinal recess, which lies on both sides behind the sternum and on either side of the vertebral column, also serves for expansion of the lung. The inspiratory enlargement of the lung increases caudally and ventrally. In the thoracic type of respiration the upper lobes are better ventilated than during abdominal respiration.
Q.22. Elaborate the structure and function of adrenal gland. (2013, 14)
Related Question -
Q. Write short note on adrenal gland. (2015)
Ans. Adrenal gland: -
In mammals, the adrenal glands (also known as suprarenal glands) are endocrine glands that sit at the top of the kidneys; in humans, the right adrenal gland is triangular shaped, while the left adrenal gland is semilunar shaped. They are chiefly responsible for releasing hormones in response to stress through the synthesis of corticosteroids such as cortisol and catecholamines such as epinephrine (adrenaline) and norepinephrine. These endocrine glands also produce androgens in their innermost cortical layer. The adrenal glands affect kidney function through the secretion of aldosterone, and recent data suggest that adrenocortical cells under pathological as well as under physiological conditions show neuroendocrine properties; within the normal adrenal, this neuroendocrine differentiation seems to be restricted to cells of the zona glomerulosa and might be important for an autocrine regulation of adrenocortical function.
Anatomy and physiology: -
The adrenal glands are located in the retroperitoneum superior to the kidneys, they are quadrilaterial in shape and are situated bilaterally. The combined weight of the adrenal glands in an adult human ranges from 7 to 10 grams. They are surrounded by an adipose capsule and renal fascia.
Each adrenal gland has two distinct structures, the outer adrenal cortex and the inner medulla, both of which produce hormones. The cortex mainly produces cortisol, aldosterone and androgens, while the medulla chiefly produces epinephrine and norepinephrine. In contrast to the direct innervation of the medulla, the cortex is regulated by neuroendocrine hormones secreted from the pituitary gland which are under the control of the hypothalamus, as well as by the renin-angiotensin system.
Cortex: -
The adrenal cortex is devoted to production of corticosteroid and androgen hormones. Specific cortical cells produce particular hormones including aldosterone, cortisol, and androgens such as androstenedione. Under normal unstressed conditions, the human adrenal glands produce the equivalent of 35–40 mg of cortisone acetate per day.
The adrenal cortex comprises three zones, or layers. This anatomic zonation can be appreciated at the microscopic level, where each zone can be recognized and distinguished from one another based on structural and anatomic characteristics. The adrenal cortex exhibits functional zonation as well: by virtue of the characteristic enzymes present in each zone, the zones produce and secrete distinct hormones.
Zona glomerulosa (outer): -
The outermost layer, the zona glomerulosa is the main site for production of aldosterone, a mineralocorticoid, by the action of the enzyme aldosterone synthase (also known as CYP11B2). Aldosterone is largely responsible for the long-term regulation of blood pressure. Aldosterone’s effects are on the distal convoluted tubule and collecting duct of the kidney where it causes increased reabsorption of sodium and increased excretion of both potassium (by principal cells) and hydrogen ions (by intercalated cells of the collecting duct). Sodium retention is also a response of the distal colon, and sweat glands to aldosterone receptor stimulation. Although sustained production of aldosterone requires persistent calcium entry through low-voltage activated Ca2+ channels, isolated zona glomerulosa cells are considered nonexcitable, with recorded membrane voltages that are too hyperpolarized to permit Ca2+ channels entry. However, mouse zona glomerulosa cells within adrenal slices spontaneously generate membrane potential oscillations of low periodicity; this innate electrical excitability of zona glomerulosa cells provides a platform for the production of a recurrent Ca2+ channels signal that can be controlled by angiotensin II and extracellular potassium, the 2 major regulators of aldosterone production. Angiotensin II originates from plasmatic angiotensin I after the conversion of angiotensinogen by renin produced by the juxtaglomerular cells of the kidney.
The expression of neuron-specific proteins in the zona glomerulosa cells of human adrenocortical tissues has been predicted and reported by several authors and it was suggested that the expression of proteins like the neuronal cell adhesion molecule (NCAM) in the cells of the zona glomerulosa reflects the regenerative feature of these cells, which would lose NCAM immunoreactivity after moving to the zona fasciculata. However, together with other data on neuroendocrine properties of zona glomerulosa cells, NCAM expression may reflect a neuroendocrine differentiation of these cells.NCAM expression may reflect a neuroendocrine differentiation of these cells.
Zona fasciculata: -
It situated between the glomerulosa and reticularis, the zona fasciculata is responsible for producing glucocorticoids, such as 11-deoxycorticosterone, corticosterone, and cortisol in humans. Cortisol is the main glucocorticoid under normal conditions and its actions include mobilization of fats, proteins, and carbohydrates, but it does not increase under starvation conditions. Additionally, cortisol enhances the activity of other hormones including glucagon and catecholamines. The zona fasciculata secretes a basal level of cortisol but can also produce bursts of the hormone in response to adrenocorticotropic hormone (ACTH) from the anterior pituitary.
Zona reticularis: -
The inner most cortical layer, the zona reticularis produces androgens, mainly dehydroepiandrosterone (DHEA), DHEA sulfate (DHEA-S), and androstenedione (the precursor to testosterone) in humans.
Medulla: -
The adrenal medulla is the core of the adrenal gland, and is surrounded by the adrenal cortex. It secretes approximately 20% noradrenaline (norepinephrine) and 80% adrenaline (epinephrine). The chromaffin cells of the medulla, named for their characteristic brown staining with chromic acid salts, are the body’s main source of the circulating catecholamines adrenaline and noradrenaline. Catecholamines are derived from the amino acid tyrosine and these water-soluble hormones are the major hormones underlying the fight-or-flight response.
To carry out its part of this response, the adrenal medulla receives input from the sympathetic nervous system through preganglionic fibers originating in the thoracic spinal cord from T5–T11. Because it is innervated by preganglionic nerve fibers, the adrenal medulla can be considered as a specialized sympathetic ganglion. Unlike other sympathetic ganglia, however, the adrenal medulla lacks distinct synapses and releases its secretions directly into the blood.
Cortisol also promotes epinephrine synthesis in the medulla. Produced in the cortex, cortisol reaches the adrenal medulla and at high levels, the hormone can promote the upregulation of phenylethanolamine N-methyltransferase (PNMT), thereby increasing epinephrine synthesis and secretion.