The lungs re-oxygenating our blood is what pulmonary circulation is all about. We have a cardiovascular (heart and blood vessels) system composed of two closed circuits. The pulmonary circuit takes de-oxygenated blood, containing low levels of oxygen and large amounts of carbon dioxide, from the heart to the lungs. It then returns oxygenated blood, high in oxygen and low in carbon dioxide, from the lungs back to the heart. The heart then pumps oxygenated blood through the arteries of the systemic circuit to where it is needed throughout the body, at speed.
It is the “at speed” that is important. The two-circuit cardiovascular (CV) system is more efficient than the single circuit one of fish. The heart of a fish pumps de-oxygenated blood to the gills where it is re-oxygenated but slowed down before passing through the rest of the body and back to the heart. By being able to pump the oxygenated blood faster to where it is needed, for example muscle tissue, the two-circuit CV system enables fast responses with less damage to the muscle cells. The quicker arrival of oxygen when needed reduces the “wear and tear” on our bodies, which generally results in longer life. If people had only a single circuit like fish, they would either need to be more sedate or would have shorter lifespans.
While the heart is the pump at the center of the CV system, it is what occurs at the capillary webs or beds that gives the CV system its purpose. The heart has four chambers, the two upper chambers are called atria and they receive blood from veins; the two lower chambers are called ventricles and they pump the blood back out into arteries. The arteries simply carry the blood from the heart to the capillary webs and the veins return it from the capillary webs to the heart. The capillary webs of the pulmonary circuit are laced throughout the lungs; all other capillary webs are part of the systemic circuit.
Lungs are made up of lobes; in humans the left lung has two lobes and the right three. Dogs and cats have two and four lobes respectively. This is because the left lung is smaller to give room to the heart which is positioned slightly to the left of center in the thoracic cavity (chest). The functional part of the lobes are the alveoli, there are approximately 300 million alveoli in total.
The alveoli are formed in small clusters, like bunches of grapes. Each alveoli is a small air-sack connected to an alveolar duct through which air moves in and out. The alveolar ducts of each cluster merge into a respiratory bronchiole. Several respiratory bronchiole merge to form terminal bronchiole, these combine to make bronchiole which in turn merge to make tertiary bronchi. The tertiary bronchi in each lobe merge to make a secondary bronchi, there is one secondary bronchi per lobe. The secondary bronchi of the right lung merge to make the right bronchi and the two of the left lung join to form the left bronchi. The left and right bronchi join at the bottom of the trachea, commonly know as the windpipe. The top of the trachea connects to the oral cavity (mouth) and nasal passages through the pharynx. Basically, these are all pipes connecting the air of the outside right through to the alveoli. The air in the alveoli is refreshed by a process called pulmonary ventilation, more commonly known as breathing.
At the bottom of the thoracic cavity is a large muscle called the diaphragm. When we are breathing in we are contracting the diaphragm and other muscles in the thoracic cavity, when we breath out we are relaxing them. The contraction increases the volume of the cavity, lowering gaseous pressure outside the lungs to below air pressure. This results in air pushing into our lungs to equalize the pressure difference between the inside and outside of the lungs, even though to us it feels as though we are sucking it in. When we relax the muscles it reduces the volume of the thoracic cavity, increasing the pressure outside the lungs to greater than air pressure. This causes the lungs to compress, expelling some of the air.
External respiration is the name given to the movement of oxygen and carbon dioxide between the air in the alveoli and the blood in a capillary. Each cluster of alveoli has its associated capillary web with each alveoli having a closely associated capillary. The amount of carbon dioxide in the capillary is higher than in the alveoli; air currently has approximately 38 molecules of carbon dioxide per 1 million molecules on average. Through a process called diffusion, these amounts attempt to equalize. Carbon dioxide actually passes in both directions, from the blood in the capillary to the air in the alveoli and vice versa. But because there is a much higher concentration in the blood than the air, much more passes out of the blood than into it.
Some of the carbon dioxide is dissolved in the blood plasma, some is attached to hemoglobin proteins within erythrocytes (red blood cells) and some has reacted with water (H2O) to form carbonic anhydrase attached to the erythrocytes. As the dissolved carbon dioxide passes through the blood vessel wall, that attached to the hemoglobin detaches and that contained in the carbonic anhydrase splits into carbon dioxide and water, making it available to pass through into the air within the alveoli too.
The same principle applies to the flow of oxygen from within the alveoli to the blood stream. Air is approximately 20 to 21 percent oxygen. The oxygen level within the de-oxygenated blood entering the lungs is lower. Oxygen passes in both directions, but much more passes in than out. Some oxygen dissolves in the blood plasma, but most attaches to the hemoglobin proteins within the erythrocytes once the carbon dioxide that was attached to them has detached.
The above two processes are independent of each other, except that oxygen cannot attach to hemoglobin that still have carbon dioxide molecules attached. External respiration is sometimes called gas exchange, and indeed gases are being exchanged between the blood in the capillaries and the air in the alveoli, but oxygen and carbon dioxide are not being exchanged one for the other.
External respiration is the primary purpose of the pulmonary circuit, however, the lungs also play a role in the body’s renin-angiotensin-aldosterone system (RAAS), a part of the endocrine (hormonal) system that regulates blood pressure and fluid balance. The lungs produce an enzyme called angiotensin-converting enzyme (ACE) that is central to RAAS. When blood pressure is low or there is a loss in blood volume, perhaps from a wound, the kidneys produce a hormone called renin. Renin cleaves (splits) an organic molecule called angiotensinogen produced by the liver. This produces a molecule called angiotensin I that travels within the blood system. When it reaches the lungs, ACE converts it to angiotensin II, which then triggers blood vessel constriction and the production of aldosterone by the adrenal glands, which work to maintain blood pressure. It is complex, but it means that the active substance, angiotensin II, effectively spreads out from the heart onwards.
Hormones play a crucial role in maintaining homeostasis. They interact with the nervous system to coordinate cellular metabolism. The majority of hormones are produced by endocrine glands. Endocrine glands are categorized into three groups; amino acid based, steroids, and eicosanoids.
Amino acid based hormones include amines and proteins. Since these hormones are not lipid soluble, they rely on a second messenger system to enter cells. In cyclic AMP system, the amino based hormone (primary messenger) binds to a receptor on a plasma membrane. This action causes the receptor to change shape, thus forming a bind with a G protein and activating it. The G protein is the signal transducer and binds to the efffector enzyme, adenylate cyclase (AMP), which is able to enter the cell via ATP. Once the cAMP is in the cell, a chemical action is triggered.
Steroid hormones are classified by their four carbon ring structure and are synthesized from cholesterol. They are a lipid and can pass through the cell membrane. The hormone will enter the cell and bind to a receptor protein, which activates a gene on the protein by transcription via mRNA.
Finally, eicosanoids are lipid based and synthesized from arachidonic acid. They are considered paracrines, which effect tissues within the same area. They are classified into two groups: 1) leukotrienes- mediate inflammation and allergic reaction. 2) prostaglandin- can increase blood pressure and blood clotting. It is also found in semen, resulting in unnoticeable contractions to force semen up the vaginal canal.
The main glands that produce hormones include:
1. Hypothalamus: It is considered the master gland by dictating the actions of the pituitary gland. It is connected to the anterior lobe of the pituitary gland by the infundibulum and hypophyseal portal system. The hypothalamus also connects to the posterior lobe of the pituitary gland by the hypothalamus/hypophyseal tract, which is a nerve bundle running through the infundibulum. Nerve stimulation results in the pituitary releasing hormones from its posterior lobe
2. The pituitary gland: Originally thought to be the master gland due to the vast array of hormones released, until scientists discovered the release of these hormones were due to the dictation of the hypothalamus. The anterior lobe produces six hormones, all of which are amino acid based and require the cAMP messenger system to enter a cell.
Growth Hormone (GH) is made by somatotroph cells of the pituitary gland. This hormone stimulates cells to increase in size and divide, speed up protein synthesis, and the breakdown of glycogen to glucose in the liver ( fat as fuel). GH levels are highest during sleep and decrease with age. GH is regulated by hormones GHRH and GHIH, both released by the hypothalamus. Growth hormone releasing hormone (GHRH) is released by the hypothalamus into the blood via the hypophyseal portal system, which then stimulates the release of GH from the pituitary gland.
Thyroid stimulating hormone (TSH), also known as Thyrotropin, is produced by the thyrotrope cells of the pituitary gland. They stimulate the thyroid gland to release Thyroid hormone (TH). Thyroid Releasing Hormone (TRH) is released by the hypothalamus, which stimulates the release of TSH from the pituitary gland, and eventually the release of TH from the thyroid. As one can see, the levels of some hormones directly affect the release or inhibition of other hormones.
Adrenocorticotirotropic hormone (ACTH), stimulates the adrenal glands to release corticosteroid hormone. ACTH is produced by the corticotrope cells of the pituitary gland. Once again the hypothalamus regulates the release of ACTH based on the levels of CRH, cortico releasing hormone, are in the blood stream. The end result of the adrenal gland releasing corticosteroid hormone is used in situations of “fight or flight”. These hormone levels rises is stressful situations of survival.
Prolactin, PRL, is produced by lactotropic cells of the pituitary gland and stimulates milk production.
Both follicle stimulating hormone (FSH) and lutenizing hormone (LH) are produced from the gonadotropic cells of the pituitary gland. FSH stimulates the production of gametes; eggs/sperm, while LH produces gonadal hormones. In females it would be estrogen and progestergen, resulting in follicle maturation. For males, LH is also called ICSH. Interstitial cell stimulating hormone, found in the testis and results in testosterone.
Now onto the hormones associated with the posterior side of the pituitary gland.
Both oxytocin and ADH are released due to nerve impulses from the hypothalamus. Oxytocin stimulates uterine contractions and the “let down” reflex for milk. In other words, it aids in squeezing milk out of the breast, but does not produce the milk. This hormone enters cells through a secondary messenger system called PIP/Ca
Antidiaretic hormone (ADH) inhibits urine formation. Osmoreceptors of the hypothalamus monitor solute concentrations. When the concentration is too high, nerve impulses result in the release of ADH, which target the kidneys and water is reabsorbed back into the blood stream, resulting in a higher blood volume and blood pressure. Alcohol inhibits the release of ADH as does a high intake of water. Both these situations lead to a high urine output.
Thymus gland: This gland is located anterior of the trachea. It is made of follicular and extra follicular cells. These cells produce T3 and T4 (assoc. with iodine) and aid in metabolism. T4 is usually converted to T3. T3 is more active and affects all cells except for those of the brain, spleen, testis/uterus, and thyroid. T3 stimulates enzymes associated with glucose. The thymus also releases calcitonin, produced by the extra follicular cells. It decreases blood calcium levels. It inhibits the breakdown of bone and leads to calcium being reabsorbed into the bone.
Parathyroid gland: Consists of four tiny yellow glands on the posterior aspect of the thyroid. PTH is involved with increase calcium in the blood. It leads to the breakdown of bone by calcium being released into the blood stream from the bones. It is essentially the opposite of calcitonin.
Adrenal glands: located on top of the kidneys. Consists of an inner medulla of nervous tissue and an outer cortex consisting of three layers, each which release a different hormone.
The zona glomerulosa (outer layer), produces mineral corticoids, which help maintain electrolyte balance. The most potent of these is aldosterone, which stimulates the kidneys to reabsorb sodium and release potassium. The end result is that water follows the sodium, and leads to a higher blood volume and blood pressure. If blood pressure or sodium levels are low, the kidneys release renin, which converts angiotensinogen into angiotensin II. The presence of angiotensin II stimulates the release of the aldosterone, whose effects include vasoconstriction of the blood vessels.The inner layer of the cortex, the zona reticularis, produce adrenal androgens such as testosterone, who’s primary function is secondary sex characteristics in males. Testosterone also aids in the female sex drive. High levels in females can lead to an enlarged clitoris and facial hair growth.
The zona fasciculate (middle layer), produces glucocoticoids, which effect cell metabolism and help resist stressors. The most abundant is cortisol, which stimulates the formation of glucose from non carbs such as fats and proteins. The hypothalamus release CRH, stimulating the release of ACTH from the pituitary gland, then finally the release of cortisol. The lowest levels occur at night, and levels are interrupted in fight or flight situations. Energy is stored.
The pancreas is located in the abdomen and acts as an endocrine and exocrine gland. The endocrine aspect of the pancreatic organ releases alpha and beta cells, both located in the islets of Langerhans. The Alpha secretes glucagon, which increase blood glucose levels, targeting the liver. Glucagon is released when glucose levels are too low. The liver breaks down glycogen into glucose. The beta cells secrete insulin, which decreases blood glucose levels. It inhibits the breakdown of glycogen and the conversion of fats into sugars. Its role is to convert glucose into glycogen.
Finally, the pineal gland located in the epithalamus, produces melatonin, which is a hormone that regulates sleep/wake cycles. Light receptors from the eye trigger the release of the hormone when it senses dark. The circulation of the hormone in the blood results in sleepiness.
It is evident that hormones play a crucial role in homeostatic balances such as blood pressure and electrolye balances. Some hormones regulate the release of other hormones. Although many hormones and their roles have been mentioned, there is still a huge library of chemicals not mentioned and their roles of homeostasis and effects on other hormones.
