Functions of the Liver and Other Vital Organs
Functions of Liver
The liver is the largest gland and one of the vital organs of the body. It performs various metabolic and homeostatic functions, which are summarized below.
1. Metabolic Function
The liver is the organ where maximum metabolic reactions such as metabolism of carbohydrates, proteins, fats, vitamins, and many hormones are carried out.
2. Storage Function
Many substances like glycogen, amino acids, iron, folic acid, and vitamins A, B, and D are stored in the liver.
3. Synthetic Function
The liver produces glucose by gluconeogenesis. It synthesizes all the plasma proteins and other proteins (except immunoglobulins) such as clotting factors, complement factors, and hormone-binding proteins. It also synthesizes steroids, somatomedin, and heparin.
4. Secretion of Bile
The liver secretes bile, which contains bile salts, bile pigments, cholesterol, fatty acids, and lecithin. Functions of bile are mainly due to bile salts. Bile salts are required for digestion and absorption of fats in the intestine. Bile helps to carry away waste products and break down fats, which are excreted through feces or urine.
5. Excretory Function
The liver excretes cholesterol, bile pigments, heavy metals (like lead, arsenic, and bismuth), toxins, bacteria, and viruses (like that of yellow fever) through bile.
6. Heat Production
Enormous amounts of heat are produced in the liver because of metabolic reactions. The liver is the organ where maximum heat is produced.
7. Hematopoietic Function
In the fetus (hepatic stage), the liver produces blood cells (Chapter 9). It stores vitamin B12 necessary for erythropoiesis and iron necessary for the synthesis of hemoglobin. The liver produces thrombopoietin that promotes the production of thrombocytes.
8. Hemolytic Function
The senile RBCs after a lifespan of 120 days are destroyed by reticuloendothelial cells (Kupffer cells) of the liver.
9. Inactivation of Hormones and Drugs
The liver catabolizes hormones such as growth hormone, parathormone, cortisol, insulin, glucagon, and estrogen. It also inactivates drugs, particularly fat-soluble drugs. Fat-soluble drugs are converted into substances that are excreted through bile or urine.
10. Defensive and Detoxification Functions
Reticuloendothelial cells (Kupffer cells) of the liver play an important role in the defense of the body. The liver is also involved in the detoxification of foreign bodies. Foreign bodies such as bacteria or antigens are swallowed and digested by reticuloendothelial cells of the liver by means of phagocytosis. Reticuloendothelial cells of the liver also produce substances like interleukins and tumor necrosis factors, which activate the immune system of the body (Chapter 15).
Liver cells are involved in the removal of toxic properties of various harmful substances. Removal of toxic properties of the harmful agent is known as detoxification.
Detoxification in the liver occurs in two ways:
- Total destruction of the substances by means of metabolic degradation
- Conversion of toxic substances into non-toxic materials by means of conjugation with glucuronic acid or sulfates.
Other Vital Organs
Endocrine Glands
Pituitary Gland
The pituitary gland or hypophysis is a small endocrine gland with a diameter of 1 cm and a weight of 0.5 to 1 g. It is situated in a depression called ‘sella turcica’, present in the sphenoid bone at the base of the skull. It is connected with the hypothalamus by the pituitary stalk or hypophyseal stalk.
The pituitary gland is divided into two divisions:
- Anterior pituitary or adenohypophysis.
- Posterior pituitary or neurohypophysis.
Both divisions are situated close to each other. Still, both are entirely different in their development, structure, and function. Between the two divisions, there is a small and relatively avascular structure called pars intermedia. Actually, it forms a part of the anterior pituitary.
The anterior pituitary secretes six hormones:
- Growth hormone (GH) or somatotropic hormone (STH).
- Thyroid-stimulating hormone (TSH) or thyrotropic hormone.
- Adrenocorticotropic hormone (ACTH).
- Follicle-stimulating hormone (FSH).
- Luteinizing hormone (LH) in females or interstitial cell-stimulating hormone (ICSH) in males.
- Prolactin.
Hormones of Posterior Pituitary
Posterior pituitary hormones are antidiuretic hormone (ADH) or vasopressin and oxytocin.
The posterior pituitary does not secrete any hormone. ADH and oxytocin are synthesized in the hypothalamus and transported to the posterior pituitary through the nerve fibers of the hypothalamo-hypophyseal tract (Fig. 63.3), by means of axonic flow.
■Hyperactivity of Anterior Pituitary
1. Gigantism
Gigantism is a pituitary disorder characterized by excess growth of the body. The subjects look like giants with an average height of about 7 to 8 feet.
Causes of Gigantism
Gigantism is due to hypersecretion of GH in childhood or in preadult life before the fusion of the epiphysis of the bone with the shaft. Hypersecretion of GH is because of a tumor of acidophil cells in the anterior pituitary.
Acromegaly
Acromegaly is the disorder characterized by the enlargement, thickening, and broadening of bones, particularly in the extremities of the body.
Causes of Acromegaly
Acromegaly is due to hypersecretion of GH in adults after the fusion of the epiphysis with the shaft of the bone. Hypersecretion of GH is because of a tumor of acidophil cells in the anterior pituitary.
Acromegalic Gigantism
Acromegalic gigantism is a rare disorder with symptoms of both gigantism and acromegaly.
Cause for Acromegalic Gigantism
Hypersecretion of GH in children, before the fusion of
epiphysis with shaft of the bones causes gigantism and if hypersecretion of GH is continued even after the fusion of epiphysis, the symptoms of acromegaly also appear
HYPOACTIVITY OF ANTERIOR PITUITARY
1. Dwarfism
Dwarfism is a pituitary disorder in children, characterized by the stunted growth.
Causes of dwarfism
Dwarfism is caused by hyposecretion of GH in infancy or early childhood.
Causes for hyposecretion of GH in infancy or childhood
1. Tumor of chromophobes: It is a nonfunctioning tumor, which compresses and destroys the normal cells secreting GH. It is the most common cause for hyposecretion of GH, leading to dwarfism.
2. Deficiency of GH-releasing hormone secreted by hypothalamus.
3. Deficiency of somatomedin C.
4. Atrophy or degeneration of acidophilic cells in the anterior pituitary.
5. Panhypopituitarism
OVERVIEW OF THYROID GLAND
Thyroid is a butterfly-shaped endocrine gland situated at the base of neck in front of trachea. This gland plays a major role in metabolic activities, growth and development of body. It also helps to regulate functions of many vital organs such as heart, brain, kidneys and liver.
Thyroid has two lobes, which are connected in the middle by an isthmus (Fig. 64.1). It weighs about 20 to 40 g in adults. Thyroid is larger in females than in males. The structure and function of thyroid gland change in different stages of the sexual cycle in females. Activity of thyroid increases slightly during pregnancy and lactation, and decreases during menopause.
HORMONES OF THYROID GLAND
Thyroid gland secretes three hormones:
1. Tetraiodothyronine or T₁ (thyroxine)
2. Triiodothyronine or T3
3. Calcitonin.
Thyroxine forms about 90% of the total secretion, whereas T, is only 9 to 10%. Details of calcitonin are given in next chapter.
HYPERTHYROIDISM: THYROTOXICOSIS
Hyperthyroidism refers to excess synthesis and release of thyroid hormones by thyroid gland resulting in increased level of hormones in blood.
Thyrotoxicosis is defined as high level of thyroid hormones in blood. It is the condition caused by not only excess secretion by thyroid glands, but also release of stored hormones.
Causes of Hyperthyroidism
1. Graves’ disease
Graves’ disease is an autoimmune disease and it is the most common cause of hyperthyroidism. Normally, TSH combines with surface receptors of thyroid cells and causes the synthesis and secretion of thyroid hormones. In Graves’ disease, the B lymphocytes (plasma cells) produce autoimmune antibodies called thyroid stimulating hormone receptor antibodies (TSHRAb).
These antibodies act like TSH by binding with membrane receptors of TSH and activating cAMP system of the thyroid follicular cells. This results in hypersecretion of thyroid hormones.
Antibodies act for a long time even up to 12 hours in contrast to that of TSH, which lasts only for an hour or so. High concentration of thyroid hormones caused by the antibodies suppresses TSH production also. So, concentration of TSH is low or almost zero in plasma of most of the hyperthyroid patients.
Myxedema
Myxedema is the hypothyroidism in adults, characterized C by generalized edematous appearance.
Causes for myxedema
1. Diseases of thyroid gland.
2. Genetic disorder.
3. lodine deficiency.
4. Deficiency of thyroid-stimulating hormone or thyrotropin-releasing hormone.
5. Hashimoto’s thyroiditis.
Hashimoto’s thyroiditis
It is an autoimmune disease common in late middle- aged women (Chapter 15). In most patients, it starts with glandular inflammation called thyroiditis caused by autoimmune antibodies. Later it leads to destruction of the glands.
Cretinism
Cretinism is a congenital hypothyroidism characterized by stunted growth, physical deformity and mental retardation.
Causes for cretinism
1. Congenital hyposecretion or absence of thyroid gland.
2. Genetic disorder affecting synthesis of thyroid hormones.
3. Lack of iodine in mother’s diet during pregnancy.
4. Use of antithyroid drugs during pregnancy.
5. Failure of pituitary to stimulate thyroid gland.
GOITER
Goiter is the noncancerous enlargement of thyroid gland giving the appearance of swelling in neck. It occurs both in hypothyroidism and hyperthyroidism.
Goiter in Hyperthyroidism: Toxic Goiter
Toxic goiter is the enlargement of thyroid gland with increased secretion of thyroid hormones, caused by thyroid tumor.
Goiter in Hypothyroidism: Non-toxic Goiter
Non-toxic goiter is the enlargement of thyroid gland without increase in hormone secretion. It is also called hypothyroid goiter (Fig. 64.6).
Based on the cause, the non-toxic hypothyroid goiter is classified into two types called endemic colloid goiter and idiopathic non-toxic goiter.
Vitamin A
Functions: Vitamin A maintains eye health, muscle and bone growth, strong immunity, healthy skin and teeth. In addition, it is an antioxidant that lowers the risk of certain cancers.
Sources: pumpkins, sweet potatoes, broccoli, carrots, dairy products, eggs, cod liver oil, spinach, etc.
Deficiency causes Night blindness and Xerophthalmia.
Vitamin D
Functions: Vitamin D maintains the normal levels of phosphorus and calcium in the blood. It thereby aids in forming bone tissues by assisting the body in absorbing calcium.
Sources: It is the only vitamin the body can adequately synthesise from sunlight. It can also be found in trace amounts in foods such as fish, fish oil, and fish oil.
Deficiency causes Osteoporosis and Rickets.
Vitamin E
Functions: Vitamin E is a type of vitamin that aids in producing red blood cells while also lowering oxidative stress. In addition, it is an antioxidant that safeguards vitamin A and lipids against damage.
Sources: Almonds, eggs, vegetable oils, kiwis, and leafy greens are good sources.
Deficiency causes neuropathy and haemolytic anaemia in children.
Vitamin K
Functions: Vitamin K helps in blood coagulation or clotting after a wound has formed by stimulating proteins and calcium.
Sources: Milk, tomatoes, eggs, green and leafy vegetables including kale, cabbage, spinach, etc.
Deficiency causes haemorrhage and bleeding diathesis.
Vitamin B1
Functions: Vitamin B1 plays a role in synthesising the enzymes involved in converting carbohydrates to energy. It is also essential for muscle, skin, hair, and nerve functions.
Sources: Brown rice, whole grain rye, kale, eggs, potatoes, pork, oranges, etc.
Deficiency causes Beriberi.
Vitamin B2
Functions: Vitamin B2 is required for maintaining healthy skin and blood. In addition, this vitamin plays a vital role in food metabolism and the synthesis of erythrocytes (red blood cells).
Sources: Bananas, dairy products, eggs, meat, green beans, asparagus, etc.
Deficiency causes mouth inflammation and fissures.
Vitamin B3
Functions: Vitamin B3 aids in digestion and is needed to maintain healthy skin and nerve functions. In addition, at high doses, it exhibits cholesterol regulating effects.
Sources: Salmon, milk, eggs, carrots, nuts, tofu, tomatoes, chicken, lentils, etc.
Deficiency causes Pellagra.
Vitamin B5
Functions: Vitamin B5 plays an essential role in food metabolism and hormone production.
Sources: Yoghurt, whole grains, meat, avocado, etc.
Deficiency causes Paresthesia.
Vitamin B6
Functions: Vitamin B6 is necessary for the formation of serotonin, the synthesis of red blood cells, and cognitive and immunological functions.
Sources: Beef liver, nuts, chickpeas, bananas, squash.
Deficiency causes anaemia and peripheral neuropathy.
Vitamin B7
Functions: Vitamin B7 or Biotin aids in the metabolism of nutrients like the structural protein keratin, which aids in the maintenance of healthy skin, nails, and hair.
Sources: Spinach, liver, broccoli, egg yolk, cheese.
Deficiency causes Dermatitis.
Vitamin B9
Functions: Vitamin B9 assists in the normal synthesis of RNA and DNA in foetuses and protects these genetic components against malignant transformation.
Sources: Peas, legumes, liver, sunflower seeds, etc.
Deficiency causes Megaloblastic anaemia. Lower levels of folate may also cause congenital disabilities called spina bifida.
Vitamin B12
Functions: Vitamin B12 is an essential vitamin for regulating the body’s metabolism. It helps synthesise red blood cells and DNA production and aids fatty acid breakdown. Cobalamin also maintains the proper functioning of the central nervous system.
Sources: Milk and other dairy products, meat, shellfish, fish, eggs, etc.
Deficiency causes different types of anaemia and neuropsychiatric disorders.
Vitamin C
Functions: Vitamin C, often known as ascorbic acid, has anti-ageing properties. It is because this vitamin aids in wound healing, blood vessel strengthening, bone-building, iron synthesis, and immune system maintenance by promoting collagen creation.
Sources: Citrus fruits, cabbage, Brussels, spinach, tomatoes, etc.
Deficiency causes Scurvy.