Understanding the Thyroid Gland: Functions and Anatomy

  1. Introduction of the Thyroid Gland
  2. Relation of the Thyroid Gland:
    1. Blood supply
    2. Lymphatic drainage
    3. Nerve supply:
    4. Variations
  3. Development of the gland
  4. Secretion of the thyroid gland
  5. Iodine metabolism
  6. Mechanism of action
  7. Functions of the thyroid gland
  8. How are these hormones metabolized?

Introduction of the Thyroid Gland

The thyroid gland is a butterfly-shaped endocrine gland located superficially in the neck. It consists of two lobes, the right and left lobes, connected in the lower two-thirds by a thin band of tissue known as the isthmus.

Shape: It is a butterfly-like endocrine organ. In the 1600s, Thomas Wharton named this gland for its resemblance to an ancient Greek shield, or ‘thyos.’

Size: The gland weighs 25 grams. Each lobe is 5 cm long, 3 cm wide, and 2 cm thick. The isthmus is 1.25 cm in height and breadth. The thyroid gland is more prominent in women than in men and increases during pregnancy or any other condition with anxiety and tension. It is very vascular and the largest endocrine gland.

Location: The thyroid gland is present in front of the larynx and trachea.

Relation of the Thyroid Gland:

Above: The thyroid and cricoid cartilages lie above it.

Below: The Adam’s apple lies below the gland.

Front: The infra-thyroid muscles lie in front.

Behind: The larynx, lower pharynx, and esophagus lie behind it.

On the sides: The sternocleidomastoid muscles and carotid arteries are present bilaterally.

Four parathyroid glands are present, two on each side. They lie between the two layers of the thyroid capsule at the back of the thyroid lobes.

A thin, two-layered fibrous capsule covers the gland. The inner layer invaginates the glandular mass, forming septa that divide the thyroid tissue into macroscopic lobes. The outer layer fuses inferiorly and continues into the pretracheal fascia, forming the posterior suspensory ligament of the thyroid gland. This ligament is known as Berry’s ligament.

Due to this ligament, the thyroid gland moves with the cartilages during swallowing.

Blood supply

Ø Arterial blood supply is by the superior and inferior thyroid arteries.

Ø Venous blood drainage: Through the superior, middle, and inferior thyroid veins.

Lymphatic drainage

The lymph drains into the pre-laryngeal, pre-tracheal, and paratracheal lymph nodes.

Nerve supply:

Ø Sympathetic nerve supply: From the superior, middle, and inferior cervical ganglia of the sympathetic trunk.

Ø Parasympathetic nerve supply: via the superior and recurrent laryngeal nerves.

Variations

There are many variations. For example, a third lobe is sometimes present, known as the ‘pyramidal lobe’ or ‘Lalouette’s pyramid.’

Development of the gland

At three weeks of gestation, the gland develops in the pharyngeal floor at the base of the tongue. It then grows in front of the pharynx and migrates to the bottom of the neck over the next few weeks. During migration, it remains connected to the tongue by a narrow tube known as the thyroglossal duct. At the end of the 5th week, the duct degenerates, and over the following two weeks, the detached gland migrates to its final position. In some cases, the thyroglossal duct remains patent even in adulthood.

Histology of thyroid gland:

The thyroid is made up of multiple small follicles. A follicle measures only 0.02-0.9 mm. About 40 small follicles form a lobule. A single layer of cuboidal cells surrounds the lumen of a follicle. When the gland is active, cuboidal cells change into columnar cells; when the gland is inactive, they become flat epithelial cells. The follicular cell is also known as the thyrocyte.

Some neuroendocrine parafollicular cells, also known as C cells, are present among the follicular cells and secrete calcitonin.

Secretion of the thyroid gland

The thyroid gland secretes three hormones:

(i) Thyroxine (T4)

(ii) Tri-iodothyronine (T3), and

(iii) Calcitonin.

The follicular lumen is filled with a transparent, gelatinous, pink colloid containing iodine, known as thyroglobulin.

Thyroid cells are highly active and contain abundant mitochondria, rough endoplasmic reticulum, and a Golgi apparatus. Their microvilli project into the follicular lumen. They synthesize protein and secrete it into the lumen by exocytosis. The rough endoplasmic reticulum of thyrocytes synthesizes thyroglobulin, which enters the follicular lumen by exocytosis. It is stored as a colloid and reacts with iodine in the follicular lumen.

Iodine metabolism

Iodine is a trace element essential for thyroid hormone production. In blood, iodide (I-) is present. Thyrocytes absorb iodides against the electrical gradient through an ion channel. In the sodium-iodide symporter channel on the cell membrane, two sodium ions move out of the cell, and one iodide ion moves into the cell. Finally, the iodide passes from the cell into the follicular lumen through pendrin, an iodide-chloride antiport.

In the follicular lumen, iodide is oxidized to iodine and becomes very active. In the presence of thyroid peroxidase, tyrosine is activated, and the enzyme combines with it to form MIT and DIT.

At first, MIT (monoiodotyrosine) is formed, then DIT (diiodotyrosine).

MIT+MIT=DIT

DIT plus MIT equals Triiodotyrosine (T3).

DIT+DIT=Tetraiodotyrosine (T4).

The follicular cells reabsorb thyroglobulin from the lumen. The iodinated tyrosines are cleaved, yielding MIT, DIT, T3, T4, and reverse triiodothyronine.

T3 and T4 enter the blood. In blood, 80-90% is T4 and 20-30% is T3.

MIT contains one atom of iodine per molecule, DIT contains two iodine atoms per molecule, T3 contains three iodine atoms per molecule, and T4 contains four iodine atoms per molecule.

Only a small percentage of thyroid hormone (0.3%) is free and active. In circulation, T4 produces 85% of T3.

70% are bound to thyroxine-binding globulin.

15% are bound to albumin, and

10% are bound to transthyretin.

C cells present between the follicular cells secrete calcitonin, which regulates blood calcium concentration. When blood calcium levels rise, calcitonin decreases calcium release from bone and increases its absorption. It helps regulate blood calcium levels.

1. Reduces the release of calcium from bone.

2. Increases calcium absorption by bone.

Calcitonin is not as potent as the parathyroid hormone secreted by the gland and opposes the effect of parathyroid hormone.

Mechanism of action

Thyroid hormones enter cells by crossing cell membranes and binding to nuclear thyroid hormone receptors TR-α1 and α2, and TR-β1 and β2.

These receptors bind with hormone response elements and transcription factors to modulate DNA transcription.

In addition, thyroid hormones act with various enzymes in the cytoplasm, including calcium ATPase, adenylyl cyclase, and glucose transporters.

Functions of the thyroid gland

Hormones of the thyroid gland are not essential for life, but their removal in adults leads to impaired temperature regulation and mental and physical health problems.

The thyroid hormones have many functions and affect all body tissues. These include

Thyroid hormones are especially required for normal fetal CNS development and fetal bone maturation.

The thyroid hormones

Ø Increase the basal metabolic rate and protein synthesis.

Ø Stimulate oxygen consumption of most body tissues, help regulate fat and carbohydrate metabolism, and are essential for tissue differentiation.

Ø Stimulate absorption from the gut, uptake by cells, breakdown of glucose, breakdown of fats, increase the number of free fatty acids and thyroid hormones, and decrease cholesterol levels.

Ø The cardiovascular effect is to increase the rate and strength of the heartbeat, increasing the rate of breathing, intake, and consumption of oxygen.

Ø The thyroid hormones are essential for maintaining normal sexual functions, including libido and a regular menstrual cycle.

Ø During development, a deficiency of thyroid hormones in children will cause intellectual disability.

Euthyroid indicates normal thyroid function.

Hyperthyroidism: when there is an excess of thyroid hormones.

Hypothyroidism: when there is a low amount of thyroid hormone.

Thyroiditis: when there is inflammation of the thyroid gland.

Goiter: when there is an enlargement of the thyroid gland.

Other conditions related to the thyroid glands are:

Thyroid nodules, thyroid cancer, autoimmune disorder Graves’ disease, etc.

The gland and its various diseases have been noted and treated for centuries.

How are these hormones metabolized?

Deiodinase enzymes in peripheral tissue remove iodine from MIT and DIT, converting T4 to T3 and RT3.

Thyroid hormones exert negative feedback on TSH and CRH. High levels of thyroid hormones suppress TSH production.

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Understanding Cystometry: A Comprehensive Guide

Cystometry:

The study of the relationship between bladder volume and intravesical pressure is called cystometry, which measures the bladder’s contractile force during urination.

Cystometry can help diagnose problems with bladder filling and emptying. Also known as flow cystometry, it evaluates bladder function — the capacity to contract and expel urine.

Cystometrogram

A cystometrogram (CMG) is a plot of volumes against intravesical pressures. In other words, a Cystometrogram plots the volume of urine passed from the bladder against intravesical pressure.

Importance

Cystography can diagnose bladder problems.

By cystoscopy, we can inspect the interior of the bladder.

  1. Cystometry:
  2. Cystometrogram
  3. Importance

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Understanding the Spleen: Structure and Function Explained

Table of Contents:

  1. Introduction
  2. Location
  3. External features Macroscpic Feature 
    1. Relation of the Spleen
  4. Histology
    1. Blood supply
    2. Lymphatic drainage
    3. Venous drainage
    4. Nerve Supply
  5. Functions
  6. Clinical

Introduction

The spleen is the largest wedge-shaped, dark purple-colored lymphatic organ of the lymphoid system, and it is connected to the vascular system. It is highly vascular. Its structure is similar to a large lymph node.

Measurements –length-5inches, breadth-3 3 inches,thickness-1 inch. Weight(average) is only 7 ounces.

Spleen performs multiple functions, although it is not essential for survival ,it is vital for healthy life. Dysfunctions of spleen will produce serious problems.

Location

The left hypochondrium houses the spleen between the fundus of the stomach and the diaphragm.

Behind the mid-axillary line opposite the 9th,10th, and 11th ribs.

External features Macroscpic Feature 

Two ends, two surfaces, two angles, and three boundaries.

Two ends: (1) anterior or lateral end that faces downward and forward, (2) posterior or medial face that faces upward and backward.

Three boundaries: 1. superior,2.inferior, and 3. intermediate.

The superior border is convex and faces and is in contact with the undersurface of the left side of the diaphragm. This has one or more notches called the splenic notch.

The inferior border is rounded and corresponds to the lower border of the eleventh rib.

The intermediate border is round and separates the gastric impression from the renal impression.

Two surfaces-1. The diaphragmatic surface is smooth, convex, and directed upward, backward, and to the left.

1. Visceral surface is concave and irregular and has four impressions 1. Gastric impression is the most significant impression and is due to the fundus of the stomach. It lies between the superior and intermediate border of the spleen.

Renal impression is due to the left kidney. It is present below and behind the gastric impression and lies between the intermediate and inferior boundaries.

The left colic flexure produces a colic impression. It is triangular and situated in front of the lateral end. and

The tail of the pancreas causes the pancreatic impression, which is located between the hilum and the colic impression.

There is a groove through which the splenic artery and nerves enter, and veins and lymphatic exit.

Relation of the Spleen

The spleen is surrounded by peritoneum and suspended by four ligaments.

1. Gastrosplenic ligament- This ligament extends from the hilum of the spleen to the upper one-third of the greater curvature of the stomach. Short gastric vessels are present in this ligament.

2. Splenorenal ligament- This ligament extends from the hilum of the spleen to the anterior surface of the left kidney. Tail of the pancreas, splenic venules, and pancreatosplenic lymph nodes are present in this ligament.

3. Colicosplenic ligament

4. Phrenocolic ligament

The visceral surface of the spleen is related to the fundus of the stomach, the anterior surface of the left kidney, the left colic flexure, and the tail of the pancreas.

Near the middle of the spleen, a long fissure is present, which is known as the hilum.

Penicilliary radicles are arterioles that supply germinal centers.

Develops from mesenchymal tissue, while most of the gut organs are derived from endoderm.

Histology

The outer capsule covers the spleen, made up of thick fibrous tissue.

Beneath the capsule, there is a subcapsular space. The spleen contains two different types of tissues, white pulp and red pulp.

The white pulp is white in colour and contains growing immune and blood cells. White pulp is composed of Malpighian corpuscles. These include lymphoid follicles rich in B lymphocytes and periarteriolar lymphoid sheaths rich in T lymphocytes.

The red pulp is red in colour and contains sinuses filled with blood, reticular fibers, and a marginal zone bordering on the white pulp.

Blood supply

The Splenic artery, the largest branch of the coeliac artery, provides the main blood supply. At the hilum, it divides into five or more branches and enters the spleen. Within the spleen, it divides repeatedly into straight arterioles called Penicilliary radicles. The penicilliary radicles supply the germinal centers.

Blood supply of the Spleen

Close theory: capillaries open directly in the venous sinusoids and lie in the red pulp.

Open theory: capillaries open into red pulp from where blood enters sinusoids through their fenestrated wall.

Compromise theory: according to this theory, circulation is open in the distended spleen and closed in the contracted spleen.

Lymphatic drainage

The Spleen has only efferent lymphatic vessels. There are no lymphatics in the proper splenic tissue. The tissue fluid formed in the spleen freely enters the venous sinusoids. However, the splenic lymphatics are present in the capsule, trabeculae, and visceral peritoneum, and drain into the pancreatocoliecal lymph nodes.

Venous drainage

The splenic vein is formed by the confluence of venules at the hilum, which join the superior mesenteric vein to form the portal vein. Its tributaries are 1. the short gastric vein, 2. the left gastroepiploic vein, 3. the pancreatic vein, and inferior mesenteric vein.

Nerve Supply

Sympathetic fibre derived from the celiac plexus supplies smooth muscles present in the blood vessels, as well as in the trabecular cord.[ splenic plexus, which connects a branch of the celiac ganglion to the vagus nerve.

Functions

1. Filtration of blood

2. Reservoir of blood cells

3. Active immune response: cell-mediated and humoral immunity.

4. Produces opsonins, properdin, and tuftsin.

5. Hematopoiesis in the embryo and in case of emergency.

6. Maintains homeostasis.

Clinical

An enlarged spleen is splenomegaly.

Hyperactive spleen is hypersplenism.

Microscopic picture of the spleen ; by Dr.P.Kr.

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Understanding the Limbic System: Key Functions and Components

Table of Contents

  1. Introduction
  2. Components of the Limbic System
  3. Histology
  4. Connections
  5. Functions

Introduction

The limbic system is derived from the word ‘Limbus’, meaning a ring. The word limbic was first used to describe the border structures around the basal region of the cerebrum.

Now the limbic system describes the entire neuronal circuitry that controls emotional, behavioral, and motivational drives and more.

Components of the Limbic System

1. The limbic lobe and the related subcortical nuclei.

The limbic lobe includes the cingulate gyrus, isthmus, hippocampal gyrus, and uncus.

2. The related subcortical nuclei are the hypothalamus, amygdala, septal nuclei, portion of the basal ganglion,paraolfactory area, and anteriorthalamic nucleus.

The hypothalamus is located in the middle of all these structures and plays a vital role in the limbic system.

The limbic cortex surrounds these subcortical limbic areas. The limbic cortex is composed of a ring of cerebral cortex on each side of the brain. The limbic cortex begins.

1. In the orbitofrontal area on the ventral surface of the frontal lobes,

2. Extends upwards into the subcallosal gyrus,

3. Then over the top of the corpus callosum onto the medial aspect of the cerebral hemisphere in the cingulate gyrus, and finally

4. Passing behind the corpus callosum and downward onto the ventromedial surface of the temporal lobe to the parahippocamal gyrus and uncus.

The neocortex sits with the legs on each side of the limbic system, like a rider on a horse without complete control. It can modify emotional behavior, but cannot initiate or terminate emotional behaviors.

Histology

The limbic cortex is made up of Allocortex (allo means old). The limbic cortex is the phylogenetically oldest part of the cerebral cortex.

Connections

The limbic cortex functions as a two-way communication and association linkage between the neocortex and the lower limbic structures.

1. Reticular nuclei in the brainstem and their associated nuclei

2. Medial forebrain bundle- main

3. Through short pathways among the reticular formation of the brainstem, thalamus, hypothalamus, and other areas of the basal brain.

Functions

1. Controls autonomic functions

2. Emotional behaviours like pleasant or unpleasant, reward or punishment, satisfaction or aversion.Emotion of rage, fear.

3. Motivation.

4. Memory.

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Why Anatomy is Essential for Medical Professionals

In this article, we will explore anatomy, which encompasses various aspects of the human body and life sciences.

My mission is to provide trustworthy, recent health information to the general public, patients, and professionals worldwide.

Table of contents

  1. Introduction
  2. The history
  3. Who conducted the first systematic, documented human dissection?
  4. The Present
  5. The Future:
  6. Why study Anatomy?
  7. Importance
  8. Scopes in anatomy
    1. Take Home
    2. Conclusion:
    3. References

Introduction

Human anatomy is a branch of science that explains the structure of the human body. In human anatomy, we study the macroscopic and microscopic structures of the human body, how it is formed, and how these structures work synergistically.

The human body has long fascinated lovers, poets, scientists, physicians, and philosophers. Everyone wants to know what is inside the skin and an innocent face. Anatomy clears doubts and produces a detailed picture of the human body.

Anatomy originates from the Greek words’ ana’, meaning ‘up’, and ‘tome’, meaning a cutting. The study of anatomy involves dissecting an organism.

The study of human anatomy has been ongoing for time immemorial; however, documents indicate that the study of anatomy began approximately 2000 years ago.

Anatomy evolved from beliefs about the human body, based on earlier texts with limited observation, to evidence-based facts, dissections, and modern digital models. This field not only underpins medical education but also plays a vital role in diagnostics, surgery, and biomedical research.

The study of anatomy is divided into

1. Gross anatomy: Studies the macroscopic structure of the body, which is divided into

Regional anatomy

Surface anatomy and

Systemic anatomy.

2. Microscopic anatomy: Examines the microscopic structure of tissues, histology, and Cytology, the study of cells.

3. Embryology –Development of the embryo.

4. Developmental anatomy: Explores the development and growth of the human body.

5. Comparative anatomy: Compares the anatomy of different species.

6. Surgical anatomy.

The history

The study of human anatomy has been ongoing for time immemorial. Early records from Egypt (around 1600 BCE) describe anatomical knowledge from mummification.

However, it was the Greeks who first began systematic studies. Hippocrates stressed the importance of observation. Later, Galen, a Roman physician, advanced anatomical understanding by dissecting animals. His work dominated European medical thought for over a thousand years.

Who conducted the first systematic, documented human dissection?

Herophilus of Chalcedon, with his junior Eresistraus of Ceos, performed the first systematically documented human dissection. And challenged prevailing beliefs about the human body, based on earlier texts with limited observational data. Their findings lay the foundation for the future of anatomical studies and influence the study of anatomy for centuries to come.

In India, Pandit Madhysudan Gupta (1800- 1856) performed the first human dissection at Calcutta Medical College in 1836.

The actual breakthrough came during the Renaissance. Andreas Vesalius, a Flemish anatomist, challenged long-held Galenic views. In 1543, his landmark work De humani corporis fabrica revolutionized anatomy by advocating for human dissection and the use of accurate illustrations. This period marked the birth of modern anatomy as a scientific discipline. Leonardo da Vinci and others made significant contributions to the field of anatomical illustration and understanding.

Vesalius is considered the father of anatomy.

The Present

Dissection of cadavers is the foundation of medical education, providing hands-on experience and an appreciation for human variation.

A histological study of slides reveals the microscopic structure of an organ.

Modern imaging techniques, such as ultrasound, MRI (Magnetic Resonance Imaging), and CT (Computed Tomography), have improved the teaching method of anatomy. These non-invasive tools enable the detailed visualization of internal structures in living patients, thereby enhancing diagnostics and surgical skills.

Virtual anatomy labs, 3D models, and augmented reality (AR) tools now supplement traditional learning. Platforms like the Anatomage Table and apps like Complete Anatomy provide interactive ways for students to explore the human body. Moreover, the plastination technique preserves biological tissues to offer lifelike specimens for study and display.

The Future:

The future of human anatomy promises exciting developments driven by technology and interdisciplinary collaboration.

Artificial intelligence (AI) is being integrated into radiology and surgical simulation, enabling real-time anatomical recognition and predictive modelling.

3D bioprinting is another frontier. Researchers are already printing tissues and organ prototypes using living cells, potentially revolutionising transplantation and regenerative medicine. This requires detailed anatomical precision, linking anatomy directly with biotechnology.

Genomic and molecular anatomy will gain importance, as microscopic structural changes are linked to genetic expressions and diseases. Functional and systems-level anatomy will also expand, integrating physiology and anatomy for a holistic understanding of the body.

In education, fully immersive virtual reality (VR) environments may soon replace or augment cadaveric dissection, especially in regions with limited resources. Global access to high-quality anatomical education through online platforms could help bridge disparities in medical training.

Why study Anatomy?

1. Medical education: Anatomy is a fundamental subject in medical education.

2. Surgery: Anatomical knowledge is crucial for surgical procedures.

3. Research: Anatomy provides various fields for research.

4. Clinical practice: Understanding anatomy is essential for diagnosis and treatment.

Importance

1. Understanding human structure: A Clear concept of Anatomy provides a foundation for understanding the human body’s structure and function.

2. Informing medical practice: Anatomical knowledge informs medical practice, from diagnosis to treatment.

3. A clear concept of anatomy is essential for aspiring surgeons.

Scopes in anatomy

Anatomy is a primary subject of medical degrees worldwide. Many universities offer bachelor’s, master’s, and doctoral degrees in the field of anatomy.

Anatomy is an essential subject in medical education for M.B.B.S. degrees and is included in the first professional year of the M.B.B.S. curriculum.

B.Sc. Programs. M.Sc. and PhD anatomy courses are available in many universities.

An anatomist may get posted in medical colleges as a tutor, assistant, associate Professor, or Professor and dean.

An anatomist may pursue research work and may start private fitness clinics. Anatomists can work in various clinical settings, such as fitness centers, community organizations, and corporate settings.

As anatomy advances, several jobs are likely to increase in demand. Many career opportunities exist in physiology.

Promising career

An anatomist has a bright career. Anatomy has a promising future, with the advancement of science and technology, and it is developing rapidly. The human body is a highly complex system, and simulations may not fully capture its intricacies.

Take Home

The scope of anatomy is broad, and its applications are diverse, making it a fundamental discipline in medicine and biology.

Conclusion:

As we move forward, anatomy will remain a pillar of medicine, constantly reshaped by innovation, ensuring better care and deeper insight into the human form.

Thank you very much for reading. I appreciate that you took the time to spend with us. If you enjoyed reading, I’d appreciate it if you could do me a small favor. Please leave a review or a comment. Your comments will encourage other folks to read. Thanks for reading.

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References

Thanks to AI,

Thanks to my teachers and the authors of the books I have studied, from which I have learned a great deal.

Disclaimer:

All possible measures have been taken to ensure the accuracy and reliability of the information; however, ‘learn-and-fly.co.in’ does not accept any liability for the use of the information supplied by the website by its viewers. The information is provided as an educational service and for public awareness. It is not medical advice. We recommend reviewing a reference book in case of any doubt, for more accurate and advanced knowledge.

Written by Priynka Kumari, M.Sc.(Anatomy) Facts verified by Dr. B.K. Prasad, Professor.

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Bijay Krishna

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