OESOPHAGUS, TRACHEA,

THYROID AND PARATHYROID GLANDS

OESOPHAGUS

· It has 3 parts. Cervical, thoracic and abdominal parts.

· Extends from pharynx to stomach

· passes through the diaphragm at T 10 vertebral level to enter the abdomen.

· It continues with the stomach at the gastroesophageal junction.

The is made of: an outer connective tissue sheath of areolar tissue.

Muscular layer of external longitudinal and internal circular fibres which are striated in the upper two-thirds and smooth in the lower one-third.

Submucous layer containing mucous glands; Mucosa of stratified epithelium passing

abruptly into the columnar epithelium of the stomach.

OESOPHAGUS STRUCTURE

The oesophagus, is 10in (25cm) long, extends from the level of the lower border of the cricoid cartilage C6 vertebra to the cardiac orifice of the stomach.

It enters the abdomen, passing forwards through the opening in the right crus of the diaphragm (T10).

It comes to lie in the oesophageal groove on the posterior surface of the left lobe of the liver, covered by peritoneum on its anterior and left aspects. Behind it is the left crus of the diaphragm.

OESOPHAGUS

It is narrowed at three sites: at the beginning, behind the cricoid

cartilage of the larynx; where the left bronchus and the arch of the

aorta cross the front of the oesophagus; and where the oesophagus enters the

stomach. These three sites may offer resistance to

the passage of a tube down the oesophagus into the stomach

NARROW AREAS OF THE OESOPHAGEAL LUMEN

Superiorly: level of cricoid cartilage, juncture with pharynx

Middle: crossed by aorta and left main bronchus

Inferiorly: diaphragmatic sphincter

Oesophageal Constrictions

BLOOD SUPPLY

Blood supply is from the inferior thyroid artery, branches of the descending

thoracic aorta and the left gastric artery. The veins from the cervical part drain into the inferior

thyroid veins, from the thoracic portion into the azygos vein and from the abdominal portion partly into the azygos and

partly into the left gastric veins.

The lymphatic drainage is from a perioesophageal lymph plexus into the

posterior mediastinal nodes, which drain both into the supraclavicular nodes and into nodes around the left gastric vessels.

It is not uncommon to be able to palpate hard, fixed supraclavicular nodes in patients with advanced oesophageal cancer.

LYMPHATIC DRAINAGE

In oesophagoscopy, measurements are made from the upper incisor teeth; the three important levels (17cm), (28cm) and (43cm) corresponding to

The commencement of the oesophagus, The point at which it is crossed by the left bronchus

and its termination respectively. These three points also indicate the narrowest parts

of the oesophagus: the sites at which, swallowed foreign bodies are most likely to become

impacted and strictures to occur after swallowing corrosive fluids

APPLIED ANATOMY

The anastomosis between the azygos (systemic) and left gastric (portal) venous tributaries in the oesophageal veins is of great importance in portal Hypertension.

These veins distend into large collateral channels, oesophageal varices, which may then rupture with severe haemorrhage.

The oesophagus is crossed solely by the vena azygos on the right side. This is therefore the side of choice surgically to approach the oesophagus.

APPLIED ANATOMY CONTD

A flexible tube also called windpipe. The trachea is about 4.5in (11.5cm) in length

and nearly 1 in (2.5cm) in diameter. It commences at the lower border of the cricoid

cartilage (C6) It terminates by bifurcating at the level of the

sternal angle of Louis (T4/5) to form the right and left main bronchi.

TRACHEA

Trachea Extends through the mediastinum and lies

anterior to the oesophagus and inferior to the larynx.

Anterior and lateral walls of the trachea supported by 15 to 20 C-shaped tracheal cartilages.

Cartilage rings reinforce and provide rigidity to the tracheal wall to ensure that the trachea remains open at all times

Posterior part of tube lined by trachealis muscle Lined by ciliated pseudostratified columnar

epithelium.

Trachea At the level of the sternal angle, the

trachea bifurcates into the right and left primary bronchi.

Each primary bronchus projects laterally toward each lung.

The most inferior tracheal cartilage separates the primary bronchi at their origin and forms an internal ridge called the carina.

RELATIONS OF TRACHEA Cervical

Anteriorly— the isthmus of thyroid gland,

inferior thyroid veins, sternohyoid and sternothyroid muscles;

laterally—the lobes of thyroid gland and the common carotid artery;

posteriorly—the oesophagus with the recurrent laryngeal nerve lying in the groove between oesophagus and trachea

Anteriorly:commencement of the brachiocephalic artery and left carotid artery, the left brachiocephalic vein and the thymus;

Posteriorly—oesophagus and left recurrent laryngeal nerve;

To the left— arch of the aorta, left common carotid and left subclavian arteries, left recurrent laryngeal nerve and pleura;

To the right—vagus, azygos vein and pleura .

RELATIONS (THORACIC)

Bronchial tree It originate from the left and right primary

bronchi. Progressively branches into narrower tubes

before terminating in terminal bronchioles. Incomplete rings of hyaline cartilage support

the walls of the primary bronchi. Right primary bronchus is shorter, wider,

and more vertically oriented than the left primary bronchus.

Foreign particles are more likely to lodge in the right primary bronchus.

Bronchial tree

The primary bronchi enter the hilus of each lung together with the pulmonary vessels, lymphatic vessels, and nerves.

Each primary bronchus branches into several secondary bronchi.

The left lung has two secondary bronchi. The right lung has three secondary bronchi.

They further divide into tertiary bronchi. Each tertiary bronchus is called a segmental

bronchus because it supplies a part of the lung called a bronchopulmonary segment.

Bronchial Tree Secondary bronchi tertiary bronchi

bronchioles terminal bronchioles with successive branching amount of cartilage

decreases and amount of smooth muscle increases, this allows for variation in airway diameter during exertion and when sympathetic division active leads to bronchodilation

mediators of allergic reactions like histamine leads to bronchoconstriction

Epithelium gradually changes from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium in terminal bronchioles 

Respiratory Zone of Lower Respiratory Tract

Respiratory Bronchioles, Alveolar Ducts, and Alveoli Lungs contain small saccular outpocketings

called alveoli. They have a thin wall specialized to promote

diffusion of gases between the alveolus and the blood in the pulmonary capillaries.

Gas exchange can take place in the respiratory bronchioles and alveolar ducts as well as in the alveoli, each lung contains approximately 300 to 400 million alveoli.

The spongy nature of the lung is due to the packing of millions of alveoli together.

APPLIED ANATOMY Tracheostomy is limited

to patients with extensive laryngeal damage and infants with severe airway obstruction. Cricothyroidotomy is preferred. In cricothyroidotomy, a tube is inserted in the interval between the cricoid cartilage and the thyroid cartilage.

THYROID GLAND It is the largest endocrine gland in adult

It normally extends from ~ C-5 through ~ T-1

It Weighs ~20 - 30 grams it is larger in women

Lobes are cone (pyramidal) shaped

Apex extends to oblique line of thyroid cartilage

The Isthmus crosses tracheal cartilages 2-4

Base located ~4-5th tracheal cartilage

Pyramidal lobe is present in ~ 33% of population it lies mostly to the left.

It extends upward from isthmus and lies anterior to thyroid cartilage.

It is covered by a fibrous capsule which sends septa deeply into it.

Introduction

The Thyroid Gland

Located in the anterior region of the neck

This gland has an accessory (pyramidal) lobe

usually on the left side, which represents a remnant of the embryological descent of the thyroid.

Strap Muscles

Muscular landmarks a. Sternocleidomastoid

muscles lie laterally b. Longus colli

(prevertebral) muscles lie posteriorly

c. “Strap” muscles lie anteriorly

RELATIONS

The gland is enclosed in the pretracheal fascia, covered by the strap muscles

overlapped by the sternocleidomastoids. The anterior jugular veins course over the

isthmus. It lies anterorolateral to larynx and trachea With the pharynx and oesophagus behind

and the carotid sheath on either side. Lobes connected by an isthmus

RELATIONS

RELATIONS In the groove

between the trachea and oesophagus lies the recurrent laryngeal nerve

and deep to the upper pole lies the external branch of the superior laryngeal nerve passing to the cricothyroid muscle.

Three arteries supply and three veins drain the thyroid gland

Superior thyroid artery—arises from the external carotid and passes to the upper pole;

Inferior thyroid artery—arises from the thyrocervical trunk of the 1st part of the subclavian artery and passes behind the carotid sheath to the back of the gland;

Thyroidea ima artery (10%)—if present, it arises from the aortic arch or the brachiocephalic artery; usually unpaired.

BLOOD SUPPY

Superior thyroid vein— drains the upper pole to the internal jugular vein;

Middle thyroid vein—drains from the lateral side of the gland to the internal jugular;

Inferior thyroid veins—often several—drain the lower pole to the brachiocephalic veins.

VENOUS DRAINAGE

Lymph vessels lie In interlobular connective tissue between lobes, it connects with network in wall of gland and terminate in thoracic duct and right lymphatic ducts.

LYMPHATICS

Innervation

Principally from ANS Parasympathetic fibers – from vagus Sympathetic fibers – from superior, middle,

and inferior ganglia of the sympathetic trunk

Enters the gland along with the blood vessels.

The thyroid gland is invested in a sheath derived from the pretracheal fascia. This tethers the gland to the larynx and the trachea and explains why the thyroid gland follows the movements of the larynx in swallowing.

Hyperthyroidism (thyrotoxicosis, Grave’s disease), is common in middle-aged women, the thyroid gland is diffusely enlarged.

Hypothyroidism occurs in two forms: cretinism in infants and myxedema in adults.

APPLIED ANATOMY

The developmental anomalies of the thyroid results in a rare occurrence of the whole or a part of the gland remaining as a swelling at the tongue base (lingual thyroid) and for the much commoner occurrence of a thyroglossal cyst or sinus along the pathway of descent.

Descent of the thyroid may go beyond the normal position in the neck down into the superior mediastinum (retrosternal goitre).

APPLIED ANATOMY

A benign enlargement of the thyroid may compress or displace any of its close relations; the trachea and oesophagus may be narrowed, with resulting difficulty in breathing and swallowing.

A carcinoma of the thyroid invades rather than displacing them — eroding into trachea or oesophagus, surrounding the carotid sheath and occasionally causing severe haemorrhage

APPLIED ANATOMY

The recurrent laryngeal nerve and the cervical sympathetic chain may be involved, producing changes in the voice and Horner’s syndrome (droopy eye, dryness, small pupil) respectively.

APPLIED ANATOMY

APPLIED ANATOMY Treatment considerations: The two main arteries

supplying the thyroid gland are closely related to important nerves that can be damaged during thyroidectomy operations

a. Superior thyroid artery is accompanied by external laryngeal nerve

b. Inferior thyroid artery is accompanied by recurrent laryngeal nerve

c. In partial thyroidectomy, the posterior part of the thyroid gland is left undisturbed so that the parathyroid glands are not damaged.

Agenesis of the Thyroid: Failure of development of the thyroid gland may occur and is the commonest cause of cretinism.

Ectopic thyroid tissue is occasionally found in the thorax in relation to the trachea or bronchi or even the oesophagus.

Persistent Thyroglossal Duct related to a persistence of the thyroglossal duct usually appear in childhood,

Thyroglossal Cyst may occur at any point along the thyroglossal tract

Thyroglossal Sinus (Fistula) Occasionally, a thyroglossal cyst ruptures spontaneously, producing a sinus

CONGENITAL ANOMALIES

Parathyroid glands Usually four – two on each side (2-8)

ovoid or lentiform structures

Measures 3-10 mm x 2-6 mm x 1-4 mm

It is yellow-brown in colour. Lie on the posterior surface of thyroid but may be

embedded within the gland. Regulate calcium/phosphate levels Total weight of parathyroid tissue is about 150mg

(each weighs ~ 50 mg.) Parathyroid hormone (PTH) is made by these

glands

Parathyroid Glands (Post. view of thyroid)

Parathyroid Glands are located on the posterior aspect of the thyroid; sometimes the tissue is embedded within thyroid tissue.

Parathyroid Gland Anatomy

Four Parathyroid glands are usually found posterior to the thyroid gland

Position of superior glands is more predictable

Aberrant glands may lie between the trachea and thyroid

Biosynthesis, Storage & Secretion of PTH

PTH is synthesized as the preprohormone

(Preproparathyroid Hormone) by parathyroid gland chief cells

The active form of PTH is cleaved from the preprohormone before release from the gland

PTH is synthesized continously (it is either released from the gland or degraded)

PTH is released by exocytosis in response to reduced plasma calcium

Vitamin D feeds back to reduce PTH secretion as a secondary mechanism

Biological Activity of PTH BONE

◦ PTH stimulates bone osteoblasts to increase growth & metabolic activity

◦ PTH stimulated bone resorption releases calcium & phosphate into blood

KIDNEY◦ PTH increases reabsorption of calcium &

reduces reabsorption of phosphate◦ Net effect of its action is increased calcium &

reduced phosphate in plasma INTESTINE

◦ Increases calcium reabsorption via vitamin D

Hypoparathyroidism This condition is most commonly caused by injury to or removal of the glands during surgical procedures on the thyroid gland.

The main symptoms and signs of low blood calcium are numbness and tingling in the fingers and toes and cramps of the muscles in the hands and feet. These cramps are called carpopedal spasms.

Hyperparathyroidism This condition is caused by a benign adenoma or hyperplasia of the parathyroid glands. It may cause the repeated occurrence of renal stones.

APPLIED ANATOMY