45. Portal Circulations

Question 1

What is the definition of a portal circulation?

Answer 1

A portal circulation is one in which
blood from the capillary bed of one organ
structure drains into the

capillary bed of another

organ structure through a larger vessel,

usually a vein or venule

(hence they are also known as portal venous systems).

Question 2

Name some portal circulations?

Answer 2

Examples of such circulations include the
hepatic portal

(see Chapter 22, ‘Liver physiology’),

placental,
hypothalamo-hypophyseal and

renal circulations (see Chapter 27, ‘Renal blood flow’).

Question 3

Describe the anatomical organisation of the pituitary gland.

Answer 3

The pituitary gland is a pea-shaped

structure that lies within the

sella turcica of the sphenoid bone.

It is approximately 1–1.5 cm in diameter
and is attached to the hypothalamus
via the infundibulum.

The pituitary gland is made up of
two anatomically and
functionally separate portions –

the anterior pituitary
(or adenohypophysis)

and the posterior pituitary
(or neurohypophysis).

Question 4

The anterior pituitary:

Answer 4

> Synthesis and release of hormones
is under the control of the hypothalamus.

> Hypothalamic hormones either
stimulate or inhibit the release of
hormones from the anterior pituitary.

> Hypothalamic hormones form an
integral link between the nervous
system and the endocrine systems.

> These hypothalamic hormones
reach the anterior pituitary via portal blood vessels –

the hypothalamo-hypophyseal
portal circulation, which directly
connect the two regions.

> This allows hormones synthesised
by the hypothalamic neurones to be
transported rapidly and directly
to the anterior pituitary, avoiding dilution
or destruction in the systemic circulation.

> These hormones diffuse into capillaries
of the primary plexus
(a capillary network located at the
base of the hypothalamus)

and are carried by hypophyseal portal veins

(which run on the outside of the infundibulum)

into the secondary capillary
plexus of the anterior pituitary gland.

Question 5

Posterior pituitary:

Answer 5

Posterior pituitary:

> It contains axons and axon
terminals of over 10 000 neurosecretory cells
whose cell bodies lie within the hypothalamus.

> These hypothalamic neurosecretory cells
produce two hormones;

oxytocin and antidiuretic hormone,
ADH.

> Oxytocin and ADH are packed
into vesicles and transported to the axon
terminals in the posterior pituitary.

> Nerve impulses propagated along 
the axon trigger exocytosis of these
vesicles and the released oxytocin 
and ADH then diffuse into 
the nearby capillaries.

Question 6

What hormones are released by

the hypothalamus?

Answer 6

Hypothalamic hormones Effects on anterior pituitary hormones

Growth hormone-releasing hormone
(GHRH)
Stimulates release of growth hormone (GH)

Thyrotropin-releasing hormone (TRH)
Stimulates release of thyroid-stimulating hormone (TSH)

Gonadotropin-releasing hormone
(GnRH)

Stimulates release of follicle-stimulating hormone (FSH)
and luteinising hormone (LH)

Corticotropin-releasing hormone (CRH)
Stimulates release of adenocorticotropic hormone
(ACTH) and melanocyte-stimulating hormone (MSH)

Prolactin-releasing hormone (PRH)
Stimulates release of prolactin

Question 7

What are the effects of the anterior pituitary hormones?

Answer 7

Table 45.2

The effects of anterior pituitary hormones

Anterior pituitary hormone

Effect

GH

Stimulates liver to synthesise and release insulin like
growth factors (IGFs).
These stimulate protein anabolism,
lipolysis,
tissue repair,
cell growth and elevate plasma glucose levels

TSH

Stimulates thyroid gland to release
thyroxine (T4) and
triiodothyronine (T3).

FSH

In females stimulates production of oocytes and
secretion of ovarian oestrogen.

In males stimulates sperm production.

LH

In females, 
stimulates ovulation, 
corpus luteum
formation, 
secretion of ovarian oestrogen and secretion
of corpus luteum progesterone.

In males stimulates secretion
of testicular testosterone.

ACTH
Stimulates release of
glucocorticoids (mainly cortisol)
from the adrenal cortex.

MSH
Stimulates darkening of skin.

Prolactin
Stimulates milk production.

Question 8

Describe the renal portal circulation.

Answer 8

The kidney is interesting because it
contains two portal circulations.

An afferent arteriole enters the
Bowman’s capsule and divides into multiple
capillaries to form the glomerulus

(this is the primary capillary bed).

The efferent venule leaving the glomerulus
enters two distinct secondary capillary beds:

a capillary bed surrounding
the cortical tubular system

and a capillary bed surrounding the
loop of Henle (the vasa recta).

The function of these portal circulations is to maximise the reabsorption of water and electrolytes
filtered at the glomerulus.