Introduction to the cardiovascular system

Question 1

How do single celled organisms get their O2 and nutrients?

Answer 1

Diffusion.

Question 2

Why do we need a cardiovascular system?

Answer 2

The human body has around 10^14 cells.
Most cells are far away from source of O2 and nutrients, and diffusion is too slow to reach all the cells in the body (diffusion works well over short distances only).
Larger organisms need a gas exchange and circulatory system
A system is required to carry O2 and nutrients to cells and carry waste products away.
A mechanism for transporting substances close to the cells is essential to allow diffusion to take place.

Question 3

What, briefly, does the cardiovascular system consist of?

Answer 3

Pump – the heart
Distribution system – vessels and blood
Exchange mechanism - capillaries
Flow control – arterioles and pre-capillary
sphincters
Capacitance- veins

Question 4

How are O2 and CO2 transported in the body?

Answer 4

Blood transports O2, metabolic substrates, CO2 and waste products around the body.
Exchange of these substances between the
blood and cells of the body occurs by diffusion.
The cardiovascular system provides the
correct conditions for diffusion to take place
at the tissues and lungs.

Question 5

What are capillaries?

Answer 5

Capillaries are composed of a single layer
of endothelial cells surrounded by basal
lamina.
Capillaries provide the site for diffusion of gases, both at tissues and at the alveoli where gas exchange takes place.

Question 6

Describe diffusion at the capillaries.

Answer 6

Some molecules e.g. O2 and CO2 can diffuse directly through the lipid bilayer.
Others molecules such as glucose, amino acids and lactate are hydrophilic and diffuse through small aqueous pores in the capillaries between the endothelial cells.
All molecules will move down their concentration gradient.

Question 7

What factors affect diffusion?

Answer 7

Rate of diffusion depends on:
– area (larger area for diffusion is faster than a narrow area)
– diffusion ‘resistance’ (nature of the molecule, nature of the barrier, path length)
– concentration gradient

Question 8

How is diffusion optimised in the body with regards to area?

Answer 8

Area for exchange between capillaries and
tissues is generally very large.
It depends on capillary density- a tissue which is more metabolically active will have more capillaries per unit volume.

Question 9

How is diffusion optimised in the body with regards to resistance to diffusion?

Answer 9

Resistance to diffusion depends on
– nature of the molecule- e.g. lipophilic or hydrophilic, size
– nature of the barrier e.g. pore size and number of pores for hydrophilic substances
– path length- depends on capillary density. Path is shortest in the most active tissues.
Diffusion resistance is mostly low, and is not the rate limiting factor for diffusion at capillaries.

Question 10

Why is concentration gradient essential to gas exchange at capillaries?

Answer 10

The greater the concentration gradient the greater the rate of diffusion.
The concentration which matters is between capillary blood and tissues.
For exchange to continue, the concentration gradient between the capillary blood and tissues must be maintained.

Question 11

Describe the concentration gradient in the cardiovascular system.

Answer 11

A substance which is used by the tissues will have a lower concentration in capillary blood than arterial blood
How much lower depends on
– rate of use by the tissue
– rate of blood flow through the capillary bed.
The concentration gradient can be maintained better if blood flow is increased.

Question 12

How is the concentration gradient maintained in the cardiovascular system?

Answer 12

At any rate of use, the lower the blood flow, the lower the capillary concentration.
The rate of blood flow must be high enough to
maintain a sufficient concentration gradient for
diffusion.
i.e. rate of blood flow determines the
concentration gradient driving O2 diffusion into
the cells.

Question 13

What is perfusion rate?

Answer 13

The rate of blood flow to tissues.

Question 14

Why is the control of blood flow to tissues important?

Answer 14

Supply and demand:
Blood flow must match the tissues’ metabolic needs.
The higher the rate of metabolism the greater
the demand for O2 and nutrients.
Increases in metabolism must be met by increases in blood flow (increased perfusion rate).

Question 15

Describe the blood flow to different tissues.

Answer 15

Brain needs high, constant flow
– 0.5 ml.min-1.g-1
Heart muscle needs high flow which increases
during exercise
– 0.9 to 3.6 ml.min-1.g-1
Kidneys need high, constant flow
– 3.5 ml.min-1.g-1
Blood flow to skeletal muscle can be very high
during exercise and gut blood flow is high after a meal.

Question 16

What are the minimum and maximum flow rates to different tissues in an ‘average’ 70kg man?

Answer 16

brain 0.75-0.75l.min-1
heart 0.3-1.2l.min-1
kidneys 1.2-1.2l.min-1
gut 1.4- 2.4l.min-1
muscle 1.0- 16.0l.min-1
skin 0.2- 2.5l.min-1
other 0.2- 0.2l.min-1
TOTAL 5.0- 24.5l.min-1

Question 17

What is blood flow measured in?

Answer 17

Litres per minute (can also be ml per gram per minute)

Question 18

What is the ‘specification’ for the CVS?

Answer 18

The cardiovascular system must supply between 5 and 25 l.min-1 of blood to the tissues whilst at all times maintaining perfusion to vital organs such as the brain, heart and kidneys.

Question 19

How is blood flow regulated, and why is regulation important?

Answer 19

•If a pump is just connected to a network of
vessels blood will only flow to the parts that are easiest to perfuse:
The brain is harder to perfuse due to gravity.
•To regulate blood flow you need to add
resistance to the system:
Reduce the ease with which some regions are
perfused in order to direct blood flow to the more difficult to perfuse regions.
• Arterioles are the resistance vessels.

Question 20

Very briefly, how does the heart work?

Answer 20

Heart is two pumps in series:
• The left heart pumps blood around the systemic circulation (runs in parallel)
• The right heart pumps blood around the
pulmonary circulation

Question 21

Give an overview of the flow of blood in the CVS.

Answer 21

• Heart pumps blood to arteries
• Arteries supply arterioles
• Arterioles supply capillaries
• Capillaries drain into venules
• Venules drain into veins
• Veins return blood to the heart

Question 22

How is blood flow able to be altered in the CVS?

Answer 22

• The total flow in the system has to be able
to change. This requires a temporary store of blood which can be returned to the heart at a different rate.
• Veins have thin walls which can easily
distend or collapse enabling them to act as
a variable reservoir for blood.
• The capacitance of the veins provides the
temporary store

Question 23

Where is the highest blood pressure in the CVS?

Answer 23

Left ventricle and arteries.

Question 24

Where is the biggest drop in blood pressure in the CVS?

Answer 24

In the resistance vessels (arterioles).

Question 25

Where is the lowest blood pressure in the CVS?

Answer 25

In the capacitance vessels (veins) and the right atrium.

Question 26

Describe the distribution of blood in the CVS.

Answer 26

Arteries and arterioles- 11%
Capillaries- 5%
Heart and lungs- 17%
Veins- 67%