Topic 14: Circulatory System

Question 1

Describe the characteristics of human circulatory system and its component

Answer 1

Human circulatory system is a closed system, physically separated from the rest of the body.
It consists of vessels, pump (heart), and blood.
This is a double circulatory system because the heart is divided completely into right and left sides.

Question 2

Describe the flow of blood and list each chamber and structure that it passes through the circulatory system

Answer 2

Blood flows unidirectionally.
The left and right sides of the heart is divided by septum.
Oxygenated blood is pumped from left atrium, down to left ventricle and into aort, the largest artery, into the systematic circulation.
Blood then flows through arteriole, small branch of artery, leading to capillary bed for exchanging nutrients and gases.
In some organs like kidneys where there is high exchange occuring, there is portal connecting capillary beds together.
Blood that is deoxygenated now goes through venule, small branch of vein, then via vena cava (superior or inferior) to back into the right atrium, then right ventricle.
The blood is now pumped through pulmonary artery to go into the lung, receives oxygen, and pumped back into the heart through pulmonary vein. This is the pulmonary circulation.

Question 3

List and describe the functions of different layers in blood vessels

Answer 3

• Endothelium: present in all vessels
• Elastic tissue: allows vessels to contract and recoil
• Smooth muscle: allows for contraction and relaxation
• Fibrous tissue: maintain structure

Elastic tissue and smooth muscles modify the structure of blood vessels to allow blood flow

Question 4

Identify and describe differences between different types of blood vessels

Answer 4

• Artery: responsible for pumping blood into systematic circulation
  + lots of elastic tissue and smooth muscle, allowing vessels to contract and recoil with the pressure from blood flow
• Arteriole: a branch of artery, resistance vessel, helps regulating how much blood flows into or out of the underlying tissues
  + no elastic tissue and fibrous tissue
• Capillary: exchange site (nutrients and gases)
  + only endothelium
• Venule: a branch of vein
  + no elastic tissue and smooth muscle
• Vein: brings blood back to the heart
  + thinner elastic tissue and smooth muscle compared to artery because of lower pressure

Question 5

Describe the composition of blood

Answer 5

Blood is connective tissue made up of cellular elements, suspended in fluid matrix (plasma).
Serves as the buffer between the cells and external environment.
92% water
7% proteins
1% dissolved organic material

Question 6

Describe how RBC are made

Answer 6

RBC is made through a process named Erythropoiesis. This process is controlled by glycoprotein erythropoietin (EPO) and some cytokines, produced by kidneys.
Hypoxia triggers EPO release in a negative feedback loop. EPO helps erythrocytes to be synthesised in bone marrow, which are then released into blood stream.

Question 7

Describe the components of the heart and structure

Answer 7

The heart is workhouse of the body, generating pressure to force blood flow continuously around the body. The heart is composed mainly of cardiac muscles.
It has atrium (upper compartment) and ventricle (lower compartment).
There are 4 valves which help maintaining the unidirectional flow of the blood.
- tricuspid valve (right): divides right atrium and ventricle, making sure blood is not flowing backward into the vena cava
- bicuscpid valve (left): divides left atrium and ventricle, making sure blood is not flowing backward into the left atrium
- 2 semilunar valves: at the start of pulmonary vein and start of aorta

Question 8

Describe how the heart contracts

Answer 8

Electrical stimulus, generated by autorhythmic cells on their own, triggers action potential which then spread through conducting pathways.
- During ventricular diastole, both sets of chamber relax.
- Then, atrial systole occurs in which sinoatrial node generates action potential which spread down the two atriums and pumping blood into these chambers.
- Then, atrial systole stops, ventricular systole starts when atrioventricular node generates action potential which spreads down the bundles of His, to Purkinje fibres and contracts ventricles from bottom upwards

Question 9

Why does the atrioventricular node generate action potential slower than the sinoatrial node?

Answer 9

Because the contraction of atrium needs to be completed before the contraction of ventricle starts, making sure blood is flowing in the right direction.

Question 10

Explain stroke volume, heart rate, cardiac output and their relationship

Answer 10

• Stroke volume is the volume of blood ejected after each contraction.
• Heart rate is directly affected by stroke volume.
• Cardiac output measures the heart performance and is directly affected by stroke volume.

CO = SV * HR

Question 11

How is heart rate altered? Which system is responsible for this and how?

Answer 11

Heart rate is altered by autonomic NS by changes to the depolarization of autorhythmic cells (sympathetic and parasympathetic).

Question 12

Describe the pressure in arterial and venous systems

Answer 12

Pressure in arterial system is higher than venous system.
Blood flows from high to low pressure.

Question 13

Why does the blood pressure decrease when going away from the heart?

Answer 13

Due to friction against the blood vessels, pressure will go down along the way.