Circulatory system and blood vessels

Living things must be capable of transporting nutrients, wastes and gases to and from cells.

Humans’ main transport system is called the Circulatory System

  1. Works with the Respiratory system
    • Exchanges CO2 and O2.
  2. Works with the Digestive system
    • Carries nutrients and wastes to/ from the cells.
  3. Works with the Excretory system
    • Carries waste to kidney for removal

Multicellular vs Unicellular organisms

Organisms do not always require transport systems, and most have much simpler ones than us – so why the complexity in the mammalian transport system?

  1. Multicellular – do not have most of their cells in contact with the external environment
  2. Size – High oxygen and nutrient requirements because mammals are:
  • large (m0re cells + increased distance from the nutrients and the cells requiring them)
  • have a high metabolic rate
  • have a high level of activity (need to generate heat as well)

3. Waste – Produce a relatively large amount of waste that has to be removed

>> Thus developed complex circulatory systems with pump to transport nutrients, oxygen, carbon dioxide and metabolic wastes

*Why can’t humans have the same transport system as the amoeba?

Amoeba are single-celled organisms. They use their cell surface as a point of exchange with the outside environment.

As substances can easily diffuse in and out of the whole amoeba, it has no need for a transport system.

Humans are multicellular organisms. so most of our cells are not in contact with the external environment and are far away from the organs supplying/producing required substances (e.g. oxygen).

Due to our size and higher activity level, we also need a greater proportion of these substances compared to the amoeba.

>> Diffusion would be too slow in supplying cells with these substances.

Therefore we need a complex transport system.

Open and closed circulatory systems


  • Found in molluscs and arthropods (insects)
  • Pumps blood into a hemocoel (A cavity or series of spaces between the organs through which the blood circulates) with the blood diffusing back to the circulatory system between cells. Tissues are surrounded by the blood.
  • The resulting blood flow is sluggish


  • Found in echinoderms (sea stars, sea urchins etc.) and vertebrates
  • Blood is closed at all times within vessels of different size and wall thickness. Blood is pumped by a heart through vessels, and does not normally fill body cavities.
  • Blood flow is not sluggish.
  • Haemoglobin molecules in vertebrate blood cells transport oxygen. It also causes blood to turn red in the presence of oxygen.

Single vs Double circulatory system

Double circulatory system has 2 routes/circuits:

Heart—lung—heart (pulmonary circulation)
Heart—body—heart (systemic circulation)

>> Mammals, birds, reptiles and amphibians have double circulation.


  • Four-chambered heart. Some reptiles have partial separation of the ventricles.
  • Disadvantage: Less efficient absorption of substances from mixed blood.


  • Amphibians have a three-chambered heart with two atria and one ventricle. Blood exiting the ventricle is diverted, some to the pulmonary circuit, some to systemic circuit.
  • Disadvantage: Oxygenated and deoxygenated blood are mixed when exiting the ventricle
  • >> Less efficient absorption of substances from oxygenated blood.

Mammals and birds

  • Four-chambered heart with complete separation of ventricles (by the septum)
  • Advantageous due to maximised efficiency of substance absorption
  • >> allows for higher activity level and larger body size


  • have single circulatory system as they do not have lungs (with the exception of lungfish).
  • Heart—gills—body—heart (systemic circulation)

Pathway of blood through the body


Pulmonary circulation

Part of system: heart—lungs—heart


  1. Deoxygenated blood enters the right atrium through the superior and inferior vena cava (collectively known as vena cavae).
  2. The blood enters the right ventricle through the tricuspid valve.
  3. The blood is pumped out of the heart through the pulmonary valve and pulmonary arterytowards the lungs.


  1. The artery branches into smaller arteries, arterioles and finally collections of capillaries known as capillary beds. These beds are wrapped around the alveoli (air sacs) in the lungs.
  2. In the capillary beds, carbon dioxide diffuses out of the blood and oxygen diffuses in.
  3. The oxygen-rich blood then enters the venules, smaller veins and finally pulmonary veins.


  1. The blood re-enters the heart through the left atrium and passes through the mitral valve into the left ventricle.
  2. The left ventricle contracts, forcing the blood into the aortic valve and aorta.

Systemic circulation

Part of system: heart—rest of body—heart

  1. The aorta branches out into
  • carotoid artery (to brain)
  • hepatic artery (to liver)
  • mesenteric artery (to small intestine)
  • renal artery (to kidneys)
  1. These arteries branch out into smaller arteries, arterioles and capillary beds. Oxygen and nutrients diffuse out of the capillaries while waste products diffuse in.
  2. The deoxygenated blood flows into a progressively larger series of venules that join to form veins.
  • jugular vein (from brain)
  • hepatic vein (from liver)
  • renal vein (from kidneys)

3. The veins transport blood back to the heart through the vena cavae. The cycle repeats itself.

Phases – renal circulation

  • Blood passes through the kidneys.
  • During this phase, the kidneys filter much of the waste from the blood.

Portal circulation

  • Blood passes through the small intestine.
  • During this phase, the blood from the small intestine collects in the hepatic portal vein which passes through the liver. The liver filters sugars from the blood, storing them for later.

Coronary circulation

Part of system: throughout the heart tissues

  1. Oxygenated blood flows into the coronary arteries through the aorta during diastole.
  2. Oxygen and nutrients diffuse into the myocardium (heart muscle) while carbon dioxide and waste products diffuse out.
  3. The deoxygenated blood flows through the coronary veins into the right atrium.
  4. Read more here

Blood vessels


  • Function: Carry blood away from the heart
  • Structure: 3 layers of thick walls, no valves
  1. Outer covering of connective tissue, collagen and elastic fibers
  2. Middle layer of smooth muscle
  3. Inner layer of elastic membrane
  • Adaptations:
  1. Thicker walls + tough outer layer to prevent arteries from bursting under high pressure.
  2. Smooth inner layer to reduce friction
  3. Elastic inner layer evens out blood pressure. When pressure is high they expand, so pressure decreases, and when pressure is lower they recoil, causing an increase in blood pressure.
  4. Inner layer and muscle layer allow the artery to expand and recoil, propelling blood through it. Arteries further from the heart have a thicker muscle layer as blood pressure is decreased.

Read more

Important arteries:


  • Small arteries that connect larger arteries with capillaries
  • Small arterioles branch into capillary beds


  • The largest artery in the body
  • Located at the top of the heart muscle. It contains three openings or branches that take blood through the body.

Pulmonary artery

  • The only artery that transports deoxygenated blood.
  • Located below the aorta.

Coronary arteries

  • They supply the heart cells with blood and nourishment.
  • Lie over the walls of the heart.


  • Structure:
    • Walls are only one cell thick.
    • No valves.
    • Nerve-controlled sphincters control blood flow.

    How the sphincters work

How substances diffuse in and out of capillary

  • Function: Connects arterioles and venules. Allows for exchange of materials between blood and body cells.
  • Adaptations:
  1. Concentrated into capillary beds to reach all cells of the body.

    Capillary bed

  2. Very thin wall to allow quick diffusion of substances in and out of bloodstream.
  3. Some capillaries have small pores between the cells of the capillary wall, allowing materials to flow in and out of capillaries as well as the passage of white blood cells.
  4. Narrow lumen forces red blood cells to travel in single file – Maximises amount of substances diffusing in and out of bloodstream.


  • Structure: 3 layers of walls, has valves
  1. Outer covering of connective tissue, collagen and elastic fibers
  2. Middle layer of smooth muscle
  3. Inner layer of elastic membrane
  • How it works:
  1. Pressure in veins is low, so they depend on nearby muscle contractions to propel blood along.
  2. Veins pass between skeletal muscles.
  3. The contraction of skeletal muscle squeezes the vein, thus increasing blood pressure in that section of the vein.
  4. Pressure causes the upstream valve (furthest from the heart) to close and the downstream valve (the one closest to the heart) to open.
  5. Repeated cycles of contraction and relaxation, as occurs in the leg muscles while walking, effectively pumps blood back to the heart.
  • Function: Transports blood from tissues back to the heart
  • Adaptations:
  1. Wide lumen increases volume of blood flow, as high pressure is not required.
  2. Endothelium (inner layer) reduces friction of flowing blood.
  3. Valves prevent backflow and mixing of blood.

Important veins:


  • smaller veins that gather blood from capillary beds into veins

Vena cavae

  • Superior: Discharges blood from head and arms
  • Inferior: Discharges blood from the trunk and legs
  • Both are located on the right side of heart

Pulmonary vein

  • Carries blood to the heart from lungs
  • Located just beneath the pulmonary artery on left side of heart.

Comparison between veins and arteries





Direction of blood flow Transports blood away from heart Transports blood towards the heart
Quality of blood Carries oxygenated blood

(except pulmonary artery)

Carries deoxygenated blood

(except pulmonary vein)

Structure/size of lumen Have relatively narrow lumen Have relatively wide lumen
Wall/tissue Have relatively more muscle/elastic tissue Have relatively less muscle/elastic tissue
Blood pressure Transports blood under higher pressure (than vein) Transports blood under lower pressure (than artery)
Presence of valves No valves

(except semi-lunar valve of pulmonary artery + aortic valve)

Has valves (throughout main veins of body)

The mammalian heart

Diagram of human heart

The four-chambered heart, along with the pulmonary and systemic circuits, completely separates oxygenated from deoxygenated blood. This allows for the higher metabolic rates needed by warm-blooded birds and mammals.

Parts not shown in diagrams:

  • Septum – separates the left and right ventricles
  • Coronary arteries >> read more here
  1. Branch out of aorta and wrap around the heart
  2. Transports oxygen and nutrients to atria (pl. of atrium), ventricles and septum

Diagram of heart with directions of blood flow

3D model of heart

Anatomy of a sheep’s heart


Parts of the heart and functions

Heart’s functions and how it works


How the heart pumps blood – cardiac cycle

Animation of how heart pumps blood >> from this website

The cardiac cycle consists of two parts:

  • systole (contraction of the heart muscle)
  • diastole(relaxation of the heart muscle).
  • Atria contract while ventricles relax.
  1. Deoxygenated blood flows from the body into the right atrium, through the superior and inferior vena cava.
  2. Blood flows from right atrium into right ventricle. (Atrium: systole, Ventricle: diastole) Valve: Tricuspid valve.
  3. Blood is pumped out of right ventricle to the lungs, through the pulmonary artery. (Ventricle: systole, Atrium: diastole) Valve: pulmonary (semi-lunar valve)
  4. Oxygenated blood flows from the lungs into the left atrium through pulmonary veins.
  5. Oxygenated blood flows into left ventricle. Valve: Mitral valve
  6. Oxygenated blood is pumped out of left ventricle to all parts of body through aorta. Valve: Aortic valve.
  7. Oxygenated blood is also transported to the atrium, ventricle and septum through coronary arteries.

Heart muscle contraction is due to the presence of nodal tissue in two regions of the heart.

  • The SA node (sinoatrial node) initiates heartbeat.
  • The AV node (atrioventricular node) causes ventricles to contract. The AV node is sometimes called the pacemaker since it keeps heartbeat regular.
  • Heartbeat is also controlled by nerve messages originating from the autonomic nervous system.



– under construction –