Chapter 31: Human circulatory system

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The human circulatory system demonstrates the organisational complexity of the human. It is made up of a number of different tissues organised into organs such as the heart, arteries, arterioles, capillaries, venules and veins.

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Open versus closed circulatory systems:

  • Open Circulatory System: blood is pumped from a simple heart and flows out of blood vessels and around tissue cells of the organism, e.g. invertebrates, such as insects. The blood is then returned to the heart via small pores called ostia.
  • Closed Circulatory System: blood flows around the body enclosed in blood vessels and does not leaves the blood vessels, e.g. human.

Structure of the human circulatory system

The human circulatory system consists of:

  • Blood vessels
  • Heart
  • Blood

Blood vessels:

  • Arteries: blood vessels that carry blood away from the heart in pulses. It has a thick wall and small lumen.

The thick wall of arteries contains a tough outer layer of collagen that gives strength to the artery that supports the pressure the blood is under from the heart.
It also contains a layer of smooth (involuntary) muscle that contracts pushing blood along. The internal layer of the artery is composed of a layer of cells called the endothelium.

  • Veins: blood vessels that carry blood towards the heart in an even flow. They have thin walls, a large lumen and valves.

Blood pressure in veins is much lower than arteries, hence the thinner wall. They also have smooth muscle to push blood along in one direction and have valves to prevent back flow of blood.

  • Capillaries: blood vessels with walls one cell thick that carries blood from arterioles to venules through tissues, releasing nutrients and taking away wastes.

Systemic and pulmonary circuits The human circulatory system consists of two blood circuits: the systemic circuit and the pulmonary circuit. This is why the human circulatory system is described as a double circulatory system.
The systemic circuit carries blood to all the major organs of the body, except the lungs.
The lungs have their own blood circuit, called the pulmonary circuit.
The diagram below shows all the arteries and veins emanating from, and returning to, the heart and internal organs.

Portal system

  • A portal system is a network of blood capillaries that connect two organs or tissues, e.g. hepatic portal system connects the small intestines to the liver via the hepatic portal vein.

Structure of the heart

Aorta: largest artery in the body carrying oxygenated blood away from the left side of the heart to all the major organs of the body (except the lungs).

Pulmonary artery: carries deoxygenated blood away from the right side of the heart to the lungs to excrete carbon dioxide and absorb more oxygen.

Pulmonary vein: carries oxygenated blood towards the right hand side of the heart from the lungs.

Left atrium: upper left chamber of the heart that receives blood from the lungs and contracts pumping blood into the left ventricle.

Bicuspid valve: allows one-way flow of blood from the left atrium into the left ventricle – prevents back-flow of blood.

Chordae tendineae: connective tissue holding the heart valves in position.

Left ventricle: strongest of the four heart chambers and pumps blood into the aorta.

Septum: divides the heart into two separate pumps.

Right ventricle: Pumps deoxygenated blood to the lungs via the pulmonary artery.

Papillary muscle: contracts preventing the heart valves prolapsing backwards.

Inferior vena cava: carries deoxygenated blood from the lower half of the body back to the heart.

Tricuspid valve: allows one-way flow of blood from the right atrium to the right ventricle – preventing back-flow of blood.

Right atrium: upper right chamber of the heart that receives blood from the vena cavae and contracts pumping blood into the right ventricle.

Superior vena cava: carries deoxygenated blood from the upper half of the body back to the heart.

Semilunar valves: allow one-way flow of blood out of the heart – prevent back-flow of blood into the heart.

The cardiac cycle
The cardiac cycle is controlled by the pacemaker – which is located in the top of the right atrium. The pacemaker is a type of nervous tissue.

Atrial contraction:

  • Pacemaker sends an electrical signal to the cardiac muscle of the atria.
  • The atria contract
  • Blood moves into the ventricles through the heart valves

Ventricular contraction:

  • The electrical signal that came from the pacemaker then travels onto the cardiac muscle of the ventricles.
  • The ventricles contract.
  • Blood is pushed out of the heart via the semilunar valves

The cardiac cycle (detailed)
The cardiac cycle is controlled by two pacemakers in the heart:

  • The sino-atrial node (SA node) located in the top wall of the right atrium
  • The atrio-ventricular node (AV node) located in between the right atrium and ventricle.

Both pacemakers are types of nervous tissue.

Atrial systole and ventricular diastole:

  • Atria receive blood from the major veins (vena cavae and pulmonary veins).
  • The SA node sends an electrical signal to the cardiac muscle of the atria.
  • The atria contract (systole).
  • The bicuspid and tricuspid valves open.
  • Blood flows into the ventricles, which are not contracting (diastole).

Ventricular systole and atrial diastole:

  • Electrical signal reaches the AV node, which relays the signal onto the cardiac muscle of the ventricles.
  • The ventricles contract (systole).
  • The bicuspid and tricuspid valves close and the semilunar valves open.
  • Blood flows out of the heart via the aorta and pulmonary artery.

Pulse and blood pressure

  • Pulse: expansion of an artery as blood passes through.
  • Blood pressure: force blood exerts on the walls of blood vessels.

Blood pressure is measured as two values: systolic pressure and diastolic pressure.
Normal blood pressure is 120/80 mmHg, where mmHg stands for millimetres of mercury.
Blood pressure is measured using a sphygmomanometer.

Effects of smoking, diet and exercise on the circulatory system
Smoking:

  • Cigarettes contain many addictive chemicals, of which the most important is nicotine.
  • Nicotine raises heart rate and blood pressure putting a strain on the circulatory system.

Diet:

  • A diet high in saturated fats increases blood pressure and the risk of atherosclerosis (or hardening of the arteries).
  • Salt in the diet also raises blood pressure by increasing thirst and water intake.

Exercise:

  • Exercise stimulates a temporary increase in heart rate and blood pressure.
  • It strengthens the heart and promotes healthy blood vessels.

Practical activity: to dissect, display and identify an ox’s or a sheep’s heart.

  • Wash the heart (to get rid of any clotted blood).
  • Place the heart on a dissection tray.
  • Distinguish between the left and right sides of the heart (left side firmer than the right).
  • Distinguish between the front and back sides of the heart (back side is flatter).
  • Identify the coronary artery on the front of the heart.
  • Identify the four major blood vessels at the top of the heart.
  • Examine the flaps of tissue on top of the heart (auricles).
  • Sketch a labelled diagram of the external structure of the heart.
  • Turn the heart so that it is lying with its front surface facing up.
  • Make two incisions either side if the coronary artery into the atria and ventricles.
  • Open up the incisions and identify the four chambers of the heart, tricuspid and bicuspid valves, papillary muscles and chordae tendineae.
  • Insert a probe down through the pulmonary artery (should come through into the right ventricle).
  • Leaving the probe in place, cut down through the pulmonary artery, identifying the semilunar valve.
  • Insert another probe down through the aorta (should come through into the left ventricle).
  • Leaving the probe in place, cut through the wall of the aorta, identifying the other semilunar valve.
  • Label as many internal structures as you can find using pins and labels.

Practical activity: to investigate the effect of exercise on pulse rate.

  • Pulse rate is the number of pulses (or heart beats) per minute.
  • Measure your pulse by placing your second and third fingers on your wrist or on your neck.
  • Measure your pulse three times while at rest.
  • Calculate your average pulse rate.
  • Walk slowly for 5 minutes and measure your pulse rate again.
  • Walk briskly for 5 minutes and measure your pulse rate once again.
  • Finally, run for 5 minutes and measure your new pulse rate.
  • You can also see the effect of vigorous exercise on pulse rate by running as fast as you can for 3 – 5 minutes.

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