Understanding Human Circulation and Excretory Systems


They take the blood from the heart and carry it towards other organs. They have thick elastic walls with smooth muscle tissue, to resist the high pressure of the heart pumping and control its diameter and capacity. As they branch into thinner vessels, they’re called arterioles.

Blood Plasma

Water with minerals, proteins, and a lot of other dissolved substances (nutrients, wastes, hormones…). About 55% of blood.

  • Erythrocytes (red blood cells): shaped as biconcave disks without a nucleus, they act as bags to contain hemoglobin, a protein that binds oxygen (or carbon dioxide) in order to carry it all over. 5 million / μl.
  • Thrombocytes (platelets): fragments of cells involved in clotting (coagulation) and wound healing. 300,000 / μl.
  • Leukocytes (white blood cells): whole cells with nucleus, they have defensive functions as part of the immune system, and there’s a wide variety of them (lymphocytes, monocytes, neutrophils…). 7,000 / μl.


After the semilunar valves in the arteries close, the whole heart relaxes and sucks blood coming from the veins.

  • Atrial systole: the atriums contract and blood is pushed into the ventricles through the atrioventricular valves.
  • Ventricular systole: after the atrioventricular valves close, the ventricles contract, propelling the blood into the arteries to leave the heart.

Systemic Circuit

Long journey over the whole body to redistribute substances; it leaves from the left part of the heart and comes back to its right side.

Pulmonary Circuit

Small trip of the blood to the lungs and back, just to recharge oxygen thanks to gas exchange in the alveoli (from the right to the left side of the heart).

Venous System

Veins carry blood from elsewhere back to the heart. They have thinner walls with valves which prevent the blood from going backwards or staying away when the pressure gets lower. They are formed when smaller vessels called venules come together.


Microscopic blood vessels, with only one layer of cells (endothelium) as their walls, since their function is precisely to allow substances to pass through, exchanging nutrients and wastes with the interstitial fluids on the tissues. They’re between arterioles and venules.



Excretory System

The main organs in charge of the excretory function are:

  • Lungs: they remove the carbon dioxide from the blood thanks to gas exchange in the alveoli.
  • Liver: it cleans the blood and eliminates the residues of hemoglobin and toxic substances (such as alcohol and medication), releasing the wastes into the duodenum together with the bile.
  • Sweat glands: found all over the skin, they produce sweat (of similar composition to urine, but more diluted), which, besides removing excess minerals and wastes, also cools the surface when evaporating, contributing to temperature regulation.
  • Urinary system: the most important excretory system; let’s study it further.

Urine is a yellowish liquid composed of water, mineral salts, and other wastes, particularly urea, and some uric acid. The process of urine formation mainly has two stages:

  • Glomerular filtration: passage of blood content from the glomerulus into the Bowman’s capsule, including a lot of water and both wastes and useful substances (vitamins, glucose…).
  • Tubular reabsorption: throughout the tubule, the useful substances previously filtered into the nephron are returned to the blood of surrounding capillaries, to avoid its loss; most water is also reabsorbed in the loop of Henle, to concentrate urine and avoid dehydration.

Lymphatic System

Part of the circulatory system, but with some distinctive features: it has a unidirectional flow, starting gathering fluid from the tissues, and discharging into the blood, without a pump of its own. The components of the lymphatic system are:

  • Lymph: the circulating fluid, in this case, is mainly composed of plasma and lymphocytes (so, almost like blood, but without the red blood cells nor the platelets).
  • Lymphatic capillaries: closed small vessels that are found all over the tissues, being very permeable so that the excess of interstitial fluid can penetrate and be gathered. A special case is the one of the lacteals we studied in the digestive system, which absorb the lipidic part of chyle nutrients.
  • Lymphatic veins: result of capillaries coming together into wider vessels, they look like a chain of beads, and contain valves to prevent backflow. All lymphatic veins end up in the thoracic duct or in the right lymphatic duct, large collecting vessels which pour the lymph into the blood in the subclavian veins.
  • Lymphatic organs: they generate lymphocytes and trigger immune responses. The main ones are bone marrow, thymus, spleen and lymph nodes. Lymph nodes are prominences found in lymph vessels in specific parts of the body (neck, armpit, groin…), where lymphocytes mature and accumulate, fulfilling other immune functions.

Human (blood) circulation has several characteristics:

  • It’s closed, since the blood never leaves the vessels (unlike some other animals).
  • It’s double, since blood needs to travel twice through the heart to complete a cycle.
  • It’s complete, since oxygenated and deoxygenated blood coming from different circuits don’t mix when they pass through the heart (it’s completely separated).

Hence, we have two different circuits for each of the times blood goes through distinct parts of the heart:

  • Systemic circuit: long journey over the whole body to redistribute substances; it leaves from the left part of the heart and comes back to its right side.
  • Pulmonary circuit: small trip of the blood to the lungs and back, just to recharge oxygen thanks to gas exchange in the alveoli (from the right to the left side of the heart).

The excretory function consists of getting rid of the wastes produced by the metabolism of our own cells, cleaning the body of toxic or excess substances.