Blood, Circulatory Vessels, Kidneys and Heart Function

Blood and the Circulatory System

Blood is a viscous, salty liquid that runs inside the ducts of the circulatory apparatus. Every human being has approximately 5.5 L of blood. This fluid has multiple roles:

• It carries nutrients and oxygen to all our cells.
• It collects waste products formed in cellular metabolism.
• It participates in the body's defense against infections.
• It carries hormones, molecules important in controlling the functions of the organism.
• It is involved in temperature regulation, as body heat is distributed similarly to a heating system, from warmer to cooler regions.

The yellow component, blood plasma, is mostly water in which several types of molecules are dissolved: proteins, minerals, nutrients and waste substances.

Blood Vessels and Pathways

Arteries carry blood away from the heart. They branch gradually into vessels of smaller diameter that penetrate the organs and branch further into finer vessels called arterioles.

Capillaries are microscopic blood vessels that branch from arterioles and permeate the organs. The wall of each capillary is formed by a single layer of flat cells, which allows easy and rapid exchange of substances between the blood circulating through them and the tissue cells they irrigate.

Venules and Veins: Capillaries come together to form venules; venules combine into veins. Veins have thicker lumens and carry blood back to the heart. Inside some veins there are valves (sometimes described figuratively as "swallow's nests") that allow the advancement of blood toward the heart but prevent backflow.

Urinary System and Kidneys

The urinary system removes wastes and maintains fluid and electrolyte balance.

Kidneys

The kidneys are bean-shaped organs located at the rear of the abdomen on both sides of the spine. Each kidney is composed of an outer renal cortex, an inner medulla, and a cavity known as the renal pelvis. Each kidney is made up of over a million nephrons, which are the filtration units responsible for filtering the blood and forming urine. Nephrons drain into ducts or tubes called collecting ducts (manifolds).

Urinary Tract

The urinary tract is the outflow pathway for urine. Ureters are two tubes, each one connecting a kidney to the bladder. The bladder is an elastic bag that holds urine before it is expelled to the outside.

Urethra is the external tube that conducts urine out of the body. At its initial part there is a sphincter that remains closed until the micturition reflex triggers urination.

Heart Movements and Cardiac Cycle

• Atrial systole: The atria contract and blood is pumped into the ventricles through the atrioventricular valves, which open to allow flow.
• Ventricular systole: The ventricles contract and the blood contained in them is ejected into the pulmonary artery and the aorta, opening the semilunar valves. Simultaneously, the atrioventricular valves close to prevent backflow into the atria.
• Diastole: The heart relaxes and blood from the venae cavae and pulmonary veins enters the atria. Blood that has been pumped into the arteries cannot return to the ventricles because the semilunar valves close.

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