Human Blood, Heart, Circulation and Excretory Systems

Composition of Blood

Blood is made up of cells suspended in a liquid called blood plasma. Blood plasma is a yellowish substance made up of water containing various dissolved molecules: minerals, nutrients, waste products, proteins and hormones.

Three types of blood cell:

• Red blood cells: These are the most common type of blood cell. They contain hemoglobin and have no nucleus.
• White blood cells: There are different types: neutrophils, eosinophils, basophils, lymphocytes and monocytes; these cells help to protect the body from infections.
• Platelets: These are not true cells; they contain substances that allow the blood to coagulate, which prevents hemorrhages.

Functions of Blood

• It transports nutrients and oxygen to cells.
• It collects waste products.
• It transports hormones.
• It helps to regulate temperature.
• It protects the body from infections.
• It helps to prevent blood loss when a blood vessel is broken.

The Heart

In order for the blood to move around the circulatory system, it needs to be pumped by an organ. Cells depend on the heart to receive a constant supply of nutrients and oxygen. The heart is an organ made of muscle tissue. It is divided into two halves, right and left, by an internal wall. The two halves are not connected in any way; each half is divided again into a small upper chamber, the atrium, and a lower chamber, the ventricle.

Between the atrium and the ventricle there is the atrioventricular valve. Several veins lead to the atria: two venae cavae (right) and four pulmonary veins (left). Two arteries lead away from the ventricles: the pulmonary artery (right) and the aorta (left).

Pulmonary and Systemic Circulation

Pulmonary circulation: The blood starts in the right ventricle, where the pulmonary artery leads away from the heart and splits into two arteries which each lead to a lung.

Systemic circulation: The blood starts in the left ventricle and flows along the aorta, which splits into arteries, arterioles and then into capillaries.

Functions of the Lymphatic System

• It maintains the fluid balance between the interstitial fluid and the blood plasma.
• It helps protect the body from infection.
• It collects the products of fat digestion in the small intestine through lymphatic vessels called lacteals.

Respiratory System

The respiratory system removes carbon dioxide from the blood and expels it from the body.

Sweat Glands and Skin

Sweat glands are found in the skin and they produce sweat which evaporates to cool the skin.

Liver and Hemoglobin Breakdown

The liver eliminates products resulting from the breakdown of hemoglobin from old red blood cells.

Urinary System

The urinary system eliminates waste products in the form of urine.

Kidneys and Nephrons

Two organs are at the back of the abdomen. Each kidney is made up of over a million nephrons, small tubes with one open end surrounded by a capillary. Nephrons filter the blood to produce urine.

Ureters

Ureters are two narrow tubes connecting the renal pelvis of the kidney to the bladder.

Urethra

The urethra is the duct that leads to the outside of the body. At the beginning of the urethra there is a sphincter which stays closed.